8 inch mirror project, nearly there!

[Garyh]

Thanks Mark, that sounds like a better method with the lines marked and the mask. Shall give it a go in the next few days.
I did try some more figuring (trying to fix the edge) but only a little progress.
First 5 minute session with the www just made the center and middle zones overcorrected while the outer zones stayed the same..

Rechanneled the lap, pressed and done some TOT for a few minutes with a 1/3 w stroke which brought it back to slightly under corrected and about 1/3 PV. Done a little TOT figuring for a few minutes which actually improved the edge slightly. Figure XP now gives me 1/7.5 PV.
Tried to find information on figuring with a full size lap TOT but I can`t seem t find any useful info....
Last time I borrowed Jean Texereau`s book from the library but someones nicked it!

Shall give a update and image in the following days..
Cheers Gary

[Satchmo]

Originally Posted by Garyh

Figure XP now gives me 1/7.5 PV.
Tried to find information on figuring with a full size lap TOT but I can`t seem t find any useful info....

Thats because a full size tool on top should push the edge spherical: not ideal. If you have 1/8 wave *wavefront* then you are done.

[Garyh]

Ah that explains why with the TOT it fixed the over corrected center!

Tonight I pulled out my mirror for another go.
First I tried something I found on the net and that was TOT working the edge with small W strokes to lower the outer zones. Done this a few laps around but no real improvement, actually made the wavefront drop to 1/6 PV.
Pressed for another hour then done a few minutes the normal figuring strokes but with a few big strokes right over the center and brought the center of the mirror right to the edge!
This has worked great and the edge now looks much better....no TDE and the mirror looks very smooth right to the middle.
Done two tests with the mirror with the coudermask and put the info into figureXP..First lot of data gave me 1/8.8 PV...(20min after figuring) and the second test done 30min later gave me a even better readings and FigureXP now gives me close to 1/20 PV. I shall have to give it a few more tests when it cools down some more to get a more accurate reading.
But it looks like it shall be better than 1/8 PV..
Shall get some images posted tomorrow after some tests with the ronchi mask and the coudermask.
cheers Gary

[Garyh]

Hi,
Well finally got around to giving the mirror some tests with a mask like Mark suggested and done some knife edge readings with a 4 zone mask.
Looks pretty good to me... Edge is still under corrected but with the eye its not really noticable.
Have attached a image which shows 1: lines marked on the mirror at center of white bands..2: knife edge nulled at around 80% 3: ronchi outside COC and a ronhi shot inside COC....Not the best pics but its not easy getting nice pics with my setup..
Mirror looks fairly smooth through the zones but I notice a bright ring/line on the same side as the light source when the outer zones are nulled..
Would this be a diffraction ring? or be caused by the very edge being under corrected? Also the edge of my mirror in my image outside COC be from diffraction? as it doesn`t look quiet like this by eye.
Have attached a image of the results from a average of three readings from FigureXP.

cheers Gary

[Satchmo]

Yes you can see the edge is a little flat ( turned up ) on the foucaultgram. The bright edeg on the same side as the light source would confirm a slightly steeper turn up at the very edge. While I'd always treat wave ratings on a limited sample of points cautiously I 'd say you are done. It is always a good idea to star test before aluminising just to check for astigmatism.

[Garyh]

Thanks Mark for your input!
Always handy to get advice from someone who knows what he is doing!
Just one more question,
Is a TUE as bad as a turned down edge of the same magnitude?
Everyone talks about how bad a TDE is on image quality but not much is mentioned about the effects of a TUE?..
Shall put the mirror away till I get the OTA finished to do that star test!
Thanks again!
cheers Gary

[Satchmo]

The very narrow flatter edge looks to me very ninimal ( <1/10 surface and the very edge doesn't look that bright. Its hard to calculate such impacts but keep in mund the relative surface area is very small. Zambuto mentioned on his forum that many of their machine finished mirrors have some small turned up edge. I would just give it a star test before you aluminise and etablish you are happy with the infocus image: at the end of the day that is what counts.

[Starkler (Geoff)]

Perhaps much of that could be removed by beveling the edge a bit more

[Garyh]

Thanks Mark, yes it is only a very narrow edge maybe 1mm or so wideand would be only a very small part of the surface area....
I suppose it would be easy to bevel out with care but knowing me I will take a big clamshell out of it!
I shall consider that Geoff if it shows on the startest!
cheers and thanks Gary