[Aavso-photometry] RE: Linearity test puzzle

[Tom Krajci]

>From: Vello Tabur <vello at pcug.org.au>

> 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

I have not built and tested this circuit, but:
http://www.edn.com/archives/1997/071797/15di_02.htm
may give you a more stable light source.  It doesn't look like a complex
circuit.

If it turns out to be more stable, it may help identify other behavior
patterns of your CCD setup.

I hope this helps.

Tom Krajci
Cloudcroft, New Mexico
http://overton2.tamu.edu/aset/krajci/



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