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
