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Hi all, maybe someone can help me with my problem: The flat correction with the optlong L-Enhance filter doesn't work right and creates a colored vignette in the final image. For example, here is an image (taken with a Skywatcher 150PDS newtonian) stacked with Siril wich shows these artifacts:  The strange thing however is that with a different telescope (in this case the Samyang 135mm lens) but the same filter the calibration works just fine (just ignore the linear gradient in the image):  I have tried flats with an LED light pad as well as sky flats with the same results. All flats were corrected with flat-darks. I used a ZWO ASI 533MC Pro controlled by NINA (for the flats I used the built-in flat wizard) Does anyone have an idea why this is happening? |
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Hi Falke! Excellent shots! If you process your images in PixInsight, try skipping flats completely and use Dynamic Background Exrtaction process instead. I've struggles so much with the flats. Never got them right, even once. And, as you say, it may not work for a particular scope. Then a professional astrophotographer suggested to stop using the flats and rely on DBE only. And it worked perfectly (at least for me). Clear skies, D |
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| Colour fringing in a flat-correct stacked light frame such as you have isn't so unusual. Sky flats are usually the best but must be taken at the right conditions, i.e. at dusk and with the right level of signal. For lens I've found EL panels work quite well and, for short FLs, avoid issues relevant to sky gradients. You can always removed the non-uniformity of the sky background using a process akin to DBE in PI. If I'm not mistaken there is something similar in Siril but probably not as good as DBE in PI. Without having the raw frames and flats at hand I can't comment any further. |
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Hi Falke, i use L-Enhance and my stacked are like your second picture, but no problems i resolve this gradient with tool "Remove Light Pollution" in Astro Pixel Processor. |
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| Try shooting your Flats without a filter. It works for me. |
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Hi, i am using the Optolong L-Enhance as well, but never had Problems with Flats. Flats should always be taken with Filter and same orientation like the lights, how are you doing flats and how does the Histogram look? Taking my Flats with a self-made LED Flatbox. Regards, Rolf Example(Streched after preprocessing, no further processing so far):  |
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Thank you for your answers. @RolfW : I do my flats exactly like you described (I also use a LED light pad). Using the Background Extraction tool in Siril I am able to remove most but not all of the color artifacts. It appears that the blue channel isn't calibrated correctly (look also at the dust donut in the lower left part of the image): Red:  Green:  Blue:  The Background Extraction tool removes most of the vignetting but the dust donut isn't removed. Here's the histogram of a faulty flat (L-Enhance + 150PDS):  And here the histogram for a correct flat (L-Enhance + Samyang 135):  |
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| Make sure your offset settings match your lights. I had the same problem and found out that ASI AIR pro uses a standard ascom 65 setting for offset but I took my flats previously at offset 50. Looked like your image exactly. I re did flats with Asi Air Pro and all good. |
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| Are you using bias frames ? , how long exposures are your flats , I use a 8” Newtonian and a really dim light source seems to work better for me using an El panel which requires a longer exposure so the refresh rate doesn’t interfere , I use APT FLATS AID and a exposure of 3.3 sec my EL panel has double layer of white cotton and a layer of opaque acrylic over it so it’s quite dim , I found to bright a light source just creates issues fingers crossed since using a zwo533mc all seems better , if using a tablet for a light source dim the screen to its lowest setting . |
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| As you are using a Newtonian, perhaps some of the light from your LED pad is entering the focuser draw tube directly rather than all of the light reflecting off of the primary, to the secondary and then into the draw tube. What finish is there on the inside of the drawtube? If it has some reflectivity then light entering the draw tube directly from the LED pad could potentially reflect up the draw tube and cause a bright area on your flats, thus causing the ineffective correction you are seeing in the final image. |
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| Since your flats are undercorrecting I would guess at a problem with a light leak during acquisition of the flats (as described above) or when taking your darks. When you acquire the flat darks and darks are you sure there is no light entering the camera? Just capping off the OTA is not enough, especially with a Newtonian. I use a metal cap on the camera and put it into an aluminum enclosure and then in the fridge to create the dark library. Even just the metal cap was not good enough. Another possibility is reflections or a light leak during acquisition. When flats do not calibrate correctly, it is always a hint on a problem with the setup (or method of calibration, but SiriL makes this rather unlikely, I think). |
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Thanks for the answers, I take the flats after focussing on a bright star, so its already quite dark during the flat aquisition. This should minimalize the problem of light leaks. I also make sure that I cover the back of the newtonian (the material on the inside of the tube is the original one, so no special anti-reflective material). The flat darks (biases) are taken directly after the flats, with the telescope cap on, and match the flats exposure time (automated by NINA). Exposures are usually around 0.03 seconds (I read that flat/bias exposure time doesn't matter with the 533), with a few layers of white T-Shirt between the light pad and the tube. But I'm still wondering why the problems only occur when using the L-Enahnce filter, because with the L-Pro, the calibration works just fine. |
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| If your flats are 0.033 seconds then I would say there may be your problem,if you used a flats wizard and it gives 0.03 seconds then your light source may be too bright try dimming panel use nina flats wizard as an experiment and see if you can get at least 1-3 sec exposure. |
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Try shooting your Flats without a filter. It works for me. That can't be right. Flats must always be taken with the same equipment as the lights. If the filter has dust motes, then not using the filter for flats will not remove those dust motes. |
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| One possible reason for the color vignette could be an incompatibility of your LED panel and the narrowband l-enhance filter. If your panel has no continuous spectrum (like an EL foil) but has only the three R, G and B LED peaks, the l-enhance might block necessary parts of the flat panel light. This would lead to an "overcorrection" in single channels. Try a continuous light source such as the sky (t-shirt method) or get an electroluminescence panel like the ones from Gerd Neumann: https://www.gerdneumann.net/deutsch/astrofotografie-parts-astrophotography/aurora-flatfield-panels/uebersicht-aurora-flatfield-panels-overview.html |
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| I thought of that too, but the same filter and LED pad in combination with the Samyang 135mm F2 lens work fine. I also tried sky flats, but this didn't work either (also produced colored vignette). |
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| Hi Falke @falke2000 , did you solve your problem? I feel I'm having the same issue with my L-eNhance filter coupled to a Redcat 51. |
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Adam blocks three part video on YouTube is definitely worth a watch even if you don’t use pixinsight , adams consensus on flats is that they should be between 3-5 secs using cmos cameras , but he explains very well why . https://youtu.be/Kkg441UBNpo |
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| Never had a flat master longer than 1 sec. Ever. |
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Hi, no, I didn't manage to solve the problem  But with my new Apo telescope and using the exact same procedure for taking flats (exp. time ~0.25s) I don't have any problems with flats. So I thinks it's something about the specific scope+filter combination, as my current method for taking flats worked for literally any other setup I own. I just don't have any clue why this problem occurs. This year I haven't tried using the L-Enhance with my Newt yet, but because of the disappointing results from last year I'm not willing to sacrifice any more time with an consistently not working equipment combination.
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Automated sky flats are what I've switched to using and they produce excellent flat masters that calibrate perfectly. One scope in particular, the e160-ED does not take panel flats very well. I've had another Epsilon user tell me the same. The trick with sky flats is that you really need automation in place to get the flats properly exposed and for them to be dithered, etc. For this, I use Voyager's Flat Sequencer, which can handle this task at dawn or dusk without my intervention. There are likely other solutions available that do something similar. |
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The issue you are seeing is due to vignetting, something in your light path is blocking light. If the vignetting is too extreme, there is too much loss of light, and it is very difficult to correct it completely with flats. Just depends how much fall-off you have in the edges of your frame. If you have no problem without the filter and problems with the filter, then the filter is likely the source of the problem. Note you can do this without flats, just compare your images with and without the filter, uncorrected, is the vignetting at the edges more severe with the filter? Two possibilities, one is the filter is to small for where it is located, the farther away from your sensor the more likely you have vignetting problems, move it closer if you can. Is it the larger 2" filter, if so I would think it should not be an issue, though not familiar with your scope geometry. Look at the geometry of your light path is the filter just too small? Second might be reflections from the filter, you could try moving it farther from the sensor if that doesn't cause more severe vignetting. I have an LeNhance filter with a Celestron 925 SCT and reducer which does have significant vignetting with or without the filter, the 2" filter doesn't add any additional vignetting--so definitely something to check on your system. I use a cheap flat panel (meant for tracing) with no issues and < 1 second exposure. Though some cameras might be more sensitive than my CMOS. There is a color difference in your vignetting with the flats. Make sure you use darks at all the same settings as your lights, flat darks at all the same settings as your flats. I do darks in a totally dark room to make sure there is no light leakage anywhere, as well as capping the objective. I also do flats in a totally dark room and block as much extraneous light as possible from the light panel to avoid stray light. As long as the R, G and B channels in your flats are not clipped and are showing reasonable signals your light source is not the problem--the histograms look good to me. Make sure temperature is the same for flats/flat darks as well as for you lights/darks, but flats can be done at totally different settings than lights of course. Note in your other system you may not have much vignetting, so your flats don't have to do much except remove dust spots, thus you don't see the problem there. It doesn't mean your flats are better there, just means your other system is less sensitive. Good luck. Rick |
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So I am using the APS-C clip filter variant of the L-Enhance, which sits more or less directly in front of the camera. As the sensor of the ASI533 is much smaller than APS-C, I don't think that vignetting from the filter is the issue here. The scope's imaging circle is also definitly large enough for APS-C (tested this with my DSLR), so again, the tiny sensor of the 533 shouldn't have any problems with too strong vignetting. I haven't tried checking for reflections yet, so I may want to do that in the future. When taking flats, I make sure that I use the same gain and offset settings as for my lights, and my darkflats also have the matching exposure time. Changing the light source (e.g. LED panel vs. skyflats) didn't change anything, I still had the same problem that some channels (for example the blue) weren't corrected, but the othere were. Interesting enough, if I run the Extract-Ha-OIII script in Siril and do the gradient removal on the two individual Ha/OIII mono images, I get much better results than with the RGB image. So that's my workaround for now, but I still want to know where the problem is coming from... |
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I don't pretend to understand the phenomenon but these threads over at CN seems plausible to me. The recommendation is to go a ?little against convention and use long exposure, lower ADU flats and dark flats e.g. 2-5s; 4,000-8,000 ADU. Note, as with most of these issues, the 'solution' doesn't appear to work for everyone but I'll be giving it a try. https://www.cloudynights.com/topic/661685-word-of-warning-asi294mc-pro-and-opt-triad-and-nb/page-3 https://www.cloudynights.com/topic/792863-asi294mc-pro-strange-recurring-color-pattern/ |
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This is all VERY new to me but some limited testing I did comparing flats taken with no filter, a mounted 31mm L-3 UV-IR Block filter and a 1.25" L-eXtreme does tend to suggest to me that there is some sort of non-linear colour response in my ASI294MC Pro sensor and the L-eXtreme when the RGB channels are scaled that I don't see in the other two cases and that this could be causing the problem. It mainly affects the red channel as others have noted. I understand that while this issue is common with the ASI294MC Pro, it's not limited to this camera or this specific sensor. The tests were done using a suprisingly well colour balanced led tracing panel, acrylic diffuser and 2 sec exposures with brightness of the light adjusted to get the targeted ADU count. Note, I had the same patterning problem with sky flats earlier. Using bulrichl's flat's stretching and colour channel comparison approach from the PI forum, I compared low ADU flats for each case against relatively high ADU flats at levels similar to that suggested by the ASIAir's Auto Flats function for each. Roughly, that was about 12,000ADU vs 32,000ADU and noted in the screenshot using (L)ow and (A)uto prefixes in the various images. https://pixinsight.com/forum/index.php?threads/flats-dont-look-correct.18417/post-112145 The images in the screenshot are layed out with the no filter case on the LHS and the L-eXtreme case on the RHS. I've left out the L-3 case here due to space but the results were very similar to the no filter case. The 1st column for each case has images of the debayered and streched flats, the 2nd column the 'standardised' RGB flats produced using bulrichl's short PixelMath equation. The 1st row is the L ADU case; 2nd row A ADU case; 3rd row A - L; 4th A / L; and the bottom row, seperate RBG components of the 'standardised' A / L image. As I understand it, if the sensor - filter combination was behaving normally, there should be no colour shift between flats targeting different ADU levels (within the sensors linear range, something like ?2,000 - 50,000 ADU at a guess but I haven't looked that up). That's what we see here in the debayered and stretched flats for all cases, including the L-eXtreme where no scaling of the RGB channels has happened (4th row, columns 1 and 3). I would think that if there was a fixed colour pattern across the linear range that it should be able to be taken care of by current processing techniques. A problem could be emerging due to this ?non-linearity when the single channel flat is split and seperate scaling factors applied to the RGB channels as part of the image calibrate and registration process such as PixInsight's Weighted Batch Preprocessing script (WBPP) when "Separate CFA scaling factors" is enabled for Flats processing. As we can see in the image (4th across, 4 down) showing the ratio of the 'standardised' L-eXtreme at high and low ADU's there is a significant colour shift between the low ADU flat and the high. If we look at the seperate RGB channel data for it in the row below, we can clearly see the anomaly in the red channel that many others have noted. The green and blue channels seem to be unaffected by this ?etalon effect in the sensor but interestingly in my case the corners are overcorrecting in both the low and high ADU cases. These effects could well be occuring in broadband imaging too but is likely to swamped by the much wider (and stronger in aggregate) response outside of the narrow wavelengths targets by the (multi-)narrowband filters. If this is right, it suggests to me that it's very difficult to remove these effects using the current norm of processes such as WBPP using "Separate CFA scaling factors". Ideally there would be another approach, something better than splitting out all the flats and lights (?others) into their own RGB channels before calibrating and registering to treat as their own narrowband datasets, if even that works. @falke2000's earlier post where he highlights much better results from independent processing of the narrowband channels does suggest it could. I have had improvements by using lower than suggested ADU levels for my filters with the ASI294MC Pro, dark flats, following the PI recommended WBPP process and using DynamicBackgroundExtraction as suggested by Shawn from VisibleDark in his YouTube video linked below. https://www.youtube.com/watch?v=eB0J4-O5WUQ I have read that others have had some success using alternative processing approaches e.g. calibration in Siril but I haven't tested that myself yet.  |
