Mitchell's Telescope Making Website

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4.5" Homemade Telescope

A Work in Progress

With my 10" Mirror done I want to do something fun while I am designing and building the scope structure. A few months ago I made my first trip up the the Springfield Telescope Makers Convention in Vermont. It is known as Stellafane. There at the swap tables I grabbed up an old 4.5" commerical mirror and its cell, along with a 4.25" pyrex tool. It was a "finished" coated mirror. Total cost was 17$. I let it sit on the back burner till now. This will be my first re-figuring project. 

1/6/07: The actual diameter of the mirror is 4 & 7/16", and the tool is 4 & 3/16". So both are 1/8" undersized from what is stated above. I will refer to it as a 4.5" though for simplicty. It is just over 1/2" thick and made of PLATE glass. Also, this mirror is fast, F/3.8. It is spherical to within .006 under foucault. I believe it must have been part of an old mak or something. The cell had no way to collimate so I may end up building a new one.

Two days ago I stripped the coating with Ferric Chloride. Took an hour or two. Yesterday I made the pitchlap. The first attempt was catastrophic when I tried to separate the mirror and tool too early. The second came out fine, and the facets are a little odd shaped and sized but they should be okay. The pitch seems to want to peel off but I think pressing will solve all this. I dont know wether the 1/4" difference in size will prove a problem since I am going right to parabolizing, but I guess I will find out. After making the lap I did a few times around just to get the feel. With such a small blank I have to slow down otherwise my stroke is too fast and too long.

This should prove to be a fun project! With a mirror this fast the tollerances are *extremely* tight. And while the foucault shadows are very distinct, they have to be read very, very, precisely. I hope to make a grab and go scope for rich-field scanning of the milkyway, etc. I am not really pushing for better than 1/10 Wave like my first two mirrors have been because this one is so fast and will be used at low power. But I would like to have some confidence that is is better than 1/4 Wave.

1/30/08: Well its done!!! I know there haven't exactly been many updates here. I spent the last few weeks parabolizing away. I had a little trouble getting correction on the edge but I got it using some TOT (turns the edge down slightly unfortunatly) and long COC with a little W swing, which really takes correction away from the center so it was a little bit of a chasing your tail game to get it just right. But I got it! Of course I said I wasn't pushing for a really great mirror but I couldn't stop till it was as good as I knew I could make it. And the results are great, 1/45 P-V Wavefront and a Strehl of .998! Pretty darn good for such a fast mirror. Now I just have to build the scope. Time to switch into building mode as I have to build structures for my ten inch mirror and this mirror.

2/18/08: Well after finding a small error in my inputs to Figure XP my mirror is not quite as good as I thought it was. I found out Couder Mask which generates the mask used to test will give slightly different zones radii from Figure XP. So I beveled the TDE a little bit, cut out a new mask, measured the ROC very carefully, and took 4 sets of readings. Two on each diameter, one going from the inside zone outward the other going from the outside zone inward, for each axis. After this careful re-testing the mirror is a little over-corrected because my perfect numbers were a little off. They were slightly higher than what they really were supposed to be. The mirror is just over 1/9 Wave with a strehl ration a little better than .96! Still a pretty darn good mirror! I am still going to call this mirror done and apply my gained experience to my future mirrors.

4.5" Test Images

0 Hours, IN

With a mirror this fast it is hard to image the entire surface but I managed to get this INSIDE ROC image. As you can see it is basically a sphere, with a tiny amount of correction. About 10% of the overall needed correction for a parabola. It also has an extremely tiny TDE. Just barely noticable because the diffraction ring under foucault is not visible as well on one side as it is on the other. This is before I did any work on the mirror other than remove the coating. Because the mirror is polished on the back you can see the reflection of the LED in the center as a tiny green dot.

1 Hour, 35 Minutes IN

The two images above show the mirror about 1/2 way through figuring. Most of the correction has been added but the edge is lagging behind a little bit. Tiny bit of light roughness going on in the center but overall nice and smooth. Progressing nicely!

3 Hours, INSIDE, FINAL TEST IMAGES

All three images above are of the finished mirror. They are all inside of focus starting close in with three lines and moving out to show four and five lines. The overall correction is smooth. There is a little belly button in the center, which will be completely concealed by the secondary. Ever so slight micro ripple is present on the surface as well from dropping the center fast. There is also a very tiny TDE but it is minor. It is even less apparent than in these photos because I beveled about .1" off. Overall these defects are quite minor and shouldnt harm the image at the eyepiece too much. This mirror was a challenge to figure with the tollerances as tight as they are. But it came out very nice with an overall good figure!

My sets of Foucault Data together all average out to yeild 1/9th P-V Wavefront Error, Strehl Ratio of .964 and a Surface RMS Error of just over 8nm!

All in all, Another great mirror!!!

Heres the FigureXP File:

Now I just have to build a scope for this thing!

3/25/08: I went down to Delmarva Mirror Making Event and it was quite fun! I brought this little mirror just for fun and took a look at it under one of the auto-collimators. It looked darn good! It was nice to have a confirmation on my readings. After I build my 10" scope I will work on this little guy so it might be a little while, but stay tuned!

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