Strange limitation on dynamic range with ASI120MM

jamesf

I've not noticed this before because I've only been imaging at night on relatively dim objects, but recently I've been using the ASI120MM with my PST for solar imaging. I've noticed that the maximum pixel value returned either in SharpCap or FireCapture is 191. At that point the histogram stops.

I've tried pointing the camera at a light and get the same behaviour. The histogram fills to 191 and then stops.

It is perhaps meaningful that 192 would be 11000000 in binary. That seems like rather too much of a coincidence.

Does anyone else see this behaviour or know why it occurs. I've tried the 2.3 beta version of FireCapture, the 2.2 release and SharpCap 1.5.0.344.

James

jamesf

Forgot to mention, this is with the 2.0.0.507 drivers, but also happened with the .302(?) version.

James

jamesf

More information on this...

It appears to be related to low gain settings, which are what I need to use for imaging with the PST. The following figures are from FireCapture 2.3 beta 08:

At a gain of 9 I can utilise the full 8-bit dynamic range. At lower gains the maximum value drops as

follows. I've added the hex and binary values of (max + 1) afterwards because it seems too coincidental to be a random problem for just me:

gain 8: 239 0xf0 11110000

gain 7: 223 0xe0 11100000

gain 6: 207 0xd0 11010000

gain 5: 191 0xc0 11000000

gain 4: 175 0xb0 10110000

gain 3: 159 0xa0 10100000

gain 2: 143 0x90 10010000

gain 1: 128 0x80 10000000

gain 0: 128 0x80 10000000

Driver/firmware bug, perhaps?

James

Sam@ZWO

Hi JamesF

seems you find it :)

gain range

0-8 equal to 0.5X-1x

9-100 equal to 1x-64x

that's why it cannot reach 255

we set it below 1x because sometimes its too bright at daytime when use it as a all sky camera

jamesf

Surely you'd still want to use the full dynamic range of the camera though, presumably? Having a pixel value range of 0-191 (or even lower at very low gain settings) is obviously not as good as maintaining the full range of the camera even if the gain settings are non-linear.

James

Sam@ZWO

Yes,James

that's the best option

but the sensor has such limit.

So you'd better set the gain biger than 8 if you can get a resonable brightness