wingeing Pom 697 Report post Posted January 18 Mizar & Alcor in Ursa Major taken with the Sky90 array. Greg 2 Share this post Link to post Share on other sites
Radar 595 Report post Posted January 18 Very noiseless and smooth Greg. May I ask how you process these to have what appears as zero grain. Ray Share this post Link to post Share on other sites
wingeing Pom 697 Report post Posted January 19 16 hours ago, Radar said: Very noiseless and smooth Greg. May I ask how you process these to have what appears as zero grain. Ray That's just a case of getting the signal to noise ratio up in the first place which means LOTS of subs. When I first started imaging with the original Hyperstar I used to aim for 100 subs (although they were only 1-minute long). I used to get glassy clear completely noiseless images but they didn't go very deep. So it's a trade off. For things like star piccies you don't need a long exposure time - for things like galaxies and nebulae you do - this is one reason I prefer star piccies Greg Share this post Link to post Share on other sites
Radar 595 Report post Posted January 20 19 hours ago, wingeing Pom said: this is one reason I prefer star piccies I can't argue with that. I'm looking at ways to bring down my noise. I also take a vast number of subs, and through my C14 at 4000mm at F11, the tolerances for focus, tracking and guiding can become difficult during dozens of 5 minute exposures ( I bin (throw away) at least half of my subs). I'm going to start using dither more during my imagining runs as well. Ray Share this post Link to post Share on other sites
wingeing Pom 697 Report post Posted January 21 100 subs was massive overkill, your images should be effectively noise-free with anything over 50 subs. I had one imaging "expert" berating me for not knowing anything about signal to noise ratio as I didn't take into account the gain in signal to noise with exposure time. Problem with signal to noise increase with exposure time is that the increase is not all that big, and also it flattens off rather quickly especially if your skies are not too good. So in my location the exposure time signal to noise ratio increase flattens off in about 5-minutes meaning that I am getting no (further) S/N ratio increase in the 20-minute exposures I need to take to get faint stuff and the only way I can get the noise down in galaxy or faint nebula images is to take LOTS of subs. The reason why I have often said you need at least 8-hours of good data to get a first class image is that if you take a typical sub length as 10-minutes and then have 50 of them, you're talking just over 8-hours. It only took me about 10 years to work this stuff out Greg P.S. Oh - and by the way - all your 50 subs have to be GOOD. You have to go through every sub and check they are o.k. as I now from experience you can actually see the degradation in the stacked image with just ONE bad sub in 100. P.P.S. And that is also the reason why I run multiple imagers in parallel - I increase the number of subs in a given time by the number of imagers. P.P.P.S. Dithering is essential. So is good image calibration with good flats and bias frames. 1 Share this post Link to post Share on other sites
Radar 595 Report post Posted January 27 Great advice Professor. Well noted. Ray Share this post Link to post Share on other sites
