[Aavso-photometry] CCD Linearity testing

[Tom Krajci]

RICHARD MILES wrote:
> Hi Tom,
>
> One problem with the top end of the range (say >50,000 adu) is that if 
> you select a large area of the CCD then you start getting more pixels 
> reaching 65535 'early' as you increase the exposure time and this 
> makes it look as though the response is dropping off even though it 
> isn't.  You refer to this effect as well.  I would be particularly 
> careful to select a uniform part of the CCD frame which is not too 
> affected by hot pixels then you can calibrate closer to the 65535 
> cut-off.
Yes, I made it a point to place the light source/diffuser at least 12 
inches from the CCD.  This makes the 'cosine error'/drop off at the 
corners of the chip fairly small.  Off the cuff I think I chose that 
part of the CCD frame that had only about a 1% - 2% drop off from the 
brightest portion to the darkest out at the corners of this cropped part 
of the whole frame.

And if I wanted an even smaller drop off at the corners, I could easily 
increase the distance from CCD to light source/diffuser.

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Tom Krajci
Cloudcroft, New Mexico
http://picasaweb.google.com/tom.krajci

Center for Backyard Astrophysics (CBA)
http://cbastro.org/ CBA New Mexico

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