Flip-Flat Cover Impacting Flats

Jerry Gerber:
Dear Astrophotographers,

I saw first light last night on my remote telescope.   Other than forgetting to dither, images look good.   But there's an issue with flats.  I have a Wanderer Astro V4-EC flip-flat cover/panel and it is impacting the entire bottom edge of the flat.   How best to correct this?  I'm 900 miles away from the site so the tech person there will have to make the adjustment.  Is the cover not closing all the way? 

Thanks,
Jerry

Jerry Gerber:
I think it's probably that the OAG is too close to the sensor.  Either that or I can try rotating the OAG so that the offending shadow might be moved off the image path, although that seems a bit unlikely.   The other option is to put a spacer, maybe 21mm, between the filter wheel and OAG, increasing the distance from the OAG to the sensor.  I can only do that if I have enough scope focus drawtube headroom to bring the focuser inward to compensate for the increased distance.  Since the refractor is a quadruplet flat-field design, backfocus isn't an issue, but of course focus drawtube length is.

Jerry Gerber:

Ashraf AbuSara:

Jerry Gerber:
I think it's probably that the OAG is too close to the sensor.  Either that or I can try rotating the OAG so that the offending shadow might be moved off the image path, although that seems a bit unlikely.   The other option is to put a spacer, maybe 21mm, between the filter wheel and OAG, increasing the distance from the OAG to the sensor.  I can only do that if I have enough scope focus drawtube headroom to bring the focuser inward to compensate for the increased distance.  Since the refractor is a quadruplet flat-field design, backfocus isn't an issue, but of course focus drawtube length is.

You just need to try to bring your prisim outward towards the periphery more. Putting a 21mm between the OAG and the EFW also means you have to add 21mm between your Guide camera and your OAG prisim, which is probably too much to get your OAG camera in focus.

Oops, yep, you're right.  I had thought of that but then forgot about it.  

Do you think Ashraf from the flat image I posted above that the shadow could be coming from only the pick-off mirror in the OAG?  I would think that the mirror's shadow would be smaller and wouldn't cross the entire width of the bottom of the image.

Jerry Gerber:
That's a good idea.  The fact that the lights are fine, now makes me think it is the flat panel.  The image above of NGC 6823 wasn't cropped at all, so if there were a problem like the OAG mirror, the stars at the bottom of the image would be masked by the shadow.  This isn't the case.  A defect in the flat panel itself however doesn't explain why the shadow is so sharp.  Any obstruction to the light at that distance from the sensor would be blurred, yes?

Jerry Gerber:

Ashraf AbuSara:

Jerry Gerber:

andrea tasselli:
Try to take a dusk flat. If it is without shadow then you know it is the flat panel.

That's a good idea.  The fact that the lights are fine, now makes me think it is the flat panel.  The image above of NGC 6823 wasn't cropped at all, so if there were a problem like the OAG mirror, the stars at the bottom of the image would be masked by the shadow.  This isn't the case.  A defect in the flat panel itself however doesn't explain why the shadow is so sharp.  Any obstruction to the light at that distance from the sensor would be blurred, yes?

It is difficult to see the shadowing from the OAG in the single light sub frames, especially if they are narrowband images and regular stretch. You will see it if you stack the frames without calibration and do a stretch.

There is no amount of obstruction in front of the objective that will cause this sharp demarcation on your image. We often image with scopes with massive central obstructions. My AGO has 57% of the the front of the scope obstructed. You will not see a well demarcated dark circle in your flats. I don't expect your flat panel will cause this at all, especially if you are taking flats at the same focal point as your lights.

What your flat panel could do if it is protruding infront of the objective, is it could cause weird star spikes and/or dim your image a little.

Maybe post one of your light frame fit files here and will take a look.

Hi Ashraf,

I posted it above, NGC 6823.  This image did not employ any cropping at all.   The original subs show no sign of the shadow, same with the processed image.  The stars in the lowest part of the image show no obstruction of any kind.  Though it makes sense that the most obvious culprit is the pick-off mirror of the OAG, if that were the case, the shadow of the mirror would also be obstructing the stars.  Yet if the shaow were caused by the flip-flat cover, it would not be as sharp as it is, too far from the sensor.