[vsnet-chat 1720] BVRI Photometry

["William S G Walker" <astroman@voyager.co.nz>]

Greetings, All,

I have been attempting to do some BVRI photometry of bright stars this
summer, mainly of Cepheids and SR stars with a few Miras thrown in. The
main aim is to obtain seasonal epochs of maxima for Cepheids to check for
period changes. This project carries on one normally done with UBV
photometry using a pm tube, using a Meade 25cm f6.3 telescope while my
normal telescope is being repaired. There have been a few problems and I
would be interested in comments from anyone who has useful experience or
ideas.

The people at Mt John suggest that CCD photometry of this type has an
accuracy limit between 1% and 2%. Presumably this is based upon LMC
photometry with 0.5 an 1 metre telescopes. What do others find? For what
it's worth the BVRI response of the telescope with an ST6B using BVRI
filters supplied by Michael Bessell of Mt Stromlo is relatively flat,
although the B response is down about a magnitude and we don't use U.

One problem I find is that the exposure times are embarrassingly short. For
a number of objects I can only use 2 and 5 seconds before saturation. The
ST6 is used in the low-res mode which effectively increases the saturation
rate by four times. But it reduces the download time by a similar factor
which is critical - it triples the number of stars one can measure in an
evening. I have a vignetting problem but this can be overcome by reducing
the telescope to a 10cm aperture which will also increase exposure times by
a factor of 5 to reasonable levels. But I can't go randomly changing
apertures all night - once I've selected one for the evening that's it.

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. 

Another puzzle is when a star is measured for different times - say 1 and 2
seconds. The magnitudes between these should always vary by a similar
amount but don't.

Some questions which arise are:

1. How much variation or noise is expected in exposures at the 0.2, 1, 5,
10, 20, 30 second levels?

2. Is there any suggestion that the exposure ST6 timing system is variable?

3. Is there any possibility that the filters don't always reposition
themselves precisely?

4. Should we flat field for each filter? This would make the data reduction
a little more complex but wouldn't be impossible.

I could, of course, write all this off to poor observing conditions but the
nights in question looked fairly good to me. Cirrus was not a problem -
it's not a region affected much by this - but I do have a large and shallow
harbour about 100 metres to the east.

I hope that there's someone out there doing small telescope colour
photometry. If so, some comments would be welcome.

Regards,
Stan Walker

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