[ATM] Sun Test During Rough Grinding
Thomas Dobbs Jr
tomdobbs at gmail.com
Fri Apr 9 23:49:51 JST 2010
I see your points and agree with you, especially since you are a far more accomplished ATM than I am, but let me clarify my laziness: I bet you are able to get more accurate results with your spherometer than I am able to get with my spherometer. And I agree if it's oh, bigger than a 14" piece of glass, it's nicer not to schlepp it out of the work area.
On Apr 8, 2010, at 4:40 PM, Berthold Hamburger wrote:
>> I personally like the flashlight test and the sun test. It's easy to do,
>> more accurate than fiddling with a spherometer, and you don't have to
>> build anything to do it. All you need is a good tape measure and the sun
>> or a flashlight. I like rinsing off and moving the glass around anyway.
>> Any excuse for a little break from grinding will do...
> All you say is correct, except that I would argue your statement about
> higher accuracy compared to Spherometer. I routinely test with a simple
> spherometer of the 2 point + outrigger type as Bob suggests on his web site.
> Readings from it during grinding coincide routinely +-10mm with Foucault
> confirmed final ROC after polishing. Construction is so simple that anybody
> who can build a telescope can put one together in an hour.
> It is sensitive enough not only to show you the depth of sagitta, but also
> how you are doing in relation to sphericity and astigmatism.
> I don't do any other test during grinding except a sharpie to decide when
> the tool is broken in and has good contact. I just find it too cumbersome to
> stand in the garden with a 16"/16kg chunk of glass trying to catch the sun
> Berthold Hamburger - Cellist/Spain
> Email: behambu at artinso.com
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