[AZ-Observing] Re: Problems With Star Test - Need Advise

Jim,
If you are getting oval shapes that flip orientation 90 degrees when you go 
from 'in focus' to 'out focus' then that could be astigmatism in the primary 
mirror. I doubt that that is what you've got since you did not have it before, 
and also because you did not state that you were getting oval shapes.  

However, I wonder if you have a pinched SECONDARY mirror. It might be worth 
checking how the secondary is mounted - make sure that it has no hard contact 
with any part of the secondary holder.

Another possibility is that your focuser is protruding into the light path of 
the optical axis. This will be especially possible if you are using a sight 
tube - those sight tubes can extend many inches out past the focuser tube and 
into the light path inside the OTA.  Rack the focuser enough to where it does 
not protrude inside the OTA and see if the defect goes away.

Let me know what you find.

Silvio.


  
----- Original Message -----
From: Jim
Sent: Saturday, November 23, 2002 9:52 PM
To: Az-Observing@xxxxxxxxxxxxx
Subject: [AZ-Observing] Problems With Star Test - Need Advise
  
I just replaced my secondary mirror holder and adjusted my spider so it's 
centered in the upper cage assembly.  I now have a distorted image when doing a 
star test. When I rack-in to view the image I get what I would expect but one 
side of the circle is distorted. It's a perfect circle but the side opposite 
the focuser is distorted like the circle has a larger radius. It looks pushed 
in.  When I rack-out it changes to the other side.  Its like the light cone is 
being cut off or I have tube currents. It can't be tube currents because it's 
on the side of the image not the bottom. The mirror is centered in the mirror 
box and the secondary is centered with respect to a sight-tube. Any advise?  

Jim
Phoenix, AZ
18" Obsession

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