Smart Histogram - Stack Time vs Exposure Time vs Gain?

Somewhere to ask questions about the best way to use SharpCap
Post Reply
LookoutLane
Posts: 5
Joined: Thu Jul 12, 2018 2:32 am

Smart Histogram - Stack Time vs Exposure Time vs Gain?

Post by LookoutLane » Thu Jul 12, 2018 10:07 pm

Robin or anyone,

I had been using a DSLR but now have a ZWO 178 mmc and so I am experimenting with the Smart Histogram (SH). In order to use it to it's potential I want to make sure I understand the minimum exposure result and how it relates to the total stacking exposure time. When I recently ran SH with a goal of obtaining a 2hr total exposure it recommended an Optimal Gain of 42 and an Optimal exposure of 477s per frame so that would be 15 frames total. Using the graph I could also see that if I set the gain to 250 the exposure per frame would be about 240s which would be 30 frames for a 2 hour total Stack time exposure.

My question:
According to the Smart Histogram result, other than dynamic range, would these two settings be approximately equivalent as far as the final 2hr stacked image?
Setting A: 477" x 15 frames at 42 gain
Setting B: 240" x 30 frames at 250 gain
OptimalGainExpForASI178mmc.JPG
OptimalGainExpForASI178mmc.JPG (66.35 KiB) Viewed 169 times
Thanks.

Scott

User avatar
admin
Site Admin
Posts: 1887
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Smart Histogram - Stack Time vs Exposure Time vs Gain?

Post by admin » Fri Jul 13, 2018 6:08 am

Hi Scott,

yes, any point on the green curve will have the same noise level in terms of shot noise (noise due to random fluctiations in number of photons detected) overwhelming read noise. That said, there may be other effects such as amp glow and fixed pattern noise that the smart histogram doesn't consider that may mean that some parts of the curve give better final results than others. Considerations like these and the capabilities of your mount tracking/guiding should help you choose which section of the curve to pick your gain/exposure from for ultimate best results.

You have the smart histogram set to aim for 'Unity Gain', which is why it has chosen the gain level of 42 - you can set that option to max dynamic range which will give another suggestion (possibly with a higher gain and lower exposure).

cheers,

Robin

LookoutLane
Posts: 5
Joined: Thu Jul 12, 2018 2:32 am

Re: Smart Histogram - Stack Time vs Exposure Time vs Gain?

Post by LookoutLane » Fri Jul 13, 2018 12:30 pm

Thanks. If I set at max dynamic range do I need to run it again if I have just run it for with Unity gain or does it only need one measurement to determine both?

Scott

User avatar
admin
Site Admin
Posts: 1887
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Smart Histogram - Stack Time vs Exposure Time vs Gain?

Post by admin » Fri Jul 13, 2018 6:58 pm

One measurement of the sky brightness can be used for both types - no need to measure again.

cheers,

Robin

Post Reply

Who is online

Users browsing this forum: No registered users and 0 guests