vlaiv

If that star is spot in the center - I'd say you are bit off with your secondary mirror. That doughnut is not perfectly concentric.

Interestingly enough - image that you linked shows galaxy ~740px long. Needle has about 14.5' in width - which is 870 arc seconds. 870" / 740px = ~1.17"/px Far cry from 0.39"/px

Attainable resolution will depend on Airy disk size (for diffraction limited scopes) or spot diagram RMS, guiding RMS and seeing FWHM. 100mm diffraction limited scope (over whole spectrum imaged) with 0.6" guide RMS and 1.5" FWHM seeing will produce 2.4" FWHM result. In any case - math is as follows: FWHM = 2.355 * sigma (RMS) RMS total = square root ( RMS1^2 + RMS2^2 + RMS3^2 ....) Guide RMS is as is Airy disk to RMS goes like arcsin(0.42 * lambda / diameter) (this returns value in radians, both wavelength of light and diameter need to be in same units - be that mm or um or inches or whatever). Alternatively you can use spot diagram RMS if you have optics that is not diffraction limited. seeing RMS is seeing FWHM / 2.355 I would recommend using at least 6" of aperture when aiming for 2.4" FWHM stars as that leaves some room for guide RMS and seeing. 6" needs 1.6" FWHM seeing and 0.7" RMS guiding for 2.4" FWHM By the way, without seeing and guiding/tracking influence, 100mm of aperture can happily resolve for ~0.52"/px imaging although Dawes limit is 1.16" for 100mm scope (Dawes limit is not particularly useful measure for imaging).

Maybe best thing is not to think in terms of focal length, but instead to think in terms of achievable resolution? Set yourself a simple goal - create image at 1.5"/px that has 2.4" FWHM stars for example (or with any other resolution for that matter - it is not 1.5"/px that counts but rather 2.4" star FWHM). When you manage that - think of going to a higher resolution setup. By the way, you need only ~515mm of focal length with 3.75um pixel sized camera to image at 1.5"/px.

Yes, for some reason, preview looks very strange, I get the same when I look at .cr2 with Irfanview. It is oriented in portrait, has green cast and is only 1288 x 1936 px. When I import it into Fitswork - it looks ok. Maybe it would be good idea to download Fitswork (it is free software) and convert your cr2 files to fits prior to stacking if you run into any sort of issues with stacking .cr2 directly

They are the same, it is just preview that messes things up with .cr2. Use software like FitsWork to save .cr2 file as fits file and you will get the same thing. There are still some differences like - ASCOM fits being scaled to 12bit range while .cr2 decoded with fits works - is 16 bit. There might be color decoding going on as well. I would personally prefer true raw data over anything else, so check what you get (you might get calibrated sRGB data or true RAW color information depending on driver used).

You can but you will gain nothing and even make life very hard for yourself. At the moment, you have 73mm aperture telescope and 4.3um pixel size camera. With 430mm focal length of telescope you are at 2"/px sampling rate. That is about perfect for that aperture size. You can't get any more detail than that in your image. Any attempt to "zoom" in more than that - and you will get blurry image. What you see above is size of FOV compared to size of target. Your sensor has very big resolution in terms of pixels - 5184 x 3456. To put things in perspective - "width" of M51 is only about 15 arc minutes. That is 15 * 60 = 900 arc seconds across. Even with large telescope and very good mount and excellent seeing - best you can hope to do is around 1"/px - which would make that target 900px across. That is again going to be small compared to full FOV of your sensor - which is x6 larger in width than galaxy at this sampling rate. What you can do is image with setup you have, try to maximize sharpness on what you already have (very good focus, very good seeing and guiding) and simply crop your image to frame the target the way you want. You can still get very detailed image of the galaxy that way - for example, here is M51 at ~2"/px. Image is only 860x670px, but there is no point in making it larger as target is not large enough and effective detail is just not enough to justify higher sampling resolution.

Depends on your budget. When you move away from the lens and go into telescope astrophotography, cost increases by at least factor of say 3-4 or even more. Even small setup like StarAdventurer star tacker + small refractor telescope can end up costing close to £1000.

Yes, that is about minimum equipment that you'll need. Camera, lens, tripod, some sort of tracker. It really does not need to be expensive if you go second hand / DIY route. DSLR can be found for very fair price second hand, and for star tracker - do internet search for "Barn door" tracker. It is very cheap DIY solution. By the way - welcome to SGL.

I would put it before quark. Quarks have blocking filter at the eyepiece side that is limited in size. Further you put sensor - more vignetting you will have from blocking filter (and possibly rest of the assembly).

My guess was about 110mm - so pretty much inline captain said above.

There seems to be something wrong with your master flat. I took only single flat - which looks good and calibrated single sub with it. I had to bin it quite extensively to get good snr - but here is result after calibration, binning and background gradient removal: I see absolutely no trace of issues with flat calibration. Here it is again with black point adjusted to maximally show nebulosity vs background: (this is linear stretch so nebula is blown completely - but point is to look at background around it). But if we inspect single flat vs master flat - we get interesting thing: Median and mean values differ by about ~3.1 (3.1 and 3.15) between single sub and master. This should not happen - they should contain roughly the same average and median values. Can you inspect all your flat subs for average value and see if there are significant differences? Maybe you should discard some of your flat files from master flat - those that are significantly different from the rest?

What is that? Quick search did not return anything useful.

Are you sure, you are doing it right? Flats should not be subtracted - they should be divided with. calibrated file = (light - dark) / flat Ignore the above (I did not properly read the post). Can you post single light, single flat and master flat cr2 files for inspection?

https://www.altairastro.com/starwave-80-ed-triplet-apo-travel-refractor-465-p.asp currently out of stock, but there are other options like: https://www.teleskop-express.de/shop/product_info.php/info/p3881_TS-Optics-PHOTOLINE-80-mm-f-6-FPL53-Triplet-Apo---2-5--RAP-Focuser.html should be about the same price after shipping and any import tax. It is same scope, only different branding. (Field flattener may be necessary for imaging - so it might be over budget if that is considered).

That is some serious blue ring of fire!

Only in case that FOV depends on aperture - and it does not. It depends on focal length (think "zoom" - although it is not quite the same - longer FL, more zoom there is). ST102 has longer FL than Redcat (twice as long).

If you want simplest approach - then just shoot a star of known temperature. Do it on the night of shooting your target and in same general direction and at same altitude (to get same effects of atmosphere). Use this calculator to get RGB ratios: http://www.brucelindbloom.com/index.html?ColorCalculator.html Say you image F2 class star - it has temperature of 7020K. Set calculator like this: (reference white to D65 and Gamma to 1.0) Put star color in CCT field and press CCT. This will produce RGB triplet values that you can use to get scaling factors - in this case 1.004, 0.987, 1.112 (rounded). You take recording of your star and measure RGB values from it and from that - you get R_scale and B_scale values that you'll apply to your planetary recording. Here is how to calculate those using simple proportion. (scaled measured red to measured green has to be equal to above calculated red to calculated green) R_scale * R_measured : G_measured = 1.004 : 0.987 R_scale = (G_measured * 1.004) / (0.987 * R_measured) You do blue scale similarly and then apply R_scale and B_scale in channel mixer of your processing software. Add gamma of 2.2 at the end (set middle slider in levels to 2.2)

My planetary color camera (ASI178) gives distinct green cast because green QE is highest. Reddish color with such camera can happen if I use inappropriate bayer matrix order. So that would be first thing to check. Do have a look at this thread about color calibration and results:

Yes. If you want true color - you should do proper color handling / calibration. This does not have to be very involved process - if you are using planetary camera and have access to small lens (they often come with small all sky lenses) - you can do simple "white balance" in controlled conditions to find best red and blue ratio (compared to green). You can also do this while shooting the planets. Shoot an image of a star of known temperature (make sure you don't saturate / clip the recording). From star temperature you can get RGB ratios and then you can calibrate your recording. There are also advanced ways of doing it by creating color correction matrix and so on, as well as doing correction for effects of atmosphere.

I agree with you, and personally don't advocate use of gain 0, but OP asked for gain 0 and as long as read noise is treated appropriately with adequate exposure length there is no reason not to choose it either.

And why is dynamic range important in AP?

Quite right - it increases like that due to way noise adds, and it is precisely what ensures that ratio of LP noise to read noise remains the same when binning. LP noise is square root of LP signal LP signal is x4 when you bin 2x2 (add 4 pixels together), hence LP noise goes up by x2 - square root of 4. Read noise goes up by x2 when you bin 2x2. Ratio of LP noise and Read noise remains the same as both go up by same factor.

That only works for CCD sensors where binning happens prior to read noise. With 183mm - it will make no difference and time needs to be calculated for single pixel even if you bin (binning effectively adds read noise from all binned pixels so camera has higher read noise per binned pixel and LP goes up - but so does read noise - relevant ratio remains the same)

@900SL Faster the optics - it is "more OK" to use lower gain settings. If you want to use lower gain settings just to get "dynamic range" or to stop stars from being clipped - well - there is not much point in doing that as there will always be some stars that clip and dynamic range is really not important in single exposure - as stacking increases dynamic range with each added sub. As far as stars clipping - there is universal way to solve that that works for every well depth and every camera - use shorter "filler" exposures that you'll use to get the data for clipped regions of long exposures. There is really simple method of determining gain setting and exposure length. Lookup read noise for particular gain setting you want to use and measure what sort of light pollution you have (measure background ADU with that particular setup you have - from any of the images you've taken with that setup - just be sure to convert to electrons from ADU after you measure and that you measure on linear calibrated data). Ratio of read noise to background noise should be at least 3 but preferably 5 or more. According to ZWO website ASI533 has about 3.9e of read noise at gain 0: This means that you need to make your exposure long enough so that background signal (LP level) reaches about 380e (3.9 x 5 all squared). If you are comfortable exposing for that long (and that should not be a problem with short focal length and low working resolution) - then by all means, use gain 0.