Making an 8 inch F 8 Telescope, C

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Figuring the 8 inch F 7.8 telescope mirror continued.
( More of the same )

	Now that I have some idea of the parabolizing stroke I decide to work more on the turned edge. 45 minutes mini weegee lap at the edge. There seemed to be a scratch along the direction of the lap motion. It turned out to be CeO and pitch. I removed it with paint thinner. 30 minutes weegee lap 1/4 inch inside the edge. Light press 15 minutes lap on top as before. 30 minutes mirror on top smoothing as before.
	Inside. If you look at the arrow on the further inside frame you see that the outermost edge still turns down. The focal length at the very edge seem to be almost the same as the mirror center area.
	Outside. The area 1/2 to 3/4 inch inside the edge is quite disturbed. This will require more smoothing. On the further outside frame the contrast at the very edge is almost zero.
	30 minutes on the lazy Susan guiding the weegee lap around the edge with my finger. Light Press 15 minutes lap on top as before. 45 minutes smoothing stroke with the mirror on top and the sweat band.
	Inside. Very oblate. Just the reverse of parabolic. The curves are fairly smooth except for some asymmetrical waviness. The waviness shifted after a while and I decided it was heat caused by leaving the telescope in the sun while I polished.
	Outside. Something is there at the edge but it seems the turned edge is gone. The parabolizing stroke should smooth the edge. Time to parabolize.
	Warming both mirror and pitch lap after a cool night.
	The lap has become very smooth and hard and is grabbing a bit. Here I use the plastic window screen to texture the lap again. The mirror is wrapped in aluminum foil in case there is something on the screen that might deposit on the mirror.
	Light press.
	60 minutes parabolizing stroke with 5 inch circle in the mirror center. About 20 degrees mirror rotation during the end of each push stroke. Across and back twice at each of the six lazy Susan positions.
	Inside. The mirror is almost spherical. Actually still slightly oblate.
	Outside. The camera causes the mirror edge to be less exposed making the bars appear to narrow more than they actually narrow outside of the mirror focus.
	Inside. The edge is slightly turned up except for maybe the last 3/32 inch which seems to be flat. Its tempting to try thumbing on the high ring of the turned up edge but I don't think I could thumb precisely enough. After processing the image in PhotoDraw I can see some faint zones further in. I suspect ( without having a way to measure ) that these zones are less than 1/10 wave. They were probably made by the weegee laps.
	At the radius of curvature of the center. Often there is a bright ring around the mirror edge called the diffraction ring. If the knife edge is on the left as in these pictures and the diffraction only shows on the right it indicates that the edge is turned down.
	Light Press
	45 minutes parabolizing stroke with 4 inch circle in the mirror center.
	Inside. Using the parabolizing stroke with the 4 inch circle deepened the center too much.
	Outside.
	Near center ROC.
	Outside of center ROC. The outer 3/8 inch is slightly turned up.
	5 minutes, pressing on the weegee lap while rotating the lazy Susan under the stationary weegee lap. Light pressure.
	Time to touch up the pitch lap with the carpenters saw again. If I hadn't heated the pitch lap under the hot water faucet first there would have been big flakes missing from the facets now.
	Light Press. 10 minutes lap on top 'W'. 3/4 inch overlap on the sides. 1 1/2 inch on the push pull.
	Inside. The high 50 percent zone has softened slightly and the turn up at the edge is less.
	Outside.
	Near the ROC of the 70 percent zone.
	10 minutes, pressing on the weegee lap while rotating the lazy Susan under the stationary weegee lap. Light pressure.
	Light Press. 15 minutes lap on top 'W'. 1 inch overlap on the sides. 2 inch on the push and pull.
	The center is still a bit deep and the edge slightly high. The center will be in the shadow of the diagonal.
	Outside. We will not need to use the outside picture further except for a quick check for turned edge.
	Looks much smoother than last time. ( Testing at the ROC. )
	Time for a reality check. Presumably the Ronchi program is giving me good information. But just to make sure I check using the laser off the reflector. Using the formula for the minimum distance to the artificial star I calculate a minimum distance of 31.5 feet. The reflector is about 200 feet away so that is definitely far enough. The mirror looks good with straight Ronchi bars. Inside of focus when I push the scope slightly I can see the bar move more quickly across the mirror center than at the 70 percent zone. This means the center focus is closer to the grating. ( The center is slightly low. )
	The Ronchi test is not good at finding astigmatism. Using the telescope on the artificial star with a high powered eyepiece I check for astigmatism and ripple. You must be careful to rotate the mirror every few strokes at this stage. Problems can be caused by forgetting to rotate the mirror when polishing lap on top. Probably best not to use the weegee lap anymore because it may leave ripples in the completed mirror.
	I trimmed the lap with the saw again and then widened the grooves with the utility knife so they won't fill in so quickly. The lap face seems hard and I'm worried about dust contamination so I scrape the lap face with the utility knife blade. I don't want any 'dog biscuit' from grabbing so I textured the lap with the plastic screen again.
	Its the end of October and the mirror is ten degrees F warmer than the air so no need for the sweat band. Warmed the cold CeO water mix for 30 seconds in the microwave. 10 minutes smoothing stroke. 1 inch overlap on the sides. 2 inch overlap on the push and pull. Mirror on top. Light pressure. 10 minutes lap on top 'W'. 1 inch overlap on the sides. 2 inch on the push and pull. Light pressure and not too fast. Actually I'm lifting the lap slightly.
	The center looks slightly better and the edge looks more smooth but the outermost edge is really spherical and not sloped down as with a paraboloid.
	15 minutes oblating stroke to bring the center up. This also brings the edge up but I will get to that. The parabolizing stroke with 4 inch circle that made the deep close to the center would have probably been fine on an F6 or F4 but an F8 is more subtle. 10 minutes lap on top 'W'. 1 inch overlap on the sides. 2 inch on the push and pull. Light pressure and not too fast. Actually I'm lifting the lap slightly.
	Its looking better now. Seems to me the edge is still a little high.
	10 minutes lap on top 'W'. 1 1/2 inch overlap on the sides. 2 1/2 inch on the push and pull. Light pressure and not too fast. Actually I'm lifting the lap slightly. ( Sure don't want to turn that edge again. ) The mirror polish is really bright.
	Seems about the same. This picture is a bit closer to the center ROC than the last picture.
	10 minutes lap on top 'W'. 1 1/2 inch overlap on the sides. 2 1/2 inch on the push and pull. Light pressure and not too fast. Actually I'm lifting the lap slightly.
	Looks just like the print out of the Ronchi program to me.
	Testing at the ROC
	Light pressure and not too fast. 5 minutes parabolizing stroke with 5 inch circle in the mirror center. About 20 degrees mirror rotation at the end of each push stroke. Across and back once at each of the six lazy Susan positions. 5 minutes lap on top 'W'. 1 1/2 inch overlap on the sides. 2 1/2 inch on the push and pull. Light pressure and not too fast. Actually I'm lifting the lap slightly.
	It looks like the simulation picture to me. Better slightly under corrected than over corrected and all the lap on top polishing might turn the edge again.
	I decide to do a final check on the artificial star. Its a cool windless overcast evening after a cool day and the testing conditions are better than usual. The only defect I can detect in the straight Ronchi lines is a turned down edge at the outer 1/8 inch of the mirror. I will mark off the outer 1/8 inch with a marking pen.
	The most sensitive test I have is the knife edge on the artificial star. The picture to the left is a simulation of what I saw. There was still a minor turn up inside the turned down. To this picture add the silhouette of the spider. And turbulence with about five times the contrast of the turn up shadow on the mirror. The shadow modulated the turbulence slightly as the turbulence swept past.
	Defocus test
	Late to be thumbing. 15 revolutions of the lazy Susan sliding a bit of CeO paper towel 1/4 inch inside the edge.
	10 minutes lap on top 'W'. 1 1/2 inch overlap on the sides. 2 1/2 inch on the push and pull. Light pressure and not too fast. Actually I'm lifting the lap slightly.
	Inside at ROC.
	Rained last night and tonight its clear and cool. The tube is wet from dew after only 15 minutes of cooling outside. The knife edge test on the artificial star shows some improvement. The contrast of the turned up zone is now about 1/10 the contrast of the turbulence. The eyepiece test shows a very sharp focus.
	During normal operation the air will be able to circulate behind the mirror. It seems the edge is turned up more when the back of the mirror is warm. So I poke holes in the masking tape to cool the mirror more quickly.
	Late to be thumbing. 10 revolutions of the lazy Susan sliding a bit of CeO paper towel 1/4 inch inside the edge for 6 rev and 1/8 inch inside for 4.
	10 minutes lap on top 'W'. 1 1/2 inch overlap on the sides. 2 1/2 inch on the push and pull. Light pressure and not too fast. Actually I'm lifting the lap slightly.
	5 minutes parabolizing stroke with 5 inch circle in the mirror center. About 15 degrees mirror rotation during the end of each push stroke. Across and back once at each of the six lazy Susan positions.
	Inside near ROC. It looks like the simulation picture to me. The grating was slightly closer to the mirror than in the simulation. Better slightly under corrected than over corrected.
	Outside near ROC
	Another knife edge test. When the turbulence is better I can see the edge has no shadows. The light thumbing didn't seem to leave any grooves. I tried to look for the deep center but if there remained a problem there it was lost in the turbulence coming off the diagonal.
	A frame of the de-focus test is shown at left. I also did a sequence of in focus images to evaluate the seeing. It seems that unless I could find a location with less turbulence further polishing won't be possible. I suspect the mirror is an ellipse with a focal point at about 150 feet. If so it is probably within 1/8 wave of a perfect paraboloid. If figuring could be continued I suspect that polishing sessions would only be a few strokes at a time. I suspect the small differential contraction of the Pyrex as it cools would become a major factor.