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Hi everyone, I have traditionally used my 2600mm with the Antlia narrowband filters but have recently tried my hand at more RGB-L photos using the baader filters. I noticed when using my Astro-tech AT130EDT with the .8 reducer that I'm getting really bad vignetting in the corners of the shot and it's causing some bad light artifacts after background extracting and stretching, but only with my RGB-L filters. I don't see this with the narrowband filters with 300s of exposure. I recalibrated the filter wheel and I'm waiting to see if that fixes the issue but has anyone else had this problem with a large focal length scope with their ASI2600 and shots that are less than 300s? See pics below for examples. Any help would be greatly appreciated, thanks! -Chase NGC 3628 raw Baader Lum filter  M78 using 2' Baader RGB filters post stretching    |
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| Have you taken flats to correct it? |
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Quinn Groessl: I take 30 flats per filter after every session using the AsiAir flats feature. I have never noticed the vignetting in the flats either. Should I do more? Maybe an additional white shirt over the scope to get more accurate flats? |
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What is the light fall-off in the flats? Just read it off in ADU. When I used a Tak FSQ106N I would be at about 24000 in the centre and 19000 in the corners. This would calibrate out easily enough. I did a lot of mosaics with it and the panels went together fine as well. I can't see exposure time as likely to affect the vignetting. Are the filters the same format - ie mounted or unmounted? Mounted will vignette slightly more. Olly |
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| I don't think more flats are going to do anything. I've always read that 20 flats should be enough per filter, more won't hurt, but isn't necessarily going to help either. I would say something is wrong with the flats you have, or the program you're using to stack isn't using them correctly. |
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Olly Penrice: All filters are mounted. I will go back and take a look at the flats and update you on the readouts asap. The more I think about it, I feel this may be the issue. I've been taking flats using my desktop on a white screen and the scope with a white shirt up against the screen. Maybe I'm overexposing the flats? |
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Quinn Groessl: Agreed. I'll give you an update once I take a look at them or how my wbpp is set up. Thanks for the help! |
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Chase, As long as the filters are being centered by your filter wheel, the kind of edge vignetting that you are seeing has nothing to do with the filter itself--or the exposure time. The effects may manifest slightly differently with respect to glancing stray light at different wavelengths but that's a red herring. What you are seeing is due solely to the geometry of the apertures in the optical system, which determines how much light makes it from the entrance pupil onto the sensor. In your case, the size of the apertures or the component spacing is simply blocking too much light in the corners of the sensor. Flat calibration removes a lot of this stuff but it can only handle so much vignetting before noise issues and small errors in the calibration data start to cause problems. My own rule of thumb is to try to keep the amount of vignetting in the extreme corners below about 30%. With values over 50%, there is almost always going to be trouble. You can measure the amount of vignetting that you have simply by measuring the ratio of the signal in the corners of your master flat to the signal in the middle--obviously using linear data. General rules of thumb to avoid this problem are to use the largest possible diameter for adapters, adhere to the correct back working distances for each component, and make the spacing between components as tight as possible. For example, the spacing between your FW and sensor should be as small as you can possibly make it--and you should double check with a drawing (or a calculation) that the marginal rays at the edge of the field (called y + y_bar) are not being blocked by the filter. I also want to mention that the other thing can will screw up image calibration is stray light. Remember that flat calibration only corrects for multiplicative effects, which are: Vignetting, cosine to the fourth radiometric fall off, and PRNU. Stray light is not multiplicative. It is additive. Therefore if you have small strays that are generated from glancing reflections from inner surfaces of your adapters, they will not be removed. In general, image calibration tends to make stray light look even worse. So, it is extremely important to make sure that the interior surfaces of your adapters have glare-stops along with highly efficient anti-reflection paint. For most amateur gear, glare stops often consist of threaded surfaces (which are quite effective) but many use standard hardware store level flat black paint that isn't very good. Finally, you can assess the level of vignetting in your system by pointing the telescope at the sky during the daytime. Remove the camera and put your eye roughly where the sensor sits and look backwards through the telescope. If you move your eye to the edge of field, you can often see exactly what is blocking the light. You can see what that looks like when I did the same thing with my 20": https://www.astrobin.com/xfedon/I/. In fact, that might be a good read to better understand this issue and how I solved it when I ran into the same problem you are having. John |
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It also happens to me with my ASI2600MM Pro, being a larger sensor (APS-C), it usually happens, but it is corrected with the flats. See a screenshot of VdB 16 Luminance and the stacked master.  |
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Jaume Zapata: Out of curiosity, how are you taking your flats? I'm still fairly new to AP and my method came from some research I did a few years ago. I haven't noticed any issues with my method of white shirt and white computer screen with my narrowband filters but maybe there is a better way? |
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Chase Davidson:Jaume Zapata: Hi Chase, Using an EL Panel, I bought the electroluminescent panel and the foamed PVC cover and a colleague (electronic physics professor) prepared everything for me.  |
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| Chase, you mentioned the ASIAIR flats feature... if this is the feature that chooses the exposure for you, then you may want to try a manual approach and use the AIR's histogram to define your preferred ADU. When I first got the AIR I also used the AIR's auto flats feature and had to go manual when I saw that the flats were not calibrating well in PI. |
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Hi Chase, The first thing I noticed in your image at the beginning of this post is that your vignetting is not symmetrical - vignetting is stronger on the right side of the image than it is on the left. In your post-stretch images, vignetting is stronger on the left than it is on the right. Assuming your flats and lights have the same orientation, this suggests that something has changed between taking your flats and capturing your lights. This can happen if the camera is removed / replaced between the flats and the lights, or if your camera or filters are not aligned exactly with the OTA's optical centre - although this usually causes the vignetting in the calibrated images to be lighter on one side and darker on the other. The first thing I would do is try to work out why your vignetting is not symmetrical. Cheers, Gary |
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Just a thought -- it looked like maybe the vignetting was not centered the same in some of your examples. Are you using a ZWO filter wheel? If the filters are unbalanced the wheel can slowly drift due to gravity, causing vignetting and even drift enough to occult part of the frame. If you changed some filters maybe they are heavier or lighter than the filters on the other side of the wheel. I solved this by distributing the filters so as to maintain approximate balance at any position and angle based on mount position. Wayne |
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I see from your Astrobin profile that you have a ZWO EFW 7 x 2" filter wheel. All your filters are 2" mounted, so that should not cause any vignetting with the AT130 and ASI2600. If you are using NB (assuming SHO) filters as well as RGBL, have you loaded all 7 filters into the FW ? That would eliminate balance issues. The asymmetrical vignetting is the main clue. Try recalibrating your FW and shooting another set of flats, then check to make sure the vignetting is symmetrical. If not, then something is definitely amiss, most likely with your OTA/OAG/FW/Camera adapters. Cheers, Gary |
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Just to clarify my previous post: I suspect the off-center vignetting in your images may be due to the filter wheel rotating so the filter is not centered. I experienced this and it went so far as to gradually mask off one side of the image. The rotation was slow and gradual, about 1/3 of the filter diameter in 30 minutes. I confirmed it by removing the camera from the scope when I saw the symptom during an imaging session. The wheel does not seem to apply any holding current to the servo motor, or not enough, and it is free to rotate with only a little resistance. I had only four filters at that time in slots 1-4 and 5-7 empty. I moved them to slots 1, 3, 5, and 7, and that was balanced enough to put a stop to the rotation. Wayne |
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| Although I haven't used this sensor before, I strongly suggest you take the flat frame shots using blue sky as light source at dawn or dusk, as I reviewed in my Youtube video (https://youtu.be/oK0MhhIQBuI) |
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| In theory, when you use a 2 'filter, such a dark angle will not occur on the APS-C sensor. You need to check if the installation position of the filter is too far from the sensor, or if there are smaller connecting rings between them that are obstructing it. If none of the above are true and this phenomenon is limited to LRGB filters, is it possible that the uneven coating on the filter surface is the cause? |
