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Hey Guys, until now I had a tendency to use darks for my lights and bias or flat darks for my flats. to make things easier I would like to use only bias from now on. thing I don't know is: do I need to calibrate the flats and the light frames with the bias frames or is it enough to only correct the flat frames with it? thx and cs Andi |
3.21
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Hallo Andi, only the flat frames should be corrected with the bias and then the light frames with the corrected master flat. Otherwise an overcorrection would result. I do this with flat and flat darks. As far as I know, it doesn't matter whether bias or darkflats are used for the correction. Best regards |
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Hi Andi, you just calibrate the flat frames using the bias frames, as the flat is then used to calibrate the light. Once is enough. |
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| While I strongly diagree of not using darks, especially on long integrations, if you restrain yourself to master bias then you should calibrate *both* flats and light with said bias. Me, I'd calibrate with just master bias-darks for lights and flats-darks for flats. |
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andrea tasselli: Whether darks are absolutely necessary also depends on whether a DSLR or an astro camera with cooling is used, or am I mistaken? edit: ...but I just saw that both a CMOS camera and a Z6 are being used... Dark frames are highly recommended for the CMOS camera... |
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Markus Gorski: In my line of imaging they are always required but, if your *integrated* dark signal is very low and its noise profile is also well mannered, there is no amp-glow and you do dither aggressively (the more so if your image scale is high) then you could just ignore it and use the bias master frame instead. |
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thank you for your answers. 2 different opinions on that. from testing I would say that the stacks with bias on light and bias on flat look better corrected. |
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All subs have bias. Darks (normal darks and flat darks) have dark signal in addition to bias. Lights and flats also have "pure" signal in addition to dark and bias. LIGHT = PURE_LIGHT+DARK_LIGHT_SIGNAL+BIAS FLAT = PURE_FLAT+DARK_FLAT_SIGNAL+BIAS DARK_FLAT = DARK_FLAT_SIGNAL+BIAS DARK_LIGHT = DARK_LIGHT_SIGNAL+BIAS Ideally you want to divide the pure light signal from lights with the pure flat signal from flats. You want to calculate PURE_LIGHT/PURE_FLAT, that's the calibrated frame. Therefore, the best thing to do is to subtract dark lights from lights and dark flats from flats respectively, removing all of the unwanted stuff before dividing. If you subtract just bias, you are making an assumption, that dark signal is negligible, i.e. DARK_FLAT_SIGNAL ~ DARK_LIGHT_SIGNAL ~ 0. If you subtract bias only from flats, you are making a further assumption: that the unwanted stuff in the numerator (lights) are negligible but in the deniminator (flats) they are not: LIGHT~ PURE_LIGHT but FLAT ~ PURE_FLAT+BIAS. These assumptions are always wrong from a mathematical point of view (you are always introducing inaccuracy, however small), but may be good enough in practice, depending on sensor type, sensor temperature, sub duration, average brightness of the subject and of course personal taste. That's where opinions tend to differ. Some people like their results without darks, some don't. Even with everything else being the same. |
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It's in the math of the calibration. Simply, when you calibrate using darks for lights and biases for flats, your equation looks something like this: (light - dark) / (flat - bias) If you expand that out a bit, you'll see that every sub (light, dark, flat) contains the bias. So, in reality the above would expand to: ((light + bias) - (dark + bias)) / ((flat + bias) - bias) Thus, when you are using darks to calibrate your lights, you are inherently subtracting out the bias from both. If you skip the darks, then your equation becomes: (light + bias) / ((flat + bias) - bias) That's where you're going to run into trouble... because you've bias-subtracted your flats, but your lights still contain the bias signal. Thus, if you are going to skip darks (which, I do not suggest, but that's a whole other can of worms), then you would want to calibrate your lights AND your flats by subtracting the bias. |
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This question comes up every so often and the math is basically what @dkamen said. I feel like it would be useful to categorize and pin or store some of these threads in some fashion so they serve as useful reference. |
