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Linearity Limit for ST-9xe

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astrolopitec's picture
Linearity Limit for ST-9xe

Hi I understand that the ST-9xe is a bit of a work horse in photometry. So what ADU linearity limit would be recomended for this NAB camera with 65,000 wells ? Thanks Juan

HQA's picture

Hi Juan,

Typical SBIG cameras have about 2.5e-/ADU gain, which would mean that 65535ADU would be equivalent to 164K electrons.  The full well on most ST-9 cameras is less than this, so it is unlikely that your camera is linear to 65,535ADU.  The ST-9 on AAVSOnet W30 becomes nonlinear about 55,000ADU as an example.

The best approach is to test your own camera.  Point your telescope at any flat surface for which you can adjust the illumination.  Set the illumination level so that you get about 10,000ADU in a 10 second exposure, and then take a series of exposures from about 1 second to 100 seconds.  Plot the mean pixel value for the central 100x100 pixels in your image against the exposure time.  You will see a straight line that starts curving up near the top; that will tell you where the end of the linear region resides.


KTC's picture
Another way to plot - shows deviations better

Plot the mean pixel value for the central 100x100 pixels in your image against the exposure time.

I recommend you determine the ADU rate per second of that central region.  (Mean pixel value divided by exposure time).

Your plot will be a horizontal line...and deviations from linear will show as deviation from the horizontal line...real obvious.

Please let us know the results of your testing.

Good luck, and thanks in advance.

PS.  If you want to be hard-core in your testing...set up a script overnight...image your stable flat light source all night in the basement/closet...a 'dense' range of exposures from about 1/2 second to 100 seconds...take a thousand images or so...then analyze them with automated tools.  (I have some Orchestrate scripts that do this, if you use Orchestrate, just ask and I'll send them.)  Your plot should be high-confidence (although you may be surprised to see that your 'stable' light source has some drift over the night).

astrolopitec's picture
Thanks a lot guys! Will

Thanks a lot guys!

Will give it a try.


astrolopitec's picture
 I would have preferred to

 I would have preferred to spend the evening taking time series of TT ari but it was raining dogs and cats. So instead I attempted to measure the linearity limit on my ST-9. It is not easy. These NAB chips remain very linear all the way to saturation. But I believe that I was able to detect a bit of a knee starting at around 58,000 ADU. But the suggested 55,000 for the ST-9 at W30 is a safer figure which I intend to adhere to. If I ever feel masochistic during another rainy night I might capture a higher resolution run around the knee area. But I feel quite confident with a limit of 55,000 ADU.

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