vlaiv

All sorts of things come into play when choosing guiding setup. It is not only focal length that counts, it is also the size of pixels both in imaging and guiding equipment. With increased focal length of guider scope you get more precise guiding provided your mount is up to it and the skies allow but trade offs are: Smaller field of view to pick guide stars from, longer exposure needed to get good SNR on guide star. Many people use off axis guiders that operate on same focal length as imaging train but use guider cameras with smaller pixels. I have read somewhere that you should aim for 1/3 - 1/4 in arcsec/pixel ratio between imaging and guiding setup if using guide scope. For example if using scope of 800mm focal length and have same pixel size on both imaging and guiding camera you would like to stay above or equal to 200mm focal length for guide scope. You can guide 1000mm focal length scope with 160mm guider provided that your imaging camera has twice as large pixels as guide camera. Sensitivity of guide camera and level of read noise also play part in this equation because of way that guide programs work. I'm in favor of fast guide scope with large FOV. This can have benefit of having multiple guide stars to guide in the same time. Not sure if this is implemented in any of guiding programs but I feel that it would be of great help in beating the seeing - perturbation of star position due to seeing is rather local so if average drift across multiple stars is taken - one expect for seeing effects to be smaller.

According to ZWO, Gain of 350 should give you the least read noise for that model. Any gain setting above 200 is good - aim closer to 200 to get better dynamic range. Set exposure time as low as possible to get at least 50% (you can go lower than that but at expense of SNR), higher values are better (like already suggested 75% of histogram, provided you don't clip histogram or have too long exposure for given seeing). I usually leave gamma at 50 (it is only digital so no real benefit, and certainly so for planetary imaging), white balance at 50 for both blue and red (again no benefit since it is digital control and can be adjusted later after stacking). One thing to do as well - there is brightness setting (at least for 185mc, but I guess it is same for 224), when you decide on exposure length, cover telescope (as if taking dark frames) and look at the histogram. If it is clipping to the left, increase brightness setting. Minimum values of your dark frames should be just above 0. Do take at least 256 dark frames and process your recording in pipp (to do dark frame subtraction - this will remove both dark and bias signal). For DSO there is no real benefit in going over 135 with Gain, so keep under. For shorter exposures stay above 60 for gain, for longer exposures you can go below 60 with gain. There seems to be a switch in read mode around gain value of 60 that has effect on read noise. Last tip - don't use bias frames - take as many dark frames as you can at exactly same settings - without touching any of controls - just cover telescope with a cap. It seems that these sony cmos sensors have some internal calibration that is applied whenever you change either gain or exposure length.

This implies that even in sky noise dominated scenario there will be difference between hardware and software bin, maybe not significant one, but still, even in this case hardware bin will be better than software. Only when there is no read noise they are the same. Vladimir

Can you elaborate on this one? Noise is noise, no matter which source it comes from (sky flux poisson process or read noise or thermal noise), it may have different statistical characteristics, but in general it will be random unwanted addition to signal. My view on things including original question is as follows: Prefer mosaic + software binning (of stacked and stitched mosaic) over focal reducer (at least over any reducer that has not been specially designed for particular scope and of highest quality). Pros of mosaic + software binning: Smaller FOV to work with - less off axis aberrations of any kind (coma, astigmatism, field curvature) which are usually amplified by focal reducer. Making mosaic in general does not require much more time to achieve target SNR (not really true due to read noise, but close enough). For example: you consider to take 16 subs over target area. In case of mosaic (2x2) you would use 4 subs for each mosaic segment. 16 subs would give you x4 SNR after stacking, while for other case: 4 subs per segment give you x2 SNR reduction for segment, and then you bin 4 segments (mathematically it is equivalent of stacking 4 subs) - x2 SNR increase again - 2x2 = x4 SNR, same as in first case. SNR has quadratic dependence, but so does area.

Here is my best of 2015. All images taken with SW 130/900 newton and qhy5L-IIc on eq2 mount (SER recording, AS2! stacking, Registax wavelets, iMerge for moon mosaic)