clouzot

That’s essentially what the RST-135E does (my understanding is it uses a Renishaw encoder, albeit not an absolute one)

This blog entry explains how you can align your darkflat exposures with those that the AAP determines automatically for flats I don’t know if the trick works for those short exposures though https://eastwindastro.blogspot.com/2021/03/asiair-autoflat-frame-exposures.html?m=0

Thanks @vlaiv for the thorough examination of that small dataset. The difference between the two comes as no real surprise: this is obviously not a bit-exact process, by any means. The presence of a residual ampglow may also have to do with the masterdark itself being slightly off (I'm always suspicious about my calibration frames, and sometimes rightly so to be honest). That skewed grid-like pattern is puzzling indeed! Surely it has to do with the little signal there is in each sub, which sometimes causes Siril (and some other stacking software I've used, including APP) to create those moire-like patterns after registration. I could hardly use more exposure that night, as stars were already saturating big time, though. The differences you found may also be related to the (additive) normalization step that is applied by default at the end of stacking process. As the two stacks can't have have the exact same pixel values, additive normalization will probably behave a bit (pun intended) differently. And most probably every other calibration and staking step may be offset by the slight difference in pixel values found in the calibrated subs: registration, rejection stacking, you name it. However, I'm not trying to shrug off your case, and intuitively speaking, I get your point about those small non-random differences building up as the integration time increases. That could a true concern. As I'm not in the 28-hours-per-channel league - yet - I'd truly appreciate if someone could test that approach with a more decent dataset. Anyway, let's assume the synth-bias technique appears to be working correctly for casual stacks (I've only done up to 3 hours for the moment), and its further applicability to longer stacks remains to be proven or falsified/disproved... depending on how one sees it.

That's really interesting. Is that what those two companies (falsely) called "ampglow reduction" then? Altair (and probably all the white label cameras that share the same hardware: RisingTech, etc) is in the same boat then, as my old 294c had the same behavior. Here you go: https://1drv.ms/u/s!AmemdJT-7GC6hB000TreEx9SZ1jT?e=dbQUil This was luminance data only as you noted (captured during a live EAA session, hence the absence of RGB data and the short 120s exposures). Please let me know what you find out, because I'm genuinely interested in what kind of devil could be hidden in that offset-VS-darkflat controversy.

Here's my take on this technique. I can publicly share all subs (save the individual darks and offsets, which I don't have anymore) if you want to replicate the experiment I had those 11 sub-exposures of NGC891 done on my C6 (those who have this little scope will recognize the well-known looping star flare!) and my ASI294MM Pro bin2. Gain was 120 (unity), exposure set to 120s, offset set to 30 (default). So there's 22 minutes worth of lights, which is admittedly not much by AP standards. Sadly, good weather is also a myth down here. I also had 19 inidividual flats, with the following settings: exposure 5s, gain 120, offset 30, bin2, mean ADU around 32k (16 bits, so 65k max) Those individual flats were then assembled with the following processing chain in Siril: - calibration with either a true 5s master darkflat, or a synthetic offset (ADU = 1920) - stacking and multiplicative normalization of said calibrated flats ...yielding two different master-flats. Said master flats were then used to calibrate the subs, along with a 120s master dark. The two results were registered, stacked in Siril. No background extraction process was applied, hence the slight gradient The renderings below show how the the final stack look like in false color and "histogram equalization" display mode that stretches thing a lot in order to see any substantial defect. I also added their respective image stats. TRUE DARK FLATS SYNTHETIC OFFSETS To further explore things, I did a pixel-math subtraction of the two stacks, and while they're definitely not bit-equal, the difference is mainly in the... bright stars, oddly enough. Anyway, no trace of the ampglow can be seen in that direct subtraction between the "synth offset"-calibrated stack and the "true dark flat"-calibrated one. For further reference, here's what the true 5s master dark flat looks like for my camera. The ampglow of the 294 can already be clearly seen (star burst pattern of the right, and top and bottom rows as well) So at this level of ampglow, synthetic offsets don't make a visible (at least, to me!) difference in the final product. Of course, things would perhaps be different if the ampglow were much more present, ie with 30s+ flat exposures. I'm yet to test that, perhaps with my upcoming 3nm Antlia filter which, I suspect, would need more exposure.

It's a real miracle you guys can do any imaging from that fierce, ever-cloudy sea fortress (that's the one thing I don't miss about the UK, to be honest. The rest was -almost- pure fantasy from start to end).

It's not crystal clear to me but I think I got your point. So, if I understand well, you're saying that ZWO is artificially tampering with the signal in order to keep a constant median ADU value? And that what we see with ZWOs (said constant median value) isn't generally applicable to other brands, sensors etc? If that is the case, then I also have similar measurements made on Altairs and QHYs, with sensors ranging from IMX290 to IMX533. All show the same behavior, where the offset level is only a factor of the offset/brightness setting, and doesn't depend on the exposure. If someone is fiddling with the signal, then it's Sony , not ZWO. That is unfortunately directly dependent on the way the camera timing logic behaves. I've seen really odd things with the IMX294 (be it the color or mono version), with flat exposures of 0.999s being really 0.999s (as shown by the mean ADU value of the flat itself), and exposures set to 1.00s being all over the place between 0.8 and 1.2s. That's another quirk with that one sensor, something you don't see with more recent offerings, and something I didn't see with my IMX183 either. That's where I beg to disagree. I've seen too many different cameras having that same behavior (yielding a constant offset current whatever the exposure) to deem it silly. I find it more odd to see a more or less recent sensor whose median value changes substantially with exposure. But I agree strictly with the second part of your sentence: yes, calibrating flats should theoretically be done with the exact same offset signal as what is in the flat proper. That's the math, agreed. However, a constant offset frame is, as was amply shown above, a very good model of said offset signal... (unless one has your ASI185MC, apparently), ampglow or not. It may not be mathematically exact, but the difference is so negligible (and hidden in other types of noise) that I don't see the point in painfully capturing hundreds of matched dark flats anymore. Unless one has to capture long-exposure flats (that may still be the case with 3nm filters), that one "synthetic" calibration frame will work for all flat exposures. That's a substantial reduction in imaging hassles, so why not give a try?

Here are some measurements I made with my 294MM, bin2 mode (11 Mpix), gain 120, T=-10°C, offset set to 30 (default value). I took a bunch of darks (or "bias", but real bias frames are a no-go with that camera) at various exposures, and measured some statistics (with no other software than Siril, of course, but ASTAP reports the same values) First, the median ADU value. All exposures are in ms, so we have measurement points from 10ms up to 180s (180000ms). Despite the well-known ampglow with that sensor, the median ADU value stays pretty much the same. What changes is the amount of noise, as measured by the dispersion. Here is a plot of the standard deviation (sigma) for the same frames. The phenomenon at stake is mainly the gradual build-up of thermal noise (low on this sensor) and ampglow (substantial, as you all know). So what does that mean from a calibration perspective? - under 5 or 10s, the thermal/ampglow signal is pretty much negligible compared to the flat signal level (which should be bright enough to swamp any kind of noise there may be). - still under 5 or 10s, the median ADU value of an offset (or "dark flat") frame stays stable around 1920 - If the 294 were behaving nicely with all exposures, that would mean that any flat exposure under 10s could be calibrated with a fixed, constant offset value, and that's pretty much what synthetic offsets are about. - However, the 294 has timing issues that prevent the capturing of very short flat frames. That is exactly why people started capturing those long flat frames, and started taking dark flats to calibrate them, but in lots of cases it's imho worthless. From my experience, I can say the technique works pretty well with flats whose exposure stays between 0.5s and 10s. - while the amount of noise that flatdarks bring back to the images may be negligible, one big advantage of that technique is that you have a single "master offset for flats" that you can reuse for any flat exposure between 0.5 and 10s. In most cases, even with restrictive filters, that should be enough. In my case, I can always keep my flat exposures under 10s, be it with a 7nm SII filter or the IR-UV cut I use for luminance. That's really the main point for me. All of the above applies to my specific model (ASI294MM Pro), but that should also be applicable to more recent sensors (ASI533/2600 etc).