From: "Wolfgang Renz" <w_renz
To: "Tan Thiam Guan" <tgtan
References: <003201c65be1$7f270270$6400a8c0
Date: Sun, 9 Apr 2006 21:11:07 +0200
Date: Sun, 9 Apr 2006 21:11:07 +0200
Organization: privat
	login:3dee00f7b36e90fde3f9fae8583d3574
	k39JB73u014001

Hello Tan

> 1. Does this sound like a problem with the quality of the filter? 

That sounds pretty much like astigmatism. See:
http://www.vanwalree.com/optics/astigmatism.html
http://www.olympusmicro.com/primer/java/aberrations/astigmatism/index.html (Java Tutorial)
http://www.olympusmicro.com/primer/images/aberrations/astigfigure2.jpg
http://aberrator.astronomy.net/html/astigmatism.html
http://aberrator.astronomy.net/assets/auto_generated_images/img_475a63a0.jpg

Are you sure that its not caused by the refractor ?
Stress on a lens often causes astigmatism. E.g. when a lens
is mounted under tension or the lens isn't cooled out well,.
Have you tested your refractor defocused also without filter ?
Was the refractor cooled out well when you made the test ?

If its not the refractor, the filter might not be polished plan parallel
or is mounted under tension. But I doubt that it can cause major astigmatism when the filter issue isn't severe.

Many optical designs show increased aberrations when used out
of focus (and off axis). So you might have to l live with it if you cann't
resolve it

> 3. Will this affect the precision of photometry?

You might get a slightly worse SNR if you have to choose the
aperture a bit wider.

4.  Should I ask the vendor for a replacement?

First ensure that it wasn't the scope.

Clear skies
  Wolfgang

Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO



From: "Tan Thiam Guan" <tgtan
Sent: Sunday, April 09, 2006 4:25 PM


Hi all,

I recently bought a Schuler V filter (2") from Adirondack.  In using
the filter for the first time last night, with defocused stars, I noticed
that the star images are not round (as they are with other filters I
am using from Astronomik).  Star images are severely oblong,
with the long axis switching direction depending on whether the
telescope was inside focus, or outside.  I was using an 80mm
refractor and CCD.

registration of images with IRIS shows that the sequence of
images does not register as accurately as with round star images.
I use IRIS to do aperture photometry.  The precision I am aiming

Questions:
1.  Does this sound like a problem with the quality of the filter? 
2.  Has anyone else encountered the same problem with Schuler
   filters?
3.  Will this affect the precision of photometry?
4.  Should I ask the vendor for a replacement?

TG Tan
Australia

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