[Shiraz (Ray)]

Originally Posted by glend

Great stuff Ray! Your tables are spot on for what i am using for broadband and narrowband in my mag20 and full moon sky. Obviously this builds on the chart you created last year for broadband sub lengths.
Very useful contibution for people just starting out with the ASI1600MM-C and for old hand to check their mode of operation. I agree absolutely with you comment about not getting fooled by single sub appearances. I have played around with various gain settings and different targets but i always seem to come back to Unity (139).
I have your old chart on the wall in the observatory over my laptop, this will go up now.
Thanks for all your valuable work on this topic.

thanks Glen. I have pretty much settled on 100 gain for broadband.

Originally Posted by Atmos

Great work Ray, coincides well with what I see (albeit with 3nm and not 7nm filters).
What I really take out of it though is that I should never be imaging from my red zone back yard

I Have not tried imaging in anything approaching red - must be soul destroying.

Originally Posted by Slawomir

Thanks Ray. Actually I also coincides very well with my experience with ICX814 (gain 0 and narrowband). Colin, don't complain, I image from an infra-red zone;best SQM I measured was about 18. Usually it is worse than that.

thanks Suavi - blimey 18!!

Originally Posted by lazjen

Nice tables - wish I had them last night as it looks like I was overdoing it.

What's interesting is that some of these exposure times are less than dithering times. I think I'm going to have to look at optimising the dithering more to not lose as much time.

For LRGB imaging I've started doing LLRGB then dither, repeat. I might have to extend that to something like LLLRRGGBB then dither, repeat.

Originally Posted by lazjen

I do it so in theory I have at least something to work with when a few cycles go through - the weather is so variable I've had very limited time for imaging.

Another reason is that I'm concerned about the dithering time when taking a lot of short images - as I mentioned previously. I know I need to dither to stop any potential fixed pattern noise appearing and with a large number of subs that could be an issue.

Tonight I'm going to put these tables to the test (for broadband) before the moon comes up. I think I'm in an orange zone, maybe red, but I think it's safe enough to start working off orange and go from there. I was using gain 75, but I'm either going to go to gain 0 or 200 - haven't decided yet.

I have to think about the narrowband values more because I wonder how much amp glow will affect things with such long exposures?

yep, dithering is an annoying overhead - as is refocus. I haven't tried alternative dither concepts - be interested in what you find. Your idea of cycling through the filters has a lot going for it - thanks for posting. I don't think that the amp glow issue is as significant as it seems - the amount of glow signal is really very low and it can be easily calibrated out.

Originally Posted by glend

Ray, looking at your exposure suggestions for f10 (where my unreduced Edge Hd sits), it occurs to me that the guide problems I presently have with DSOs at f10 may not be an issue, provided I shoot at normally unguided sub durations (but just do alot of them). Am I correct about that? Seems like a modified DSO Lucky Imaging strategy.
For example, look at the Orange Sky chart, 20mag, Gain 200, f10, I can shoot 35s subs. Given my precise PA, I don't expect any noticable drift at that sub length.
Are there other factors at play in this?
Even though my location is in your Green zone chart, I see no reason that I could not shoot using the Orange zone durations but doing many more subs.

I would think that you can shoot very short subs with some loss in sensitivity. FWIW, I tried 20 second subs one good night and got some blindingly good resolution, so it definitely might be worth seeing what you can get at f10 - provided your target is bright.

Originally Posted by lazjen

If I understand correctly, you'd just need more total exposure time to get the image quality. There's a about 4:1 difference in the times from the Orange to the Green, but I don't know if you'd need > 4 times the number of subs?

If you're doing it unguided and you believe you've got good PA, are you going to dither at all? Are you concerned about possible fixed pattern noise if you don't, especially given the number of images you'll be taking?

I think that you roughly need 4x the number of subs if they are 1/4 as long - will probably be a bit more, since the shorter subs may not be strictly shot noise limited and you could have additional read noise to deal with.