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I am seeing an issue that I don't understand, which is a single-corner vignette that appears to be changing over time. Below are a sampling of images I've taken with generally the same setup. Relevant details are included. I would greatly appreciate any help that you all can offer, as this is really driving me mad. Details:
In chronological order: The Crescent Nebula. No field flattener. 5m exposure @ 300 gain. This was overexposed, and you can see some amp glow in both the right corners.  The Bubble Nebula. No Flattener. 4m exposure @ 200 gain. Here you can see an obvious vignette in the top right corner.  The Tadpoles. We added an Astrophysics Quad TCC .72x reducer. 4m exposure @ 200 gain. It actually looks like the amp glow creeped back in. My suspicion is that I didn't account for the lower F-stop after adding the TCC, so it's overexposed again.  Veil Nebula. Quad TCC. 105s exposure @ 200 gain. Here you can see it's now the bottom right corner that has the issue.  Horsehead. Quad TCC. 25s exposure @ 139 gain. Finally, some consistency. The bottom right corner looks off again.  The kicker is that I don't own the telescope, so I can't completely test this on my own. A friend owns the scope, I own the rest of the imaging equipment, and we meet when there's a clear night to image. So this weekend I've been testing what I can at home, sans-telescope, basically pointing the camera at a white wall with the filter wheel attached, but nothing else. Taking 1s exposures through the O-III filter and 0.0058s exposures through the L filter, which looks... interesting. I have no idea if these are of any value, but you can pretty clearly see the lower right corner looks darker in both. Again, these have a PixInsight auto-stretch applied. Of note: I have tried these with multiple filters and multiple orientations of the camera with respect to the filter. The corner in question is always the bottom right. The lone exception is the Bubble Nebula above, which I cannot explain. O-III:  L:  My questions are: 1. Can anyone explain what's going on? Is this a "feature" of the camera sensor, or something else? 2. In your opinion, would this calibrate out if we took flats? We don't typically take flat frames, but I'd be more than happy to make time for them if they would calibrate out these horrid corners. Happy to try anything people would suggest given the limitation of not having the telescope in my physical presence. Also happy to take notes for test shots to take next time we have the scope out. Thanks much for any ideas or suggestions! |
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| For the stripes in the L filter flat, the exposure is too short so you pick up the 120Hz flickering of your light source (or 100Hz if you live in the place where the electricity is 50Hz). |
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i have the same filterwheel,you need to put the asi as close in the wheel. with flats you can get it away. |
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| I had similar vignetting issues and it was that my filter wheel needed to be calibrated. Basically, the filter wasn't square in the center of the sensor. I have the ZWO EFW, which I was able to calibrate and lined up the filter. Maybe your filter wheel has the same feature? Worth a shot. |
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John: In my testing, I noticed the filters were closer to one edge of the wheel than the other and that my camera was mounted on the side where they were farther from the edge. So I tried moving the camera to the other side so that the sensor was closer to the filters. I had to remove the center screw on the back of the wheel, but it seems to be fine without it. Tommy Lease: Interesting. I can actually see that the filters don't rotate perfectly in alignment with the sensor. BUT - the filters are 36mm which more than cover the ZWO's sensor, even with the slight misalignment. Do you suspect that could still cause problems? If so, how did you calibrate your wheel? It sounds like it may have a calibration feature built in... I'm not aware of mine having such a feature, but will definitely check it out. |
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| I think its a simple alignment issue of the image train. |
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Jeff Bennett: Can you elaborate? How would one go about correcting such an alignment issue? Sorry if this is a basic question. |
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Taking flats and dark flats will fix your image clear skies |
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John: If you have 36mm filters, then the filter wheel misalignment might not be your issue. But again, worth a shot to calibrate it. |
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Jeff Bennett: Many scopes have "tip/tilt" push/pull screws on or outside the visual back or focus tube. My Televue 129is does, all quality RC's do. You just have to adjust the screws, take images like these, and see which way the vignetting moves, readjust, image again and repeat. I doubt it has anything to do with the FW or FW filters. Honestly I don't think what you have here is that big of deal. For corners, edges etc, just dynamic crop out the corner. If that removes an important part of the image, start over with new frames. |
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Here's a single frame of a recent integration. I clearly have vignetting, but I don't worry about it. |
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But the final is corrected: https://astrob.in/27zr6v/0/ Good luck |
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In your crescent nebula, you describe amp glow in the right corners... This is actually a large Ha cloud, not amp glow. your data seems ok. clear skies. |
