From: "Dennis Hohman" <dennishohman
Date: Mon, 10 Apr 2006 17:46:57 -0400

Hello TG,

I also use 48 mm Schuler filters and have noticed the same phenomena. The
focusing pattern is exacerbated when the filter is being used at F6 or
faster. My first setup was an F10 8" SCT , F6.3 reducer and then the filter
wheel and camera and I observed the same kind of focusing pattern that you
are seeing although round stars (more or less) did appear right at focus.

My current configuration is the F10 8" SCT, Filter wheel, Optec F5 reducer
and then the camera. I hardly see the strange focus pattern I saw before.
The filters work much better when the light cone is not steep and passes
through at more or less right angles to the plain of the glass. 

I suspect your refractor is either F5 or F6 and the light cone is pretty
steep so the light comes through the filter at an angle and results in
oblong images out of focus. I believe these filters are of the interference
type and depend on light of certain wavelengths traversing or reflecting off
the layers in a certain phase relationship which only occurs at right
angles. 

   Dennis Hohman (HDF)
   Stone Edge Observatory
   Orchard Park, NY  USA 
    




 
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


_______________________________________________