[Aavso-photometry] Linearity test puzzle

[Radu Corlan]

On Tue, Jan 17, 2006 at 09:14:14PM +1100, Vello Tabur wrote:
> Thanks Radu. I believe this explains the drift in the LED rather well. 
> Smith (R.M. 1998, Experimental Astronomy, 8, 59) quotes -1%/C drift, 
> rather more than your figure above, but in the same ballpark. FYI, I am 
> using a green LED.

of course, the exact chemistry of the led affects this greatly. 

> Forgetting the lamp stability issue, do you have any idea why the first 
> exposure of each group might show a lesser flux compared the rest of the 
> group? Here's the plot again for reference:
>    http://users.bigpond.net.au/vtabur/lintest.jpg
> BTW, I tried taking groups of 30 exposures to bracket the linearity test 
> and found that still only the first exposure in the group was affected, 
> so the effect appears to be  independent of the number of exposures in 
> the group. In all cases, the camera was idle (and shutter closed) for 
> 30-60 seconds before a test was started.

did you take bias frames before the flat frames? it is possible that the 
electronics of the camera self-heat when reading frames. with a 1V swing 
at the sensor output, 1LSB is only 20microvolts - it's easy to drift by 
1mV in a circuit that is not particularly optimised for this. the output 
stage of the sensor, which is just a simple source follower is 
particularly prone to do something like this. CDS cancels that drift, 
but only to the first order.

radu

> 
> Thanks,
> Vello
> _______________________________________________
> 
> Aavso-photometry mailing list
> Aavso-photometry at mira.aavso.org
> http://www.aavso.org/mailman/listinfo/aavso-photometry

-- 
Radu Corlan      

You pay now, or you pay later, but you always pay the entropy tax.