[Aavso-photometry] Linearity test puzzle

[Vello Tabur]

Ben Davies wrote:

> Hi Vello,
>
> I saw a similar response when testing my ST-10.  It wasn't as 
> consistent and repeatable as your results seem, but I saw the anomaly  
> frequently enough that I came to be very suspicious of the first image 
> in a series.
>
> http://ben.davies.net/st-10a.htm
>
> There are 5 minutes between images in the series sets I was making.  
> My first guess (and that is all it is) is that the the acquisition 
> software is somehow doing this.  I have been using SBIG's CCDOps, but 
> also have CCDSoft and AstroArt.  Sometime this week I will give them a 
> try and see if I can make any  improvement.
>
> By the way, It takes 24-36 hours for my led source to settle down to a 
> reasonably stable state.  There is a thread on the AIP site "constant 
> light source" from a few weeks ago where this was discussed.  Bruce 
> Anderson found an interesting feedback circuit which purports to 
> produce a stable led output.  
> http://groups.yahoo.com/group/AIP4WIN/message/6537  I may give this a 
> try soon.
>
> Ben Davies
>
Hi Ben,
Thanks for the confirmation. I too was wondering if the software was to 
blame. In my case, I've written it myself using the published APIs. I 
was going to re-test using CCDOPS, but since you see a similar effect, I 
don't think I'll bother. Taken together, our results would seem to rule 
out a client side software bug. It still leaves open the possibility of 
some common code, perhaps in the SBIG USB driver or camera's firmware. 
That fact that it happens to an ST-8 and an ST-10 leads me to speculate 
that it might be more generic in nature, although I have yet to see a 
convincing hypothesis to explain the effect.
Regards,
Vello