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David Moore: I do not appear to need flats as I have no dust bunnies as I have no filter wheel and I have coma correctors permanently attached so dust cannot get in. I have also found that GraXpert etc removed vignetting adequately. I wasn't always successful doing flats and making them was a nuisance and sometimes made things worse. More skill needed probably.My 2600 cameras do not produce amp glow so that is not a problem. I have a 2600MM mono and 2600MC colour camera. I had used darks a lot but I processed some data with and without darks and could not see any difference. I do not claim to have done extensive testing or to be an expert so I am making no claims but at the moment at least I am not using darks and happy with the results. It does mean that I can adjust the sensor temp to the lowest that seems sensible instead of being bound by my dark library. Regarding bias I think I will repeat some more tests to see if I need them after reading this thread as making them is so easy and I see the point of taking them. I do not have problems with pattern noise and I dither every 3rd frame. Something else that put me off calibration frames was that my cameras kept on being replaced due to oil of the sensor plus another issue and that meant I had to start again with dark libraries plus having 2 cameras doubled the amount of work. I started with the colour camera and later bought the mono to increase resolution, sensitivity and for narrowband and I did not want to sell the colour camera and buy a filter wheel. I am not saying it is a great way to work it is just what I do and I am happy with that. They are great cameras when they work properly. I am also not saying John Hayes advice is wrong, he is the expert, it just works for me at the moment at least. Thinking about it I should probably try darks for worse case i.e. narrowband imaging, gain 250 300s exposure and see if that makes any difference. Most of the time I use gain 100 and 180s exposure. David Moore |
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I use the 533 mcpro and the 071 mcpro. I have a dark library that consists of 30 darks per exposure. And I take 20 flats and 20 flat darks per imaging target. Mike |
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| 500, end of story. |
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Telescopes 8” and 10” Newts Cameras 2600MC and 2600MM Full suite of Calibration is essential in my case a my TS GPU coma corrector introduces Vignetting into the image train Dust or no dust , your crazy if you don’t take advantage of Calibration, it will always improve your images , how much ? Depends on many factors , but it DOES !!! 40 x Flats for each filter ( captured for each imaging session) 40 x Darks ( Dark library replaced every 6 months or so ) 60 x Bias ( Library replaced every 6 months or so ) |
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25 Flats 100 Flat-Darks 100 Darks With camera cooling it is super easy for me to create a master dark library at home and reuse that. |
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andrea tasselli:Miguel Morales: Dithering improves the signal quality in other ways, not just "preventing" walking noise. Without dithering you'll often (if not usually) get a bit of that mottling (orange peel like effect) in the background. Its just a consequence of integrating undithered frames with a gaussian distribution. Sufficient dithering will usually eliminate that. You can do other things to optimize the time cost of dithering. Either don't dither every single frame, or if you can set up things like accurate filter offsets, you can even interleave LRGB(HaOIIISII) filters, and dither once per set. You can also very accurately dial in the settling time, which is one of the things people often configure far too long. Settling can usually take just a matter of seconds, but it is one of those things people rarely optimize, and when its not optimized it can result in non-trivial (and with remote imaging, even costly) inter-frame overhead. |
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I am weird when it comes to frame counts. I like to know the exact reduction in noise that my masters will have. So I usually use counts that have whole-number square roots. ;P If I use BIAS, then its 100 frames. Just because they are super easy to get, and SQRT(100) is a 10x reduction in noise (read noise, in the master bias.) For DARKS, I usually use either 36 (6x reduction) or 49 (7x reduction) for the master. For normal imaging, 25 frames should be enough, but I have often stacked hundreds of light frames in the past, and a more meticulously crafted mater dark can help when you are stacking a LOT of frames. For FLATS, I usually also use 49, but flats are interesting in that they have a lot more shot noise most of the time, and technically of all the masters, these can benefit from integrating more frames. If all you want to do is correct mots and vignetting and other shading), then 50 frames will do. That will usually net you around a million ADU, which is generally recommended as the bare minimum. If your goal is also to correct FPN due to PRNU, then you want 10 million ADU or more, to average out the photon shot noise. For that, you might need a hundred to hundreds of flat frames. |
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| I do recollect some post where darks should be at least half of lights number (say 50 lights-25 darks). Is it mathematically proven or just a joke??? |
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533MC Pro I have been doing 35 darks, flats, darkflats. After reading this thread it seems I could go with less darks. I tend to reuse my darks for months at times trying to save time on imaging nights. Sometimes I reuse my flats and darkflats for multiple sessions as well. I understand every session should have calibrations done in a perfect world. Unfortunately I think taking 18 subs at 5 minutes is still too much time along with calibration to take from my nights imaging. This will save a considerable amount of time creating my darks libraries though. |
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Adel Kildeev: I don't see any reason you would need to do that. Its a matter of remnant read noise. The purpose of subtracting a master dark is to subtract the fixed pattern of pixels that represent the sensor bias signal and any dark current signal. Subtract a single dark frame, which contains one "unit" of read noise, and you add that read noise to each light frame you calibrate. So, if you have 3e- read noise, your light has 3e- and your dark has 3e-, so the effective read noise that remains in the light AFTER calibration would be 4.25e- (from: SQRT(3^2 * 2)). In other words, calibrating with a single dark FRAME increases read noise by an undesirable amount, and then since you calibrate every light frame, you effectively end up with a higher read noise level than your camera is rated for. By stacking many frames to create a master dark, you are averaging down the read noise that remains (what I call remnant read noise) in the MASTER DARK frame. If you integrate 25 dark frames into a master, you are reducing read noise by a factor of five. That means your master dark has a remnant read noise of 0.6e- rather than 3e- (in our hypothetical.) When you subtract that master dark from each light, your read noise will still increase, but minimally so: SQRT(3^2 + 0.6^2) = ~3.1e-. Very small difference. If you integrated 49 frames into your master dark, then you would end up with 3.03e- read noise in each calibrated light. If you integrated 100 frames into your master dark, you would end up with 3.015e- read noise in each calibrated light. So for most people, unless you are stacking a lot of lights, 25 darks in the master should be fine. If you are doing short exposures (and are more likely to be stacking lots of them), then its easier to get more dark frames, so you could more easily create a 49 frame master dark. If you are doing long exposures (and ar emore likely to be stacking only say tens or maybe a hundres), then you should still be fine with just 25 darks in the master. You also need to consider that each light frame contains not just read noise but also photon shot noise. So, the difference between say 3e- an 3.1e- (25 vs. 49 dark frames) is going to be drowned out (swamped) even MORE by the shot noise. FWIW, the remnant read noise itself, is actually a form of fixed pattern noise. The pattern of read noise in the master dark does not change. So, while technically, its "read noise", in practice that same pattern is being applied to every light during calibration. As a FIXED structure, if you integrate enough light frames, that pattern could potentially become a limiting factor on background SNR. You would need to stack a lot of frames for this to actually become an issue, but that is the ultimate consequence. If you intend to stack LOTS of light frames, then integrating more than 25 dark frames into you master may be warranted. However, beyond 100, the benefits are going to become very small, even with deep light stacks. I used to stack hundreds of light frames, hence the reason I often used 49-frame master darks. But as I got into doing more and more narrow band, and shifted to 10 minute subs, I shifted back to using 36 frames, then even 25 frames. When it comes to DEEP stacks with LOTS of frames, IMHO, dark calibration is less often a problem than flat calibration. Or rather, DFPN due to DSNU is less of the limiting factor on SNR than FPN due to PRNU. A well constructed flat (millions, tens of millions of ADU, and very accurate as well) will correct the FPN due to PRNU which can become the key limiting factor for deep stacks of hundreds to thousands of subs. Ironically, when you see an image crafted from say 20,000 or 50,000 short exposures... There is likely very, very little benefit from stacking that many frames. Most of the time, FPN is going to limit you WAY before stacking that many frames would provide any benefit. Unless you were meticulously calibrating all of those frames (often NOT the case...imagine calibrating tens of thousands of light frames!), then it is highly likely that by 500-1000 frames you are already FPN limited, and any further stacking beyond that point is probably not going to improve SNR. |
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Jon Rista:Adel Kildeev: *** Jon, you made it loud and clear-great thanks, no more doubts and questions! I asked my former classmate, he is a captain now: What did actually happen in Baltimore.... He gave me so detailed answer and even his Chief Mechanic explanations about the poor diesel fuel quality and the problems with the main engine - I have now no doubts and questions! I love to hear real professionals, it is always a pleasure, no bullshit and evasive answers, this is why I hate all politicians and lawyers (being one of them too). Thanks again! Clear skies! *** |
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ASI183MC Pro I always do 60 darks  |
