Question about Exposure and Gain settings

[peng155]

Hi Folks,

I got a question about general exposure and gain settings in the Allsky program.... I think I already know the answer, but I thought I'd ask any ways...

I understand the relationship between exposure setting and the gain setting.... from a very rudimentary level... if I take a picture at a certain gain and exposure setting... and then take another picture of the same object, but increase one, and decrease the other... I should get basically the same looking picture.. not taking into account for change in lighting, or what have you....

I understand that if i increase gain.. I start to increase noise into my picture.. so I basically want to increase exposure time if I de-crease gain level...

So currently here. are some of my nighttime settings:
Auto-Exposure: Yes
Max-Auto: 30000
Manual: 3000
Mean Target: 0.500
Mean Threshold: 0.005
Delay: 30000
Auto-Gain: No
Max-Auto: 300
Gain: 125
Binning: 1x1
Auto Stretching in config.sh is "false"

Below is a sample of the image I'm currently getting, What I'm trying to achieve is to capture as many stars as I can..
With a 30 sec exposure and gain set at 125, the brightest stars are just becoming exposed in the picture.. but since the Moon is on it's way to full moon phase, I'm assuming that my images will continue to be washing out until the moon passes full and begins to wane... and it's a waste of time for me to dink around with settings until it's completely dark at the new moon phase...

Would that be the correct assumption??

Thanks.....

[EricClaeys]

@peng155, you are correct in your knowledge of gain versus exposure. "gain" is like ISO on a normal camera. Doubling the ISO and halving the exposure gives the same brightness. Not sure if ZWO gain is linear, i.e., if you double it and halve the exposure I don't know if you'll get the same brightness.

Take a look at the Histogram Box setting. By default Allsky looks at a 500 x 500 box at the center of the image to determine the mean brightness. On the ASI 224 that box is about half the total image so it's probably getting some of the moon so the sky is darker than normal. Try shrinking the box or moving it down some. You can also display the box if you temporarily set Overlay Method to "legacy" then enable the Show Histogram Box setting. I did this on one of my cameras where the Sun and Moon were going through the center of the image and made it too dark.
If you physically move the camera you may need to change the box size and/or location.

[peng155]

@EricClaeys

Thanks for the info on being able to activate the histogram box, having a visual aid will make it a lot easier make to adjustments