[Aavso-photometry] CCD Linearity and anti-blooming....

[Michael Koppelman]

Yup, you can test your camera and stay in the linear range. It is  
harder, but possible, to "correct" non-linearity.

Here is my test from a while ago:

http://www.lolife.com/astronomy/lintest/

You really have to look at the residuals (i.e. the difference for  
your data and a perfect line) to see the effects.

M.


On Jul 6, 2007, at 1:49 PM, Darrel Moon wrote:

> I understand the traditional wisdom dictates using ccd chips which  
> do not have the anti-blooming feature if one desires accurate  
> photometry. However, with the newer chips that have micro-lensed  
> configuration, the light loss due to the anti-blooming gate  
> structure has been greatly reduced thereby eliminating one of the  
> negative elements in using such chips. The biggest remaining issue  
> seems to be the absolute linearity of these anti-blooming chips.  
> When one examines the linearity graph provided by Kodak of their  
> KAF-09000 chip, it seems to indicate linearity to within 1% to a  
> fairly high saturation level. See page 16 here: http://www.ccd.com/ 
> pdf/ccd_9000.pdf  Since all ccd chips (anti-blooming or not)  
> display some loss of linearity at high saturation levels, one must  
> know the highest level of saturation allowed  to produce good  
> photometry.
>
> Two groups of questions:
>
> 1. To what degree must the linearity of a chip be to provide  
> reliable photometry (say to mili-mag precision)? Is 1% enough? If  
> not, how much deviation from absolute linearity is considered  
> acceptable?
>
> 2. Presuming 1% is not enough, cannot the linearity be quantified/ 
> characterized so as to be able to "subtract out" the deviation from  
> perfect linearity during the calculations required to present a  
> photometric measurement? I would presume this quantification/ 
> characterization would have to be performed using the entire  
> optical train: telescope, filters, integration time, binning,  
> various chip temperatures, etc. Have I left out any other important  
> parameter?
>
> I have posed these questions to various commercial camera  
> manufactures and they all have approximately the same response:  
> "don't use anti-blooming chips for photometry." That seems like a  
> rather canned answer; no one has addressed the proposition of  
> testing the camera/system to produce a characterization to be  
> applied against the photometric measurement. None of the  
> manufactures could tell me how much deviation from absolute  
> linearity was acceptable for mili-mag photometry or what the actual  
> deviation from absolute linearity any of their cameras.
>
> Thanks for any input,
>
> Darrel Moon
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