HI Here is my first post about the camera ZWO 6200 MM PRO First of all , carefully and without contaminating both Masters, apply the same value to both. TOPIC#1 ( BIAS VS DARKFLAT) TESTED 09/23/2004 MASTERS DATA INFO : MASTERS LIGHT HA 35 X 300S 780MM F/6.5 FILTER CHROMA 3NM 50MM 6200 MM FULL FRAME MODE, 0C TEMP SENSOR ,GAIN100 ,OFFSET 50 SKY QUALITY SQM 16.55 , TEMP 84 F ,87 HUMIDITY ( MIAMI, FL USA ) BIAS INFO : MASTER BIAS EXP 0.01/0 C /100 BIAS DARKFLAT: MASTER DARKFLAT EXP 3S/0C/100 DARK FLATS MASTERDARK : MASTER DARK EXP 300S/0C/100 DARKS MASTERFLAT: MASTER FLAT EXP 3S/0C/100 FLATS GAIN 100 OFFSET 50 SOFTWARE INFO: ASTRO PIXEL PROCESSOR , PIXNSIGHT IMAGING PROCESSING INFO ( FOR BOTH MASTERS ) CROP ( SAME) DBE : CORRECTION: SUBTRACTION , SMOOTHING 0.200 BLURXTERMINATOR : SHARPEN STARS 0.25. SHPARPEN NONSTELAR 0.90 NOISEXTERMINATOR : FACTOR 0.50 STF : TARGET BACK GROUND 0.15 . SHADOWS CLIPPING -3.1000 END RESULT ( DATA) SADR HA ( With master dark , flat and bias )SNR :1.392E+06 61.44dbBackground Mean (normalized) = 0.14516760, (16-Bit) = 9513.55862008 Noise Evaluation: Ch | noise | count(%) | layers |0 | 1.813e-01 | 31.80 | 4 | SADR HA ( With master dark , flat and Dark Flat )SNR :1.723E+06 62db BackgroundMean (normalized) = 0.14435297, (16-Bit) = 9460.17168219 Ch | noise | count(%) | layers |0 | 1.649e-01 | 25.99 | 4 | IMAGING SADR HA ( With master dark , flat and bias )
 SADR HA ( With master dark , flat and Dark Flat )
 CONCLUSION I don't see a difference that can be detected by a human eye, much less that it affects or influences that final image I would appreciate your comments Thank you Clear Skies Brian
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i would apply gradient correction only, then measure the noise. NoiseX (and blurX) is potentially going to do something different to each image. the STF applied won't change anything in terms of the analysis but of course it changes how the image looks to the eye.
you can also LinearFit one gradient-corrected image to the other (assuming they have the same registration reference frame) and then divide one by the other to see if there is any hint of vignetting in the result. you might need to invert the result as most pixels will be very nearly 1.0 after that division.
having said all that i've only ever used bias frames to calibrate my flats with the 6200MM since i take sky flats and the exposures are all slightly different.
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thank you for your comment
the blur, noise , was apply after result , just for final imaging view
thank you
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I did some testing a year or two ago, same camera. I could not see any significant difference (I thought I could zoom way in and find something I thought was a difference but could easily be confirmation bias).
Because I use a flat panel and can do fixed exposures, I use flat-darks, because it is more correct in theory. in other words, I do not think it can ever produce results that are worse, and it MAY be better.
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Hi
thank you so much , for you comment , linwood
i could not see any difference too
cs, Brian
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Brian Diaz:
I don't see a difference that can be detected by a human eye, IMHO, what else matters? These are, after all, images for viewing and not for science. I think sometimes that maybe too much in astroimaging comes at things from a "scientific-only" POV. This is probably because the hobby attracts tech types by it's nature. I would certainty not argue that theory and the supporting science and math are not important - clearly they are very much so. But at some point one gets so far into the mathematical and theoretical weeds that one can fail to see the esthetic and/or practical garden.
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It may be worth explaining for those who stumble on this thread...
Calibrating a flat with a dark is "correct" in the same sense as a light, in fact a flat is a form of light.
However, unlike most astro lights, a flat is taken at very short exposure, and so the difference in noise from a bias at 0.001 seconds (or whatever) and one at 1 second is pretty negligible, you might really see a difference in 0.001 and 300 seconds, but not 1 or 2 seconds.
So the theory is if your flats are dynamic and have exposures which vary a lot, it's a pain to make darks at the same shutter speed, and using bias darks are a big win for a tiny (maybe invisible) loss.
The same logic applies to those whose flats are at consistent exposure times, but just don't want to bother taking matching flat-darks for something that may not make a difference, just the logic is a bit less compelling.
And doing matching flat-darks if your flats are consistent is, in my mind, not much work for one less thing to worry about.
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hi, bill
thank you for comments
i agree , however there are something very important there
it is CURIOSTY , "I don't have any special talents, but I am deeply curious." Albert Einstein
cs, Brian
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Bill McLaughlin:
Brian Diaz:
I don't see a difference that can be detected by a human eye,
IMHO, what else matters? These are, after all, images for viewing and not for science. I think sometimes that maybe too much in astroimaging comes at things from a "scientific-only" POV. This is probably because the hobby attracts tech types by it's nature. I would certainty not argue that theory and the supporting science and math are not important - clearly they are very much so. But at some point one gets so far into the mathematical and theoretical weeds that one can fail to see the esthetic and/or practical garden. As a counter-point, a lot of what we do with raw images and final stacks are specifically aimed at magnifying very, very small differences so they become visible. Practically every image we take starts off with invisible data, brought out by subtracting noise and stacking signal and as we stack trying to deal with everything from pixel-to-pixel variations in the sensor to starlink trails. There is much we can't see that ends up making a difference upstream by final processing. It's hard to know that this is for sure not one. I THINK it is as well. But...
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I have never used dark flats. I don't think it's worth the
extra work.
I use a dark master library that I create in the Spring and Fall for NB and RGB at several temperatures (-5C, -10C, -15C and - 20C. and exposures ( 60, 90, 180, 240, and 300sec.).
I take fresh flats and bias for each refocus filter every night I shoot.
I'm very satisfied with the results.
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hi, steven
thank you so much for your comments
cs, Brian
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Steven Rosen:
I take fresh flats and bias for each refocus filter every night I shoot.
. Why do you take biases nightly if darks are only every 6 months?
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If you were to remove the "regular" darks from the equation as well, you'll find that they're also completely unnecessary, and I would argue you're better off not using them. According to the ZWO website, "At a cooling temperature of 0 °C, the dark current noise is only 0.003e/s/pixel" For your 300sec image that would be 300x0.003e/sec/pixel = 0.9e/sec/pixel This is less than the read noise. You risk introducing more noise with your dark than dark current you're removing. Of course if you're comparing a masterbias made of 100 frames and a masterdark made of 100 frames, you'll get a similar result. But you can easily do a masterbias of 1000 frames and you'll even out all randomness. Also, the camera supports captures of 0.00032sec, so why not get them as close to a true bias as possible? See my reasoning in more detail in this video I made on this very topic: https://www.youtube.com/watch?v=EQGWr34Fu0c (start at 6:39) Darks are not meant to remove noise. They're meant to remove dark signal, which you don't have.
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Now the argument to use NOTHING for a flat dark to me is much more interesting than using a bias vs matching exposure dark. It's related to the argument never to use bias and dark together for a light (with CMOS when not scaling darks), the extra math operation does more harm than good (plus your dark - bias my go negative).
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Linwood Ferguson:
Now the argument to use NOTHING for a flat dark to me is much more interesting than using a bias vs matching exposure dark. this couldn't work because you have this baseline you're subtracting from your lights (be it bias or dark) and you have to subtract it from your flats as well as flat calibration is a division rather than a subtraction. With a great camera like the 6200mm you may not notice a visible difference but it is definitely there, and if you want to have a correctly calibrated image (e.g. for photometry) this is a must
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thank you , luca
for you comments
that was just a test about the zwo 6200 with the most important eyes view
i have done different test with dark , without dark , with flat and dark flat and all version
i will unload a different final imaging , one with dark flat and dark flat , flat and bias only and flat and darkflat
and is impossible for human eyes see the different
nobody can see the different and to be honesty that is the most import the final or not
personally i take more care when processing my imaging
CS, BRIAN
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I haven't used darks in 2 years, just flats and dark flats entered into WBPP as bias.
scott
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thank you so much for your comment , WSG
my next test is with out dark
only flat and dark flat and also flat and bias
cs, Brian
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I've noticed that the astro community has slowly trended toward the use of flats & dark flats (or biases) exclusively while skipping the use of darks. This of course coincides with the use of newer cmos cameras with their back illuminated chips. Something like the IMX571 has negligible dark current (at 0C and below). That being said, as soon as someone implies you don't need darks, you will surely hear from the other camp. Some might say it's generational - old timers with PTSD from the early CCD cameras are sticking with what works for them. If you poke around online you might even find mathematical formulas that supposedly prove that you are literally spinning your wheels every time you add dark frames into the mix (again, with the newer cameras). I'm agnostic on the issue so I won't try to sell either idea here. If you maintain a dark library, it probably won't matter much in terms of time expenditure. If you feel better taking a rigorous approach to calibration, then why not? Sometimes "best practices" are not about having a clear win, it's just something you do to cover the bases.
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Steven Rosen:
I have never used dark flats. I don't think it's worth the
extra work.
I use a dark master library that I create in the Spring and Fall for NB and RGB at several temperatures (-5C, -10C, -15C and - 20C. and exposures ( 60, 90, 180, 240, and 300sec.).
I take fresh flats and bias for each refocus filter every night I shoot.
I'm very satisfied with the results. That sounds like far more work than using dark flats. If you use the same exposure time for your flat frames each time then you can just built the dark flats when you build your dark library and keep them for the same time frame you keep your darks for. Using a flat panel or light box allows this to be done quickly and easily, even fully automated should you wish.
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Nice post Brian. I agree with you, both images are great. If you really want to know the difference, just subtract one image from the other (with renormalization). From the statistics and level of detail in the subtraction image, you might get a technical answer, but I think any difference is small. I am still believer in a dark/bias library that I apply to both flats and lights and only update 1/year, but this requires a cooled camera.
Cheers,
Dave
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thank you so much, David
CS, Brian
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I really can't tell the difference at all. (there's more trained eyes for that around here)
I should flats and darkflats with every period (1-2 weeks of clear skies it seems), but darks every 3-4 months, if that. I've never done bias frames as I wasn't sure they were needed and never got to it.
If someone tells me adding bias frames to my calibration will improve quality, I definitely do it. I just haven't heard that before.
Is it a holdover practice from ccd cameras? (I only have cmos)
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Nick Grundy:
If someone tells me adding bias frames to my calibration will improve quality, I definitely do it. I just haven't heard that before. I don't think anyone is suggesting ADDING bias to a cmos workflow that includes darks (whether for flats or lights). In CMOS days bias was used with darks to scale darks (maybe for other reasons). The idea being that if you think conceptually of a light has having a (Signal + Bias + Dark), and the dark component was proportional to length of exposure, then you could calibrate a light with a different exposure by Light - Bias - Dark * (T / T0) where T was the actual exposure and T0 was the exposure of the dark. Because there's so little difference in dark and bias this works very poorly for CMOS, at least in my experience, and I think it's generally accepted you should never use both bias and dark. Someone please correct me if I'm wrong.
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the point is that in the end there is no different that it can be detected by the human eye, so it doesn't matter
wish variant you use no one will know
for me , i prefer to spend my time processing the final image in the end it is what counts and we see 100%
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