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.