From: "Wolfgang Renz" <w_renz
To: "TG Tan" <tgtan
References: <003201c65be1$7f270270$6400a8c0
	<008001c65c09$5bea9880$0b01a8c0
	<002401c65cab$565e2010$6400a8c0
Date: Tue, 11 Apr 2006 00:13:12 +0200
Date: Tue, 11 Apr 2006 00:13:12 +0200
Organization: privat
	login:3dee00f7b36e90fde3f9fae8583d3574
	k3AMDBkk016599

Hello Tan

Have you checked, if the filter and the nosepiece have been screwed in
correctly ?
If one of them was screwed in off by one thread, it isn't perpendicular
anymore.

Have you inspected the filter mounting cell ?
If the cell and/or the securing ring aren't plain it might cause astigmatism
by tilt or by stress on the glass.
The securing ring has not to be tight ! Just as tight to avoid that the filter
will tilt in the cell. Some vendors like Baader don't use a threaded securing
ring anymore for their fine polished filters, but just a springy ring to ensure
a stress relieved mounting.

Clear skies
  Wolfgang

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



From: "TG Tan" <tgtan
To: "Wolfgang Renz" <w_renz
Sent: Monday, April 10, 2006 4:30 PM


> Wolfgang,
> 
> Some good links there...
> 
> The Airy pattern I get looks like the ones on the Olympus site illustrating 
> 
> So it looks like the filter.   Visually it looks flat, but I don't know if, 
> as mounted, the glass is square to the holder.  Seems strange that a bit of 
> 
> TG
> 
> 
> 
> From: "Wolfgang Renz" <w_renz
> To: "Tan Thiam Guan" <tgtan
> Sent: Monday, April 10, 2006 3:11 AM
> 
> 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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