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member
Reged: 04/05/06
Posts: 10
Loc: New Mexico
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Hi,
I've got about 13 hours of polishing in on my 8" f/5.2. I'm wondering how much pressure to use. I place my right palm in the middle of the tool, TOT, and use a gentle pressure. The glass is very clear, even under an 8x loupe, but with my green laser, I still see quite alot of surface anomalies.
I'm wondering how much polishing pressure, hand placement, and stroke type that people in the forum use and how many hours of rough polishing they do before moving on to sphereizing.
Under a green laser, after finishing polishing, do you still see reflections in the glass surface?
Thanks,
Eric
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Home-made home-ground 8" f/5.2 Newtonian
Stellarvue 80/9D
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Televue Nagler T5 31
Televue Nagler T4 22, 17, 12
Zeiss Abbe Ortho II 10, 6, 4
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1671
Loc: Fort Worth TX
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My 20 millidollars,
Yes, the fully polished surface will reflect laser light very well. But if by "reflected" you mean being able to see residual "salt and pepper" (incompletely polished pits) at the surface, the laser is a very severe test for polish quality, able to detect surface defects far below levels visible at the eyepiece. The old saying "a day's dust" means that you can polish your heart out and get an absolutely pristine surface that shows no laser beam spot on the surface, then have it coated and mounted in the scope, and one day's dust undoes all your hard polishing work. But it doesn't affect the image contrast or resolution one bit. You can stand a few tiny remaining pits here and there, as long as the surface isn't grayish. Aluminization is too thin to fill in pits; it only lines them very nicely and makes them scatter more light.
Regarding down pressure during polishing but before figuring, 1/4 to 1/3 PSI is probably a good down pressure rule of thumb. Your 8" is a little over 50 in^2, so bearing down between 10 and 15 pounds will speed up the polishing. Lighten up when parabolizing to 5-10 pounds max. Smooth figures are the result of thousands of blending strokes, not deep fast polishing in short spells with excessive downpressure, which makes zones and irregularities.
Mark/Mark? Comments?
Mike
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56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and others.
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2316
Loc: salem, OR
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Yeah, you can always blow the dust off!  Oh for a can of "Pit-B-Gone"!
Pits can do more damage than dust, I expect, because they can be very small but numerous, but I can't prove it. I'm a fanatic, I don't like to see any residual pits on the glass and I'll polish until they're gone. It's easier by machine, though. I also see an effect where the so-called "black polish" appears, and without having SEM imagery prepared it seems to me that there's a level of micro-surface smoothness beyond strictly polished out that results in a higher reflectivity on the uncoated surface. It's the same time that the contrast starts to peak, and that a light source off axis shows no scatter off the surface. I doubt that adding some dust to that surface does much more than absorb a tiny fraction of the incident - and look bad under a flashlight.
I run polishing pressures of right around 0.3 lbs per square inch, although at fairly high speed by machine, averaging maybe 30 inches/second. Figuring uses a bit less, around 0.2 PSI and about 8 inches/second, and down to half that when needed (for small correction with good blending).
Bottom line, for polishing out by hand, you really can't use too much pressure. Use whatever the lap will sustain in terms of heat - then lighten up when polished out and do a couple hours of gentler work to prepare for figuring.
Best,
Mark
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3401
Loc: Northeast
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I like to see the tiny pits just about -GONE- with a red laser, straight on incidence. It's a rather unforgiving test, but for offering customers less than that, and being that I finish polishing on the machine, why take the chance?
As to pressure, I use -feel- more than pressure as a guide. A nice smooth drag, and it makes me happy. Mark
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So many projects, so little time!
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what?
member
Reged: 04/05/06
Posts: 10
Loc: New Mexico
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Ok, sounds like I'm not using enough pressure by all accounts. Under the green laser, I've got way more than a few pits here and there, so I'm going to try and polish them out, hopefully the extra pressure will help alot. You guys have given me the confidence to forge ahead.
Thanks much,
Eric
--------------------
Home-made home-ground 8" f/5.2 Newtonian
Stellarvue 80/9D
---
Televue Nagler T5 31
Televue Nagler T4 22, 17, 12
Zeiss Abbe Ortho II 10, 6, 4
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3401
Loc: Northeast
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One thing I noticed lately with BK7 lens... in polishing with a pitch polisher, the darn thing would -NOT- get smooth action until practically all the pits were gone. After that, the fit was a piece of cake, with a gorgeous clean surface. I used the laser to detect the residual 5 micron pits. Quite a suprise how noticeable this was. It's not the only glass that acts this way, either.
Mark
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So many projects, so little time!
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