Greyhoundman
sage
Reged: 11/20/07
Posts: 283
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Everything I read is for 6" and up mirrors.
What would be a good size to make the squares on the lap for a 4.25" mirror?
Can you tell I'm new to this? 
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http://www.flickr.com/photos/greyhoundman/
http://greyhoundman.blogspot.com/
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kfrederick
sage
Reged: 02/01/08
Posts: 212
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doesnot matter much maybe one inch square or bigger
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Mark Harry
Carpal Tunnel
Reged: 09/05/05
Posts: 2470
Loc: Northeast
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My preference for the smaller mirrors- 3/4"~, but as Kevin says, not really critical. For a long focus sphere, I'd use the whole surface as solid, with concentric grooves spaced 1/2" apart, and one radial groove to prevent a partial vacuum from forming. (both types with light scratches for microfacetting)
M.
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Scopes in the works-
Too many for putting down here! Favorites- 8" F/6, 8" F/4.72, 4.5" F/5.4, 14" F/4.455, all completed.
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Greyhoundman
sage
Reged: 11/20/07
Posts: 283
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Would window screen work in place of scratches?
I've been researching a bit.
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http://www.flickr.com/photos/greyhoundman/
http://greyhoundman.blogspot.com/
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Gary Fuchs
professor emeritus
Reged: 05/22/06
Posts: 612
Loc: Easton, PA, USA
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I used 1" and 3/4", both with onion bag microfaceting.
Gary
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2253
Loc: Courtice, Ontario
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Quote:
Would window screen work in place of scratches?
I've been researching a bit.
In my (very limited) experience I've found scratching with a sharp blade much nicer than window screen or onion netting. It doesn't disturb the nicely pressed face nearly as much and it only takes a minute to scratch up the surface of the lap rather than pressing for a lengthy time to press in the pattern of the screening - and then a longer cold press afterward to get good contact compared to the scratching.
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Ryan
Antares 200mm f/6 Dob & 130mm f/5 Travel Dob.
Projects on the go...
- a couple 80mm SS refractors on the back burner.
- a few small mirrors awaiting polishing
- 260mm f/7.15 mirror... still polishing
Member of DRAA
 My house.
 DRAACO
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Greyhoundman
sage
Reged: 11/20/07
Posts: 283
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Thank you all for the ideas. Right now the project has gone back on the shelf. Guess I'm not the grinding type.
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http://www.flickr.com/photos/greyhoundman/
http://greyhoundman.blogspot.com/
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Beri
sage
Reged: 06/29/05
Posts: 269
Loc: Croatia
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I use an old automotive rubber carpet that has a square 20x20 mm pattern on it. Pour pitch on the lap, smear some CeO on the mat so it doesnt stick, and press in.
Wait a few minutes to cool down and peel of the mat. Remove the pitch that went over the edges and you are ready to press it against the mirror
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15x70 Binoculars
no scope right now
http://www.scopemaking.net
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Adventurer
professor emeritus
Reged: 11/22/07
Posts: 703
Loc: Planet Earth
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I use window screen when warm pressing, so I get better patterns.
if you are not familiar with using blade to scratch the pitch, you might ship it.
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"man is not made for defeat....A man can be destroyed but not defeated"
Regards
Adventurer
Meade ETX125 Telescope.
Home made 10 inches reflector.
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kfrederick
sage
Reged: 02/01/08
Posts: 212
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you wont to use fiberglass window screen if you use screen/ i use a plastic pet fence to hot press in the pattern . i donot cold press much just start polishing after i press in the screen pattern it makes contact fast but i like to polish for at least a hour non stop /then get the lap of the mirror donot leave the lap on the mirror when you are done polishing . also with my hard water i clean the mirror when done polishing or i get a film on the glass that will not come off/ kevin
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Greyhoundman
sage
Reged: 11/20/07
Posts: 283
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Mark mentioned a set of concentric grooves with a radial.
I'm considering 3 concentric and radials to divide those every 45 degrees.
Maybe throw in some scratches for good measure?
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http://www.flickr.com/photos/greyhoundman/
http://greyhoundman.blogspot.com/
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 1937
Loc: Boechout, Belgium
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A long time ago we tested this pattern. I thought it would polish faster. Did not work, it gave us 'nice' zone...  I had to trow this siliconenmold away.
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Chris
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Stefan Rostyne
scholastic sledgehammer
Reged: 10/19/04
Posts: 937
Loc: Assenede, Belgium
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We don't use square lap tiles anymore. We pour our lap as a whole, then cut it with a soldering iron in a pattern like below. It takes only 5 or 10 minutes. We allways use the same pattern, now matter the size of the lap, just an upscaled version. It works very fast and yields a very smooth surface
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Stefan Van de Rostijne
4.5" F4.5 newt 5°widefield/finderscope
8" f/5.6 travel dob
old 12.5" F5 dob (used to look better...)
30 cm f/30 Classic Cassegrain (polishing primary)
23" f/4 dob project
Edited by Stefan Rostyne (08/22/08 05:36 AM)
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Mark Harry
Carpal Tunnel
Reged: 09/05/05
Posts: 2470
Loc: Northeast
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One thing to keep in mind about pitch squares- the larger they are, the more force that gets applied to the glass in specific areas.(pitch doesn't have anywhere to go) It's not critical if you are working by hand, and vary the position randomly. But with machine polishing, that can be a problem with reinforcement of certain actions on the glass.
Also, the wider/more numerous the grooves, the more slope change the polisher will accomodate, and the pitch will act softer. Particularly in a design like Chris's. You could have "saved" your mold if you used harder pitch, or faster polishing speed. We used a pattern identical to that with very hard pitch to do flats, mounted about 50 on a large block.
Mark
Mark
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Scopes in the works-
Too many for putting down here! Favorites- 8" F/6, 8" F/4.72, 4.5" F/5.4, 14" F/4.455, all completed.
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 1937
Loc: Boechout, Belgium
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Hi Mark,
We've tried different techniques with that pitch and always ended with very strange zones.
In my opinion it has to do with the shape of that pitch in combination with the size of that mirror. We had the same problem with polishingpads. Using larger mirrors there is no problem, no zones at all.
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Chris
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1365
Loc: salem, OR
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Looks familiar...  This is a 6" figuring lap.
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1365
Loc: salem, OR
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And this is a 12" polishing lap (Acculap). I press the channels in the polisher while the pitch is still soft with the narrow side of a 1/8x1/2" aluminum bar, then use the wide side to form the overall surface before final shaping with the mirror. Very fast.
Notice there are more channels in the larger example? Otherwise the pitch flow rate isn't as good, that's why.
Best,
Mark
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