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Use plate solve to give you exact coordinates and rotation and then match that with the one in above image. You have coordinates of the center - tell your software to goto there and verify with plate solve (or tell software if it supports automatic centering with plate solve - to just make its way to those coordinates). Make sure you rotate your camera to match rotation in above image - again if you have rotator .... if not, you'll have to do it manually.

It should have absolutely no impact on precision of guiding. It will just give you "more correct" reading of how your guiding performs. Having ruler with wrong spacing between markings will not make your item longer or shorter - it is what it is, you'll just have more or less error in knowing how long your item is. Guiding will still have same error in pixels - only issue is that you won't be certain how much that is in arc seconds. If you want to be really precise about this - take image of known stars with guide scope / guide camera combination. Measure separation of these stars on image in pixels and divide with know angular separation of stars. This will give you exact sampling resolution. Alternatively - take an image and plate solve using Astrometry.net - it will also give you arc seconds per pixel.

Correct - you are not wrong, in general with refractors - you need to measure lens to sensor distance. Problem is knowing exactly where "center" of the lens is. You don't need very big precision in this for guiding, but there is much simpler way of doing it, here, let me "measure" that for you. F/4 scope with 32mm of aperture - my ruler says focal length is 4 * 32 = 128mm

As far as I understand it - we can still use it without yearly subscription if we stay on current 3.2 version, is that right?

I'm certainly not a great imager, and this is not per request as it was one of my first images. It also does not render Trapezium as nicely as your multi layer approach, but here it goes: Just a tad over blow core and stars, but you can still make out four main stars of the Trapezium. And yes, it's not 32bit either

You could do something like that - provided that you use IR pass rather than L filter. That way sensor will act as monochrome. You'll need filter that passes above 800nm. There are two issues with that approach however. First is that you won't get resolution that you would otherwise get. Airy disk at 900nm is twice as large as Airy disk at 450nm. Human eye finds sharpness in luminance information rather than in color. You'll be shooting luminance in region where your telescope is acting as if having almost half of aperture. There is upside to this - IR is less affected by atmosphere so it will be easier to sharpen up recording. Second issue is that luminance is related to color in human vision, or rather two different colors will have different luminance data. Green carries more luminance than red and blue for example. This reflects somewhat in QE curve of sensor. We have no way of knowing relative luminance of different colors in IR part of spectrum. What should be relatively bright like yellow color could end up having very low IR intensity and something that should be dark - like deep red might end up having quite a bit of intensity in IR - all of those will create odd looking image.

Most people think that elongated / moving stars come from polar alignment error only. This is not true, as there are two different causes of star drift in the image: 1. polar alignment error 2. periodic error Second tends to be larger with good polar alignment. This is "oscillatory" movement of mount in RA direction - happens because gears are not perfectly round, and repeats every RA worm period. Usually about 8-10 minutes for most mounts (each mount model has its own worm period). You can distinguish the two by direction - Periodic Error happens in RA direction and Polar alignment error happens in DEC direction. On your main movie (not zoomed in) - orientation is: RA direction up down, DEC direction - left right. On your zoomed in video, you'll notice rather smooth movement from left to right - this is drift due to polar alignment - slow drift in DEC (left-right). There is jumpy / seesaw motion up and down - this is RA periodic error. There is general slow drift in RA direction and this can be either due to worm wheel shape not 100% round (period 24h - one full revolution) or slight tracking speed difference to sidereal. Second thing can sometimes happen if mount electronics does not have very precise quartz crystal to keep time accurate - so it makes full revolution in 23:55 or 24:05 or whatever. You can see on your video that Periodic Error is much faster - really fast jiggling up and down on this sped up video. If you have star trails - this will be responsible rather than polar alignment error. Cures? To some extent : Periodic Error Correction - look up PEC in your mount manual, and of course - guiding.

There is no unseeing it now ... At least it looks interesting that way.

Sorry, I was not precise about what I meant. Not local time, I'm sure your local time and location are correct. I meant - for how long have you been guided before DEC RMS error became 13.7". It did not start out that high - it grew over to that value. Either we need that time, or how many samples your guiding app uses to calculate RMS error. With PHD2 you can set number of samples to display / use in calculation. Here I marked number of samples for guide graph and RMS error - in this case 100 samples were used. If your guide cycle is for example 3 seconds - that means that RMS error displayed is for last 300s of guiding. We can use this time and RMS error to calculate drift rate and hence polar alignment error - which I tried to do by counting "points" on the graph and using 3s exposure length. If you have 1' minute in altitude and 2' minutes in azimuth (these are really errors in altitude and azimuth rather than RA and DEC) - combined error is ~2.24'. This is very low polar alignment error - drift rate would be 0.6" per minute. Depending on your imaging resolution - it would take many minutes before frame would move single pixel. It would take something like 20-30 minutes before DEC RMS error would reach 13.7" If above screen shot was taken after half an hour of guiding then everything is fine - your drift rate is low, but it did accumulate to almost 14" over half an hour. Yes - I'm not talking about RA tracking error here - I'm talking about RA axis pointing direction. RA counters Earth's rotation and hence needs to be parallel to axis of rotation of Earth - and point to celestial pole. When I say that RA is out - I meant not properly aligned with axis of rotation of the Earth. Nothing to do with RA tracking error (which is difference between the speed at which Earth is rotating and the speed at which the mount is tracking).

I've been reading a lot about different lenses lately and all points to Samyang lenses for AP purposes. They are not without faults, especially fast lenses like F/1.4 or F/1.2 and often need to be stopped to F/2.8 for AP applications - but that is certainly fast as well. I believe things only improve with NB imaging.

Do уou know what time is used to calculate total / RA / Dec error from? I counted about 54 line segments above and your guide cycle is 3 seconds from the image. That gives about 160 seconds of graph and I'm going to assume that error figures are calculated from the graph - so again ~160s. You have about 14" of error in DEC from those 160s - this equates to 0.0875"/s of drift. This equates to 20' of polar alignment error. To put it into perspective - you are aiming your RA axis about 2/3 of a full Moon away from celestial pole. On the other hand - it's not bad if you consider that placing RA axis right on Polaris will create about 44' of polar alignment error - or about twice of what you have.

No, these stars are red indeed. In fact, most stars are orange / red These are main spectral types and their stats.

Hi and welcome to SGL. What you are seeing is part of the Milky way around what is called "Summer Triangle" - three prominent summer stars - Altair, Deneb and Vega. Prominent constellations are Cygnus and Sagittarius. Download software called Stellarium and you can get a lot of information about night sky from it. Here is screen shot of the same area of the sky: I marked summer triangle out for you and here it is in original image:

Ok, I'm all out of ideas. Only thing that I could suggest to you is that you leave your camera on telescope and do 120s dark that way - do a single sub for testing. Compare mean ADU value of that sub to mean ADU value of dark sub that you took in your fridge. If there is significant difference - redo your darks on scope - lights were shot on scope so darks also need to be shot in same conditions (if there is a difference compared to those shot in fridge). This could resolve the issue. I mentioned before that either light signal is stronger or flats are "weaker" for some reason. We concentrated on flats, but signal can also be "stronger" than it should be - if darks are "weaker" than they should be - having lower ADU number. Those darks shot in the fridge to have lower ADU values than expected - maybe darks on the scope will have higher mean ADU and this will solve the issue. LP is generally a bit red in color - and this can explain why only red subs were affected - higher LP background ADU in these subs compared to green and blue.

Ok, here is the think that I don't get. Circles are in left bottom part of the image: - in first sub that you posted in this thread - in master flat - in calibrated stack - in sub that you attached - in single flat that you attached. However - there is a pair of stars that is in top right corner in all the images except in full stack - they are in bottom left. It is feasible that you did meridian flip when shooting red channel and that you have some subs with double stars in bottom left and some subs with double stars in top right corner - but calibration issue is only present in bottom left corner - not top right. I would expect both corners to be affected if some subs were rotated by 180 degrees due to meridian flip Do you have any idea of why is that - did you do meridian flip at all - are some subs with stars in bottom left corner?

It could be if there was a slight light leak or something - like IR light. It is strange that all three flat darks have the same mean ADU value however - I would expect some variation if there was source of the light (or even IR radiation) present. In any case - I don't think this is causing you issues with red channel - as it is no causing issues with green nor blue and all three have same mean ADU for their respective flat darks.

No, not flats - flat darks, can you load a single green flat dark and calculate stats on it and see what is mean ADU value and one blue flat dark - do the same. Regular dark that is 120s long has mean ADU value of ~1548ADU while 4.24s long flat dark has mean ADU value of ~1574 This is wrong - shorter dark must have lower mean ADU value because both darks should have same bias / offset level and only difference should be dark current build up - longer the dark - more dark current there is - higher mean ADU value. I want to check what sort of ADU value you are getting for 2.4s and 3.3s exposures. If you are having trouble measuring this for some reason (no software support or whatever) - upload one green and one blue flat dark sub and I'll measure it.

And their mean ADU values?

How long were your blue and green flats? Could you measure mean ADU values for their flat darks?

Yes, even in single frame, there is issue with calibration: There is arc visible in bottom left corner that is due to flats There is same arc visible in uncalibrated sub - it is feature common to both light sub and corresponding flat: Which means that flat is under correcting. calibrated = light / flat and calibrated has higher value than it should have - means either light signal is stronger than it should be or master flat is "weaker" than it ought to be. Two possible explanations: 1. Light leak - any chance of that? Light leak might have happened when red lights were taken but not when flats were taken nor when blue and green channels were taken 2. Weaker master flat usually means stronger flat darks. Now, flat darks were taken at same settings as flats, correct? I have found rather interesting discrepancy. It is something that I would otherwise expect but under different circumstances - so it might be important or it might not be important. This shows that 4.24 second flat darks have higher mean adu value than 120s regular darks. Such thing should not happen, but it sometimes happens on CMOS sensors. It usually happens when one has very short exposure - like less than one second because these exposures are timed by sensor it self (to enable high fps) - longer exposures are sort of bulb mode - drivers or external electronics signals start and stop of exposure. It also means that internal calibration of sensor is different and different bias level is used - hence stronger bias in such short exposure - one of the reasons we don't use bias subs with CMOS - they are unreliable. Here, I believe 4.24 seconds is too long exposure to have this effect. Could there have been a light leak while you took red flat darks? How long were your blue and green flats? Could you examine blue and green flat darks to see their mean ADU value and how it compares to this value? If you find that both green and blue flat darks have smaller mean value - smaller even than regular 120s darks (as they should have because we have less time for dark current build up) then you should retake red flat darks to eliminate possibility of light leak there.

You have red lights, you shot red flat field and corresponding flat darks and you are using matching darks for your red lights and you are having issues with calibration? Could you post one of each: 1. red channel light fits 2. red channel dark fits 3. red channel flat 4. red channel flat dark (darks don't need to be specifically for red channel of course as long as they match corresponding lights).

Taken from here: https://www.kenrockwell.com/tokina/11-16mm.htm#perf In any case - stop it down to F/5.6 if you want good corner performance or just live with it as is, because most people that describe how lens is sharp - don't really shoot test charts at infinity like astronomers do.

Have no idea but my bet is on APS-C size...

Use corresponding flat for each filter. Use flat dark with corresponding flats. Use dark that match your lights for each filter. Turn off all "enhancements" and "optimizations" and you should be fine.

That always happens with B masks whether mask is clear or not. Same reason spikes on stars in RGB are rainbow colored. Angle of diffraction of light depends on wavelength of light. This is how diffraction gratings in spectroscopes work. Same happens here and what you see is first, second, third, fourth, .... order lines smeared by seeing with narrow band and mask.