[Aavso-photometry] Flat Stuff

[Chuck Pullen]

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