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Re: [ATM] Focal Length Accuracy
Good morning Bob and all,
Bob thanks for your reply. You made comments regarding stroke length.
You pointed out that a longer stroke length will work the edge more. And
you commented that shorter stroke lengths will bring the mirror back to
a sphere.
Well, I wasn't going to bore the list with my simple project. However,
I'm so happy with having learned something, I figure I'll go ahead and
put everyone to sleep.
I actually concluded your exact comments empirically. I reverted to 120
grit with TOT in the hopes of quickly decreasing the sagitta. I also
maintained the standard 1/3 stroke length. However, the progress
remained very slow. Maybe ~ 0.0005 inches of sagitta depth per 15
minutes of work.
I mulled over the geometries for a while and decided to try a longer
stroke length. Sure enough I quickly overshot my target depth in the
space of 15 minutes. Awesome...
I was then suspicious of the tool to mirror contact. So I ran a quick
sharpie test. The test readily revealed no contact in the center.
Hehehe... I then fell back to a more conservative stroke length and the
tool and mirror quickly came back into good contact. Lastly, a test
against the sun showed the focal length to be in the 36 to 37 inch
range. Whew...
So in conclusion, I learned that TOT along with longer stoke length
quickly shallows the mirror. And, shorter strokes bring the mirror back
to an approximate sphere. Very cool...
Anyway, I think I've completed 220 grit again. I need to clean
everything up and move on 25u.
Anthony
Round Rock, TX
> -----Original Message-----
> From: atm-bounces@atmlist.net
> [mailto:atm-bounces@atmlist.net] On Behalf Of Bob May
> Sent: Wednesday, June 06, 2007 1:44 AM
> To: atm@atmlist.net
> Subject: Re: [ATM] Focal Length Accuracy
>
> You'rede going the right way about things. Use longer
> strokes to hog out the edges more and the ROC will change
> faster. Shorten the storkes to insure that you get more back
> to a sphere after the ling strokes. At that radius, you can
> get to within a tenth of an inch if you so desire by
> correcting as you go down through the grits.
> For a pulley, take any old zinc cast pulley of the right size
> and grind for a few moments against a piece of glass. This
> will even out the working edge and you can refine that by
> filing the edge to a sharper point. You don't have to ha the
> dial indicator exactly in the middle for a larger pulley
> although the closer, the more accurate. Check the math for
> how much of an error you get when you're a little bit off.
> I've found that mounting the dial indicator rigidly to the
> rest of the tool is more imporant than getting it exactly in
> the middle of the disk as any tilt or wobble of the dial
> indicator will make for larger errors.
> Bob May
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