From: "Robin Leadbeater" <robin
Date: Mon, 10 Apr 2006 01:01:08 +0100
Date: Mon, 10 Apr 2006 01:01:08 +0100

Hi TG,

How are you?   A good trick to see if  the filter is the problem would be to
rotate it. If the elongation follows the rotation it is the filter or its
holder (are the threads square to the filter?) I find these photometric
filters much thicker than usual filters so small errors in squareness can
give large astigmatism problems. I have still not decided if it is best to
do exoplanet work filtered or unfiltered. The filter loses a lot of photons
which makes it tougher to get good precision with a small aperture. it
should help counteract drift due to differential extinction though.

Robin
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Robin Leadbeater
54.75N 3.24W
http://www.leadbeaterhome.fsnet.co.uk/astro.htm
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From: "Tan Thiam Guan" <tgtan
Sent: Sunday, April 09, 2006 3:25 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
milimags (for exoplanet transit detection).

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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