Hi Stargazer66207, Elmiko and Francesco,
I've had an ASI-071 MC PRO for about a year now, and have been getting pretty good results so far.
I've just started exploring the pros & cons of using different settings, and trying to understand the tradeoffs between gain, noise and dynamic range.
I finally managed to work out that, although increasing gain reduces noise, it also reduces the Full Well Depth (FWD), meaning that the pixels saturate with a lower level of light, resulting in a correspondingly reduced dynamic range.
I often wondered what was meant by 'Unity Gain' - I though that just meant Gain = 1 ... fair enough ??
But I couldn't understand why 'Unity Gain' was significantly different to Gain = 0, particularly when the gain can be adjusted 0-240.
Then I wondered why the graph of GAIN(e-/ADU) in the camera's documentation had an arrow at Gain = 94, then suddenly realised that this is actually the 'Unity Gain' setting. That is, it's the gain where 1 electron = 1 ADU - which is completely obvious when you look at the vertical axis of the chart.
So, 'Unity Gain' refers to the gain in
e-/ADU - NOT in
db.
What is 1 ADU ?
That's one Analog-to-digital unit, the minimum increment the camera's internal Analog to digital converter is capable of resolving.
So, at a gain of 240 the Read Noise is minimised, but so is the FWD.
At Gain = 240, the FWD = 3k, which is 1/16 of the maximum of 48k, so the dynamic range drops from about 14 stops to about 10 stops,
the difference being -4, and 2^-4 = 1/16.
AHA !
It all makes sense now.
So - I will start shooting at gain settings higher than 'unity = 1' from now on
In terms of 'Offset', I'm still not certain what that actually is - can someone enlighten me - but in the ASI-071MC Pro it is fixed at a value of 65.
The other thing is 'Dark Current' which is reduced by cooling the sensor.
The ZWO documentation is not very clear on this point, and a lot of ZWO users have asked whether the 'Target' setting in the ASIAIR software is the
absolute temperature, or the
delta from ambient ... it is definitely the absolute temperature.
To minimise noise, the best thing is to cool the sensor to at least -5°C, and if you go to -10°C you get a marginal improvement.
The maximum cooling achievable is 35°C below ambient, so you can easily get to -10°C if the ambient is anything up to +25°C.
Whether you select a low temperature will depend on your setup - for example, if you are battery-powered you might prefer to minimise power consumption,
which increases exponentially with the temperature differential.
The last thing to think of is the DEW Point, which can create issues with fogging.
You can find the Dew Point at your location using websites such as
Clear Outside.
Basically, if you cool your sensor below the Dew Point, you may need to turn on your Dew Heater, which consumes about 4 Watts and is only ON or OFF - another thing to think about if you are battery powered.
Anyway - I hope some of this info is helpful
Gary
(Sydney, Australia)
