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Imaging galaxies in a heavy LP environment - cmos asi1600

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#1 jpbutler


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Posted 15 March 2017 - 11:17 AM



This is my first season of imaging galaxies, as I started about NEAF time last year.

I have tried to stay away from LRGB imaging because of the heavy light pollution in my city.

I think about going to a dark site, but I am too comfortable at home and I keep my rig outside almost permanently and don't want to move it.


A couple of weeks ago I imaged the markarian chain at gain 200 without really thinking about it too deeply.

My darks were all for gain 200, so that is what I did.

My Lums were at 4 seconds and at the other end red was 45 seconds.


Processing out the LP in Pixinsight was pretty hard and I felt like I just needed to leave a certain amount in the image in order to not degrade the resolution.


THis week I took a bunch of darks and biases and made masters at unity gain and at gain 75.


What I am wondering and hoping to get out of this post are ideas on the best gain settings to use that would possibly minimize the impact of LP, if that is even possible.


I would love to also hear about techniques for processing that people use that image RGB in a heavy light pollution environment.




#2 Jon Rista

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Posted 16 March 2017 - 10:42 AM

The best way to combat LP is to gather a lot of light. As much as possible. This usually means acquiring many hours if not tens of hours of integration when in a heavily light polluted zone. When I was still doing OSC/DSLR imaging, I would routinely get 8-14 hours of data, and in a few cases acquired around 20. Some people routinely acquire 20-40 hours. 


If you are imaging RGB (I assume this is the ASI1600MC? MC-Cooled?), then you might want to try using a lower gain setting. High gain settings are great for dark sites, but if you are in a red or white zone, you can very easily swamp both the read noise and the high quantization error of even Gain 0. At Gain 200 you will have about 10.5 stops of DR, while at Gain 0 you will have at least 12 stops (technically speaking the camera has 12.5 stops clamped to 12 bits by the ADC). The extra dynamic range will help you get some longer exposures...at least 30 seconds, maybe 60 seconds around a red/white zone border, longer if you are closer to a red/orange zone border.


With some longer subs, you should be able to get some more object photons per sub, and yet still be able to stack a sufficient amount of subs to improve the precision of the final stack. With this camera, being 12 bit, you want to stack at least 100 subs to improve the precision and dynamic range of your final integration. There is little harm in stacking 250 subs or more either...so long as you are sufficiently dithering (dither aggressively) and properly calibrating your subs. With good calibration and dithering, you can get extremely clean results, even with very high count stacks. This is a 1120 frame stack (was actually an experiment to see how much SNR was impacted when not dithering with very deep stacks, so it's a combination of 560 undithered subs with 560 dithered subs, as once I had all that data, I figured...why not stack it all! tongue2.gif) of 10s Gain75/Offset 15 L subs with the ASI1600MM-Cool @ -20C from a red/white zone border:



Edited by Jon Rista, 16 March 2017 - 10:52 AM.

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