Mitchell's Telescope Making Website

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8" Homemade Telescope

A Work in Progress

Wayne Petko, a friend from the NJAA Observatory, was kind enough to donate a mirror to me that he had not finished. It is an 8" F/8.25. The mirror is about 3/4 polished out and half way corrected. I am going to pour a lap and give it a thorough polish out. Then I will switch to my finest polishing compound (.3 Micron Black Rouge), and give it the best figure I have ever done on a mirror to date. I will settle for nothing less than a well polished out mirror with a perfect edge, smooth surface free of zones, and an excellent figure consistently read at better then 1/20 Wavefront. I plan to also build a tracking mount and make this into a really great planetary killer type scope.

8" Test Images:

0 Hours, INSIDE

This is the mirror INSIDE of the Radius Of Curvature (ROC). Shows the mirror as slightly parabolic, a pretty good edge and generally smooth surface, only a tiny bit of roughness in the center.

0 Hours, OUTSIDE

Same story as in the first image, except OUTSIDE of ROC. The edge defect is so small it doesn't show up, and should be polished away quickly. Again, a generally smooth mirror overall.

0 Hours, Foucault

The image above is a Foucault picture. Sort of gives something to compare the Ronchi to. A little correction as seen by the slight shadowing in the center, and slight lumpiness seen by the unevenness in the center.

6/21/08: The NJAA needed to have the roof resurfaced so while there Wayne brought up the old mirror of his. We had some fun working on the roof then had lunch. When I got home I tossed the mirror up on the test stand. It is smooth without zones and has a decent edge. Even only half corrected, with such a long focal length it is about 1/6 Wavefront already, but it needs to get polished out all the way before I can figure.

6/23/08: Took some Ronchi and Foucault pictures of the mirror that I put up here before I start work on the mirror. Soon I will pour a pitchlap so I can get polishing on this puppy!

6/25/08: Got the lap poured today, came out good. After pressing in the channels I pressed the mirror and tool with about 40 lbs. of weight for a few minutes, then I just leaned on it real heavy. I'm leaving it to sit for an hour and just gravity press to ensure perfect contact and let it all cool down to room temperature. Then I trimmed it after the heavy press, and let is sit with a few pounds on top for a little while longer, contact is pretty good, after the next press is should be good to go.

6/26/08: Scored up the lap some and pressed. If properly scored with a razor it only takes about 20 lbs for just a few minutes to get and maintain good contact. Did two, ten minute sessions. After working any "crud" off the surface I can see the mirror is polished out to within about 3/4" of the edge. Did another ten minutes to smooth it out, still correction hanging on at the edge. Hopefully it won't take too long to get that last bit. Center is spherical, with correction remaining only at the edge.

Here's a Pic of the newly made lap after breaking it in. Channeled, microfaceted, and ready to go.

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