[vsnet-chat 1721] Re: BVRI Photometry

[aah@nofs.navy.mil]

Stan Walker wrote about his experiences working with Cepheid
variables.  I've been following these beasts for about 20 years
now, so may have some experience in the matter.

Some questions still come to mind.

First, you do need to check your hardware if you are concerned
about exposure time steps not giving accurate ratios.  First,
it is unclear to me from SBIG's page exactly what kind of shutter
arrangement is on the ST6B.  Could you explain?  Ratios may not
be repeatable depending on sky conditions as well.  Not only
do you have transparency variations, but scintillation on short
exposures and seeing variations all affect the raw results.
What experiments have you run to check the shutter (if one is
used) or the exposure timing?

Are you doing differential photometry in a given field?  Do you
have adequate signal/noise ratio in all objects to expect 
1 percent results?

Do you really understand the saturation level for your CCD?
What experiments have you run to check this?  2-5second
exposures are pretty short -- I usually recommend 10-second
exposures as the shortest exposure time for 1percent work.

>What I'm noticing is that there seem to be unexplained variations in the
>data. All observations are made as pairs of BVRI sets and generally these
>agree quite well. But not always - sometimes there is a difference of 5-8%
>in one or more filters. And from one star to the next there is often a
>similar difference. 
  What is the signal/noise in each image?  Reduction software?
If you get these variations between two stars in the same field, but
different images, then you have real problems.

You should definitely flatfield for each filter.  Not only will you
have a different dust pattern (and how strongly this affects your
image depends on the position of the filters with respect to the
focal plane), but the QE of each pixel is a function of wavelength.
A flat taken with a B filter will not flatten an I image.

Arne

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