[Aavso-photometry] CCD Linearity testing

[chemist9755 at gmail.com]

Wouldn't it simply be easier to use a known star field at meridian and  
take a time series of star  images then do regression for time series  
and magnitude series using a b filter
This is the approach I am taking then I don't have to worry about abs  
lumenosity etc just correct for airmass and bolometric magnitude
Or is there flaws in this simple measue

Sent from my iPhone

On Jul 9, 2009, at 8:42 AM, Tom Krajci <tom_krajci at tularosa.net> wrote:

> From: arne <arne at aavso.org>
>
>> Tom is doing linearity tests, not flatfielding.  For linearity, you
>> don't worry so much about flatness or QE peaks - what you are looking
>> for are changes with respect to total exposure on each pixel.  LEDs
>> are fine for this, with the possible exception being Residual Bulk  
>> Image,
>> an effect that increases in strength with red light.  So you may  
>> get a
>> different result with, say, red LEDs, than with white LEDs.
>
> I'm currently using a green LED.
>
> Does this mean I should get a Blue, Red, and IR LED for more  
> complete testing?
>
>> Also, with
>> linearity tests, you generally don't need to have the telescope  
>> involved
>> at all - you can take the camera to the lab and work with it  
>> there.  This
>> keeps the temperature constant, and you work with completely  
>> defocussed
>> light, since there are no optics in the path.
>
> Yes, bench testing is easier and more accurate...LED, and CCD...and  
> not much else.
>
>> One trick to guard against a systematic buildup of residual light  
>> is to
>> increase exposure until you get total saturation, and then ramp down
>> and see if there is any systematic difference between increasing  
>> exposures
>> and decreasing exposures.  I usually run several sets of "ramp-up"
>> and "ramp-down" cycles.
>
> After Richard Miles pointed out that the middle of my run appeared  
> to be the when the LED was most stable, I looked at that data more  
> closely.  I did use a ramp-up/down exposure sequence, and at about  
> the 0.1% level you can see a difference in the ramp up versus ramp  
> down exposures...for the fainter levels, and at the faintest levels  
> this affect appears greater, but the measurements also have greater  
> scatter.
>
> Also, I find that if I add about 0.0055 seconds to the stated  
> exposure time...the curve flattens out at the faintest levels (to  
> better match the rest of the curve/slope at medium ADU levels).  I  
> can't prove that I have a small amount of shutter error, but I've  
> seen this before on some CCD's I've evaluated, to include my  
> Cookbook CCD...and for that one I know that it did have a shutter  
> error (the fault of one version of the camera control software, not  
> hardware).
>
> Even with my crude/hasty test set up, it's surprising how this is  
> accurate enough to show such subtle effects in the CCD's performance.
>
> -- 
> -------------------------------------------
> 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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