[vsnet-campaign-v838mon 376] V838 Mon photometry/imaging techniques

[aah@nofs.navy.mil]

Doug West asked off-line about the best way to
observe V838 Mon.  My answer might be of interest
to other observers, so I am posting the reply here.
  V848 Mon is a highly unusual nova from at least
two standpoints:  it is *very* red, with a late
M spectral type; and it has a light-echo shell.
Both of these features make it challenging to do
accurate photometry.
  Recent photometry from USNO gave V=16.0, Rc=14.7
and Ic=11.2.  Kiyota-san gave 10.66C; Doug West
gave Ic=10.3.  All three datasets are relatively
close in time and so should agree.  Obviously
they don't, so the question is: why?
  First, you should use a standard filter if you
intend to submit your data for others to use.
The red color [(V-I)~5] means that systems with
red response will measure the object much brighter
than those that don't.  Unfiltered systems are
the most prone to this (try combining Sony interline
unfiltered CCD observations with KAF0400 CCD observations,
for example).  This is the main reason for the Kiyota
discrepancy.
   However, even filtered observations will have problems.
Cousins-I (also called Ic) is strictly defined in the Cousins
and Landolt papers with a central wavelength of 800nm and half-power
width of 150nm.  The Bessell glass prescription (3mm RG9)
does a pretty good job of matching this bandpass
except in the red, where the falloff is CCD-determined
and not filter-determined.  This means very red objects
like this one will again be measured too bright in
comparison to a true Ic filter.  Our 1.0m filterset
actually uses an interference filter for Ic to get
the proper red cutoff, which is why our measures are
fainter than others reported.  V and Rc do not suffer
from this problem (both red and blue edges are filter
defined), so I would use V or Rc for your measures
over Ic or B (a filter that can have a red leak).
  Second, the light-echo interferes with aperture
photometry.  The inner radius of the light echo is
something like 5arcsec, so if you use a maximum measurement
aperture radius of around 5arcsec, the flux recorded
comes from the central star itself.  If you use an
aperture much bigger than this, you start including
nebulosity and the measure will be brighter than reality.
Likewise, you need to set the sky annulus out beyond
the edge of the light-echo.  It currently has about
a 30arcsec radius, so you need to set the inner radius
to >30arcsec.  Otherwise, the nebula is included in
the sky annulus and the subtraction will yield a
final measure that is fainter than reality.  This
may seem like a strange combination (small star aperture
and huge sky annulus radii), but it gives the best results.
  So in summary, to do photometry of the nova itself:
     - use V or Rc filters
     - use a small measuring aperture
     - set the sky annulus outside the light-echo shell
  Now, the light echo shell itself is a lot of fun to image, especially
if you have decent seeing and good pixelization.  To image
the shell, any bandpass will do.  Using three will give
you a nice tricolor image. Using a blue bandpass will give
the maximum contrast between the star and the light echo,
but unfiltered will do a nice job of recording the maximum
extent of the nebula since it is pretty faint.  It is expanding
at about 0.1arcsec/day, so watching the light-echo shell over
a month or so will show noticable expansion.  Determining
the diameter is a bit more tricky since the echo is somewhat
asymmetric; you can take multiple radii and average them.
Arne

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