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EDIT: consensus seems to be this is dust on camera sensor cover, or on reducer, or somewhere in the image train. Solution: Clean, then shoot again and take new flats. If you think it's NOT dust motes, please let me know below. I understand calibration frames, been using them for a year. I used them with this scope on this; https://www.astrobin.com/982lw6/ I know I'll need to shoot new ones (AFTER I clean my image train), and I'm not worried about the two hours of Tulip Neb I shot here, I'll toss that, seeing was bad. I'm glad this is just dust, not something much worse. ============================ ANYONE KNOW WHAT THIS IS? These circular artifacts are the shape of the front of the scope. They've obviously some kind of multiple reflection of it. Scope is well collimated and in focus (EAF and ASIAIR, looks out because seeing was poor this night.) Not using any filters and no filter wheel. I thought it was the Moon, the Moon was out when I shot these last week. But last night hours before the Moon was up, I shot a few subs that ended up mostly clouds, and it still had these. I'm Bortle 2, with no lights shining on my scope. I've shot a lot with this same scope and camera without having this until recently, after I took my image train apart to clean everything and put it back together. I've found a couple posts on CloudyNights about this but none have an answer. Any ideas? Thank you  |
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| They are out-of-focus dust/debris, probably thereabouts the reducer, filters, etc. This, along with other optical path variation such as vignetting, is what flats correct. The dust motes tak on the shape of the aperture when out of focus, just as stars do. |
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James Peirce: Thank you. Will open all up and clean, then put back together and try again, THEN shoot new flats. Not using a filter, edited to add that. Have you seen this particular issue with your gear? |
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Michael W. Dean:James Peirce: This is normal. The slower the focal ratio, the more prominent these motes display. The size in the image is relative to the aperture, the size of the debris, and the position of the debris in the optical path. It can be tricky to remove all of them and it is easy for more to turn up when things are disassembled. Especially on a telescope like this. Note that your flats are meant to model these defects. You want the same defects in place for flats that were in place for the lights the flats correct. If you clean the optical path and take new flats, those flats will not correct these issues. Same goes if the camera was detached, rotated, etc. Even driving around can jostle dust. So always capture the flats corresponding to the imaging and capture new flats if the optical path is changed (e.g. for each filter as swapped or for each filter in a wheel, when the camera is rotated, etc). It is worth reading up on some tutorials and the like to fully understand calibration frames. Good flats and the like play a very important role in quality of results. Good luck! P.S. You mentioned the moon. You don’t want it anywhere close to where you are imaging such that it can either shine *or reflect* into the optics. That creates its whole new range of light gradient issues. So best to just pick a different target if the moon is nearby. Light leaks are also an increasingly more prominent issue with the moon out. |
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James Peirce:Michael W. Dean:James Peirce: Thank you. I'll clean and re-shoot flats. I understand calibration frames, been using them for a year. I used them with this scope on this; NGC6888, 4 hours, Celestron EdgeHD 8". BRAAAAAAAINS! ( Michael W. Dean ) - AstroBin I know I'll need to shoot new ones (AFTER I clean my image train), and I'm not worried about the two hours of Tulip Neb I shot here, I'll toss that, seeing was bad. I'm glad this is just dust, not something much worse. Whew!~ Michael |
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| The dust is probably in your glass screen protecting the sensor. Also, just to note, from the shape of the shadow, your secondary may be out of collimation (since the doughnut shape is fatter on one side than the other, granted it is near the edge of the field). |
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| Always keep your image train clean and consistent. This way you can reuse your flats. |
