[Aavso-photometry] CCD Linearity testing

[Gary Walker]

 Hello Tom

I assume that you are using a light box.? Are you using LED's or incandencent lights?? Arne has always cautioned me away from LED's, because they have a peak in the response curve.? If this aligns in frequency with a peak in the QE curve, we can have problems.? 


 


Thanks

Clear Skies
Gary Walker
Maria Mitchell Observatory
4 Vestal Street
Nantucket, Mass  02554

 


 

-----Original Message-----
From: Tom Krajci <tom_krajci at tularosa.net>
To: RICHARD MILES <rmiles.btee at btinternet.com>
Cc: John Saxton <lymmobservatory at hotmail.com>; Michael Kran <michael.kran at kran.com>; aavso-photometry at aavso.org
Sent: Thu, Jul 9, 2009 12:34 am
Subject: Re: [Aavso-photometry] CCD Linearity testing









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?
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Center for Backyard Astrophysics (CBA)?

http://cbastro.org/ CBA New Mexico?
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American Association of Variable Star?

Observers (AAVSO): KTC http://www.aavso.org/?

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