|Grinding an 8" Fritz Mirror|
4.71 Focal Length
Here are my early-on plans for the mirror cell in Acrobat 5 format.
Here's the finished Fritz telescope that the mirror ended up in.
|The mirror was
started on August 17, 2002 at the St. Petersburg Astronomy
Grinding Lab. The lab met every Saturday from Noon to
4pm. The photos on the left were taken after the second class.
8/17/2002 - I walk into class and was immediately designated a 'Mirror Virgin'. It was the nicest thing said to me that day ;). After some initial instructions...
0:45 beveling the edge
2:15 Grinding with 60 grit. Remainder of time was spent asking questions and trying to keep the big grit away from people who were polishing. When you walk around covered in 60 grit everyone wants to get out of your way! Finished the day with the mirror in the mid f/20 something range (guess by others).
1:05 Continued on the 60 grit. Took out just a little too much in the middle. Pretty close to f/6 though.
1:00 cleaned tool with dental pick and tooth brush getting ready for 80 grit.
0:54 Ground with 80 grit with tool on top to lessen curve for 25 minutes. It worked - curve matches f/6 template. Continued for 12 minutes with mirror on top and another 12 minutes with tool on top. Ready for 120 grit next week
1:45 with 120 grit. Alternated every 15 minutes between Tool on top (TOT) and Mirror on top (MOT). I was interrupted a few times by visitors asking questions, a pitfall of being in the front room by the door. Talk time was not included in the grinding time. Finished 120 about 3pm!
0:40 needed to add additional bevel the edge - a nice sit-down job. Time to go home and play with NEWT to get the dimensions for the actual scope.
1:20 with 220 grit. Alternated every 30 minutes between TOT and MOT. Given the okay to move to 320 grit.
After cleaning up another grinder suggested that I checked the actual focal length using a tape measure and the sun - 38.5" - YIKES! That's less than a f5. Decided to attempt to get closer to f6 (at least above f5) by using the 320 and TOT for 45 minutes and MOT for 15 minute cycles. Other options are to go back to 220 or even 102, or accept the low f ratio and struggle in the figuring phase. I learned that the template is a ROUGH GUESS at best and do a Sun check after each grit BEFORE cleaning up.
The target is at least a 44" (f/5.5) focal length next week. If the 320 plan doesn't look like it's paying off after 2 hours (1 more next week) I'll drop back to 220.
Paul McNabb checked the curve with the template and proclaimed that it was between f/5.8 and f/6.1. It looks like the hour of flattening out the curve last week paid off. As an added bonus I finished the 320. I would have preferred a 'sun' test for the focal length but the sun hasn't been seen around here in the past 5 days. I'll check it when the sun peeks out and finally start building the tube.
Next I started the micron grits. This also uses a different stroke. Back and forth across the center of the tool (MOT now) allowing the mirror to extend 1/3 of the diameter over the tool. I keep walking around the table and rotating the mirror as before. I also need to bevel the tool for the first time.
1:00 with 25 micron
0:30 with 12 micron - be very careful not to scratch anything.
0:10 with 5 micron
Adding cleanup time the above took about 3 hours.
On to the Pitch Lap! With the remaining hour the pitch lap was made, pressed against the mirror and I began polishing with Cerium Oxide. Note the use of the word polishing. We are leaving the grinding phase and the mirror's curve is pretty well fixed now. At this point I'm polishing off the high spots until it's time to start figuring it.
0:10 with Cerium Oxide.
The Sun came out this week and I tested the focal length. Somewhere around 36" (back to Yikes!) After talking about the options with the more experienced grinders I decided to let the mirror choose it's curve. An besides, it was a really good sphere! It was tested on the laser stand and measured in at 75.5" (radius), or a 37.75" focal length (f/4.71). I'm going to see if I can figure this. If it works this will be a great little travel scope and good for wide field photography.
2:10 with Cerium Oxide
2:35 with Cerium Oxide. Continued with MOT. The center is polished out pretty well but the closer to the edge I check with the laser the rougher it is. Not bad, just rougher than the middle. Considering TOT to cleanup the edges and even out the stroke.
Missed this week due to a much needed vacation.
2:20 with Cerium Oxide. Cold pressed for 30 minutes. MOT for 20, TOT for 2:00.
2:50 with Cerium Oxide. Cold pressed for 35 minutes. Continued with TOT. for 1:30. . MOT for 20 minutes and then TOT for 0:40.
2:10 with Cerium Oxide. Cold pressed for 30 minutes first. Continued with TOT. for 0:30. Bumped tool against the side of the mirror and cracked dome patch off. Cleaned everything and cold pressed again for 20 minutes. MOT for 20 minutes and then TOT for 1:20. May start using Red Rouge next week, maybe not.
1:45 with Cerium Oxide. Cold pressed for 40 minutes first while I mopped he floor - swab the deck - aye capt'n. Continued with TOT. for entire 1:45. Test stand shows a small hump in the middle. Probably from the TOT polishing the week before. The plan is to continue with MOT and the standard 1/2 center-over-center stroke with an occasional 'W' thrown in for randomness. That's the stroke W, not the George W.
2:10 with Cerium Oxide. Cold pressed for 40 minutes first. MOT the entire time.
1:20 with Cerium Oxide. Cold pressed for 35 minutes first. MOT the entire time. Lengthened the stroke to 1/2 Center over center with an occasional 'W added. The longer stroke was added to remove a small ridge at the outer 1/3 of the mirror. I ended up with three 'lesser' ridged (< .0005"). The longer stroke was a little too fast and caused a harmonic rippling. This is the pitch lap bouncing in rhythm across the mirror by too regular of a speed. Next week I'll slow down the 1/3 center over center stroke and see where I end up.
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