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Exposure length and star saturation - dkuchta5 - 2019-08-22

Using my ZWO ASI071, I find that if I expose more than a minute or so I end up with lots of saturated stars, which reduces star color and makes them look unnatural.  With the low read noise and low dark current (when cooled to -10C) of the camera, I've found that I get much better results if I take lots of exposures at 30 seconds to one minute.

In SkyTools, after entering all the pertinent data in for my camera, I'm finding that the exposure calculator is always trying to push my exposures to the longest time period I will allow, which I currently have set to 5 minutes.  So I guess I'm wondering - is SkyTools just looking at the best exposure for the target object only?  Or does it take into account star saturation as well?


RE: Exposure length and star saturation - GregM - 2019-08-22

I was wondering something similar question for my ASI1600MM.
I usually end up just looking at the total exposure recommended, set the max to about 180 sec for wideband filters, 240s for narrowband and use a number smaller sub exposures to achieve the total exposure recommended for the SNR.
That has the advantage of not loosing as much data if I have to ditch an exposure due to a plane etc., so I didn't think of questioning the longer exposure suggestions of ST4 earlier, but I'm also interested in the answer.

RE: Exposure length and star saturation - theskyhound - 2019-08-26


Exposure times are calculated for your target object, and for your project, the additional selections of your exposure goals. Saturation depends on how bright the stars are in that particular field. You can preview this via the Camera View of the Atlas for a given target and exposure time.

When you create an imaging project you have direct control over what exposure times you wish to use. You also have control over what exposure times are suggested in your imaging system setup. Click on Exposure Times and don't allow long exposures.

RE: Exposure length and star saturation - PMSchu - 2019-08-26

Hi Dan & Greg M.,

Check out the bottom right section of the Exposure Calculator. It provides estimates of the faintest stars detectable in the image as well as the stellar magnitudes that cause blooming and clipping of brighter stars. These values depend on the accuracy of the parameters of your camera (QE, full well capacity, anti-blooming, etc). As you change the sub-exposure length, these values will be updated. As Greg C. mentioned, the 'Camera View' will mark over exposed stars with a vertical red bar as well. Shorter sub-exposures is the main way to control star over exposure, if that's important for your image.

Phil S.