The next test I wanted to do with the ZWO ASI1600MM-cool, was to test its gain settings and compare it with my back-lit CMOS ASI178MM-cool. Also, I thought I would post images taken in daylight (albeit using an Astronomik CLS-CCD filter) to show the amount of grain or noise at each gain setting up to Gain 500.
What does an increment of the Gain settings do to the exposure time? I used the same subject lit by ambient light and only varied the gain setting, but flicking back and forth between the different gain settings to ensure that the ambient light level had not changed significantly. I allowed Sharpcap to automatically select the exposure required for each gain setting and I noted the exposure times for these and have tabulated them below.
What I found was that an increase in gain of 100 reduced the exposure time by 1/3 from Gain 0 to Gain 200. After that, the increase of 100 in gain resulted in under 1/3 reduction in exposure time. The same could be said about my ASI178MM-cooled which also showed that increasing gain by 100 reduced the time required for each image by 1/3.
Next I wanted to compare Gain 0 setting on the ASI1600MM-cool vs ASI178MM-cool. I pointed the camera at the clouds in the sky and took a screen shot of the exposure time chosen by Sharpcap for each camera, shown in the two images below.
It looks like the sensitivity at Gain 0 is pretty similar between the two cameras 1.5 ms for ASI1600MM-cool vs 1.8 ms for ASI178MM-cool. There was perhaps a slight edge to the ASI1600MM-cool. But bearing in mind that each pixel of the ASI1600MM-cool is 2.5x larger than those on the ASI178MM-cool, it actually shows how sensitive those small pixels of that back-lit CMOS sensor is.
Comparison of the different gain settings and the grain/noise in the images. I have included images from Gain 100 to 500, at 100 increments.
Gain 100 to 300 are relatively noise-free and smooth, but once you get to Gain 400 and above the images look much noisier or grainier. But the higher gain does have its uses. For example if you were wanting to adjust focus using a Bahtinov mask, at higher gain settings it should allow for focus adjustment even with stars that are less bright or for shorter exposure times to be used with larger stars.
Thus ends my tests of the gain settings on the ASI1600MM-cool, from which I’ve learnt a lot about the sensors of both the ASI1600MM-cool and my ASI178MM-cool. When I do get a chance in the next 2-3 weeks, I would like to image the same target with both cameras so the results can be compared to see if one is able to capture finer details compared to the other, as well as compare the final image after stacking to see if the 12-bit ADC does make a difference.