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Glasspusher

Making a 12 inch Mirror

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hi john , thanks for your comments , here are a few better pics please let me know what you think i need to do to finish making a better sphere , i think it shows a small edge defect and a slight hole , ? i think i have more polishing to do with perhaps mot for a while first then a little less with tot and test, do you concur? looking forward to your feedback PT

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If the ronchi grating is just inside the radius of curvature the mirror is showing a slight turned edge and the bands are bowing inwards towards the centre indicating mild under correction here. In other words the middle is a little high. Working mirror on top (MOT) with a short stroke should sort out the edge. If the middle is still a little high you can still start to parabolise from this point IF the mirror is fully polished. Are you anywhere near a full polish?

John

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i think based on polishing so far i am 75% polished out a little mistiness near outside edges and a small area in the centre perhaps 2" radius i have repressed my lap warm,and then cold overnight , then i hope to finish with another 1-2 hrs of polishing then on to parabolizing. i have a plywood lap base 6.5" dia x 1.5" to use as a lap base , i have to use the same pitch so i will melt it off my 9" lap i don't want to scrape it off in case i get contaminants off the base in the pitch, PT

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john tonights ronchi tests ,please see previous post.PT

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Edited by ponytale

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PT, if at all possible I would keep the large lap, I often find that after figuring with a sub-diameter lap (50%) I need to go back to the full size lap for a final 'smooth over' to blend any subtle zones resulting from the 50% lap. If your mirror is fully polished and based on what your ronchigrams are showing it would be safe to begin parabolising. You do not have to have a perfect shere before beginning to parabolise.

John

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a link below to my almost finished mirror , polished out and no discernable scratches whoo hoo!!. but it may need some adjustment to parabola ?:( i may also mount it in my scope to do star test, or ronchi eyepeice test if i can before sending for coating. Ponytale

YouTube - ronchi test 080211

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Odd question - do you have rotate the mirror 90 deg and check the test too?

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PT, good video, the mirror looks very nice. You really need to make some quantitative assessment of the figure now. This could be done using the matching ronchi test which I descibed in an earlier post. Alternatively, you could use a star test if you have the tube assembly made. Basically, you look at slightly defocused images of a star either side of 'ideal' focus, if they look similar the mirror is good.

NickK, rotating the mirror during testing is a way of checking for astigmatism (a condition in which the mirror does not have a regular figure of revolution), assuming gross astigmatism is not present, as is the case with PT's mirror, rotating the mirror would be carried out in conjunction with a knife edge test and zonal mask. Astigmatism in some cases can only be revealed by interferometric testing.

John

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Well done PT. I'm so pleased you've almost completed your mirror.

It looks very smooth, and a nice edge. You've done a great job under GP's guidance.

Are there anymore guys making good progress on this project?

Ron.:(

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thanks barkis and gp for your support, . i think i will star test this weekend weather permitting let you know then pt

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