A question for fellow ASI 1600mm pro users (some results! )

[simmo39]

Hi, I have had my ASI 1600mmPro for a while now and have been doing narrow band imaging with it. I have been using unity gain and the results have impressed me when comparing to others images on this site using similar scope and exposure they seem to just be that bit sharper and deeper.  I have done a little reading ( not a good idea ) and I see a few are using a gain setting of 200 for narrow band. I understand that this may allow a shorter exposure time and offers lower read noise, but is  there anything else I should consider before giving it a try at this gain setting?

Also what sort of exposure time am I looking at? at unity i have found the sweet spot to be about 240 sec.

[spillage]

How many subs are you taking? I find that things start to smooth out more after I stack more than 30. There are mathematical ways to work things out but I just take more subs if it needs it.

[simmo39]

2 minutes ago, spillage said:

How many subs are you taking? I find that things start to smooth out more after I stack more than 30. There are mathematical ways to work things out but I just take more subs if it needs it.

Hi, I average about 30 subs for each channel. I can get a smooth image, im just trying for that little bit extra. Are you using unity gain or something else?

[spillage]

So far I have just stuck with unity. I think with limited imaging time I would rather be getting known results then spend time experimenting for now.

[simmo39]

2 minutes ago, spillage said:

So far I have just stuck with unity. I think with limited imaging time I would rather be getting known results then spend time experimenting for now.

Hi Mark, I am thinking about that too but I do like to mess! lol. I was just wondering if the gain was 200 and slightly less exposure time with more subs may be a benefit. 

I dont now.

[david_taurus83]

You tried longer subs? I done 15 minute subs the other night and they came out well! Unity gain also. Might clip a bright target though.

[simmo39]

4 hours ago, david_taurus83 said:

You tried longer subs? I done 15 minute subs the other night and they came out well! Unity gain also. Might clip a bright target though.

Thats a long sub! longer than My set up could cope with. I can get away with 6 mins and 8 if I try hard. Im hoping that with gain at 200 I can use 2 to 3 minns subs but more  of them.

[david_taurus83]

Are you talking about tracking/guiding? If so thats understandable. Try the higher gain. You will need a separate set of darks for that again.

[simmo39]

28 minutes ago, david_taurus83 said:

Are you talking about tracking/guiding? If so thats understandable. Try the higher gain. You will need a separate set of darks for that again.

Hi , I have no problems tracking upto 6 to 8 mins. Im just wondering whether taking more shorter subs at a higher gain gives me a better or worse pic. I dont mind taking more subs at higher gain if it gets me a better end result. Looking on other sites I see a lot of images where the gain is set 200 and sub lenghts of 120 to 180sec. I suppose the only way im going to find out is try it. Was just seeing if anybody has any experience of doing that way.

[kens]

It all depends on whether you are shot noise limited or not. If you are shot noise limited then the main effect of increasing gain will be to reduce dynamic range but SNR wont be altered much for a given integration time. If you are still read noise dominant then longer subs would work better for SNR.

You are shot noise limited when the peak of your histogram corresponds to 5x to 10x read-noise squared or higher. At unity gain (for 10xRN^2) that equates to a 16bit value of 440 plus 16x your offset. Assuming offset of 50 that's 1240. For a gain of 200 and offset 50 its 1440. Don't be fooled by the higher number because with the higher gain the number is reached faster.

With NB, even with strong light pollution its hard to get to 10xRN^2. So you might even want to increase the gain AND increase your sub length.

[simmo39]

10 hours ago, kens said:

It all depends on whether you are shot noise limited or not. If you are shot noise limited then the main effect of increasing gain will be to reduce dynamic range but SNR wont be altered much for a given integration time. If you are still read noise dominant then longer subs would work better for SNR.

You are shot noise limited when the peak of your histogram corresponds to 5x to 10x read-noise squared or higher. At unity gain (for 10xRN^2) that equates to a 16bit value of 440 plus 16x your offset. Assuming offset of 50 that's 1240. For a gain of 200 and offset 50 its 1440. Don't be fooled by the higher number because with the higher gain the number is reached faster.

With NB, even with strong light pollution its hard to get to 10xRN^2. So you might even want to increase the gain AND increase your sub length.

Hi Ken, thanks for the info. Im not sure what shot noise is so I will do some research on that. I understand the reduction in dynamic range and from what I have read on that its only small  and barley noticeable. I will have to do more searches and read up some more. Thanks for the pointers.

[MartinB]

You can make this as complicated as you as you like but there are no magic numbers and there are plenty of variables which make a difference e.g. sky brightness, scope speed, filters and target brightness.  Ideally you will use the shortest subs which get you over the read noise without saturating too many stars.

It doesn't take long to run a few exposures of different lengths to check for star saturation at a given gain.  Try 200 gain, set your exposure guided by the above and see how you get on.

Provided your sub lengths are sufficient to get over the read noise what matters most is total exposure time rather than the number of subs.  This might vary depending on target and what filters you are using.  

So, no magic set of numbers, experiment by trying different gains and over the next few years your will discover what works best for you!

[Adam J]

On 03/02/2019 at 10:38, simmo39 said:

Hi, I have had my ASI 1600mmPro for a while now and have been doing narrow band imaging with it. I have been using unity gain and the results have impressed me when comparing to others images on this site using similar scope and exposure they seem to just be that bit sharper and deeper.  I have done a little reading ( not a good idea ) and I see a few are using a gain setting of 200 for narrow band. I understand that this may allow a shorter exposure time and offers lower read noise, but is  there anything else I should consider before giving it a try at this gain setting?

Also what sort of exposure time am I looking at? at unity i have found the sweet spot to be about 240 sec.

You would not find me using 200 gain and 60 - 120 second exposure method, to much of a pain to store and process. I use 139 gain and 240 sec exposures as you do and try to get about 50 - 100 per channel for narrowband.  

[simmo39]

1 hour ago, Adam J said:

You would not find me using 200 gain and 60 - 120 second exposure method, to much of a pain to store and process. I use 139 gain and 240 sec exposures as you do and try to get about 50 - 100 per channel for narrowband.  

Hi and thanks for the advise. From what I have been reading today going for 200 gain I will still be doing about 240 sec exposure. As we have a sort of clear night here ( fingers crossed ) and I need some Si data for an image Im doing i thought I would just give it a go. If i dont appear to be of any great advantage  ill be back to unity gain next session.Will post my thoughts when done.

[simmo39]

Well here is the first result of my experiment. Not sure what I have achieved! I have to say this is the best Si image I have ever got  ( apart from the bad internal reflections, but thats another story ) but Im not sure whether thats because the HH is Si rich or the increase in gain. I should have done a run at unity gain but once I started to get what looked like good data I wasnt willing to stop. Any way what are your thoughts?