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.
