[Aavso-photometry] Linearity test puzzle

[walt]

Vello Tabur wrote:

> I've been testing the linearity of my camera (SBIG ST8XE) using a LED 
> + voltage regulator, as per the circuit in AIP v1 (pg 201).
> Ten exposures (30sec each) were taken either side of a linearity test 
> to assess LED stability during the test.  The plot below shows a 
> gradual increase in LED brightness of 0.24% over a period of one hour. 
> A curious systematic feature appears on the plot: the first exposure 
> of each group shows an rapid increase in flux relative to the other 
> exposures. It occurs in each group and the same effect occurs in tests 
> conducted on other nights.  Any suggestions as to what could be 
> causing this?
>
>
>
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My guess is that the resistor's temperature is increasing causing the 
resistance to change. The resistor is dissipating about 13mW of power.
walt