Mitchell's Telescope Making Website

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10" Homemade Telescope

A Work in Progress

I am excited and proud to have just finished my second mirror. A 10" is a nice jump from a 6", with a ten having 3X the area. I am also went for a faster focal ratio. I was shooting for F/5 and ended up just a little shorter (by about 2") which is plenty close enough. I hope to make a really nice second telescope, applying all my gained knowledge into it. I look back and see how far I have already advanced. Bigger mirror, new testers, test stands, work stands, and everything is going much smoother with my acquired knowledge. Things can only go up from here!

10" Test Images

Test Images:

These are the test images of my 10" mirror. You might notice they look a lot better as far as the shape goes compared to my 6". My lessons learned are allowing me to control it and keep a good sphere throughout polishing so far. The first two images are OUTSIDE RADIUS OF CURVATURE, the third is INSIDE. 

20 Minutes, OUTSIDE

 

This image is very early on in polishing, after just 20 Minutes. The mirror is a little oblate but not bad and has a tiny tiny turned down edge that should disappear with further polishing. The bottom got a little cut off because the mirror was simply resting on edge on a piece of carpet. This is Outside ROC.

4 Hours, OUTSIDE

You can see the mirror is much nicer now! A nearly perfect sphere. The edge is good and the surface only has a little roughness. This is Outside of ROC.

26 Hours, 45 Minutes, INSIDE

This image is INSIDE ROC. I have a new/better camera and could only get the whole mirror inside ROC. As you can see, a well corrected mirror. The central roughness is visible, but will be totally covered by the secondary mirror. You can see where there is a slight lack of correction around Zone 4, and there is a very slight TDE. Overall a smooth mirror with just a few issues. The Mirror is 1/17.5 P-V Wavefront with a Strehl Ratio of .99!

                                                                                                                                                                 

10" Mirror Log

Here is my Mirror Log, check it out! 

3/13/07: (ATM Meeting) This project is officially started! Many thanks to the members at our ATM Meetings, and the very generous host Gordon Waite for their help in making a tile tool last night. This will be used for grinding the back flat and hogging. The tiles will probably wear too thin by then as the sagitta is the same as the tile thickness and we will make another for fine grinding. We decided it best to start with a full size tool to keep simplicity and reduce the possibility of future issues. This project was officially started tonight on 3/13/07, letís see how long it take me to finish mirror number two! I have decided to challenge myself a little and go with a focal ratio of F/5.

Here is a picture of the Mirror blank and Hogging Tile Tool before any grinding started:

3/18/07: I kicked this project off with a nice back breaking two and a half hours of flattening the back of the mirror against the tile tool. I did 24 wets with #80 carbo and then 10 wets with #180. Half MOT, half TOT (Mirror on top and Tool on top). Other than some #60 I got at the Meeting for hogging, I am using the leftover grit from my first mirror. I still even have a little bit of #80 left after all this. It took quite awhile because the back of the mirror was a little concave and when the tool was poured the tiles floated up a little bit in the center so I had to match two concave surfaces. The back was pretty rough looking with a couple of generating marks, including a raised 3" circle about .020" high in the center with the edge being about another .010" higher than that. So I had .030" worth of glass to remove height wise. The back is now only slightly concave (about .008 at most) so that should be good enough for this blank that is a little thicker than full thickness (1.75"). "Full thickness" 6:1 ratio would be 1.6" and I can now move on to actually hogging out the mirror! 

3/19/07: I continued on to grinding the front today. I did 10 wets with #60 grit just to get a little curve going. The sagitta is now a little under .035. Although the blank was a tiny bit concave to start so probably only about 3/4 of it was glass I removed.

3/20/07: 10 wets with #60. Sagitta is about .048.

3/22/07: 10 wets with #60. Sagitta is about .060. The grinding has made it to about 1/3 of the edge. There remains about a 1" ledge around the other 2/3 that tapers off toward the side being ground.

3/23/07: 15 wets with #60. Sagitta is .070. I have used up all the #60 I got at the ATM Meeting so I will have to continue with the small bit of #80 I have left after using it to grind the back flat. The stubborn ledge is shrinking both in width and circumference albeit slowly.

3/24/07: 15 wets with #80. Sagitta is about .085. I'm all out of #80 now too. I moved on and did 20 wets with #120. Sagitta is about .095. Seems #120 might be a little slow going.

3/25/07: I did 40 wets which took about 2 hours total time with #120 of hogging followed by a few COC wets to sphereize a bit and I am now out of #120. Based on the rate yesterday I figured I would still be a little shallow but I am actually over by .010! My sagitta is .135". This makes it about F/4.5. This should be good though as I want to work TOT to spread the curve out to the edge more efficiently and this should shallow it up a bit too so I can knock out both problems. The ledge around the edge is now about 1/2" wide and a little under 1/2 the mirrors circumference so I am confident sphereizing will shrink it. I have some #180 I can use for this. Hopefully I will have it reasonably spherical before the ATM Meeting which should be this Tuesday if all goes to plan.

3/26/07: 5 TOT wets with #180 to try and shallow it up a bit and sphereize but the center hasn't even been touched and the tool is getting thin, it is obvious the mirror is hyperbolic as most are after roughing.

3/27/07: (ATM Meeting) We took some more time to check out the blank. It has about .031" of wedge, which compared to the blank thickness of around 1.6" is less than 2% wedge which is pretty good compared to some blanks. It shouldn't hurt the final mirror since it is so mild, but I put in a little effort to reduce it anyway. The sphereometer reads the edge of my mirror as a little longer focus then the center with the overall being a little deeper than F/4. I did an hour and a half with TOT and 1" overhang to spread the curve all the way to the edge and make the bevel much more regular as it was a lot wider in spots. I also applied more pressure on the side with wedge for the first half hour. It is reduced to about .028". Last I did half an hour with a 2/5 W stroke TOT which though the sphereometer reading didn't change, should help make it more spherical if continued. The curve is now out to the edge and has overall flattened out to within a few inches of F/5, the edge now needing a little more flattening than the center. The bevel is almost completely regular in width. Before the night was over we poured a new tile tool as the outer 1" of tiles all the way around on my hogging tool had slowly disappeared! Surprisingly no tiles had popped off; they just kept disappearing in little pieces as I worked. This tool lasted a lot longer than I thought it would. If all goes well this new tool should get me all the way through the rest of the grinding.

4/8/07: 15 wets TOT with #80. Center looks about the same so the grinding action is at the edge where it is needed.

4/9/07: 35 Wets TOT with #80. There is a lot of glass to remove to meet the central depression so it's moving slowly. Then I did 1 hour followed by 1.5 hours all with #80 TOT and a little "W" action now and then. About four and a half hours of work done today! The sagitta is still a little too deep. ATM Meeting is tomorrow.

4/10/07: (ATM Meeting) Before the meeting I did 1 more hour TOT and used up the last of my #80. At the meeting we found I had a very good sphere but it was still only a little shallower than F/4. I started by hand but progress was very slow so we put it on Gordonís fixed post grinding (FGP) machine. Basically it allows you to set the offset you want and gravity or added weight does the rest as the tool and mirror spin at different speeds to do the grinding, it is used for grinding and gross polishing. We used a large offset to bring the edge down fast then a smaller offset to sphereize it. All of this was done TOT. Took about an hour and a half of grinding it on the machine and by the end of the night it was spherical to within 1" all across the mirror and the ROC is also within 1" of F/5. Still just a tiny bit deep; Now I can finally move on to fine grinding! I'm going to start with #180 and do that all TOT just to dial in the focal length the one last inch and then I'm good to go alternating MOT/TOT all the way through the rest of grinding!

4/13/07: I did 45 Minutes with #180 TOT with a bit of overhang to flatten out the curve just a tiny bit more and help dial in the focal length that last inch or so. It probably won't move much at all from now on. I threw in a little COC work during this. Then I did 30 Minutes of COC, TOT to made sure I still had a nice sphere, dial in the focus just a bit more and make sure all the #80 pits were gone. I cleaned up and am ready to move on to #220.

4/14/07: I picked up 5 "dispenser bottles" like old restaurant ketchup bottles. I got one with a removable cover for the small opening so I know that is always polishing compound. The other 4 bottles are for 25, 15, 9 and 5 Microns.

4/15/07: I ground #220 for 45 minutes all TOT. It didn't take long since #180 is close in size. I cleaned up and ground 25 Micron for an hour (20 min MOT, 40 min TOT) and am now done with it. I got a slight sticking if a wet got a little dry, got to remember to keep it wet. Lightened up the pressure a little bit and only separated the two when flipping from MOT to TOT and when I was done with 25M to prevent scratches from pull off. Unfortunately, spring break ends tomorrow but I got a lot accomplished on this mirror and the 6" scope.

4/16/07: Well school was cancelled due to a little flooding so I decided to keep grinding. I skipped 20 and 15 micron and went right to 9 Micron. I ground that for 1 hour and 15 minutes all TOT. Without separating them to prevent scratches I kept on going with 5 Micron for 30 Minutes. I used almost no pressure and slowed my stroke considerably. I carefully removed the tool. There are 3 VERY light scratches, most likely from pull off. They are extremely light and even under magnification they have no visible width. If I tried to keep going I would most likely get more scratches so I now pronounce grinding done! It took me 13 months to finish doing the grinding on my 6" because I went slow and took my time, but now that I have a better idea what I'm doing the grinding on this mirror, which is larger, took me only 1 month from start to finish!

4/24/07: (ATM Meeting) We first measured the ROC with a sphereometer and it read the ROC as 97.3" making the focal length 48.65" or about F/4.87. It was also spherical to within 1/10" which is easily close enough. We poured a lap and I got in about 20 minutes polishing time but the contact was bad. Threw it up on the test stand and it looked pretty good! Overall a very nice sphere, tiny bit oblate with a tiny, tiny TDE from the bad contact. Just press good and away I go polishing!

4/26/07: I tried to start polishing today. I had a fiasco with pressing and could not get contact even with hot pressing and a lot of weight. A few of the channels on this brand new lap had almost closed from all the pressing! It was suggested to me by Gordon that I wasn't waiting long enough after hot pressing and the mirror was still deformed a bit from the heat making it feel like bad contact. I managed to get about 30 minutes polishing in but nothing good.

4/27/07: I followed some of Gordonís suggestions. I heated the mirror and lap and pressed with a lot of weight for a short time then let them sit and gravity press for a few hours so all the heat could dissipate. It worked great; Nearly perfect contact! I went on and did 30 minutes polishing. A 10" may have 3X the area of a 6" but it feels like 9X the friction! It is a workout. The heart rate goes up, your arms feel the burn and you break a sweat in 15 minutes. But thatís okay, because I always keep in mind, "the harder I push the faster it is going to polish". I also picked up some 2X4's for reinforcing the polishing stand, a large sheet of wood for building a new mirror test rack, and two plastic bins, 1 for heating the lap, and 1 for storage of the mirror and tool for transport to the ATM Meetings; This way it is all in one container and easier to keep track of things. I now have in a total polishing time of 1 hour and 20 Minutes.

4/28/07: I cut the reinforcing pieces for my stand and attached them, it came out great. Much more stable now! This is mainly because I can put my foot on it and can but a board between the three cross-braces and load weight on that. I'll put some pictures up soon. I also cut the large parts for my new mirror test stand.

4/29/07: I got some more polishing in. My total is up to 2 Hours and 15 Minutes. The polish is showing considerable and steady improvement.

5/1/07: Continued on today. Under normal lighting with a laser the mirror is nearly polished out to within about 1" of the edge. Total polishing time is 3 Hours.

5/3/07: Total polishing time is now 4 Hours. With same conditions as above the mirror looks polished out to within 3/4" of the edge with the last 3/4" showing considerable improvement. After polishing for awhile the lap gets a little dry and friction really picks up, this is when polishing is most efficient, and at it's hardest physically. Got to brush the lap with a brass wire brush about every hour or it "glazes" over with old cerium and just gets slippery. I threw it under the tester for a quick peek and the mirror is a DEAD ON SPHERE; Though it has a tiny bit of roughness from not perfect contact. See the test image.

6/18/07: Been awhile since I updated. I have been working very often. Once I hit 10 Hours of polishing I considered coarse polishing done, although there was an ever so slight improvement noticeable till I got to 12 Hours. There was a small Turned Down Edge (TDE), Which is the biggest pain there is in mirror making. Luckily mine is small. I continued on with short strokes and some sphereizing strokes to keep things in check. On I went hour after hour as TDE, even a small one, takes a long time to remove. After more hours my total polishing time has now reached 23 Hours, 40 Minutes. Almost half of the time has been spent trying to remove the TDE. It has improved significantly. I was doing 2 things wrong. 1. I had my fingers too close to the bevel so it was heating it up and causing the expanded glass to be polished away and 2. I was used to the lower friction on my small mirror so when I rotated the mirror I didnít realize it was offsetting itself about 1/4" to one side so my strokes were not perfectly center over center and that also can cause a TDE. So after all these hours of work, and a little beveling (my bevel needed to be widened anyway) I have most of the diffraction ring back and the TDE is very small. For now I am going to push on and start figuring. I have the mirror at about 25% correction spread a little unevenly but I evened it out significantly. Need some more edge correction. Just needs more parabolizing at this point.

I had to make a new lap for figuring, below is a picture. This was the second attempt, and it came out very nice.

1/1/08: My Mirror is DONE!!! Let me recap my last few months first. After my last update I tried to parabolize and sphereize a few times with limited success. I got the mirror to about 1/3 wave a few times. I finally had to make a new lap, which came out bad. I tried again and got an amazingly beautiful lap. After another sphereizing attempt I had decent correction at the edge with very little at the center. I slowly added correction and evened things out. I decided to use TOT with overhang to parabolize the edge and this worked  great without turning the edge much at all. And as of just a few days ago a few little tweaks to drop the center down, and presto! The mirror is done!

Heres the Figure XP File:

The Mirror is smooth with a defect in the center which will be covered by the secondary. It has a small TDE which is not major. It is 1/17.5 P-V Wavefront with an Strehl Ratio of .99 and an RMS Surface Error of just 4.3 nm! (The picture above is ever so slightly different because I put in the correct focal length later which changed by a few fractions of an inch after polishing out.)

3/25/08: I went down to Delmarva Mirror Making Event and it was quite fun! I brought the 10" down and looked at it under one of the auto-collimators. It looked good to me and the others there! It is nice to have a confirmation of my Foucault readings. Now the building of the scope continues. So far I have cut out two rings of plywood for the upper cage with my new router, cut the "truss poles" made of 1.25" hardwood dowels, and have begun sanding and experimenting with stain schemes. Before I thought I had made a msitake with the couder mask zones in figure XP that made this mirror worse but they were right the first time! It is still the original 1/17.4 Wave.

4/16/08: Stained and polyurethaned the cage rings. They are a deep cabernet red. They look great. I also ordered the spider from Protostar, it might take awhile to get here though, stay tuned.

6/25/08: E-mailed Protostar, they are a little behind but checked my order and it should be shipped by july 8th, an extra week or two over but at least they checked. Ordered the 2.6" Secondary mirror, 1/18 Wave flat, from Antares. They seem to be the best, you can get higher quality if you want to pay for it and each comes with nice documentation. But 1/18 Wave is pretty darn good, no need to have better unless I was making the most perfect scope ever (like the plan for my current 8" Project).

7/5/08: The secondary arrived. I ordered the right size but accidently ordered one quality up. They offer 1/15-1/30 Wave flats in various steps, I ordered a 1/18 Wave flat. Oh well only a little more money and it's better quality and should match the primary well!

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