I make my telescope mirrors usually by hand using usually two equal size thick window glass like 3/8" or 1/2"and 6 or 8" diam, I lay 1 disk on a wet cloth to keep it from slipping. some grinding grit powder and water,on it, place the other disk on the first (tool) and start grinding with only circular stroking , 35 to 40% of the top (mirror-blanc) over-hanging the tool, all around, holding it with both hands circling but not twirling or spinning, that causes the edge of the tool to grind all around and the center of the mirror grinds out, keep grit and water between and in a few hours the whole mirror face will be ground dull to the edge, and you have likely a deep enough sagitta-dip in the mirror, next use some finer (or recycled grit), 20% overhang and keep stroking around. when the face becomes somewhat smooth rince with water and while dripping wet it can reflect sunlight to a focal point, measure the distance focal length if it is about what you planned, do some more finer and finer grinding until ready for polishing with a "pitch-lap" another time.
Peter-circular-stroke-mirror-on-top
Thursday, January 21, 2016
Tuesday, November 10, 2015
ATM MIRROR MAKING
ATM
MIRROR MAKING
Effective manual Telescope-mirror making
We start by choosing our first mirror to be hand made, a 6”
diameter and 36” focal-length, minimum 3/8” thick glass, (1/16th
diam.) for the mirror, and ¼” thick
glass-grinding tool. B Cut mirror piece
6.5”square from .3/8” plate-glass with a
“straight-edge” and “glass-cutter”, (picture1)removing exess glass, next with
the “Circle cutter”.and sanding all sharp edges off , clean up the waste.
C Lay the“tool disk” on a wet “Chamois” or towel on the smooth working surface
to avoid slipping, sprinkle some 80grit Sylicone Carbide and water on the
glass, place the mirror disk face
down on the tool, off-setting 2”(1/3 diam),move
the mirror-disk, holding it with both hands on 3 points with the Circular-stroke (picture 2) motion over/around the tool, keeping
continual 2” overhang without spinning/turning the mirror. The scratching noice-motion and wide
overhang causes the grinding down of the
mirror center. When noice stops, add more grit and little water, continue grinding
for 2 hrs. Rinche off the mirror and while still dripping wet,let the sun
reflect from it to measure the focalpoint distance from the mirror. If sun is
not available we can measure the F.L. with a home made 4”diam “Sphero-Meter” (pict.3)and
calculate the sagitta-depth : Divide 1
by desired F.L as 1/36=0.0277 or reversed
to find the F.L.:1/0.019= 52.6” F.L. the Sph.Mtr. has 3 small ballpoint-pen leggs
siliconed in ½” thick Corian countertop material (which can be cut and drilled
with wood-working tools)
When we reach the planned
sagitta dept and the edge of the mirror , it is time to change the overhang to
1.”(1/5 diam.)and use #120 (or recycled) grit to smooth the surface and get a
good Sphere by doing the same Circular stroking with little or no extra weight,
after 20 minutes check for surface pits, they need to be all evenly small, if
so, use finer grit #220 and again #320, then change to 25 micron Allum. Oxide,
all done with same 1” overhang. And Circular Stroking, when # 12, #9, #5 are
used,we likely will have a good sphere and pre polish surface, clean the mirror,
than dry glance on low angle over surface to horizon, if the reflected upside
down picture is clear, the surface is good for sphere testing: place a pencil
circle-mark around the rim and across the center, than with about 10 light dry
strokes remove the pencil and the Sphere is also good.
Next is the decicion how to proceed, one of
3 different ways:
THE COMMON-WAY (according to
the books) is:Pitch-lap polishing with minimum overhang the Sphere, And Foucalt
testing at Radius of Curvature. Using Rounche screen showing straigth lines and
Nulling, after which Parabola-Correction
with pich-lap is done and testing with Cauder-screen for zone-nulling and other
test
SECOND WAY is To start
Parabola Correction during Polishing
by using the polishing Pitch-lap using 40%
overhang circular stroking dareby producing
the Parabola until the whole mirror is
polished and likely corrected all at once, thereby saving the seperate polishing
time and Spherical testing, but doing
Parabola testing is commonly done
A THIRTH WAY is to keep
on fine grinding with 5 Micron All.Ox, thinned down, ligth weight, 40% overhang
Circular Stroking causing fast Correction, when penciled-edge is reached, change to pitchlap and 40% light-weight
overhang until complete polished and
corrected ready for testing
I have done both ways with exellent
result My 6-7-8
mirror (6”F7 8 hrs.) was all
done by hand in 8 hours time the 3th way,
grinding trough parabola-polishing-
corection
We
have telescope mirror-making class most Saturday mornings, if interesting to learn call
Peter
De Baan 760 745 0925 peterdebaan@gmail.com in Escondido Ca.
Wednesday, August 5, 2015
PARABOLA
The Parabola was
named 300 B.C by Appalonia 600 years later 300 AC
Menachmus described the
dimentions X^2=Y AND Y^2= 2X. and
PaPaS (290-350) \
Qualfies
parabola-focus (on vertical line) & “Directrix” a (horizontal line below), the distance from focus to parabola is allways equal to the
distance from parabola vertical to the Directrix line. The focal point is vertical above the parabola center. If you roll a parabola along a straight line, its focus traces out a catenary according to :
Wells, D. The
Penguin Dictionary of Curious and Interesting Geometry
The shape
of a Catenary (hanging cable/chain) is based
only on gravety and distance it is not
The shape of the Parabola which is based upon the reflection angle
of paralel-light to produce a Focal point. The distance from the Parabola focal point to the
Parabola face is always equal to the"Directrix" line (a line below
the Parabola 90 degree from center-line) on every point on the Parabola,
therefore the vertex is closer to focus
than the rim, (shorter FL) but since we cannot add
material to the mirror, we move the focal point minutely closer during
Parabolizing-correction. While the mirror-rim stays un-corrected, the amount of
glass that has to be removed during correction at the vertex is between 1 and 2
ten-thousants inch (0.00015”) but since parabolizing is done with polishing, it
takes as much time as the rough-grinding. Short FL mirrors need more center-correction
than long FL.mirrors, but fortunally our mirrors are usually near flat, like F5
a section around the vertex. And we start usually by grinding a spherical concave which is close to a parabola See below.
The parabola rim and the sphere rim
are the same. While the centers have different F.L.
Long Foc.Length mirrors (F12 and
longer) do not need to be corrected. Are near spherical
PARABOLA
TESTING
The first testing
for parabolic shape is done on the Foucalt tester by calculating zones
(rings)of the mirror surface called Cauder zones of equal surface area, the
Focal length of each (different) zone is computer calculated with Figure X
program, the testing correctness is done on the Foucalt tester, thereby using
not the focal length but the Radius of Curvature distance of the sphere,which
is twice the Focal-length, this makes the mirror-zones Null-test distances having
much larger numbers as they are in reallity (each seprate zone needs to (NULL) on
the R.O.C ).when correct, even as the whole Sphere nulls when correct. While
the parabola whole mirror only Nulls at Focal-Length. The Parabola and the
Sphere can both be star-tested and Auto Collimated with Null and Rounche screen
for straight lines tested
OTHER TEST AVAILABLE
For mirror-surfaces and quality are Star-test,
Auto-Collimation,Interferometry
Parabolizing is done by
hand and also by machine, by Casting, Slumping or Flexing the mirror
On a 10” F5 mirror the center to rim zone diff. on ROC is
0.114 while the real FL diff.=0.00078”
PARABOLA-CORRECTION DIFFERENT WAYS
It
is posible (and has been done)to parabola-correct a mirror by hand and
spin-table, during pre-polish at the end of fine grinding when ready for
polishing, Mark the mirror-edge again with a pencil-line around. Continue fine
grinding with 5 or 9 micron, the Mirror on Top using the Circular Stroke with
40 % overhang and light weight (the Parabola-Correction-Stroke)continually
until reaching the pencil-line, than change from fine grinding to full-size
pitch-lap-polishing with the same P.C. stroke until complete polished and
parabolized
Alternate way is: at the
end of fine grinding the sphere, start polishing with P.C.stroke and create the
parabola during the polishing
Thursday, February 26, 2015
“HOW TO MAKE YOUR OWN TELESCOPE MIRROR BY HAND”
A To
make a telescope mirror from a flat disk (of glass), the flat surface has to be
changed to a spherical / parabolic surface, because a flat surface can only
reflect light in the same opposing angle as it comes to the surface, but for a enlarged
view to be seen, the light has to come to a small focal point, at our eye or
camera lens, therefore the surface is changed by grinding and polishing to the
disired shape to produce that “focal-point”. By first rough grinding to an
aproximal shape as calculated, then by finer grinding to the proper spherical
shape, which will have a Focal point at the Radius of Curvature, (the center of
a ball .) when light is produced at that focal point it will be reflected from
the whole mirror surface back to that same point (if properly polished) If the
R.O.C. Point is more than 15 times the diameter of the mirror, the mirror may
be considered finished and does not need to be parabolized, only Aluminum or
Silver coated for better reflection, because at that distance the Focal point
from near-by and the focal point from “infinety” are nearley the same.
To make the focal
point closer to the mirror, like 12 times (or less) the mirror diameter. than
the focal point of the outer zones (“rings”) of the mirror will not be
coinciding with the mirror center. The remedy is to change the mirror-center
section zones to a shorter reflective distance and tapering-angeling the
difference to the outer edge, which will not be “corrected”, untill all zones
will again have one focal point, this is Parabolizing, measuring is done with the zonal
Cauder-screen measuring on the Foucald
testing, and ultimately with a “star test”, or “auto-collimation” becauce the
differences in surface depth/angle we can only measure with light-waves.
The measuring on the
Foucald tester is done at the Radius of Curvature
of the spherical mirror shape, which is
twice the distance of the parabolic focal point
and will show a “doughnut” shape when viewed face-on, showing that it is
not a sphere any more, the real shape is a smooth parabolic shaped surface. which can be tested and seen
with a “double-reflection” Auto Collumation. (which is a indoor star-test)
B Making your telescope mirror-disk by hand.
Start by deciding what size your telescope will be
and what material to use for your mirror.
You can purchace the glass disk or “trepan” it yoursellve.
If you have a drill-press you can quite easely “trepan” two disks, 6” (min.) x
3/8” from plain glass with a 6” diamont edged tool. Or you can make the trepan
cutter yourself: cut with a sabersaw or wood cutting router from ¾” plywood a
circular disk of the mirror size, mount around the edge a 2” wide strip of
galvanized iron, cut 4 to 6 diagonal reverced “teeth” in the cutting edge and mount a
center “driver” floor-flange for mounting on the drill, lay the glass sheet in
a water bath-tray, (block all edges from moving) start the slow turning, add grinding
carbon dioxide powder on the cutting line and some cooling water and/or
glycerine, keep an eye on the cutting process.
For the material choice, plain window glass
(soda-lime glass) and “Pyrex” are the main choices and both can produce high
quality mirror. The size of the mirror-telescope and focal length is a personal
choice, for reason where and how it is to be used, and how the user is able to
handle and use it.
Advice for the first
time telescope-mirror maker: make your mirror no larger than 6” (152mm)diameter
and focal length of 36” (914mm F6,) or 48” (1219mm F8)
minimal 3/8” thick of
soda lime glass for a Newtonian reflector telescope.
One Way to calculate the grinding-depth of the concave-sagitta-dip
is (measuring with a 4” home-made dial
indicater-sphero-meter) to divide #
1. with the desired focal length of the telescope mirror (1/36 =
0.0278” or .70 mm) or (when sunlight is
available you can dip the ground disk in water and the whet surface can reflect
the sunlight and show the focal length (the sun size reflection focal point is
about ½” diam)
MIRROR MAKING with circular strokes
When ready to make a
concave surface on the mirror you will need to have the mirror-glass-blanc and glass-grinding
tool-disk and grinding grit, place the “tool” on a wet cotton rag (to prevent
slipping” on a level work surface, sprinkle Carbo grit and little water on and grind-hogging with the mirror on top, making circular strokes with (1/3 min.
of mirror overhanging, during hogging) this produces a concave spherical sagitta
surface. (a few lb extra center weigth on the mirror causes less fatique, and
can be used troughout the process, it
simplifies even the parabola-correction)
Use the circular stroke with M.O.T.
(mirror on top) for all grinding and polishing and even parabolizing, only changing the overhang
and the center-weight on the mirror for the different functions, here is a film
showing circular stroke polishing
The Circular Polishing hand-stroke with max 20 % over edge
When grinding sounds
become muffled it is time to re-plenish the grit and water, after a few times
replenishing, clear off the waste and see the mirror center wear, grind again
until you have reached the calculated sagitta (#1/planned foc. Length, as
1/35=0.028) using a 4” spherometer, This home-made 4” diam. Indicater
spherometer on 3 ballpoint pen leggs works on all size mirrors and
focal-lengths) or to measure the foc.lenght divide 1/reading as1/0.020=50).
When Sagitta is reached you can start fine grinding with the samecircular
stroke, with only +/- 20% overhang, finer grinding grit and no extra weight,
polishing is done also with this stroke using a pitchlap for the polishing
When “Hogging ” 12 inch or larger mirrors use
a sub-diameter tool, lay the
mirror face up, on a larger ¾” octagonal or
round sealed plywood board, (with a pivot center hole), To hold the mirror in place: (with 3
screwed-on clips) and lay the board on a level work place on a short pivot and
a anti-slip wet rag mat. Sprinkle some 80 grit Carborundem (Silicone Carbide) powder
and little water on the mirror surface and start grinding with a tool (a cast
iron pipecap, SMALLER THAN THE MIRROR RADIUS) doing strokes all over the
surface onto, but not over the edge
Change from M.O.T.
to T.O.T. only to lengten the the Focal Length.of the mirror.
(bevel the sharp mirror edge before the changing to avoid edge
spalding) then work the full size tool
with the same circular stroke with 30%
overhang thereby changing the F.L. (this change need to be done to including the mirror center to avoid making 2 different
spheres) and avoiding T.D.E. Test for
good sphere with the pencil or
Sharpie- ink-test. While grinding with a
full size tool. Use a pencil, circle around the mirror and a cross over
center, a few light weight small circular strokes with #5 Al Ox should wipe the
pencilm mark from the mirror, showing perfect contact and sphere.
1A TOOL MAKING
When you used a sub size tool for hogging. You need
to make a full size fine grinding tool,
.Lay the mirror face up on a level surface (the tool is cast on the mirror face,
protected by thin plastic sheeting),Grease the top side of the plastic sheet,
and place a “wall” around the plastic covered mirror about 1” higher than the
mirror, place 1” glass or tile pieces on the grease, (to keep the tiles from moving),
mix “Dental-stone” or “Hydro-stone” (from
Tile-supply) cement powder with water to a thick liquid without lumps,
pour carefull into the tool-form ,minimum ½ to ¾” thick,without stirring or shaking, lay a pre-cut
backing board (with 3 screws penetrating trough the board for better bonding) in
the soft mix.
Clean all cement mixing equipment and wait until tool is
hardened (allow 15 min,) remove the mould ring, clean the tool edge, rinse the
tool, pour-sprinkle some #120 Silicone-carbide-grit (or recycled grid) and
water on the tool. Start grinding with maximum
20% overhang to keep the sagitta constant (no need to wait for complete drying of the tool )
THE “FINE” GRINDING:
Use a full-size tool, small weight, M.O.T., using # 120 grit (or recycled grit) , 15 to 20%
maximum diameter overhang, creating a sphere with circular
strokes for 15 or more minutes, clear the mirror, check the surface for extra
large pits with a magnifier, if none are found, check with a 4” sphero meter
the eveness of the whole surface, restart with # 220 grit / 320 going trough
the Carbo-grit sizes, then change to ALuminum Oxide micron-sizes from 25 to 5
micron, (apply 5 micron only when only magnified pits are vizable), once and when more
liquid is needed use only water ( see note below)
and possible little tooth-paste) little pressure on mirror, now just moving the
mirror with 40% overhang to avoid fine
scratches and produce a very fine pre-polish, check sagitta depth, and
spherical surface with pencil test, which
is important before beginning polishing, next look over the horizontal
mirror surface, if it reflects the distance power-pole/flag-pole or other view,
it has a good pre-polish.
Very
carefull clean all working parts and-working
area before and during pitchlap handling, to avoid time-consuming scretches in
the polished surface
NOTE: I find no need for spherical polishing before beginning parabola
correction, when pencil-test have shown a good sphere and the parabola center
will be only 0.0002” extra deep, therefore prepolishing with the Parabola
correction stroke after spherical testing, as well as polishing with 40%
overhang all to the mirror edge has produced parabola’s with much less efford,
this circular stroke avoid the over and under corrected zones, from center to
edge.
Making the Pitch-lap
Lay the mirror face up on level surface, place the vinyl
mould face up on the mirror, spray little dish-soap water on for mould release,
support mould corners, so that the
mould rest corect on the mirror, melt the pitch slowly to MAX 400 F, (with
a thermostate controlled heater) in old coffee-can, do not let the pitch
boil, and do not add any softener, (a hard pitch-lap does not change shape
and works longer) stir until no lumps are left, pour onto the mould over the
mirror area only, place the precut backer-board onto the hot pitch. (I
make the backer little smaller than the mirror) usually Corian type countertop
material for pitch-lap backing),
When pitch is “set” (hardened), place mirror with mould and
pitch in clean-cold water, when cooled remove the pitch-lap careful from mould,
clean edges from exess pitch (use hammer and chisel carefully) test lap
contact, if improvement is needed, place the mirror face up, cover with a
“screen” type (fibre-glass), spray little detergent water on screen for
release, warm the lap face with a heath lamp, when face is sligthly soft, place
face-down on the screen-covered mirror, ADD a weight ( 5 or more Lb) on it (hand
pressing is uneven) for few minutes, water cool lap, start polishing with the
fine shaped good matching lap
2 PITCH-LAP POLISHING:
For making a Spherical mirror, place the pitch-lap on the
turn-pin, poor some liquid Cerium Oxide on the lap and start with mirror on top polishing the mirror, after
20 or 30 minutes polishing with the circular
stroke using max. 15 - 20% mirror-diameter overhang and little weight, for
the Cerium works with chemical reaction, not pressure. Clean
up the mirror and see the first polish,
with sunshine you can measure your
mirror’s focal-length (with
artificial light you can measure the Radius
of Curvature, which is dubble the focal length) and see whether the whole mirror has polish, keep polishing
for a few hours, than see with a laser at 45 degree angle, (always slide the
mirror off thepitchlap, not lifting, and see how the polished surface is coming. good reflection
on the polished surface should be nearly none. (5%). If the mirror gets
“sticking” instead of sliding add more Cerium-Oxide If that does not
solve the problem, place the pitch-lap a few minutes under a heat-lamp until surface
is slightly softened and repress with the F.G. screen.When mirror surface is near
non reflective, test the Radius Of Curvature with the Foucalt knife edge for a
“null” blanc-out (indicating a sphere) and test with the Ronci screen to see
the spherical-mirror-surface quality, (when Ronci lines are straight from edge
to edge you have a good sphere as shown with the “null”,a Turn Down Edge (lines
curving near the edge)causes much longer parabola labor and can usualy be corrected
by polishing longer with 15 - 20% overhang.
ABOUT Turn.Down or Up Edge. IMPORANCE
The turn down edge
name is somewhat of a misnomer, it is the name of the uncompleted strip
on the mirror edge, caused by insufficient/incorrect grinding and/or polishing,
complete to the mirror edge.
One T.D.E. is caused by changing M.O.T. to T.O.T., even for short time.
That change causes a reverse in the
grinding as well as polishing.
Do change that way only to make the focal-length
longer, and for that the whole surface needs to change, to the mirror center,
the effect begins at the mirror edge and produces first “two sphere-portions”. In
reality the outside begins changing towards convexity in stead of
concavete. By Foucalt testing the area
involved can, even with a nice polish, null all focal-reflection as with convex
surface.
3 Parabola-correction
It is important to have a good smooth
surface spherical mirror with
no T.D.Edge when starting
the parabola correction. Use the same
full size hard micro-faced (made with the Fiberglass screen) repressed
pitch-lap (for we do want to
change the mirror shape, not the lap), done again with M.O.T. CIRCULAR HAND STROKES, overhanging
mirror by 30 to 40% avereage 35%, little extra center weight (the
mirror-center going over the lap edge must wear down mostly, 0.0002”) and
Cerium Oxide. After 75 slow circular strokes test with Foucalt to see if there
is any surface correction change. (With
circular strokes the first correction to see is a faint donut hole depression at
mirror center, the next to see is with the Rounce screen slightly centered curved
lines instead of straight.) the adjacent zones will follow all to the rim with
the continual use of the circular stroke/weight /overhang. I mark the back of
the mirror with a sharpy marker,avoiding tilting over the edge. Test often, for you do not want to OVER
CORRECT at all. When all zones are corrected, end the parabola strokes and
improve the surface with some very light-short overhang polishing strokes to
have a higher RMC surface avereage. (Note: the 1/3 to 35% overhang at hogging
will produce a sphere. the 40% avereage overhang during Parabola correction
will change the sphere to a parabola.)
3 A. PARABOLIZING MIRRORS ON THE Mirror.O.Matic-TURN-TABLE
Mirrors larger than 12…..14” to be parabola corrected on Mirror-O-Matic
machine with sub sizes Tool On Top, slow
5 rpm turntable, stroke arm 20….50 p/m
start with a small 1/3 size lap going edge to edge but not over, for 5 to 10 min, test mirror when first of
doughnut shows, continue with same lap, simular strokes not over edge and
little or no weight, for 10 minutes, keep checking every 60 to 100 strokes. On the
foucalt with zone screen from Cauder and
Figure XP program numbers
3 B. Making a
mirror on a powered SPIN-TABLE
THE SPIN-TABLE IS THE MACHINE-MODE OF THE
CIRCULAR STROKE
The spin-table machine turning 29
to 30 rpm, is more accurate than the circular hand-stroke. Mirrors to 12” diam
are best done with mirror on top. On full size tools (The mirror is “spinning
freely while dragging on the powered base ) acting like the circular stroke,
hogging with 35% overhang, fine grinding
and polishing with max 15 - 20% overhang, avoiding Fl change. Parabola
correction with 35% overhang, test often
to avoid hole or hill at center (caused
by the spinning accuratie)
(NOTE: TO AVOID THIS LET THE”PIN” MAKE SMALL SIDEWAY STROKES possible
with the MOM, or do some corection on
the spinning table by hand without the Pin
measure regular for sagitta
depth, when that point is reached, change to finer grit, following with the
same changes as done by hand-fine grinding.
All work for mirror making can be done with
this spinning from hogging to parabolizing.
3 C Larger mirrors are made on
the Spin-table with Tool on top. this is
working upside down : REVERSED IN
EFFECT, AND NEEDS A PRESSURE ON THE TOOL EDGE WHERE THE MIRROR-CENTER “HOGGING”
OR PARABOLIZING CORRECTION TAKES PLACE
For working larger mirrors use sub-dia-meter
tools on top, (minimum 70% mirror-size), during hogging, and fine grinding and polishing, when ready
for polishing use a fine pressed hard lap troughout, little or no pin-weight, maximum
20% overhang, continual supplying Cerium-water, regular testing until ready for
Parabola corrction Change to minimum 70% pitchlap, 40% “overhang”(= 40%
clear mirror) , little or no pinweight, BUT
EXTRA WEIGHT ON TOOL EDGE, ( I MOUNTED A ROLLER-SKATE WHEEL ON THE”PIN”SIDE-ARM
ABOVE THE LAP-EDGE CARRYING A FEW LB LOAD), makes smooth spinning (LESS than ½
table rpm) and continual cerium-water dripping, regular testing the zones,
until edge zone nulls, test
often paying attention to “hole” in center of mirror, if that happens, remove
the large lap and pin. Use a 1/3
pitch-lap, center over center, edge to
edge strokes, by hand on the turning mirror until the hole is gone.
TESTING THE PARABOLOID
MIRROR
THE
FOUCALT TESTER
The first mirror test for a good sphere is with a
“null” test and straight Ronche screen lines with knive adge viewing or with
Camera on TV screen.
For Parabola testing use also the Foucalt-tester now
with a full mirror-size “Cauder-screen” (calculated for Figure XP
program) on the mirror face with cut-outs to show the parabolic zones of the
mirror, each zone having a different focal-length for correct “nulling”, all
zones are of equal area calculated
MAKING THE CAUDER
SCREEN: Cut a thin cardboard disk the size of the mirror, Calculate the total mirror surface area, (r*r*3.1415)
divide area by the number of zones you planned for, calculate the center-zone
area and radius (zone 1), zone 2 radius will be calculated from twice
the center-area, etc. Draw the different
zone radius on the screen-board and
cut some (25 deg. sect) in the left and
right side of each zone area. using the zone
edge information for Figure XP program .
When all zones are on the Foucalt knive edge corrected,”NULLED”
to mach the programmed zones on the Radius of Curvature. from the Figure XP program shows a good parabola, we can do the Rounce
test which should now show the curved parabola lines. (Note: without Rounce screen a parabola will
show a “doughnut” but any doughnut is not a parabola but shows we do not have a
sphere anymore.)
Next do a Star-test. or (when an optical flat larger than the mirror size is available a test by
AUTO COLLIMATION.
We set up another stand on the test bench for the Auto-collimation test, which is done at the
mirror Focal length which is ½ the ROC
and uses double reflection, producing a artificial star test. We use on this
stand a coated“Optical-flat” mirror of same or larger size as the mirror with a
central LED light, wich shines at the
mirror and a CAMERA lookjng at the mirror, the two mirrors are facing each
other and needs to be alinged by adjusting the mirror stand XYZ axes first, so
that the single reflection from the mirror-center points at the camera beyond
the flat, next we allign both mirrors so that the (fainter) complete mirror is
projected trough the camera, by minutely changing the focal length we find the
exact focal lengt by the NULL and the
quality of the parabola. (note the Focal.Length is distance from test mirror to
camera or knive-edge. not from mirror to
opt.flat) Other test are also available for for mirrors
larger than the Opt.Flat mirror, the Ross Null test, which uses a lense instead
of the optical flat mirror.
Next step is to get the mirror Aluminized or
Silvered.
take
equaly more time for shorter mirrors and larger mirrors.
The
final quality of the mirror is found after Parabolizing, not after spherical
polishing, so end up to do some very lightweight polishing, with less overhang
for a fine surface without changing your parabobic shape.
Only
smoother with a higher RMC surface avereage
A PINHOLE LIGHT
for artificial star-testing a mirror
20 micron diameter
is the recommended size
1 remove the print-cartrige cover plate from the
front that exposes a 20m hole or
2 Use a
thin copper plate on a wood surface,
punch a dimple in the copper plate reverce the plate and sand the dimple
carefully until a 20m appears.
Test for ligth rings at 1 meter (40” )
distance star-test at 20 meter (60 ft) min
How I
came to use the circular hand-stroke
In
2008 when I was starting to fine grind a 8” mirror with M.O.T. after I hogged
it with a sub diameter tool and 80 carbo grit, now using 120 grit and stroking
as all the books tell jou: Back and fort, back and fort and going around the
barrel, and after a short while I was thinking while grinding: why this dumb
back and fort and becarefull to do it from all sides to produce a sphere, this
is like I use to do as a carpender with a hand saw or a hand plane, why not do
what I do as a modern woodworker who uses a circular saw for cutting much better
and quicker and better I will try to
stroke around circularly, using same overhang and weight, so I tried.
And
it worked more easy and surprisingly, made at the same time a very good
spherical surface, so I finished the grinding being less tired and done quicker.
So
I made the pitch-lap on the mirror with the lap-mould and being in the mood of
changing, I changed from making a softened lap, I make the pitch-lap hard,
for I want to change/polish the mirror surface
but not the pitch-lap, so I did and polished with this hard lap,
using the same circular stroke until
the surface was ready for parabolizing.
Parabolizing
is changing the surface from a sphere to a Parabolic shape by deepening the mirror-center by 10,000th of
inch, from the spherical shape so star light from the whole mirror can
focus at the same location.
But
Parabolizing is really not much unlike what is done at the start of the
hogging-grinding, only on a much more delicate scale. I started with the same
circular stroke, mirror on top, using 35 to 40 % mirror overhang and little
extra weight at the center so that the 20% center of the mirror would wear over
the lap-edge and the remainder of the surface wear less until no wear at the
mirror edge. All done with the same hard spherical pitch-lap and circular
stroke.
And
while most ATM people are somewhat reluctand of Parabolizing, it turned out
surprisingly well as planned and much simpler and quicker than before.
Starting correction at the mirror center and
spreading it continualy to the edge, going slower as the correction area became
larger, until the mirror-edge was reached without any zones over or under
corrected. Caused by the continual 40% overhang and the fine-hard lap. I also
found that the amount of time-effort for hogging equals more than the parabola-correction time.
The
Circular Polishing hand-stroke with max 20 % over edge
http://www.youtube.com/watch?v=0Nky7CF4eVI&feature=youtu.be
Tuesday, December 2, 2014
PETER DE BAAN 06/24/14
“HOW TO MAKE YOUR OWN TELESCOPE MIRROR BY
HAND”
A To
make a telescope mirror from a flat disk (of glass), the flat surface has to be
changed to a spherical / parabolic surface, because a flat surface can only
reflect light in the same opposing angle as it comes to the surface, but for a enlarged
view to be seen, the light has to come to a small focal point, at our eye or
camera lens, therefore the surface is changed by grinding and polishing to the desired shape to produce that “focal-point”. By first rough grinding to an approximate shape as calculated, then by finer grinding to the proper spherical
shape, which will have a Focal point at the Radius of Curvature, (the center of
a ball .) when light is produced at that focal point it will be reflected from
the whole mirror surface back to that same point (if properly polished) If the
R.O.C. Point is more than 15 times the diameter of the mirror, the mirror may
be considered finished and does not need to be parabolized, only Aluminum or
Silver coated for better reflection, because at that distance the Focal point
from near-by and the focal point from “infinity” are nearly the same.
To make the focal
point closer to the mirror, like 12 times (or less) the mirror diameter. than
the focal point of the outer zones (“rings”) of the mirror will not be
coinciding with the mirror center. The remedy is to change the mirror-center
section zones to a shorter reflective distance and tapering-angling the
difference to the outer edge, which will not be “corrected”, until all zones will again have one focal point, this is called Parabolizing, measuring is done
with the zonal measuring on the Foucault testing, and ultimately with a “star
test”, or “auto-collocation” because the differences in surface depth/angle we
can only measure with light-waves.
The measuring on the Foucault tester is done at the Radius of Curvature
of the spherical mirror shape, which is
twice the distance of the parabolic focal point
and will show a “doughnut” shape when viewed face-on, showing that it is
not a sphere any more, the real shape is a smooth parabolic shaped surface. which can be tested and seen
with a “double-reflection” Auto Columation. (which is a indoor star-test)
B Making your telescope mirror-disk by hand.
Start by deciding what size your telescope will be
and what material to use for your mirror.
You can purchase the glass disk or “trepan” it yourselves.
If you have a drill-press you can quite easily “trepan” two disks, 6” (min.) x
3/8” from plain glass with a 6” diamond edged tool. Or you can make the trepan
cutter yourself: cut with a saber-saw or wood cutting router from ¾” plywood a
circular disk of the mirror size, mount around the edge a 2” wide strip of
galvanized iron, cut 4 to 6 diagonal reversed “teeth” in the cutting edge and mount a
center “driver” floor-flange for mounting on the drill, lay the glass sheet in
a water bath-tray, (block all edges from moving) start the slow turning, add grinding
carbon dioxide powder on the cutting line and some cooling water and/or glycerin, keep an eye on the cutting process.
For the material choice, plain window glass
(soda-lime glass) and “Pyrex” are the main choices and both can produce high
quality mirror. The size of the mirror-telescope and focal length is a personal
choice, for reason where and how it is to be used, and how the user is able to
handle and use it.
Advice for the first
time telescope-mirror maker: make your mirror no larger than 6” (152 mm ) diameter
and focal length of 36” (914mm F6,) or 48” (1219 mm F8)
minimal 3/8” thick of
soda lime glass for a Newtonian reflector telescope.
One Way to calculate the grinding-depth of the concave-sagitta-dip
is (measuring with a 4” home-made dial
indicator-sphero-meter) to divide #
1. with the desired focal length of the telescope mirror (1/36 =
0.0278” or .70 mm) or (when sunlight is
available you can dip the ground disk in water and the whet surface can reflect
the sunlight and show the focal length (the sun size reflection focal point is
about ½” diam)
MIRROR MAKING with circular strokes
When ready to make a
concave surface on the mirror you will need to have the mirror-blanc and
grinding tools and grinding grit, use also a equal size(10% larger or smaller
works fine) glass tool-disk, and grind-polish with the mirror on top, making circular strokes with (1/3 min.
of mirror overhanging, during hogging) this produces a concave spherical sagitta
surface. (causes less fatigue, and can be used throughout the process, it simplifies even the
parabola-correction)
Use the circular stroke with M.O.T.
(mirror on top) for all grinding and polishing and even parabolizing, only changing the overhang
and the center-weight on the mirror for the different functions, here is a film
showing circular stroke polishing
When grinding sounds
become muffled it is time to replenish the grit and water, after a few times
replenishing, clear off the waste and do it again until you have reached the
calculated sagitta (#1/planned foc. Length, as 1/35=0.028) using a 4”
spherometer, This home-made 4" diameter Indicator spherometer on 3 ballpoint pen legs works on all size mirrors and focal-lengths) or to measure the focal length divide 1/reading as1/0.020=50). When Sagitta is reached you can start fine grinding with the same circular stroke, with only 20% overhang, finer grinding
grit and no extra weight, polishing is done also with this stroke using a
pitch-lap for the polishing
“Hogging ” 12” or
larger mirrors use a sub-diameter
tool, lay the mirror face up, on a larger ¾” octagonal or round sealed plywood board, (with a pivot center hole), to hold
the mirror in place with 3 screwed-on clips and lay the board on a level work
place on a short pivot and a anti-slip mat. Sprinkle some 80 grit Carborundum (Silicone
Carbide) powder and little water on the mirror surface and start grinding with a
tool (a cast iron pipe-cap, SMALLER THAN THE MIRROR RADIUS) doing strokes all
over the surface onto, but not over the edge. (bevel the sharp mirror edge before the
changing to avoid edge spalding) lenthen work the full size tool with the
same circular stroke with 30% overhang thereby changing the F.L. (this change
need to be done including the mirror
center to avoid making 2 different spheres) and avoiding T.D.E. Test for good sphere with the pencil or Sharpie- ink-test. While grinding
with a full size tool. Use a pencil,
circle around the mirror and a cross over center, a few light weight small
circular strokes with #5 Al Ox should wipe the pencil mark from the mirror,
showing perfect contact and sphere.
1A TOOL MAKING
When you used a sub size tool for hogging. You need
to make a full size fine grinding tool. Lay the mirror face up on a level surface (the tool is cast on the mirror face,
protected by thin plastic sheeting),Grease the top side of the plastic sheet,
and place a “wall” around the plastic covered mirror about 1” higher than the
mirror, place 1” glass or tile pieces on the grease, (to keep the tiles from moving),
mix “Dental-stone” or “Hydro-stone” (from
Tile-supply) cement powder with water to a thick liquid without lumps,
pour careful into the tool-form ,minimum ½ to ¾” thick,without stirring or shaking, lay a precut
backing board (with 3 screws penetrating trough the board for better bonding) in
the soft mix.
Clean all cement mixing equipment and wait until tool is
hardened (allow 15 min,) remove the mould ring, clean the tool edge, rinse the
tool, pour-sprinkle some #120 Silicone-carbide-grit (or recycled grid) and
water on the tool. Start grinding with maximum
20% overhang to keep the sagitta constant (no need to wait for
comlpete drying of the tool )
THE “FINE” GRINDING:
Use a full-size tool, small weight, M.O.T., using # 120 grit (or recycled grit) , 15 to 20%
maximum diameter overhang, creating a sphere with circular
strokes for 15 or more minutes, clear the mirror, check the surface for extra
large pits with a magnifier, if none are found, check with a 4” sphero meter restart
with # 220 grit / 320 going trough the Carbo-grit sizes, then change to Aluminum
Oxide micron-sizes from 25 to 5 micron, apply 5 micron only once and when more
liquid is needed use only water (and possible little tooth-paste) no pressure
on mirror, just moving the mirror to avoid fine scratches and produce a very
fine prepolish, check sagitta depth, and spherical surface with pencil or Sharpy pen test, which
is important before beginning polishing, next look over the horizontal
mirror surface, if it reflects the distance power-pole/flag-pole or other view,
it has a good pre-polish.
Very
carefull clean all working parts and-working
area before and during pitch-lap handling, to avoid time-consuming scratches in
the polished surface
Making the Pitch-lap
Lay the mirror face up on level surface, place the vinyl
mould face up on the mirror, spray little dish-soap water on for mould release,
support mould corners, so that the
mould rest complete on the mirror, melt the pitch slowly to MAX 400 F, (with
A thermostat controlled heater) in old coffee-can, do not let the pitch
boil, and do not add any softener, (a hard pitch-lap does not change shape
and works longer) stir until no lumps are left, pour onto the mould over the
mirror area only, place the pre-cut backer-board onto the hot pitch. (I use
usually Corian type counter-top material for pitch-lap backing),
When pitch is “set” (hardened), place mirror with mould and
pitch in clean-cold water, when cooled remove the pitch-lap careful from mould,
clean edges from excess pitch (use hammer and chisel carefully) test lap
contact, if improvement is needed place the mirror face up, cover with a
“screen” type (fiber-glass), spray little detergent water on for release, warm
the lap face with a heath lamp, when face is slightly soft, place face-down on
the screen-covered mirror, ADD a weight on it (hand pressing is uneven) for few
minutes, start polishing with the fine shaped good matching lap
2 PITCH-LAP POLISHING:
Lay the pitch-lap on the turn-pin, poor some liquid Cerium
Oxide on the lap and start with mirror
on top polishing mirror, after 20 or 30 minutes polishing with the circular stroke using max. 15 - 20%
mirror-diameter overhang and little weight, for the Cerium works with chemical
reaction, not pressure. Clean up the mirror and see the first polish, with sunshine you can measure your mirror’s focal-length (with artificial light you
can measure the Radius of Curvature,
which is dubble the focal length) and see whether the whole mirror has polish, keep polishing for a few hours, than see
with a laser at 45 degree angle, how good the polished surface is coming.
Reflection on the polished surface should be nearly none. If the mirror gets
“sticking” instead of sliding add more Cerium-Oxide If that does not
solve the problem, place the pitch-lap a few minutes under a heat-lamp until surface
is slightly softened and repress with the FG screen.When surface is near non
reflective(0.5%) test the Radius Of Curvature with the Foucalt knife edge for a
“null” blanc-out (indicating a sphere) and with the Ronci screen test to see the mirror-surface quality, (when
Ronci lines are straight from edge to edge you have a good sphere as shown with
the “null”,a Turn Down Edge (lines curving near the edge)causes much longer parabola
labor and can be avoided by polishing longer with 15 - 20% overhang.
ABOUT Turn.Down or Up Edge.
The turn down edge
name is somewhat of a misnomer, it is the name of the uncompleted strip
on the mirror edge, caused by insufficient/incorrect grinding and/or polishing,
complete to the mirror edge.
One T.D.E. is caused by changing M.O.T. to T.O.T., even for short time.
That change causes a reverse in the
grinding as well as polishing.
Do change that way only to make the focal-length
longer, and for that the whole surface needs to change, to the mirror center,
the effect begins at the mirror edge and produces first “two sphere-portions”. In
reality the outside begins changing towards convexity in stead of
concavete. By Foucault testing the area
involved can, even with a nice polish, null all focal-reflection as with convex
surface.
3 Parabola-correction with circular stroke
It is important to have a good smooth
surface spherical mirror with
no T.D.Edge when starting
the parabola correction. Use the same
full size hard micro-faced (made with the Fiberglass screen) repressed
pitch-lap (for we do want to
change the mirror shape, not the lap), done again with M.O.T. CIRCULAR HAND STROKES, overhanging
mirror by 30 to 40% avereage 35%, little extra center weight (the
mirror-center going over the lap edge must wear down mostly) and Cerium Oxide.
After 75 slow circular strokes test with Foucault to see if there is any surface
correction change. (With circular
strokes the first correction to see is a faint donut hole depression at mirror
center, the next to see is with the Rounchi screen slightly centered curved
lines instead of straight.) the adjacent zones will follow all to the rim with
the continual use of the circular stroke/weight /overhang. I mark the back of
the mirror with a sharpy marker,avoiding going over the edge Test often, for you do not want to OVER
CORRECT at all. When all zones are corrected, end the parabola strokes and
improve the surface with some very light-short overhang polishing strokes to
have a
higher
RMC surface avereage
(Note: the 1/3 to 35% overhang
at hogging will produce a sphere.
the 35% avereage overhang
during Parabola correction will change the sphere to a parabola.)
3 A. PARABOLIZING MIRRORS ON THE Mirror.O.Matic-TURN-TABLE
Mirrors larger than 12…..14” to be corrected on Mirror-O-Matic machine with sub sizes Tool On Top, slow 5 rpm
turntable, stroke arm 20….50 p/m start
with a small 1/3 size lap going edge to edge but not over, for 5 to 10 min, test mirror when first of
doughnut shows, continue with same lap, simular strokes not over edge and
little or no weight, for 10 minutes, keep checking every 60 to 100 strokes. On the Foucault with zone screen from Cauder and
Figure XP program numbers
3 B. Making a
mirror on a powered SPIN-TABLE
THE SPIN-TABLE IS THE MACHINE-MODE OF THE
CIRCULAR STROKE
The spin-table machine turning 29
to 30 rpm, is more accurate than the circular hand-stroke. Mirrors to 12” diam
are best done with mirror on top. On full size tools (The mirror is “spinning
freely while dragging on the powered base ) acting like the circular stroke,
hogging with 35% overhang, fine grinding
and polishing with max 15 - 20% overhang, avoiding Fl change. Parabola
correction with 35% overhang, test often
to avoid hole at center (caused by the spinning accuratie
(NOTE: TO AVOID THIS LET THE”PIN” MAKE SMALL SIDE-WAY STROKES possible
with the M.O.M, do some circular stroking by hand
without the Pin
All work for mirror making can be
done with this spinning from hogging to parabolizing.
measure regular for sagitta
depth, when that point is reached, change to finer grit, following with the
same changes as done by hand-fine grinding.
3 C Larger mirrors are made on
the Spin-table with Tool on top. this is
working upside down : REVERSED IN
EFFECT, AND NEEDS A PRESSURE ON THE TOOL EDGE WHERE THE MIRROR-CENTER “HOGGING”
OR PARABOLIZING CORRECTION TAKES PLACE
For working larger mirrors use sub-dia-meter
tools on top, (minimum 70% mirror-size), during hogging, and fine grinding and polishing, when ready
for polishing use a fine pressed hard lap troughout, little or no pin-weight, maximum
20% overhang, continual supplying Cerium-water, regular testing until ready for
Parabola correction Change to minimum 70% pitchlap, 40% “overhang”(= 40%
clear mirror) , little or no pin-weight, BUT
EXTRA WEIGHT ON TOOL EDGE, ( I MOUNTED A ROLLER-SKATE WHEEL ON THE”PIN”SIDE-ARM
ABOVE THE LAP-EDGE CARRYING A FEW LB LOAD), makes smooth spinning (LESS than ½
table rpm) and continual cerium-water dripping, regular testing the zones,
until edge zone nulls, test
often paying attention to “hole” in center of mirror, if that happens, remove
the large lap and pin. Use a 1/3
pitch-lap, center over center, edge to
edge strokes, by hand on the turning mirror until the hole is gone.
TESTING THE PARABOLOID
MIRROR
THE FOUCAULT TESTER
The first mirror test for a good sphere is with a
“null” test and straight Rounce screen lines with knife edge viewing or with
Camera on TV screen.
For Parabola testing use also the Foucault-tester now
with a full mirror-size “Cauder-screen” (calculated for Figure XP
program) on the mirror face with cut-outs to show the parabolic zones of the
mirror, each zone having a different focal-length for correct “nulling”, all
zones are of equal area calculated
MAKING THE CAUDER
SCREEN: Cut a thin cardboard disk the size of the mirror, Calculate the total mirror surface area, (r*r*3.1415)
divide area by the number of zones you planned for, calculate the center-zone
area and radius (zone 1), zone 2 radius will be calculated from twice
the center-area, etc. Draw the different
zone radius on the screen-board and
cut some (25 deg. sect) in the left and
right side of each zone area. using the zone
edge information for Figure XP program .
When all zones are on the Foucalt knive edge corrected,”NULLED”
to mach the programmed zones on the Radius of Curvature. from the Figure XP program shows a good parabola, we can do the Rounce
test which should now show the curved parabola lines. (Note: without Rounce screen a parabola will
show a “doughnut” but any doughnut is not a parabola but shows we do not have a
sphere anymore.)
Next do a Star-test. or (when an optical flat larger than the mirror size is available a test by
AUTO COLLIMATION.
We set up another stand on the test bench for the Auto-collimation test, which is done at the
mirror Focal length which is ½ the R.O.C
and uses double reflection, producing a artificial star test. We use on this
stand a coated “Optical-flat” mirror of same or larger size as the mirror with a
central LED light, which shines at the
mirror and a CAMERA looking at the mirror, the two mirrors are facing each
other and needs to be aligned by adjusting the mirror stand XYZ axes first, so
that the single reflection from the mirror-center points at the camera beyond
the flat, next we align both mirrors so that the (fainter) complete mirror is
projected trough the camera, by minutely changing the focal length we find the
exact focal length by the NULL and the
quality of the parabola. (note the Focal.Length is distance from test mirror to
camera or knife-edge. not from mirror to
opt.flat)
Other test are also available.
For mirrors larger than the Opt.Flat mirror, the Ross Null
test, which uses a lense instead of the optical flat mirror.
Next step is to get the mirror Aluminized or
Silvered.
take
equaly more time for shorter mirrors and larger mirrors.
The
final quality of the mirror is found after Parabolizing, not after spherical
polishing, so end up to do some very lightweight polishing, with less overhang
for a fine surface without changing your parabobic shape.
Only
smoother with a higher RMC surface avereage
Sunday, January 26, 2014
CHAPTER 1
peterdebaan@gmail.com
“HOW TO MAKE YOUR OWN TELESCOPE MIRROR”
A To
make a telescope mirror from a flat disk (of glass) the flat surface has to be
changed to a spherical / parabolic surface, for a flat surface can only reflect
light in the same opposing angle as it comes to the surface, but for a enlarged
view to be seen, the light has to come to a small focal point, our eye or camera lens, therefore the surface
is changed by grinding and polishing to the disired shape to produce that
“focal-point”. By first rough grinding to an aproximal shape as calculated,
then by finer grinding to the first proper spherical shape, which will have a
Focal point at the Radius of Curvature, when light is produced at that point it
will be reflected from the whole surface back to that same point (if properly
polished) If the RoC. Point is more than 15 times the diameter of the mirror,
the mirror is considered finished and does not need to be parabolized, only
aluminum coated for better reflection, because at that distance the RoC and the
focal point from “infinety” are nearley the same.
To make the focal
point closer to the mirror, it will have to be closer, like 12 times (or less)
the mirror diameter, then the focal point of the outer zones “rings” of the
mirror will not be coinciding with the RoC. The remedy is to change the center
section zones to a shorter distance and tapering- feathering the difference to
the outer edge untill all zones will again have one focal point, this is called
Parabolizing, the measuring of it is done with the zonal measuring on the
Foucald test, and ultimately with a star test, or “auto-collimation” for the
differences are too small for other forms of measuring.
The measuring on the
Foucald tester is done at the Radius of Curvature
of the spherical mirror shape, which is
twice the distance of the parabolic focal point
and will show a “doughnut” shape when viewed face-on, showing that it is
not a sphere any more, the real shape is a smooth parabolic shaped surface. which can be tested and seen
with a “double-reflection” Auto Collumation. (indoor star-test)
B Making your hand made telescope mirror.
Start by deciding what size your telescope will be
and what material to use for your mirror.
You can purchace the glass disk or “trepan” it yoursellve.
If you have a drillpress you can quite easely “trepan” two a “6” x (min,) 3/8”
disks from plain glass with a 5 or 6” diamont edged tool. Or you can make the
trepan cutter yourself: cut with a sabersaw or wood cutting router from ¾”
plywood a circular disk of the mirror size, mount around the edge a 2” wide
strip of galvanized iron, cut 4 to 6 diagonal reverced “theet” in the cutting
edge and and a center “driver” floor-flange to mount on
the drill, lay the glass sheet in a water bath-tray, (blocking the edges from
moving) start the slow turning, add grinding powder on the cutting line and
some cooling water and or glycerine, keep an eye on the cutting process.
For the material choice, plain window glass
(soda-lime glass) and “Pyrex” are the main choices and both can produce high
quality mirror. The size of the mirror-telescope and focal length is a personal
choice, for reason where and how it is to be used, and how the user is able to
handle and use it.
For the first time telescope-mirror maker, my advice is to
make your mirror no larger than 6” (152mm)diameter and focal length of 36” (914mm
F6,) or 48” (1219mm F8) of soda lime in the Newtonian form.
One Way to calculate the grinding-depth of the
concave-sag-dip is (measuring with a 4” (101.6mm) sphero-meter) to divide # 1.
with the desired focal length of the telescope.
(1/36 = 0.0278” or .70 mm)
MIRROR MAKING with circular
strokes
Make the mirror no thinner than 1/16” (1.6 mm) per inch
diameter (6.25%)
When ready to make a concave surface on the mirror you
will need to have the mirror-blanc + grinding tools and grinding grit, (making
a 6” x 3/8” mirror use also a 6” glass tool-disk mounted (5 smal dabs of melted
pitch on a wood backing) and grind with the mirror on top, making circular strokes with 1/3 mirror overhanging, this produces
a concave spherical sagitta surface.
1 For “hogging” 10” and larger mirrors with a
sub-diameter tool, lay the
mirror face up on a larger ¾” octagonal or round sealed plywood board, (with a
pivot center hole), to hold the mirror in place with 3 screwed-on clips and lay
the board on a level work place on a short pivot and a anti-slip mat. Sprinkle
some 80 grit Carborundem and little water on the mirror surface and start
grinding with the tool (a cast iron pipecap) doing strokes all over the surface
onto, but not over the edge. When grinding sounds become muffled it is time to
replenish the grit and water, after a few times replenishing, clear off the
waste and do it again until you have reached the calculated sagitta (#1/planned
foc. Length, using a 4” spherometer ) . When that is reached you can start fine
grinding with a full size tool.
1A TOOL MAKING
Lay the mirror
face up on a level surface (the tool is
cast onto the mirror face, protected by thin plastic sheeting),Grease the plastic
sheet, and place a “wall” around the plastic covered mirror about 1” higher
than the mirror, place 1” glass or tile pieces on the grease, (to keep the tiles
from moving) mix “dental stone” or “Hydro-stone” cement to a thick liquid
without lumps, pour carefull into the toolform ,minimum ½ to ¾” thick
without stirring or shaking, lay a precut backing board (with 3 screws
penetrating) in the soft mix.
Clean all cement mixing equipment and wait until tool is
hardened (allow 15 min,) remove the mould ring, clean the tool edge, rinse the
tool, pour-sprinkle some #120 carbo-grit (or recycled grid) and water on the
tool and start grinding (no need to wait
for comlpete drying)
We use the circular
stroke with MOT (mirror on
top) for all grinding and polishing and
even parabolizing, only changing the overhang and the center-weight on
the mirror. It causes less fatique, produces from start a good sphere
and simplifies the parabolizing. Mirrors larger than 12” diameter are done
different because of size.
When we need to change
from M.O.T. to T.O.T. to lengten the the
F.L.of the mirror. bevel the mirror edge before the changing to avoid
edge spalding) then work the tool with the same circular stroke with 30% overhang
thereby changing the F.L. and avoiding T.D.E. and Test for sphere with the pencil
or Sharpie test.
THE “FINE” GRINDING:
Begin with small weight, M.O.T., finer 120 grit (or
recycled) , 20% diameter overhang, with circular strokes for 15 or more minutes,
clear the mirror, check the surface for large pits with a magnifier, if none
are found, check with a 4” sphero meter,(3 point base, home made, with a gauge available
at very reasonable cost), restart with finer grit (or recycled) going trough
the grit sizes and Aluminum Oxide micron-sizes until 5 micron, apply 5 micron
only once and when more liquid is needed use only water and no pressure on
mirror, just moving the mirror to avoid fine scratches and produce a very fine
pre-polish, check sagitta depth, and spherical surface with pencil or sharpy
pen test, by making a cross on the
mirror and a circle on the mirror-edge, than with light-weight strokes go a few times around and if the
marks are removed, it indicates a good
sphere to the edge, which is important before beginning polishing, next look
over the horizontal mirror surface, if it reflects the distince powerpole/ flag-pole
or…… view it has a good pre-polish.
2 Making the Pitch-lap
Lay the mirror face up on level surface, place the vinyl
mould face up on mirror, spray little dish-soap water for mould release,
support mould corners, so that the mould rest complete on the mirro, melt the
pitch slowly to (350 to 400 F) in
old coffee-can, do not let the pitch boil, and do not add any softener,
(a hard pitch-lap does not change shape and works longer) stir until no lumps
are left, pour onto the mould over the mirror area only, place the precut
backer-board onto the hot pitch (I use usually Corian countertop material),
When pitch is “set” (hardened), place mirror with mould and
pitch in cold water, when cooled remove the pitch-lap careful from mould, clean
edges from exess pitch (use hammer and chisel)
2b PITCH-LAP POLISHING:
While the lap is still warm, poor some liquid Cerium Oxide
on mirror and start pressing-polishing carefully to make perfect contact, after
20 or 30 minutes polishing with the circular
stroke using 20% mirror-diameter overhang and little weight, clean up the
mirror and see the first polish, with
sunshine you can measure your mirror’s
focal-length (with artificial light
you can measure the Radius of Curvature,
which is dubble the focal length) and see whether the whole mirror has polish,
keep polishing for a few hours, than see with a laser at 45 degree how good the
polished surface is coming. Reflection on the first surface should be nearly none.
If the mirror gets “sticking” instead of sliding add more
Cerium-Oxide.
If that does not solve the problem, place the pitch-lap a
few minutes under a heat-lamp until surface is slightly softened. Place fine
fiberglass netting on the mirror, spray it with some soapy dishwater, lay the
lap with softened face down on the netting, place a 10 lb/ 5 kg weight on the
lap backing, wait 5 minutes and commence with polishing. When surface is non reflective anymore, test
the Radius Of Curvature with the Foucalt knife edge for a “nul” blanc-out
(indicating a sphere) and with the Ronchi screen test to see the mirror-surface
quality, (when Ronchi lines are straight from edge to edge you have a good
sphere as shown with the “null”,a Turn Down Edge causes much longer parabolizing
labor and can be avoided by polishing longer with 20% overhang.
3 Parabolizing/correction
It is important to have a good smooth
surface spherical mirror with
no T.D.Edge when starting
the parabolizing. Use the same hard
micro-faced (made with the Fiberglass screen) repressed pitch-lap (for we do want to change the mirror shape,
not the lap), done again with M.O.T. CIRCULAR HAND STROKES, overhang 40%, some extra center weight
(the mirror-center must wear down mostly) and Cerium Ox. After 50 slow circular
strokes test to see if there is any surface correction change. (With circular strokes the first correction to
see is a faint donut hole depression at mirror center on the foucalt, the next
to see is with the rounchi screen slightly centered curved lines instead of
straight.) the adjacent zones will follow all to the rim with the continual use
of the circular stroke/weight /overhang. I mark the outter edge of the mirror
with a sharpy marker,avoiding going over the edge Test often, for you do not want to OVER
CORRECT at all.
3 A. PARABOLIZING MIRRORS ON THE M.O.M. TURN-TABLE
Mirrors larger than 12…..14” to be Parabolized on Mirror-O-Matic machine with Tool On Top, slow 5 rpm turntable,
stroke arm 20….30 p/m start with a small
1/3 size lap going edge to edge not over, for 5 to 10 min, test mirror if first of
doughnut shows, change to larger lap ½ mirror size, simular strokes not over
edge and little or no weight, for 10 minutes, check again.
3 B. Making a
mirror on a SPIN-TABLE
THE SPIN-TABLE IS THE MACHINE-MECHANICAL WAY OF
THE CIRCULAR STROKE
The spin-table turns 20 to 30
RPM, Mirror up to 12” diam are done with mirror on top, larger mirrors with
Tool on top.
The top unit is “spinning” freely
with drag on the powered base
All work for mirror making can be
done with this spinning from hogging to parabolizing.
Start
hogging (UP TO 12” with the MIRROR overhanging the tool by 35% using grit and
moisture as done by hand, and some extra weight on the pin, but unit continual
freely spinning, measure regular for sagitta depth, when that is reached change
to finer grit and 20% overhang and less pin weight, following the same changes
as hand-fine grinding.
For working
larger mirrors use sub-dia-meter tools on top, no smaller than 70% of mirror,
when ready for polishing use a fine pressed hard lap troughout, little or no
pin-weight, 20% overhang, continual supplying Cerium-water, regular testing
until ready for Parabolizing.
Change to 40% “overhang”(= 40%
clear mirror) , little or no pinweight, smooth spinning (about ½ table rpm) and
continual cerium-water dripping, regular testing the zones, until edge zone
nulls
TESTING THE
PARABOLIZED MIRROR
THE
FOUCALT TESTER
The first mirror test for a good sphere is with a “null”
test and straight Rounche screen lines with Camera on TV screen. For Parabola
testing we use also the Foucalt-tester now with a full mirror-size “Cauder-screen”
with cut-outs to show the parabolic zones of the mirror, each zone having a
different focal-length for correct “nulling” while all zones are of equal area,
focal distances are computer calculated
MAKING THE CAUDER
SCREEN: Cut a thin cardboard disk the size of the mirror, calculate the
total mirror surface area, (r*r*3.1415) divide area by the number of zones you
planned for, calculate the radius of center-zone 1, zone 2 radius will be
calculated from twice the area, etc.
draw the different zone radius on the screen-board and cut
some (25 deg. sect) in the left and right side of each zone area.
When all zone are
corrected,”NULLED” to mach the programmed zones on the Radius of Curvature. And
the Figure XP program shows a good
parabola, we can do the Rounche test which should now show the curved parabola
lines. (Note: a parabola will show a “doughnut” but any doughnut is not a
parabola but shows we do not have a sphere anymore.) Next do a Star-test or when an optical flat of the mirror size or
larger is available going to do the next :
TEST THE
PARABOLA BY AUTO COLIMATION.
We set up another stand on the test bench for the Auto-collimation test, which is done at the
mirror Focal length which is ½ the ROC
and uses double reflection, producing a artificial star test. We use on this
stand a coated“Optical-flat” mirror of same or larger size as the mirror with a
CENTRAL LED, wich shines at the mirror
and a CAMERA lookjng at the mirror, the two mirrors are facing each other and
needs to be alinged by adjusting the mirror stand XYZ axes first, so that the
single reflection from the mirror-center points at the camera beyond the flat,
next we allign both mirrors so that the (fainter) complete mirror is projected
trough the camera, by minutely changing the focal length we find the exact
focal lengt by the NUL and the quality
of the parabola.
For mirors larger than the Flat we can use the Ross Null
test, using a lense instead of the optical flat mirror.
Next step is to get the mirror Aluminum
coated.
How I
came to use the circular stroke
In
2008 when I was starting to fine grind a 8” mirror with M.O.T. after I hogged
it with the sub diameter tool and 80 carbo grit, now using 120 grit and
stroking as all the books tell jou: Back and fort, back and fort and going
around the barrel, and after I short while I was thinking while grinding: why
this dumb back and fort and becarefull to do it from all sides to produce a
sphere, this is like I use to do as a carpender with a hand saw or a hand
plane, why not do what I do as a modern woodworker who uses a circular saw for
cutting much better and quicker and better with an electric plane, I will try
to stroke around circularly, using same overhang and weight, so I tried.
And
it worked more easy and surprisingly made at the same time a very good
spherical surface, so I finished the fine grinding being less tired and done quicker.
So
I made the pitch-lap on the mirror with the lap-mould and being in the mood of
changing, I changed from making a softened lap, I make the pitch-lap hard,
for I want to change the mirror surface but not
the pitch-lap, so I did and polished with this hard lap, using
the same circular stroke until the surface was ready for parabolizing.
Parabolizing
is changing the surface from a sphere to a Parabolic shape by deepening the mirror-center by 10,000th of
inch, from the spherical shape so starlight from the whole mirror can focus
at the same location.
But
Parabolizing is really not much unlike what is done at the start of the
hogging-grinding, only on a much more delicate scale. I started with the same
circular stroke, mirror on top, using 40 % mirror overhang and some extra
weight at the center so that the 20% center of the mirror would wear over the
lap-edge and the remainder of the surface wear less until no wear at the mirror
edge. All done with the same hard spherical pitch-lap and circular stroke.
And
while most ATM people are somewhat reluctand of Parabolizing, it turned out
surprisingly well, as planned and much simpler and quicker than before. Starting
correction at the mirror center and spreading it continualy to the edge, going
slower as the correction area became larger, until the mirror-edge was reached
without any zones over or under corrected.
I also found that the amount of time-effort for
hogging equals about the parabolizing
time, both take equaly more time for shorter mirrors and larger mirrors
The final quality of the mirror is found after
Parabolizing, not after spherical polishing, so end to do some very lightweight
polishing, for a fine surface without changing your parabobic shape, just
smooter.
Lens makers from 16th cencury used the
circular stroke also.
Parabolizing is really changing the surface angle since we want to focus the
paralel light from infinety, not the light from the ROC as with the spherical
testing
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