vlaiv

Not sure what image are you taking about. Star bloat can be due to many things - one of those is chromatic aberration. Chromatic aberration is due to refractor lens not bringing all wavelengths of light to same focus - lens has shorter focal length for some wavelengths then for other. Effect is very recognizable as it presents most times as blue (sometimes red) halo around stars (especially white/bluish ones). Take for example this image of M45: There is no mistake - stars have blue (and a bit red closer to core) halo. This does not happen with mirror telescopes. It does not happen with good APO scopes neither unless you let in IR and UV part of the spectrum. ED doublet and Triplet scopes are optimized to bring to common focus two or three wavelengths in visible part of spectrum. Further away from visible part of spectrum - bigger deviation of focus. If you don't remove these (essentially not important wavelengths as far as color and visual representation of target are concerned) - they can cause star bloat. Depending on camera used and its QE - star bloat can have color or be white (if camera is equally sensitive for each color in far IR). This does not happen with mirror telescopes. Even using corrective refractive elements in optical path don't usually lead to this. Most corrective elements don't disperse light very much in order to introduce significant amount of chromatic aberration.

As has been pointed out - UV/IR cut filter is needed to achieve true color image (or rather - to have a chance to achieve it).

Coma corrector is too close to focal plane to be able to impart significant chromatic aberration. It is also not bending light significantly to separate wavelengths much. You can check spot diagram of various models of CC - and you'll see that there is no significant separation of colors.

Yes it is - but only if you use super pixel mode. Most people use default debayer mode which is interpolation, then you retain sampling rate of smaller pixel. For full resolution one should really use bayer drizzle - like @Adam J mentioned. It only requires dithering and software support. That is by the way very good point and OSC sensors should be thought of in that way - bayer element instead of pixel. For example ASI485mc has 5.8µm pixel size and 1/4 of declared QE in red and blue and 1/2 declared QE in green. That is probably best description of "pixel unit".

If 135mm focal length is an option - then Samyang 135mm F/2.0? Stopped down to 2.8 - it will have very good definition over the surface of smaller chip like 485. Only issue is pixel size (with any of these scopes really as it is 2.9µm - too small), but you can use super pixel mode / bin.

That's one cute canis - even if it is a minor one How do you like the camera?

That is rather nice image - however - over sampled . You are right - one can make nice images with that gear (300p + ASI290) - but it is not for novice imager and it will over sample. or One of these two is original - other has been resized down by 50% and then enlarged back to 100%. See any difference? In another words - image is over sampled by at least double.

If I use constant as a bias??? Why leave all that fixed pattern noise in the image?

I did not even mention unstable / unusable bias. My ASI1600 has larger mean bias value than in short exposure. If I used bias - I would end up getting "negative dark current" in my flats.

On a 300p with 2.9µm pixel camera? Too small FOV and one would need to bin at least x3. That reduces pixel count to 640x320 and increases read noise x3 which is not very good for short exposures.

No, I don't have a light leak. Every camera is different and we are shooting in different settings. ASI294 has quite low dark current, lower than ASI1600. ASI185 might not have such low dark current, and it depends on ambient temperature what that dark current will be (for non cooled cameras). In any case - both measurements prove my point - there is dark signal that needs to be removed, otherwise you might end up with flawed flat calibration. You are right in one point - people might get away with using bias instead of flat darks, and that is fine in some cases - like short subs with set point cooled camera where there is not enough dark signal to make visible difference, but please don't make that into general conclusion - you'll be only confusing people that don't have enough knowledge to distinguish different cases. Flat darks work in all cases, they are correct way to do things - no shortcuts taken, and as such they should be advised as proper way to do things. If one knows what they are doing, then by all means - shoot in any way you want, but don't go advising others if you don't fully control their process as well.

No, in SGP I have offset set to 64 in this case (camera is 12bit so ADU values are multiplied by 16 - but that does not change anything 16*64 = 1024, so ADU "offset" is around 1024 in this case - as you can see from above table Mean values start at around 1000 and raise - zwo offset is not very precise it often "misses" by dozen or so ADU).

Yes, ASI185 is modern CMOS sensor 1944x1224 pixels from ZWO (Sony IMX185) I also have ASI1600 with Panasonic CMOS sensor and ASI178 with again Sony sensor - all behaving the same. Here, I did same test again in SGP: you have exposure length in title and rest is self explanatory ...

I can repeat this with SGP - using ASCOM driver as well instead of native - but result will be the same

Ok, so I took my ASI185 - and SharpCap and took 5 shots. bias, 1s, 2s, 4s, 8s here are reported mean and stddev values.

You are saying that standard deviation is rising as exposure time but mean / median value stays the same? What capture software are you using?

You are correct - it won't be wrong just because I say it is wrong. I've shown you very simple math that shows that you can't have correct flat if you don't remove offset. You are further claiming that bias offset is the same as dark current offset, and that contradicts my measurements on numerous occasions. It also contradicts common knowledge about dark current - why do we even have a term if it does not exist?

Can you post mean ADU of 300s, 150s and bias that you measured in 294MC camera?

Why would I take a constant to replace actual dark current that might not be constant - or rather never is? Why should I forcefully push what is essentially wrong method of calibration over right one? Did you try this on any dedicated astronomy camera with set point cooling?

It is not wrong - it is basic math. If you have flat without removed dark - you have light signal and dark signal. Only light signal determines percentage of light - dark signal must be removed. Say you have 70% vignetting at one pixel and your flat at that pixel is 7010ADU - 7000ADU being light and 10ADU being dark that you did not remove. Peak flat is 10000ADU or 10010 ADU because of dark (let's assume dark current is uniform). So your scaled flat in that pixel is 7010 / 10010 = ~0.7003 There you go - we just got wrong flat - for 70% vignetting our flat will correct with 70.03% It might not look like much - but once you stretch your data - that will show as improper correction I have - many times. All of my cameras have measurable amount of dark current - and yes it corresponds for published dark current for those sensors. Offset level is not the same as level of dark in any of my cameras.

I'd recommend choosing camera based on following criteria: 1. pixel size that will easily fit critical sampling rate 2. lowest read noise possible 3. highest QE possible 4. fastest download rates possible (USB 3.0) Problem is that you'll have hard time finding one camera that will satisfy both of your needs - lunar with F/10-F/15 scopes and solar with quark that has integrated telecentric and makes your refractors operate probably around F/30. For Ha light at F/30 - optimum pixel size is 9.84µm While for lunar at F/15 it is 3.75µm Those are vastly different pixel sizes - and you'll be hard pressed to find suitable camera for quark

Issue that you mention - one of injecting more noise with flat darks - is non issue. We control amount of noise we inject back by choosing number of subs we use to create our masters. Want less noise? Use more subs for masters. Problem with bias only approach is that you will have incorrect flat calibration. Using bias instead of flat darks can cause flats to over correct. If you don't remove dark signal - it will also be subject to flat calibration - but it was not subject to attenuation by optical train in the first place - so you'll end up with uneven offset signal (if your dark is uniform -and worse if it contains amp glow). There is a reason why flat darks are used - because it is proper calibration of data.

ASI2600mc-pro if I'm not mistaken.

Those stars are indeed much tighter - the way they are supposed to look like. I do however have a slight worry about that flaring present with some of the brighter stars. Not sure what could be causing it. In either case - not sure what FF/FR to recommend. I do know the one I'm looking into getting for my setups - one of scopes is 80mm F/6 triplet. It is this item: https://www.teleskop-express.de/shop/product_info.php/info/p11122_Riccardi-0-75x-APO-Reducer-and-Flattener-with-M63x1-Thread.html Not sure how it would work with shorter FL scope though and how it would behave on ED doublet.

No, I think you have pretty good match between camera and scope. 1.8"/px is a bit on high side for 70mm scope - using FF/FR might be better option - to get to 2"/px or slightly above - but that is nitpicking compared with issue you now have. Your scope + normal seeing + your mount and guiding that you've shown in that screen shot should match your resolution. Everything should match and you should be getting around 3.5" FWHM stars (which correspond to ~2.18"/px - and you have 1.8"/px - so slightly off but still very near perfect sampling rate). Problem is that stars in your image have FWHM of 8.6" and that is much more then expected. This means that stars are not bloated in your images just because of processing - there is something making them very large. About twice as "fat" as they should be given specs of your system. - Either you were imaging in extremely poor seeing conditions. Maybe your local seeing is very poor? Do you image over rooftops of houses that are heated so there is great differential in temperature? Maybe there is large body of water in direction where you image? - Another explanation would be that your guiding is not as good as you think it is. According to that screen show - your guiding is ok. It's not great - you could possibly go down to about 0.6-0.7" RMS with your mount, but 1" RMS is not that bad at all. In fact - it is in line with your setup and working resolution. Question is - is reported RMS accurate or not? It can be off if you entered wrong values for either pixel size or focal length of guide scope. - Third and unlikely possibility is that something in optical train is blowing up your stars. Maybe that flattener, or perhaps fact that you are using ED doublet - but I'm not seeing too much of blue fringing in your images - so I don't really think that is the case. In fact - maybe it is flattener? Looking at this spot diagram: RMS radius goes from ~7.2 up to almost 10µm - so that is two pixels up to 3 pixels for radius. This equates to 7.2" RMS diameter up to ~10" RMS diameter - with flattener. That is significant bloating of the PSF. Telescope without flattener should have 3.52" diameter of airy disk - so twice as small. If this bothers you - maybe try imaging without flattener once and see if stars in the center of the frame will be better (of course - you'll have distortion at the edges due to field curvature). Maybe get different FF/FR for your scope if this is causing issues for you?