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Old 04-12-2009, 11:04 AM
rally
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Join Date: Sep 2007
Location: Australia
Posts: 896
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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