Knowing When The Exposure is Right

[ChrisWSM]

Hi there

I was wondering if there's a scientific way to identify that you have the correct exposure settings/time for an object, using the image data readouts  provided by most capture software? I have Nebulosity, but prefer SGP. I appreciate that each set-up is different, but having taken various exposures from 30s to 10min (all guided), I'm still at a loss of how to work out my best settings.

I've tried various gain settings, and found if I used g=1 on a 10min exposure of M42, the image was super clean, but not so much detail as I hoped for. This is clearly a balancing act, but I'd like to apply some logic to the settings, if possible.

My equipment: Celestron C11 with CGEM DX Mount. ZWO ASI1600MC-COOL.

I have great polar alignment, and guiding is exceptional, giving perfectly round stars at 10min exposures. I cool the OSC to -20 to reduce noise. Culmination is good and the optics are all clean. My location is poor as I'm working from a balcony that vibrates when traffic passes on the main road, but am still able to get decent images, even if a little lacking in fine detail. 

 

Thanks in advance

Chris

[Bass007]

Good question, Interested what some of the experienced members will say

[wimvb]

There is no single correct answer for this question. It not only depends on target, but as you already noted, also on guiding/tracking, light pollution, etc.

If your guiding allows 10 min subs, then clearly any longer exposure is inferior resultwise. Otoh, if you want nb imaging, it probably is too short.

Generally, it is more common that an image is underexposed than overexposed. Unless there is very strong light pollution.

But there are several guidelines or rules to follow:

Lower gain/iso generally means lower noise and more dynamic range.

Longer exposures means more dark noise and possible amp glow.

Longer exposures means higher s/n from the incoming light.

In your case: longer exposures means more risk of traffic induced vibrations.

The only sure way to determine your optimum exposure time, is to experiment. As the ad says, "Just do it".

[Adam J]

The ASI is in a realm of its own where this is concerned. From what I have seen of other peoples photos most avoid longer exposures in faviour of lots of short exposures due to the low read noise. If what I have seen is anything to go by lots of people use 120s exposures but lots and lots of them.