Re: [ATM] Acceptable Level of Astigmatism

["Jeff Baldwin" <bald@olypen.com>]

Set up a ball bearing test in the shop.  Shine a tiny light at the ball, the
ball at the COC of the mirror, and observe it's image in an eyepiece, which
is also at the COC (both actually close to it).  Since the ROC is twice the
FL, then the magnification is twice that as used in a telescope.  Focusing
in and out will yield either round images on both sides if it is a surface
of revolution, or 'footballs' rotated 90 degree from each other in and out
of focus if it has noticeable astigmatism.  If you use your telescope with
at most a 5 mm eyepiece (for exaple), then use a 5 mm eyepiece on this test.
That would be twice the juice as when used as a telescope, and if you don't
see the astigmatism there, you won't when it is a telescope.  Works best
when the mirror is still spherical before figuring into a paraboloid.
JB

Jeff and Glenda Baldwin
Beaver  WA
bald@olypen.com
glendabaldwin@olypen.com
http://olypen.com/bald

----- Original Message -----
From: "Peter" <peter@kitgear.com>
To: "Atm" <atm@atmlist.net>; "Atm_Free" <atm_free@yahoogroups.com>
Sent: Wednesday, June 15, 2005 5:35 PM
Subject: [ATM] Acceptable Level of Astigmatism


> Hi,
>
> A fellow ATMer wanted me to ask what level of astigmatism is acceptable
> before it becomes detectable? He's working on a 10" mirror which shows
very
> a slight bit of tilt in the Foucault shadows.
>
> Is there a simple way for an amateur to measure this parameter and
determine
> what is good enough?
>
> Thanks,
>
> Peter
>
> _______________________________________________
> ATM mailing list http://www.atmlist.net/

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