[Aavso-photometry] Flat Stuff

[Greg Crawford]

> Finally, redo your flats often! When you change focus the dust donuts
> change size and your flat is invalidated. I take new flats every time I
> focus if not more often than that.

I find the taking of flats tedious, but on the other hand, absolutely necessary
for photometry. Because I tend to push all things I find tedious onto the
back-burner, I often found I was foregtting to do flats in a night's
observation. Therefore, I decided to consign flat-making to a largely automated
process. I use a light-box set to a single, particular voltage to get decent
levels of stauration across the range of filters and vary the length of the
exposures, depending on the filter used. Because Ic gets so saturated from
filament globe illumination, and Ic observations don't appear to be crucial, I
omit flats (and observations) in Ic. I end up delivering 11 volts to the light
box and exposing B for 36 seconds, V for 9 seconds and R for 2 seconds. 

All of this can be scripted, so it's simply a matter of:

1) Focus on star.
2) Put lightbox on, and turn on.
3) Run script which takes about 15 minutes.
4) Flats delivered automatically for B,V & R.

So my answer to tedium is automation and the result is the good habit of flats
every night.

I don't see why flats would need to be redone after focusing. Surely the
re-focus is adjusting for temperature changes and restoring the optical path to
what it was at the beginning?

- Greg

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Bagnall Beach Observatory

http://www.nelsonbay.com/~gc/observatory.htm
 
> -----Original Message-----
> From: aavso-photometry-bounces at mira.aavso.org [mailto:aavso-photometry-
> bounces at mira.aavso.org] On Behalf Of Michael Koppelman
> Sent: Monday, 17 January 2005 2:03 PM
> To: 'aavso-photometry at mira.aavso.org' list)
> Subject: [Aavso-photometry] Flat Stuff
> 
> Arne will tell all in his new book I'm sure, but he once told me that
> he thought a good flat should have about a million electrons. I'm at
> 2.5 e/ADU so I need 400,000 ADU to get a million electrons. If I get
> 32,000 ADU per exposure I need about 13 of them. I like to do a min/max
> clipped mean which throws out the highest and lowest value for each
> pixel so I usually take 16 or more in each color.
> 
> Another thing about flats is that one should not take too short of
> exposures or there might be an effect from the shutter. I try to stay >
> 5 seconds on my flats to avoid any chance of this.
> 
> Finally, redo your flats often! When you change focus the dust donuts
> change size and your flat is invalidated. I take new flats every time I
> focus if not more often than that.
> 
> Just some random flats thoughts that have occurred to me on the
> discussion lately.
> 
> Cheers,
> Michael Koppelman
> http://www.lolife.com/astronomy/
> 
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