Hi all.

I have a pair of ASI2600 cameras, an MC Pro and an MM Pro. I purchased the MC Pro first before the MM version was released. One of my reasons for buying was that I expected the 16 bit conversion to allow for longer exposure times before stars would begin to saturate to white, and that was proven correct in practice.

In my imaging workflow, I use gain 100 only on the mono camera and then only when shooting narrowband filters, the rest of the time I have used gain 0 for both cameras as the lowest available gain, to maximise full well capacity as I have fairly dark skies and can use longer exposures before the background becomes objectionable.

Now that extra gain settings have been enabled, is a ZWO tech able to advise what the closest gain is to true "Unity" of 1e/ADU? Is that -25? I have shot a new master dark library at gain -25 but I have not yet been able to test the results due to weather, if a different gain is true unity or closer to it than -25 I will re-shoot darks while the weather is poor.

Am I correct to assume the reason for the extra gain settings being a negative value was to preserve the original gain settings rather than invalidate dark and bias frames shot before? All the original settings remain the same and the range has been extended into negative numbers?

    6 days later

    Unity gain is not all that important in the grand scheme of things. Lower read noise is more important. The best reason to use a setting like -25 gain where noise is way higher is if you have a very bright broadband target and a maybe a fast scope and do not want to stack hundreds of images. The slight increase in DR and less chance of overshooting the FWC is then warranted. All else aside, it is just best do gain 0 on brightish targets and most broadband targets or use gain 100 for dim broadband targets and for narrowband filtered targets. I tried -20 gain at f2.8 on M31 and it was fairly good and colourful but needed to add lots of subs to reduce the noise levels.

    In my case, I am under pretty dark skies and one of my scopes is F4 and even with 120 second exposures, brighter stars can end up with white saturated centres either with the 2600MC or LRGB with my MM.

    IMO, even at about 4.2e read noise, that is still not what I would call a noisy camera (Having started out using a friends CCD at more like 8E and about two thirds of the QE of these modern CMOS sensors) I am keen to have a look at what the results end up like at true unity gain rather than the slightly oddball approx 0.7e/adu of gain 0 where basically every second electron adds a count of 2.

    What I am still not sure of is exactly what gain is true unity or as close as the camera gets to it as the image of the gain graph does not make that clear. There is no clear way to work out the gains between the 25 steps per division horizontally as even the dotted horizontal lines do not line up as a clean multiple.

      blue I understand as I am in a bortle 4 sky. When I do broadband I tried at -20 gain as that is unity and I did like the extra DR and my star colours were fabulous. I did not find noise at all that bad. M31 was great and the core was not blown out. I also use a uber fast f2.8 setup and it is necessary to use -20 gain on bright targets and 0 gain on most others except narrowband. I do think that -20 gain has the best star colours and you can’t go wrong with this camera.

        I will give -20 a try if that is unity. I will need to re shoot darks and flats to make new masters, but there is no clear sky forecast for a while so that is hardly a problem.

        It would be handy if ZWO would mark unity on the extended graph in the same way as the gain where HGC mode starts.

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