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Excess vignetting from pick-off prism from Monster MOAG

Equipment Imaging
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#1 ASIM

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Posted 12 April 2021 - 09:09 AM

I am seeing a good deal of vignetting from the pick-off prism of my Astrodon Monster MOAG on my system. Please see the attached example.

 

I have the following equipment:   

AG Optical iDK 12.5” telescope
Optec Gemini rotator/focuser
Astrodon MMOAG off-axis guider
Lodestar CCD guide camera
FLI PL-16803 CCD camera
FLI CFW-5-7 filter wheel

 

I see that Astrodon (now Farpoint) sells prism spacers, but they are meant to move the prism further into the light path.

 

Has anyone dealt with this problem? Is there a way to move the prism further out of the light path?

 

Thank you!

 

Attached Thumbnails

  • Example 1.jpg


#2 Dynan

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Posted 12 April 2021 - 09:24 AM

Looks to me like an aluminum spacer (0.125" [1/8], 0.063" [1/16]) to ADD distance from the light path would be easy to make. Install at the top of this configuration:

 

MOAG.jpg



#3 TOMDEY

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Posted 12 April 2021 - 09:43 AM

That's the one biggest detractor of Off-Axis Guiding. And your nice expansive 16803 chip exacerbates the problem. Are you collecting your flats with the prism in that exact same position and setting? That should calibrate out. Prisms oft seem unnecessarily large, but it's a Catch-22.    Tom



#4 ASIM

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Posted 12 April 2021 - 11:34 AM

Thank you Dynan for the really helpful suggestion. I will give that a try.

 

TOMDEY, unfortunately this does not calibrate out. I do collect my flats without adjusting the prism, focus or other parameters.

 

I have figured out a way to remove the remaining vignetting and other artifacts not removed by flat fielding in PixInsight using the residual layer from a Multiscale Median Transform and the Clone Stamp tool, but it needs to be applied one image at a time which is really painful but works pretty well. See https://pixinsight.c...ormation.15348/



#5 rockstarbill

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Posted 12 April 2021 - 01:56 PM

Switch to the Sagitta OAG, which has an adjustable prism that you can raise or lower without removing the imaging train.
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#6 ASIM

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Posted 17 April 2021 - 12:58 PM

It turns out that my unit had two spacers under the prism that pushed the prism further into the light path. See the attached image.

 

After removing both spacers I no longer have any vignetting from the prism assembly. I will need a clear night to see if I can still get sufficient guide stars.

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  • MMOAG_small_arrows.jpg

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#7 Dynan

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Posted 17 April 2021 - 01:11 PM

Not being familiar with that particular OAG, I couldn't tell that they were spacers when I suggested 'counter spacers'. 

 

Good catch!




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