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

[Tom Krajci]

>From: "Darrel Moon" <moon at chdao.org>

>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.

As was said before...sometimes camera electronics will make a (more or less)
linear chip behave in a non-linear matter.

See:
http://overton2.tamu.edu/aset/krajci/
specifically
http://overton2.tamu.edu/aset/krajci/st-7/st-7.htm

>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?

Once it was fixed (internal support electronics changed):
http://overton2.tamu.edu/aset/krajci/st-7-new/st-7-new.htm
...the camera still had some small non-linearity...but even when I made some
simple software to linearize the CCD data...it made little difference in the
kind of work I was doing.  (But for millimag work...that may not be the
case.)

You can bench test the CCD without the telescope/optical system.  Just use a
stable light source, such as an LED on a regulated voltage supply.

I can't comment on how it changes with different light...I've always used
unfiltered light for my tests, and for data collection.

Yes, binning can have a big impact because:  1.  the read out node can
saturate/go non-linear when you bin   and   2.  When you bin...you no longer
know if individual (1x1) pixels are saturated/non-linear.

I hope these comments help.

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Tom Krajci
Cloudcroft, New Mexico
http://overton2.tamu.edu/aset/krajci/

Center for Backyard Astrophysics (CBA)
http://cba.phys.columbia.edu CBA New Mexico

American Association of Variable Star
Observers (AAVSO): KTC http://www.aavso.org/
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