[Aavso-photometry] What to use when there are no suitable comparison stars in the field?

[arne]

Terry Bohlsen wrote:
> Dear All
> I have been taking mostly images of long period variables and these have all had a nice selection of suiatable comparison stars. 
> I thought I would like to try to take some time series of stars suggested in the AVSON southern CCD target lists. One from 24/7/08 suggested AS Ind, SZ Ind and AR Ind. They are all in the same CCD filed for my setup. I have taken a short series but the problem is a lack of comparison stars with photometry data. There is one star in the field TYC 8794-1530-1 with a tycho V mag of 9.589 but this is pretty bright compared to the variables. With a 60 sec exposure it half fills the dynamic range of my CCD but the variables are still a bit dim with a S/N giving errors of ~0.1 mag. It would be better to double the exposure but this will make the tyco star not usable.
> There are lots of other stars in the field (see chart 1013bh) but I have only got GSC or USNO-a2 mags for them that are pretty unreliable.
> I'm not up to measuring the mags using all sky photometry so what are the choices?
> I can just do differential mags but this seems to be not as good as producing non transformed mags.
> I could measure one of the mag ~13 stars in the field using the tycho star and then use that measurement as the comparison star on longer exposures but I assume this will potentially add more error.
> Any suggestions or do I not bother with this field?
> 
> Thanks in advance
> 
If you are using a V filter, I'd suggest going to the ASAS catalog:
http://archive.princeton.edu/~asas/
use the RA and DEC of possible comparison stars, and do a coordinate
search.  Look at the light curve too, to ensure that the selected
star is not variable.  That will get you down to the 13th magnitude range.

As soon as we get the Mt. John system running, we'll start taking
calibration data for the southern fields so that you have reliable
multifilter calibration.
Arne