[Aavso-photometry] RE: Dark-subtracting flat frames

[Donn Starkey]

Greg:

Flat frames improve photometric precision across the chip.  For Pretty Picture
images, they take out the dust doughnuts and any vignetting of the optical
train.

What method are you using for even illumination of your flats? [e.g. twilight,
light frame, dome flats, etc.]

The "normal" procedure for flats is to saturate the pixels on your chip to
about 50% to 60%.  Short exposure times [<10 sec] tend to have aberrations due
to shutter movement and amplifier noise.  Try taking exposure times of 15 - 30
sec of a evenly illuminated light source.  Artificial light sources have the
advantage of being able to take flats whenever you like, not just a twilight.
Also, blue filtered twilight flats can take a looooong time if you wait too
late in the evening ;-)

I have been using a 'rope light' string from Home Depot as a light source for
my flats.  It is on a dimmer and is wrapped in a circle at the end ring of my
RC-16.  The light bounces off of a white screen on the dome and back into the
scope.  The allows me to use low power for IR and R band flats and full power
for those pesky Blue band flats.

Also take bias frames.  These show the internal noise level of your camera at
zero exposure time.

Donn

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The price of freedom is constant vigilance.
Donn Starkey
starkey73 at mchsi.com
http://www.starkey.ws
AAVSO Observer SDB  -  CBA Indiana
VSNET Observing Team  -  MPC Code H63
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