Read Noise measurement of ASI432MM WRONG?

BurrellS

Hi there,
I am trying to understand why my read noise measurement of our ZWO ASI432MM doesn't match the number(s) given in the data curves. I understand that there might be minor variations from device to device but the RN value is far off. The plots show that a gain = 0 the 432MM has a read noise of 21 e-/pixel and a gain of 23.5 e-/ADU. This should translate into 1 ADU read noise in my measurement.

If I take two bias frames (with an exposure time of say 0.001s), subtract one from the other, and take the standard deviation of the difference, divide this value by root 2 I get a read noise value of ₁₅.3 ADU.
(http://spiff.rit.edu/classes/phys445/lectures/readout/readout.html)

If I subtract a master bias (average of 200 bias frames) from a single bias then the value drops to 11 ADU, still factor ₁₀ more than expected.

I am lost and would appreciate some hints in the right direction ...
Thanks!

w7ay

BurrellS I am lost and would appreciate some hints in the right direction ...

I notice that the camera that you are measuring has a 12 bit sensor.

Are you reading the data image in as a 16-bit image? If so, there is a scale factor in the ADU of a factor of 16 (4 bits).

A native ADU of 1.0 will therefore correspond to a 16-bit FITS file ADU of 16.0. So, your 15.3 ADU read noise is really 0.96 native ADU in 12-bit scale.

Shine a bright light at the sensor to see if it has an ADU of 65535 or ADU of 4095 as a sanity check.

Chen