From: "Wolfgang Renz" <w_renz
To: "Tan Thiam Guan" <tgtan
        "Dennis Hohman" <dennishohman
Date: Tue, 11 Apr 2006 23:01:18 +0200
Date: Tue, 11 Apr 2006 23:01:18 +0200
Organization: privat
	login:3dee00f7b36e90fde3f9fae8583d3574
	k3BL1RXe011211
        Don Goldman <don

Hello

I asked Don directly if he has a clue (see his attached response
below). He suspected that internal refections might be part of the
problem.

But internal reflections inside the filter, chaimber window and cover
slip and between these and the CCD are usually just in the low %
to o/oo range and should be about equally deformed like the star
image, in the center of the FOV concentric and in the corners
shifted towards the center.

camera lenses out of focus, I also get (beside some smaller halos,
prolly due to some kind of internal reflections) radially or tangentially
elongated stars. Depending on the actual focus and used filter, they
sometimes even look like crosses. As they don't look equal in all
four coners, I might have a minor tilr issue.

Clear skies
  Wolfgang

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



From: Don Goldman 
To: Wolfgang Renz 
Sent: Tuesday, April 11, 2006 3:57 AM
>
> Hi, Wolfgang,
>
> First I've heard of this.
> 
> I should really join the aavso discussion group to address these
> questions directly.  Please pass this on if you like.
>
> As you know the green filter is made from 2mm of GG495 and
> cured (Norland 61) epoxy layer.  So, the Schuler filter have no
> dielectric coatings.  They are made with 2 or 3 Schott glasses.
> Thus, they are not angularly depedent from a spectral perspective.
>
> However, the multiple substrates can and most lkely will lead to
> internal reflections that will cause halos.  These halos become
> larger with faster optics.  A rough fomula that I describe on my
> web site is:
>     Physical reflection distance (mm) =  (halo diameter in pixels
> Thus, for an f/9 system with a 50 pixel diameter halo around a
> bright star unbinned with 9 micron pixels, you get:
>     D  =  ( 50 x .009 x 1 x 9 ) / 2  ~  4./ 2  = 2 mm
> This is comparable to the internal reflection betwee the epoxy
> and either surface.
> At f/5 that same spacing would produce a halo about 88 pixels in
> diameter.
> 
> Thus, it may be possible that internal reflections may be causing
> some of these effects.  Note that the halo may not be symmetric
> around the star if the filter is canted to the optical axis. Scroll down
> at:    http://www.astrodon.com/oldsite/Reflections.html
> and see an internal reflection halo from a Schuler filter and see
> how this could distort the star shape. These are at f/10.
> 
> Also, note that the 1mm thick Astronomics will produce even
> smaller halos that may only extend beyond the star a little bit.
> 
> I don't have an absolute answer, but I think this may be part of the
> problem. As long as we are looking through multiple substrates,
> this could be a contribution. 

Don Goldman
Astrodon Imaging
www.astrodon.com



From: "Wolfgang Renz" <w_renz
To: "Don Goldman" <don
Sent: Tuesday, April 11, 2006 1:30 AM

> Hello Don
> 
> to get a wider PSF for a more precise bright star or exoplanet
> transit photometry.
> introduces severe astigmatism with fast focal ratios.
>
> Have you heard already anything about this kind of low focal ratio
>
> Can it be avoided/corrected/adjusted ?
>
> Clear skies
>   Wolfgang
>
> Wolfgang Renz, Karlsruhe, Germany
> Rz.BAV = WRe.vsnet = RWG.AAVSO



From: "Dennis Hohman" <dennishohman
Sent: Monday, April 10, 2006 11:46 PM

> 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
> 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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