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Yes, way to too complicated to formulate a single number, and there will always be that edge case. Let's say two completely same setups - one at dark site of mag21 and one in LP skies of mag19. Is there a single number that can specify the difference? No - it will depend on chosen target - fainter the target, larger the difference.

In only 120s with 72mm scope? Don't really think so ... Offset is low - so it can't be that, average value of dark is around 80e (120 gain is close to unity, so I'll just use 1e/ADU). On the other hand green background level is about 6000e. That system is sampling at just a bit over 2"/px (somewhere around 2.3 or so - not sure if it is using reducer or not). I image from mag 18.5 skies, and my sub 1 minute long, in green filter with 80mm scope and sampling of 2"/px has something like 200e background value, which means that two minutes would be 400e or about x15 less. That is 3 magnitudes difference. Or 15.5 mag skies. I don't think there is such LP - worst that I could find would be center of Rotterdam with mag16.5 Center of Melbourne is mag17.75 so not so severe. I think this points again to light leak - that can also raise levels of image.

Quite right. Strange thing is - it does not affect Flats either. Btw - I did the same thing you did above - debayer single sub without calibration, and here is my result: I guess difference is to the fact that you are wiping your image, and this one is just with aligned black point. It sort of looks like light leak maybe? Is there any chance camera/scope connection is letting some light pass in?

Yes there is a way for that to happen as well. It is in part to processing - so it is not down to scope as such - but to the way it is used. Imagine using same sensor in my above example on 6" F/10 and 4" F/5 scopes. Since we use the same sensor - it is obvious that FOV is going to be smaller on F/10 scope, but let's for argument sake make target fit smaller FOV - it will then fit larger FOV as well, so both scopes will image what we are interested in with single frame (no need for mosaics). Now we want to match sampling resolution and we decide to bin x3 pixels when using sensor with F/10 scope. That will make pixels x3 larger and paired with x3 longer FL - we will have same resolution in arc seconds per pixel. What really determines speed of imaging setup is - "aperture at resolution". We again have same resolution but F/10 scope has larger aperture (and x2.25 light gathering surface - so x2.25 more signal). If you want general "speed number" of the system it would be pixel surface * aperture surface or (pixel_size * aperture)^2 - that will give you comparable numbers whose ratio will give you roughly total integration ratio to get same SNR. Even if you use large sensor on small fast scope - you can still be faster by using even smaller sensor on slow larger scope - provided that in both cases intended target fits FOV, so difference in FOV size is not important - again it would be matter of aperture at resolution or above given (pixel_size * aperture)^2.

Here is another interesting find: - dark sub in zip above - 0 pixels that are not divisible with 4 - flat sub in zip above - 0 pixels that are not divisible with 4 - light sub - ~37.5% of the pixels have value that is not divisible with 4 (and therefore wrong value if we take it that ASI294 is 14 bit ADC and we compare to other 14bit asi cameras and flat from this camera). And now for final surprise: 0 green pixels have wrong values ~75% of red pixels have wrong value and ~75% of blue pixels have wrong values Huh, hold on, is there setting for ASI294 in drivers to adjust B and R value? Maybe this has been fiddled with if exists?

Don't mix ISO into it - it's just numeric conversion factor that will not impact signal levels or SNR. Above was true even if we made FOV and sampling resolution the same. If we don't take that into account things can get even crazier. You don't necessarily have to have very large illuminated/corrected circle on F/10 scope for this to be true. This is because one can get dedicated astronomy camera with rather small sensor these days. Sensors of 8-10mm in diagonal are not uncommon. Couple that with variety of pixel sizes, and you can see that novice astro photographer can find them selves underwhelmed by their result from their very fast F/4.8 scope in comparison to someone else's F/7 or F/8 slow imaging rig using APS-C or full format sensor. Or you can think of it even like this: 4" F/5 refractor with camera that has 2.4um pixels vs 6" F/9 RC with pixel size 12um size pixels - F/9 is going to "rule" F/5 scope in terms of speed as long as both scopes can fit target in their FOV (but sampling will be different).

I agree, there is some strange effect - but I don't see it be on front of the lens - it will not spread like that what ever shape it has on front of the lens - things will only get darker and that is about it. It needs to be somewhere inside system - flattener probably as sensor is not cooled and it is warmer than rest of the gear and it will not allow for frost / dew to form.

I see what you mean - it looks like over exposed flat is only part of the story. What do you think about odd pixel values in the image? I have ASI1600 and it uses 12 bit ADC - every number from raw image that I ever got was divisible with 16 (2^4). I also have ASI178 - it has 14bit ADC - again same thing, only this time all numbers were divisible with 4 (2^2). Never had odd pixel values in raw image.

If we go by darks (and I'm hoping that darks are matched to lights), here is histogram for darks: Minimum value is way above 0 at 68, histogram is nicely shaped and there is no clipping to the left. Even if short exposure bias has issues with offset - that is not going to impact lights and darks because both have histogram way higher than 0 and no clipping occurs (and bias is not used for calibration here). But there are couple of things that are surprising now that I've taken a look again at subs: 1. Both darks and flats have following data written in fits header: What I don't understand is camera gain and EGAIN part. Camera gain of 120 should give us less than 1 e/ADU according to ZWO data on the camera: For EGAIN of about 4e/ADU, gain setting should be at about 0 gain. This is confusing but can be bug in drivers with EGain being reported for gain 0 rather than actual gain setting (one can calculate actual e/ADU for 0 gain and any gain setting for ZWO cameras because gain is in 0.1dB units). That aside, only channel that has not got clipping in flats is blue channel, and here is calibrated single sub and blue channel extracted: It has classic LP gradient but I don't think it has anything else in there? Let me remove linear gradient to see what is left: I would call this reasonably flat fielded. There is little under correction - but that is due the fact that no flat dark was included, just to attest it is flat under correction - here is blue part of the flat: Hm, that does not explain top left corner. But here is one more oddity for the end. These are supposed to be raw files straight from the camera, right? ASI294 has 14bit ADC and 16bit FITS is written in MSB - that means all numbers in the image should be divisible with 4. There are bunch of odd valued pixels in the image and all numbers divisible with 4 are even. This should not happen - either drivers are buggy or image has been tampered with. Maybe there is issue with data transfer or storage (data gets corrupt for some reason).

As far as I could tell from supplied subs, offset is just fine - issue was due to clipping of the flats - look above histogram

Ok, here it is: F/10 scope is for example 6" (or rather 150mm to be precise) F/10 scope and F/5 scope is 4" (again 100mm) F/5 scope. F/10 scope has 1500mm focal length and F/5 scope has 500mm focal length. We put sensor A on F/10 scope and put sensor B on F/5 scope. Sensor A is 3 times larger than sensor B and has x3 pixel size of sensor B. Due to fact that F/10 has x3 focal length of F/5 scope and sensor A is three times larger (x3 height and x3 width) than B - both F/10+A and F/5+B will provide exact same FOV. Due to fact that A has x3 larger pixels than B and F/10 has x3 longer FL than F/5 - again F/10+A and F/5+B will have same sampling rate / resolution in arc seconds per pixel. So both FOV and resolution of these two setups are the same. However light gathering surface of F/10 scope is 75^2*PI centimeters squared vs 50^2*pi centimeters squared of F/5 scope or calculated this turns out to be ~17671.5 : ~7854 = x2.25 F/10 scope will gather 2.25 more photons in same amount of time as will F/5 and those photons will be spread over same FOV and sampled by same number of pixels. Measured signal will be therefore larger by factor of x2.25 and resulting SNR will be at most larger by factor of x1.5 (at most because SNR does not depend only on signal level and shot noise, but other noise sources as well). Here we have shown that F/10 scope when paired with carefully chosen camera and pixel size will be faster than F/5 scope with certain camera and pixel size. We have shown another important thing - speed of astrophotograpy setup depends on other factors than size of objective and focal length and thus those two alone can't be used to determine speed of of setup.

I don't think that clouds have anything to do with it. Clouds can make target dimmer and background brighter - but all of that is light reaching sensor and subject to any vignetting / dust shadows. Flat calibration should work regardless if one is shooting clouds or clear skies and there should be no vignetting and dust shadows visible in the calibrated sub. Perhaps uneven background but not vignetted.

Are we talking about same set of subs posted earlier? How do you explain my calibration with single sub and the fact that central part is over corrected while edges are under corrected?

I can't really recommend a good book on all of that, but I don't think it would require a whole book to put all there is to it either. I learned all I know by reading articles online and applying some logic to it all - what counts for signal level how is it related to noise, how telescopes work (focuses all incoming parallel light rays to a "single point"), etc ... Here are some guidelines for deeper understanding of it all: - understand two main types of noise: Gaussian and Poisson - understand four main sources of noise: read noise, dark noise, lp noise and shot noise (first one is gaussian others are poisson distributions) - figure out formulas for sampling (focal length / pixel size stuff) - understand how and why stacking improves SNR. And after that just think about how things vary and what you can do with them

While you can apply similar logic to lights as to flats - there are differences which in practice prevent you to do so. For lights you also want as large signal as you can get in single sub within certain we could say constraints. Flats light is way too brighter than stuff that we image and it is really easy to achieve 75% or above of histogram. With lights it is much easier to go the other way around - gather as much subs as possible then it is to push histogram past 75% or so - as that would require multiple hours for single exposure - maybe one would not even manage to do it in a single night. There is rule that will help you with understanding relationship between best SNR possible and sub duration and light pollution, but that requires you to know things about your camera - namely it's read noise and gain factor. What is always true is following - fewer longer subs will be better than more shorter subs - totaling to same imaging time (it's better to have x12 5 minute subs than x60 1 minute subs - both total 1h or imaging time). This is true always, but how much difference there will be - depends on read noise of the camera and light pollution, or rather other noise sources. Once any of other noise sources "swamps" read noise (is higher in intensity about x4-5) there will be minimal difference in end result. This is why you can use shorter subs in higher light pollution - as there will be no difference (visible) to using longer subs (shorter up to a point - there is always that point of diminishing returns). You need to know specs of your camera (not something that is readily available for DSLRs unlike dedicated astro cameras) in order to calculate optimum exposure length for your conditions. In absence of solid numbers, there is simple way to determine your exposure - expose for "reasonable" amount of time so that your guiding works well (there are no signs of star trailing and stars are round and tight), you gather enough subs for your imaging time - advanced stacking algorithms require some amount of data - they work better if you feed them 20, 30 or 50 subs as opposed to 2, 3 or 5 subs, and be careful about wasted data - if there is wind gust or cable snag or passing airplane or anything happens to make you throw away your data - it's better to throw away 1 minute of imaging than it is to throw away 20 minutes of imaging. For this reason - pick exposure times in range of 2-3 minutes (1 minute is ok, 4 minutes is ok - as long as it is "reasonable" amount of time in above sense).

Here is comparison of field of view between 8" F/6 with 32mm Plossl and 90mm heritage virtuoso with 32mm Plossl - virtually same field of view. So how can one be wide field and other narrow field when both give same field of view? Field of view depends on apparent field of view of eyepiece and magnification used. Magnification depends on focal length of scope and eyepiece. There is no scope aperture in that equation and indeed both scopes of same focal length give same field of view with same EP regardless of their speed and aperture.

And then people go around saying - don't use that scope for AP, it is F/10 and therefore slow

Interesting thing is that for 8" F/6 scope or 6" F/8 scope no one is saying it is high power narrow FOV scope - and most will qualify those scopes as ideal scopes for serious beginner - ones that can turn into life long instrument, but telescope with same focal length (ok, not the same but 50mm longer 1200mm vs 1250mm) is suddenly long focal length / high power / narrow FOV scope?

No, please, don't perpetuate that slow/fast scope - longer/sorter time to take the image thing as it is not true. It just creates confusion, as a person can come and think, "look I have F/5 scope - that is supposed to be fast, and if another person with their F/8 scope can take certain image in 2h, I should surely be able to take it in less than 2h" - but that might not be the case, and people will have a wrong idea why they are failing to meet "fast scope expected exposure time".

Maybe it would be best to first just take flats and then we can see how to best test them. I would personally use ImageJ to do all processing and testing. You can use that also - it is free software used for scientific image manipulation and it's written in Java, which means that it will work on almost any computer / operating system. Once you have your flats and bias, I'll walk you thru the procedure how to do various things with them - like check histogram, do statistical analysis (mean value, min/max and such sort of things) and how to do image math (calibrate them, stack them, etc) .... You will need one more piece of software (also free) - FitsWork. That one is used to convert DSLR raw files into fits that can be then used in ImageJ. As for lights and histogram position - it is down to two things: 1. linearity of sensor Here you want to ensure that there is no clipping left / right (low or high values - all three peaks present and looking nice in histogram). There is also concern about linearity of sensor response - it can happen that sensitivity depends on signal level (or rather gathered signal so far). Which means that doubling of amount of light, or doubling exposure length does not produce twice as high recorded signal (ADU level). This is bad thing for calibration and needs to be addressed in certain way. I think that nowadays pretty much no sensors that we use in AP behave that way, but "most linear" region was around 2/3-3/4 back then when it happened. In fact saturation and anti blooming gates cause non linearity in modern sensors - but we consider them to be "clipping" to the right. 2. Minimizing noise impact. Once you are nice linear region and that part is covered - well, you want to have the least noise in your flats as possible - that is why we take more than one flat as stacking them reduces their noise. This noise will end up being injected back in the image in a certain way so you want to minimize it. There is two ways of minimizing the noise: 1. stack more subs 2. make sure SNR of subs is high to begin with - this part relates to histogram position. SNR is signal to noise ratio or signal/noise. If we want it to be high - we want our signal to be strong. Stronger signal is right on histogram or higher value of pixels (so closer to 1 but not too close as it will star saturating - that is why 80-85% is often mentioned). bottom line - you can use histogram at even 5% but make sure you have enough flat subs to stack to minimize noise coming from subs.

In fact you can do full calibration with single sub of each - one light, one dark, one flat and one flat dark. The only reason we use multiple of each is to reduce noise, but if you are looking at general artifacts in the image - like background illumination and flat correction - you can improve signal to noise ratio in another way - you can bin image. I think I used bin factor of something like x10 on above image. Image will be tiny and massive amount of details will be lost - but we don't care for such details in the image to diagnose flat issues. Ok, then focus position is not the problem here. No, I don't think it can be issue with flats. You can check this with your scope - add central obstruction, but in principle many people provide proof or that on daily basis - scope with central obstruction acts as if frost is blocking light in central part of the lens (in fact being more severe - as it is 100% light blockage). Central obstruction or in fact any obstruction before light is bent has no effect on field illumination (that is not strictly true - there are cases where level of obstruction depends on incidence angle - like with very long dew shield that will act as aperture stop at angles). What can happen on the other hand would be dewing up corrector lens or dslr sensor (or frosting up) - any chance that happened? I mean it's a long shot since it is closed system once everything is screwed together so absolute humidity does not change, but it can happen if it was particularly humid and quite a bit warmer when you put your gear together and it cooled considerably during the night? I highly doubt it is sensor (as it is not actively cooled), but could be corrector lens (field flattener) We can test that theory - central blockage of corrector by dew/frost. I need to be careful about this one as it is easy to mix up things, so I'll write down everything to be corrected if I make mistake. Flat correction = light / flat In center we have over correction (look at that bright dust shadow) and in outer parts we have under correction - which means center - higher value edge - lower value light / flat = higher value can happen if light has higher value, or flat has lower value light / flat = lower value can happen if light has lower value or flat has higher value. I don't think it is this either. Central blockage would cause over correction in the center but it would not cause under correction at the edges (it can't boost flat value or reduce light value). I think there is something that you can try without a clear night as it just involves flats. Try flat-flat calibration to rule out flat settings as culprit for the problem. This would mean that you take bias subs (you can do just one or couple - number does not matter here except for reducing noise), at each ISO setting that you will test. Test with at least two different ISO settings. For each ISO setting - shoot 2-3 different "type" of flats (where type here only means exposure length / strength of flat panel). Aim to have proper histogram each time (no clipping either to left or right) - as you did with above flats, but have different position on histogram each time - around 20-30% around 50-60% and proper one at around 80-85% for highest peak. Each of these flats should calibrate others (once bias is removed) - meaning that you take any two flats - flat1 and flat2 and divide them, you should end up with perfectly uniform illuminated sub (no vignetting / no dust shadows to be seen). If this happens and each flat calibrates others - you can be 100% sure that your flats are working as they should - then we need to look elsewhere for the source of the issue.

Don't know if I can answer, but I can sure add to confusion How about 183mono? It has pixel size of 178mm, but the size of 174mm (or even a bit more), it will fit whole disc and people use it for DSO imaging

Please don't use fast and slow in astro photographic context to specify any sort of "speed" of capturing an image - as it is very misguiding and often wrong. Term has significance in both visual and photographic use as has been described, but its use to denote photographic speed comes from domain of daytime photography and lens operation (on a single camera where there is plenty of light and single exposure is taken). Comparing two scope in astro photographic domain by F/ratio and concluding that one will create image faster - is simply not true. F/10 scope can be faster than F/5 scope, and in here faster means - less time spent to make an image of equal SNR. Btw, to address original question, In my view 6/9 so less than F/6 is fast, F/6 to F/9 is medium and above F/9 is slow scope.

I was looking at this but always "objected" the fact that it is IR type remote and thought that I would need to point it towards the scope / mount for some reason, but you are quite right. Arduino with WIFI and simple IR detector (there is plenty of tutorials for that online) can pick up remote commands and translate those into UDP packets sent over wifi to command the mount! Excellent idea!

You should be able to see quite a bit of details on the Moon. Atmosphere will start to be limiting factor for even such a small scope. To get the idea of what you might expect, take a look at these videos: https://www.youtube.com/watch?v=wbbeU6JJE5A https://www.youtube.com/watch?v=z74yn4JdSGQ https://www.youtube.com/watch?v=vTwM1UGkEfA There are also few others on youtube - just search for Mak 90 or the scope you are interested in. This will give you rough idea what can be seen, but are not 100% correct representations because of different methods used to record and process those videos.