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Hello everyone, I am already quite some years into Astrophotography and image processing and was following the standard pre-processing workflow for my DSLR images (darks, bias, flats) because... well everyone does... But now i switched to an ASI 533 and read curious things about pre-processing with these cameras. On one hand some say to not use bias frames to calibrate, and I even found people saying quite the opposite to use bias but no darks!? Now I'm here, for the first time thinking about what the whole calibration with darks and bias actually is good for and can not figure why I would or would not leave out the one or the other from pre-processing my CMOS pictures, and if the fact that the ASI 533 has zero amp glow plays into this as well? So I hope that some of you could shed light on these particular questions and the reasoning behind the decisions or maybe link me to some literature, as many sources I found only stubbornly explain how to do the calibration and not why this way! CS Robert :-) |
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Never used that particular camera, but I would never skip bias and flat no matter which camera I use. If bias is non-zero and if you skip it, your flat will have an incorrect black point and it will not work well. Chance is good that you can skip dark if the following criteria are met: 1. you dither you exposures 2. you use some outlier rejection method (like sigma-clipping) for stacking 2. the camera has a CMOS chip as good as recent Sony CMOS 3. the chip is sufficiently cooled. |
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| Yes, modern trend is not to use bias frames and use dark flats instead. |
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You can just try. For me the best combination for narrowband (178MM) is darks + bad pixel map + flats of the same duration as lights. So darks double up as dark flats and bias frames are not needed (not that they make any perceivable difference). This minimizes the amp glow which is my number one problem in narrowband. ZWOs own recommendation against amp glow is to try without bias. It won't really go away but it's visibly reduced if you also divide by a corrected master flat with duration equal to your lights. In broadband it's darks, short flats (0.5 seconds), bpm and bias. In that case it is not so much amp glow that I care about since it's insigificant but just vignetting, dust motes and hot pixels. I add bias because why not. Again, I don'think they have much of an impact. Depending on gain and offset this might vary (I use very low offset and a gain of 120) My point is give it a try. You don't have to do a full integration + best effort processing, just load a single raw frame and compare how it looks with various calibration frames applied. Usually the differences are fairly obvious. If you can't spot a difference when applying e.g. master bias, chances are there isn't any and you can skip it. |
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For a CMOS camera, it is important to note that dark frames do not scale in the way they do for CCDs. Thus, you need dark frames that exactly match in length, sensor temperature, and gain/offset - at this point it's probably clear that it's best to choose two gain/offset combinations (one for broadband, one for narrowband), and then use a limited set of sub exposure lengths to avoid the need to capture too many exposure frames and reuse a library of calibration frames. Since dark frames do not scale, in the same vein, if you wish to use darks for flats, they also need to match exactly. As such, bias frames aren't really necessary in image reduction (since they are used to de-bias darks to allow them to scale). If you reduce the data in Pixinsight (and possibly other software), you need to uncheck the optimise darks option to stop it trying to scale the darks to reduce noise - if it does it introduces systematic noise (amp glow that isn't properly subtracted usually). (For Bias frames, you might also want to note that really short bias exposures use a different mode to longer exposures in a lot of sensors, and so the bias data from v short exposures isn't truly representative of what goes on in a real life long exposure) |
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Wow! Thanks to everyone for your explanations! This really helps a lot to understand why what is done and what options there are! I think for the next time I'll just try light+lightdarks and flats+flatdarks. flats of the same duration as lights @dkamen This is a really smart solution but I wonder how do you make flats on site with e.g. 120 Sec exposure time and not burn them out when using some artificial light source? Clear Skies, Robert |
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In my first attempt I simply defocused and used the same night sky but in narrowband they behaved pretty much the same as darks, there was no serious signal to speak of. I also tried the dusk sky because the sun is a rich Ha source but they would burn at 3 seconds. So I waited for a cloudy night and pointed the telescope towards the clouds. Living in a light polluted area (and using a relatively cheap 12nm narrowband filter) the clouds are bright enough to bring the flats at ~ 50% of maximum value. Not ideal but not too bad. It's tricky and I appreciate it wouldn't work with all combinations of sky/equipment. Not on broadband for instance. There, I would prefer using the western sky where there's Athens' light bubble, this will allow me to do perhaps 10 second flats. I emphasize the bit about the long flats because I agree with Graeme that sub-second frames seem to have a very different profile from frames that span at least a few seconds. The latter are much more similar to the long exposure lights in things like noise profile, banding and glows. In my experience this is quite important in narrowband because signal is so weak compared to all those things so you want to calibrate away as much as possible. Cheers, Dimitris |
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Hi all. My two cents here. My simple advice is not to use bias frames. My calibration routine is: 1) build a dark library at different cooling temperatures, exposure times, gain and offset 2) shoot science frames in order to match the dark (argh, this stage need to be done first...  )3) take flats at same gain than science frames but at least 5 sec of exposure time, not below that value, otherwise you will catch a non linear behaviour of the camera 4) create a master flat using dark flats (and NO bias). 5) create a master dark 6) calibrate the science frames as usual (NO optimize!) In such a way I never had ampglow problems or other issues. Hope this helps. Ciao. Lorenzo |
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Note that all this applies to DSLR too. They are CMOS sensors. I generally prefer no calibration at all, but when I have to it is (in order of importance): 1) flats of at least 3 seconds exposure 2) a couple of darks. I also think BPMs do a very good job compared to darks, if you want to correct hot pixels and not dark signal. Allow me to share a small discovery: This type of vertical banding in the Nikon D7500 was something that I just could not calibrate away (except with paranoid levels of dithering):  Even though I have stretched it beyond hope and exaggerated the saturation it is not immediately obvious but there are red (or green, I guess) vertical bands spanning the whole image. One of them goes right through M101. So, dark frames help a little in the sense that most hot pixels are red so they reduce the red bands too. Bias frames sure aren't helping at all. Normal flat frames of less than 1 second make things worse because they introduce some crazy patterns in the red channel. Actually looking at a dark frame shows So one night it dawned (heh!) on me that if incoming light plays a part, this is clearly uneven field illumination. In the red channel only, not really the shape we are used to seeing, definitely swamped if you are imaging a bright part of the sky like Sagittarius or Orion, not terribly apparent in short subs, but very much uneven and very much illumination. Therefore I took a 60 second flat using a wall (shown here with contrast and saturation exaggerated even more than in the light sub):  So now I believe (but will not be able to fully confirm until my next project) that one needs long flats to calibrate away the red banding in the D7500's IMX321 sensor. It appears to be the single most important kind of calibration sub for that particular sensor, since bias signal is practically negligible and darks only correct some hot pixels which most people can live with. If true, it would probably apply to other large modern Sony sensors too. |
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I suggest to take a look to this document from the PixInsight forum: https://pixinsight.com/forum/index.php?threads/for-beginners-guide-to-pis-imagecalibration.11547/ It has been sticked by the moderators (that I assume that is from the PTeam itself) so I consider it as the most accurate source regarding the calibration workflow. Hope it helps. |
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Thanks for the additional input, everyone! @Alberto Ibañez THATS a very elaborate documentation. Thanks! I found another very helpful source. @Adam Block talks in his amazing video tutorial regarding the new WBPP-Script about CMOS and pre-processing! Can recommend it very much! WBPP - Playlist CS Robert |
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I am new to AP and therefore can only share my short experience about preprocessing. Indeed I was also confused by what I read about using bias frames. I will test without and therefore rework my Master Dark library by not using the Master bias i made (if i get better results). What I can say about Flats : I used to take them with my iPad screen right after the lights, without moving the refractor or focus to really capture all the imperfections (dust spots / vignetting etc.). But the screen of the iPad is very bright, and I quickly noticed a kind of reverse vignetting in my calibrated lights after using my flats in Siril or Pixinsight... I dimmed down the brightness of my iPad before the next ones, but with flats between 20ms and 1s, I still observed this effect, like if the master flat was strechted (I mean gamma stretched) before « correcting » my lights. I therefore tried playing with the gamma of the master flats until I got good results on my calibrated lights ! But that’s painful and accurate. I deduced from that that indeed there is probably a non linear behavior of the sensor at short exposure times. I did not solved that issue yet, but reading you guys above, it starts to make sense. Will try making flats with exposures of 5sec or more, still wondering what light source to use and without defocusing the refractor because you lose the dust shadows by doing that... |
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| @Jérémie Ochin you may use a thick artist's paper sheet in between the iPad screen and telescope in order to further dim your iFlat . This method works like a charm with my laptop screen and my f/2 lens. I can achieve 1s exposures without reaching the nonlinear regime of my DSLR's sensor. I usually take 60 flats with different paper angles (with the paper surface always parallel to the screen of course) in order to average out any paper/light source inhomogeneities. Bias frames or dark flats are essential for flat frame calibration to work. See for example this. Also dithering is essential too. I've read somewhere that one of the professional astronomers' motto is "Dither of die!". |
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Hi AstRobert, There is a lot of great advice here already. I thought to way-in anyway since we're using the same camera. As you've mentioned yourself, the videos from @Adam Block are a great asset. And I can confirm that his instructions in the WBPP videos work perfectly for the ASI533MC-Pro. Also, have you seen the Darks and Bias frames that the camera produces? You might as well render a solid grey jpeg file in Photoshop that's how clean they are. (I'm kidding don't do that.) But even in my 600seconds 0C darks, there is no hint of amp glow. I would recommend taking longer flats, like @Lorenzo Siciliano mentioned. I do notice significant banding when I take short flats. I've chosen to create a master bias and a full dark library with different temperatures and a whole variety of exposure lengths. I pretty much always choose one of those durations for my lights. It took me a couple of days (cloudy anyway) but now I don't have to think about it. I only have to take my flats for each session. I am extremely happy with the results. So little noise. CS Remco |
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My own experience of two different CMOS cameras, a ZWO ASI294MC Pro and a newer ASI2600MC Pro, I still have both. Bias frames with the 294 are unusable. By my understanding the IMX294 uses internal timing circuitry for exposures under a threshold (I am not sure exactly what that threshold is, a couple of seconds) and over that threshold the ZWO supporting circuitry does the timing. Different circuits in action = different read noise so bias frames are not useful. With the 294, I shot flats with a light panel at a fixed exposure length of 10 seconds and shot corresponding dark-flats. I shot a dark library for the gain and exposure times I used and don't use bias frames anywhere. The ASI2600 appears not seem to suffer the issues with bias frames being different in read noise to longer exposures. With the 2600 I made a master bias from 100 bias frames (I only use 0 gain so far with this camera so I have only shot bias at that gain) and master darks at my normal exposure times. Flats, I have Voyager shoot dawn-sky flats each night, 50 flats with a mean ADU of 40K, I calibrate the flats with the master bias and nothing else to generate a master flat. Light frames I calibrate with the appropriate master dark, master flat and a bad pixel map and have been getting results I am happy with. It simplifies things a lot, not needing to use fixed length flats. |
