[Aavso-photometry] Flat Stuff

[Robert Koff]

Chuck,

Thanks for the comments.  I have recently stopped using the diffuser for 
I-band flats, and in such cases I set the drive to slew at guide speed (2X 
sidereal with the LX200).  This smears out the star trails pretty well, and 
they seem to median out.  BTW, I generally point near the zenith for the 
flats.

Bob Koff

----- Original Message ----- 
From: "Chuck Pullen" <cpullen at pacsafe.com>
To: <aavso-photometry at mira.aavso.org>
Sent: Monday, January 17, 2005 21:52
Subject: Re: [Aavso-photometry] Flat Stuff


> Hi Bob - I've tried a diffuser too, but upon reflection, it seemed to me 
> that I was just shooting a dome flat with a better non-biased illumination 
> source. Any flaws in the plastic would be a systematic error.  Don't know 
> about the spectral issues, I would think polyethylene would absorb in the 
> IR.
>
> The biggest bugaboo people have with evening twilight flats is that they 
> may not be focused in the early evening yet (I use a hartman mask and 
> focus on the "first star I see tonight" then still have time for flats if 
> I want to do evening flats). I'm already focused from working all night 
> for dawn flats.
>
> Another issue are stars starting to show up.  I've read, and been told by 
> Arne, that stopping the mount (i.e. not tracking) and doing a median 
> combine pretty much eliminates any adverse effects of faint star tracks. 
> I sure don't see any evidence of them in my median combined master flat.
>
> When I was using an ST-6, the 45 second download time was a show stopper 
> for me.  I figured I got better results with dome flats.  However, my AP 7 
> has a 12 second readout period, so is much faster, making twilight flats 
> possible.
>
> Chuck Pullen
>
> At 21:31 1/17/05 +0000, Robert Koff wrote:
>
>>I've also gone to dawn or dusk twilight flats.  I use exposures of 10 and 
>>20 seconds as the light changes.  the download time of my camera is about 
>>45 seconds, so I don't get a lot of images.
>>
>>I use a diffuser to prevent stars from interfering.  For the diffuser I 
>>use the same white translucent sign material I have used with a light box. 
>>Any thoughts on the validity of using a diffuser?  How about using it with 
>>a back-illuminated chip and I-band filter?
>>
>>Bob Koff
>
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