FreeLists / az-observing / [AZ-Observing] Re: Observing Weather this Weekend

["Steven Dodder" <sdodder@xxxxxxxxxxx>]

My reference is "Star Testing Astronomical Telescopes" by Richard Suiter. 
Defocusing and counting the rings sets a standard for opposite sides of
focus.  (Ten rings on one side, check for aberations, ten rings on the other
side and repeat.)  It also makes it easy to see zonal errors as Brian said. 
It is not necessarily in itself an optical test, but if the optics are bad,
you certainly would not be able to discern many layers of rings due to
scattering, aberations and the like.  Suiter's star test is a great
beginningpoint to find obvious optical problems such as pinching,
collimation, astigmatism, spherical aberation, tube currents and so on, and
correct them. 
Overall qualities such as flatness of field and coma are more easily
analyzedif the easy stuff is eliminated first.



Steve and Rosie Dodder sdodder@xxxxxxxxxxx Visit my web site at
http://www.stargazing.net/Astroman 


>From: Brian Skiff Brian.Skiff@xxxxxxxxxx[1]
 The number of
>rings per se isn't important, though Back suggests 10 or more.  

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