jimme100 Take a look at the curves in Section 4 of the ASI2600 Manual. The camera driver kicks in the sensor's HCG (High Conversion Gain) mode when the gain is 10 dB or over (gain = "100").
You don't have independent control of HCG today. ZWO turns it on when the gain is set to 100 or more. (Incidentally, I did not find an HCG flag in the camera's SDK for MacOS, so I suspect the 10 dB HCG threshold might be fixed in the camera's firmware itself.)
From the dynamic range curve, there is nothing to be gained [sic] by using a gain value that is between about 20 and 99. While the read noise barely budges between gain of 0 and gain of 20.
At the same time, I also don't see any advantage of using a gain of between 101 and 300, since the read noise barely improves from gain of 100.
The ASI2600 is thus almost a no brainer to set-and-forget at a gain of either identically 0 (if you need that extra bit of dynamic range) or to exactly 100 (if like me, you prefer to use shorter exposures). I suspect that is why the three ASIAIR gain buttons only let you set the gain either to 0 or to 100.
Chen
