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
