American Association of Variable Star Observers (AAVSO)
Wed, 06/10/2020 - 11:11

New member I hope this is the right place to post this question,

I have a ASI178mm uncooled camera I'm using to teach my the fundamentals of photometry.

Since there is not yet a AAVSO manual on cmos photometry I've been reading about ccd technology and trying to replace the bits that would change by using cmos technology.

I'm trying to do the conversion from ADU to electrons. My camera is 14bits with a full well depth of 15000 electrons.

So the maximum value of the ADC is 2^14 =16384 my software outputs a 16 bit number by scaling the 14 bit  by 4 IE multiply the 14 bit output by 4 to obtain a 16 bnit value.

So to do the ADU to electron conversion wouldn't I have to scale the gain by 4 as well?

The 178mm documentation shows a gain of .514 e/adu so I divide by 4 to get .1285 and this would give the correct full well depth.

Am I Overthinking this or am I correct.


Wayne Hilliard

American Association of Variable Star Observers (AAVSO)


I've got the same model, and I'm quite satisfied with it.

Not quite sure your numbers are correct, tho. The only way to use the full well depth of ca 15k electrons without being limited by saturation by the 14 bit ADC is to select the very lowest gain setting for this particular model, and for that the gain is specified as ca 0.9 e-/ADU here:

Sanity Check: So in the camera's native ADUs:

Gain * max_ADUs =  0.9 e-/ADU * (2^14-1) ADU ~= 15k  ... works...

In the scaled ADUs produced by your softwre, indeed the gain has to be divided by 4, so

Gain_scaled * max_ADUs_scaled =  0.225 e-/ADU * (2^16 -4)  ADU ~= 15k  ... works...

If you select a different gain in the acquisition software, your effective full well depth decreases accordingly as shown in the figure under the link posted above..

I would not rely on the numerical gain value selected in the software being in the same units as those in the plots, but I'm confident that the minimum setting in softwre corresponds to the minimum possible gain for the sensor at least. 

Hope this helps.


American Association of Variable Star Observers (AAVSO)
Thanks! This helped a bunch.

Thanks! This helped a bunch.

Yes I saw where the full well depth reduces as gain increases.

I wasn't relying on the chart for gain so much as a formula I found in another thread in another AAVSO forum, the Photometry forum on 4/19/20 where the equations are detailed for calculating the actual gain.

I think I'm slowly getting my head wrapped around this whole issue.

Anyways thanks again!!!!