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

[Darrel Moon]

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