Re: [ATM] astigmatism as seen via ronchi?

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

I'll check for plumbness, but I know it's very close to being vertical, and, IIRC, this phenomenon only happens on certain mirrors - ones that I suspect have a good chance of being astigmatic, and, in fact, had astigmatism pretty bad in the past (a 3/4" pyrex 16.5" diameter f/5 mirror, for example).
I think i've seen

Mike Lockwood <melockwo@uiuc.edu> wrote: Hi,

Don R Surles wrote:
> You may also want to check the ronchi screen for plumbness...if the
> ronchi screen is not very close to vertical then the lines will
> not be vertical and they will change angles from inside to outside
> of focus.

I believe this is impossible, at least as I do the Ronchi test.

There is no reason the lines would show symmetry about an arbitrary 
"vertical" axis as one goes in and outside of focus.

For a spherical mirror, the tilt of the lines on one side of focus is 
the same as on the other.  Any change in tilt is caused by the shape 
of the mirror under test.

 Mike Lockwood

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Guy Brandenburg, Washington, DC
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