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Old 04-12-2009, 12:38 AM
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wysiwyg (Mark)
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Star Burst Pattern with FSQ-106N

Hi Guys,

In my recent images I have noticed a star burst pattern on brighter stars, predominantly near the edge of the frame.

I am using a FSQ-106N with a STL-11k mono and a MMOAG.
Have tried several things now to see if I can trouble shoot the problem but I am running out of ideas.
So far, I have removed the OAG prism from the light path to no effect, took an image with no filter to eliminate the possibilty of reflections from filters.

The only thing left for me to do next is to remove the MMOAG and attach the STL directly to the FSQ-106N.

I have attached an image of the artifact I am getting.

Hoping some of you have had experience in having seen this before and might be able to shed some light on this problem.
I am reluctant to think its the optics of the FSQ as it has a very solid track record being a TAK and all.

BTW, this is a very small sub-frame of the original image, focus was dead on and it was taken in Very Dark skies (Hawker) with no moon, or near street lights and no stray sources of light.

Cheers
Mark
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  #2  
Old 04-12-2009, 07:20 AM
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multiweb (Marc)
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I've seen that on a number of FSQ106 on very bright stars on the edges. Maybe it's normal.
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Old 04-12-2009, 09:57 AM
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wysiwyg (Mark)
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Quote:
Originally Posted by multiweb View Post
I've seen that on a number of FSQ106 on very bright stars on the edges. Maybe it's normal.
Marc,

After having read your reply (thanks by the way)!
I visited some Gendler's and Pugh's websites to see their FSQ-106 images.
Guess, what I noticed the same star burst pattern on their images, predominantly on bright stars close to the edge.

It seems this artefact seems to be the norm for FSQ-106's.

Would like to have an idea of what is causing though.

Cheers
Mark
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Old 04-12-2009, 11:04 AM
rally
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Here is one explanation I found - not sure of its reliability ?

Q
Does anyone have any idea what could be causing this strange phenomenon?
The "radial shadows" appear to "rotate" with the field of view. For example, the shadow in the lower left star of Orion's belt is oriented southwest to northeast, the middle star's "shadow" is oriented west to east, and finally the upper right star is generally northwest to southeast.

A
The artifact you are seeing with your FSQ is normal.
The FSQ-106 is a petzvel optical design and as such it has an internal field stop.
This design allows for a flat field and virtually no astigmatism over a wide field. But there is a cost.
The field stop causes a fair amount of vignetting off axis as it does not allow the full aperture of light to reach the off axis image plane.
The off axis image is really an oval or cat-eye section of the 106 mm aperture.
It's the field stop that's also responsible for the dark spikes you see on bright stars off axis.
On axis stars up to about 25 mm will not exhibit this affect The dark spikes are the diffraction results of the field stop.
Basically the third lens in the system is the field stop.
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Old 08-12-2009, 10:34 AM
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That sounds like the answer Rally, and it makes sense too.
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Old 09-12-2009, 04:27 PM
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Yes sounds reasonable enough... My thoughts would have been filters, however now that you mention it I have seen those same diffraction patterns on plenty of FSQ106 images... Although not all... I have seen quite a few images through FSQ106's that do not show these... However that could be one of the differences between the FSQ106ED, FSQ106N / Fluorite models...

I suppose someone who's had both could probably tell you...
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