Unity gain means a gain setting, which results in a conversion gain of 1 e-/ADU. As can be seen from the plot "GAIN(e-/ADU)" on the ZWO website ( https://astronomy-imaging-camera.com/product/asi2600mc-pro-color ), there is no such gain setting for this camera. Even at a gain setting of 0, the conversion gain is only about 0.76 e-/ADU. This is a property of the sensor that is used in the camera. It is due to the bit depth of the A/D converter of 16 bits and the full well capacity of 50,000 e-: (50,000 e- / 65536 ADU) = 0.76 e-/ADU.
A gain setting of 100 in the ASCOM driver corresponds to a gain setting of 100 in the plot "GAIN(e-/ADU)" on the ZWO website.
My interpretation for the confined range of gain setting in the ASCOM driver:
ZWO measured the characteristic data of the camera over a wide range (range of gain setting 0 - 450). However, for the ASCOM driver, the adjustable range was confined to 0 - 100. The reason for this decision was that actually there are only two gain settings that make sense:
1) Gain setting 0: this allows to use the full well capacity completely and expose longer and with lower risk of saturating stars respectively.
2) Gain setting 100: this allows to take advantage of low readout noise. At still higher gain, the readout noise will not decrease significantly any more, but the dynamic range drops considerably. Therefore these higher gain settings are not reasonable at all.
Bernd
