# Linearity. How to assess it for your system

Forums Photometry Linearity. How to assess it for your system

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• #620096
Kevin West
Participant

If I may put upon you good people again. (Getting there)

Linearity yes. That is THE most important word in photometry. Without it your photometry may be useless. I have to assess where it lies between zero and max ADU. I think I have to take some exposures and plot a graph.
How is it practically done?

Of relevance to this is another oddity I have noticed.
I have a ZWO 1600 MM Pro cooled camera which is 12 bit.
On the AISAIR histogram when taking flats I had to be aware to try and get the maximum between 1/3 and 1/2 of max ADU which on the screen is coming out at about 65000. I think this is the max ADU for a 16 bit detector (2^16). I thought my camera has 12 bit detector so should only be 4096.

I’m confused over this.

Kevin

#620097
Participant

I’m confused over this.

The counts have been multiplied by 16 to look like 16 bits. You can see the true ADU from the manufacturers charts where a FWD of 20k e- at 5e-/ADU gain gives 4k ADU

Robin

#620128
Kevin West
Participant

Thanks Robin,
The 5e per ADU figure, is that something that can be adjusted?

It looks from the graph that this is the definition of or correspondence with zero gain, which is what I have used so far.

To clarify, each pixel has to receive 5 electrons before it outputs/registers 1 ADU
Is that what the charts are saying?
Regards
Kevin

#620139
Participant

Yes that is how gain is usually defined for camera sensors It is slightly perverse because the figure goes down as the conventional gain (how much the signal is amplified) increases !

With CCD sensors the gain is normally fixed, set so the full well depth of the sensor (the point at which the sensor becomes saturated) matches the maximum counts of the ADU.(In this case it would at 5e-/ADU for a 12 bit ADU and FWD of 20k e-) But with CMOS sensors you can increase it which reduces the maximum number of electrons you can count before you reach the maximum of the ADU. Why would you want to do that you may well ask? Well with CMOS the read noise goes down as you increase the gain so in circumstances where you cannot use the full well depth (eg a faint object or you need a short exposure) you can increase the gain and take advantage of the lower read noise.

Cheers
Robin

#620150
Kevin West
Participant

Thanks Robin,
Useful info.
Regards
Kevin

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