[Aavso-photometry] Linearity test puzzle

[Vello Tabur]

walt wrote:

> 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?
>>
>> The plot is here:     http://users.bigpond.net.au/vtabur/lintest.jpg
>>
>>
> My guess is that the resistor's temperature is increasing causing the 
> resistance to change. The resistor is dissipating about 13mW of power.
> walt


Hi Walt,
I believe that an increase in temperature would lead to higher 
resistance and a drop in LED brightness, which is opposite to what I 
see. Although the slow increase in LED brightness warrants an 
explanation, it wan't my main question. Assuming a constant rate of 
increase in brightness over the one hour period, I'm wondering why the 
first exposure of each set always has a much lower mean pixel value. 
Just by eye, the first exposure of each of the four sets is at least 
3-sigma from the linear best fit relationship.
Cheers
Vello