Cannot correct vignetting from my MMOAG off-axis guider

asimen

Member
I have had an issue getting good flat frame correction of my images with my current setup where the vignetting from my MMOAG is not being corrected.

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
My issue is that I get good flat field correction of dust donuts and most vignetting but there is some residual vignetting from my MMOAG off-axis guider in all of my images.

I just finished watching Adam Block’s really excellent series of YouTube videos on the Weighted Batch Processing Script v2 and I used this script for the two examples shown here.

Has anyone seen this problem?

Thank you!
 

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Silly question -- why not back it out an 1/8" or so to get rid of the vignetting?

Any chance it's really a shadow, i.e. too dark?
 
By "backing it out" do you mean moving the pick-off prism so that intrudes less into the light path? That makes sense, but I am not sure how to do that. In terms of shadowing, no, it is not a shadow.
Thank you.
 
I do not have one of those, but the little Celestron OAG I have has a set screw and you can just loosen it and lift it out with your fingers slightly.

You can probably see whether it's leaving a darker area with a flats panel and testing in daylight.

I guess I was being ambiguous -- vignetting from this is a shadow. What I meant is it was too much of a shadow and not enough data for the flat to correct. The other possibility is the flat panel or technique is somehow not casting the darker area of the same size, maybe due to the angle of illumination. I'd still suggest the answer is to get it out of view entirely not trying to fix it with flats. One reason for such a nice large prism is you can just stick it in the middle of the wide side of the frame and never move it, and have plenty of room to get good star images. With a full frame diagonal of about 47mm or so image circle, and narrow side of 24mm, there's plenty of room to keep the prism out of the frame (just speculating you are in a full frame camera).
 
All great thoughts. After some research it seems that Astrodon (now Farpoint) do sell prism spacers, but they are meant to move the prism further into the light path. I have contacted them to see if there is a solution. In terms of placing the prism at the narrow side of the chip, my sensor is square. I hate to lose expensive real estate on the chip!
 
Wow, for that expensive of a device you would think the prism could move in and out.

I've often wondered why more astro cameras are not square, since astro imaging circles are, well, circles not ovals. I know the answer (reuse of terrestrial sensors), but that just begs the next question, why are regular cameras not square since they also have round imaging circles.

Anyway, good luck. I guess this still begs the original circle, whether you can do something better with flats. Are you using sky flats or a panel? One thing that always confused me about flat panels theoretically is that it seems like their light would not be parallel as sky flats would, and whether the shadows from dust (etc) really was the same or different because the light cone changes shape. I use a flat planel, but I still wonder.
 
I am also surprised that the prism cannot be moved more easily. I may need to fabricate something. I agree with you regarding square sensors. I find it helpful for composing images.

I am using a flat panel. I think that the reasons that you propose for the issue make sense. I do have a method in PixInsight for removing artifacts left over after flat field correction (see https://pixinsight.com/forum/index....ning-layers-due-to-loss-of-information.15348/) but it is really labor intensive.
 
Have you checked if the Prism spacer is not already installed ? MMOAG come with on extra spacer.
 
Thank you freed, it turns out that this was the issue.

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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