[vsnet-alert 21078] Re: [vsnet-newvar 6113] ASASSN-17gw (= V1296 Sgr)

[Brian Skiff]

On Thu, 2017-06-01 at 18:46 +0200, Josch Hambsch wrote:
> Very difficult field for finding comp stars. Most are either too bright or do not have B, V values in UCAC4.
> I have first observations from last night and had to take comp stars with V around 11.5 mag.

     Instead of UCAC4 to get APASS photometry, I would recommend
instead using APASS DR9 directly from VizieR (item II/336),
although it looks like coverage has been omitted in this
region because of the extreme crowding.

     For brighter stars in V, as a stop-gap, you can use ASAS-3.
Be sure to select uncrowded stars from this catalogue, 
usually no bright stars within about 45" radius.  In this
part of the sky, you can use the UK Schmidt short-V plate
available from Goddard SkyView to see how crowded the brighter
stars are (default 'DSS' image in this part of the sky).
The regular sky-survey images of course are solid stars,
so are not very useful.

     It looks as though there is Pan-STARRS coverage down
to this Declination (I just grabbed a star-list centered
on V1296 Sgr):

http://archive.stsci.edu/panstarrs/search.php

There should not be a problem with crowding.  Pan-STARRS
provides high-quality Sloan g,r,i,z,y  photometry for
stars fainter than mag ~13.5.  V = (g+r)/2 approximately
within ~0.1 mag irrespective of color.  Be sure to look at
the various flags and uncertainties on the Pan-STARRS data.
(Typically if the error is >0.01 mag, then the star is variable.)
For telescopes of ordinary image-scale and quality I would
suggest using the Pan-STARRS 'MeanApMag' (aperture magnitudes)
rather than 'MeanPSFMag' (from point-spread fitting).
For R and I filter observations, I would tend to recommend
abandoning traditional Cousins R and I and adjust the zero-point
to Sloan r' and i' (the filter passbands are nearly the same
even though magnitude scale is different).
     With care, choosing comp stars are similar color,
adjusting unfiltered CCD data to Sloan r' is probably the
best course.
     Pan-STARRS provides coverage of the entire northern-
accessible sky from the north pole to -30 Dec (but evidently
slightly beyond!), so there is no reason any longer to lack
comparison stars in even very small CCD fields.
     Hope that helps!


\Brian