2 replies to this topic

[LoganF-1234]

Hello all, beginner here!

 

I am going to be capturing with my unmodified Nikon D500. I have been looking around a lot of sites trying to figure out what the best ISO would be for me to balance Read Noise / DR. From some of these charts, 400 ISO looks like the sweet spot, but I am unsure about that when I add other variables in. For example, I purchased a guide scope and a mount, so I will probably be shooting longer exposures (1-5 minutes or more). If I am not mistaken, 400 ISO at 1 minute would be a much darker picture than 400 ISO at 5 or 10 minutes. That being said, on Telescopius, I still see people using ISO 800 for their 800-second exposures. 

 

Assuming I want to shoot variable length long exposures, would I ever change my ISO away from 400 depending on the length of the exposure? 

 

Read Noise per ISO:

 

Dynamic Range per ISO:

 

Input referred Read Noise per ISO:

Edited by LoganF-1234, 08 June 2025 - 09:22 AM.

[scanner97]

Yes, ISO (gain) is a trade-off between DR and read noise and 800 is a good place to start for most DSLRs.  Looks like 400 is also a good spot for your camera given the big drop in read noise there.  You are correct that you'd capture more photons in a 10-minute sub than in a 1-minute, but that doesn't mean you automatically want 10 minutes.  For a given set of equipment, the best improvement in SNR comes from longer total integration.  i.e. more total time.  While longer subs can help, at some point the sky noise "swamps" the camera read noise, and shooting longer subs doesn't give you a benefit that you'll notice.  At some point, with long-enough subs, you'll blow out the brightest parts of your image and start to lose detail and DR.  There are a lot of factors that go into figuring out the optimum.  And there tends to be a fair amount of flexibility around the optimum.  For that camera at f/6, if you're shooting broadband in Bortle 3, you could start around 2 minutes and be fine for most targets.  If you're shooting in Bortle 6, it might be more like 30 seconds. 

 

Although there can be reasons to adjust ISO/gain depending on the target, it's not like terrestrial where you are constantly adjusting ISO to match f/ and time to get the best image.  Most folks shoot with one ISO/gain setting for most of their imaging.  They adjust the sub-exposure length based on things like sky conditions, filters used, and target. 

Edited by scanner97, 08 June 2025 - 10:12 AM.

[Kevin_A]

Iso 400 will be the best. Sony sensors are much better at lower iso than Canon sensors. I have had many Nikon/Sony sensors and iso400 is the best for long exposures. Since it is not cooled the sensors run best between 60-180s depending on your light pollution level and optics f ratio. The only Nikon sensor that was not between iso200-400 was my Nikon Z6 which had its HCG at iso800. Even my Nikon Z7 was at iso 400.