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Troubleshooting FAQ | Problems? PM a Red or a Green Gu.... uh, User
JohnH
professor emeritus
Reged: 10/04/05
Posts: 505
Loc: vancouver near the wilds of B...
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I wanted to start using this tool, but without a flat (like a machinists granite plate) I could only think of this method.
1. Zero it on the flatest surface available (side of fused silica blank)
2. Measure the sagitta of a polished but not parabolized mirror.
3. Measure the convex curve of the tool used to grind the above mirror.
4. Note any difference (other than sign) to figure out a correction amount.
5. Start using calibrated spherometer
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John Carruthers
Skiprat
Reged: 02/02/07
Posts: 1705
Loc: Kent, UK
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Sounds good, what's the smallest graduation you can read on your scale? I'm guessing 0.001" or so? if so hen tclean, new, float glass is well within the tolerance for a zero calibration.
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Jc
ATM 10" F6.1, 1/25th wave spec (max wavefront error +/- 1/12.6 in zone 4 of 6, sodium light )
6" F7 spec
127mm F9.4 Refractor
10 x 50 bin
ETX80 (finder)
Canon 20D
and a curious mind
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Owen
sage
Reged: 06/21/07
Posts: 353
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All I can say is float plate glass. 6mm should do...
Owen
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DAVIDG
Pooh-Bah
Reged: 12/02/04
Posts: 1451
Loc: Hockessin, De
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Here is a "trick" I learned from an old master. Zero the spherometer on either the tool or the mirror that your working on and measure the other piece. Take the reading and divide by 2 and that is your sagitta. It will be a little off because as you start out, the tool and mirror are not in perfect contact. The accuracy goes up as the grit size goes down.
Also zeroing the on the fused silica blank is going to work well.
What tolerance are trying to achieve ? Even if you zero on an optical flat you also need to corrector for the diameter of the ball feet and any error in the placement of those feet.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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