[Aavso-photometry] Linearity test puzzle

[Robert J. Modic]

Could this be due to trapped charge in the CCD?
When I take a series of flats, the ADU level of the
first frame is always a bit lower than the ones that 
follow.  This difference is about ~100 ADU if the
flats are exposed to ~30,000 ADU.

For linearity testing, I usually wait about 2 minutes
between successive frames.  This seems to eliminate 
most of the problem.

Bob Modic


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Date: Mon, 16 Jan 2006 17:31:39 +1100
From: Vello Tabur <vello at pcug.org.au>
Subject: Re: [Aavso-photometry] Linearity test puzzle
To: aavso photometry <Aavso-photometry at mira.aavso.org>
Message-ID: <43CB3DCB.5090206 at pcug.org.au>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

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