[vsnet-campaign-v838mon 407] Re: V838 Mon CCD observations by Kis

[Arne Henden <aah@nofs.navy.mil>]

These V-band values are ~0.2mag brighter than mine, and
seemingly show variation whereas my data are constant to
+/- 0.02.  There are two possible explanations:
   1) "Kis" is using either (V-R) or (V-I) to do the
      transformations onto the standard system.  Both of
      these colors are heavily influenced by the red "star"
      of the composite spectrum.  For example, (V-I) is
      approximately 5.1, whereas (B-V) is 0.7.  Looking at
      the spectrum, you should be using (B-V) as the red
      "star" spectra is not present in the V bandpass.
      This is one case where you either need to ignore
      transformations (since most V filters are pretty close
      to the standard system), or else observe with B as
      well as VRI.
   2) The measuring aperture is not small enough.  There are
      two fainter stars close to V838 Mon, and including them
      in the measuring aperture will make the star seem too
      bright.  Your aperture diameter should be less than 8arcsec.

Ic is about a half magnitude too bright.  My guess here is that
a Bessell prescription Ic filter is being used.  These filters
work fine for normal stars, but they have a red tail that influences
measures of red stars.  The spectrum is so cut up with molecular
bands that even small differences between filters will be amplified.
   Tough object on which to do photometry!  The best filters, if
you can go that deep, are B and V.

Arne


Taichi Kato wrote:
> V838 Mon CCD observations by Kis
> 
> WWW: http://vsnet.age.ne.jp/x/meineko/index-e.html
> Instruments:  0.25-m +Apogee AP-7 + Optec Maxfilter 2"
> 
> object         YYYYMMDD(UT)   mag  code
> MONV838        20021103.788  9.46Ic  Kis.VSOLJ
> MONV838        20021103.792  15.89V  Kis.VSOLJ
> MONV838        20021103.802  14.34Rc  Kis.VSOLJ
> MONV838        20021104.659  9.40Ic  Kis.VSOLJ
> MONV838        20021104.667  15.88V  Kis.VSOLJ
> MONV838        20021104.677  14.24Rc  Kis.VSOLJ
> MONV838        20021109.776  9.58Ic  Kis.VSOLJ
> MONV838        20021109.780  15.98V  Kis.VSOLJ
> MONV838        20021109.788  14.24Rc  Kis.VSOLJ
> MONV838        20021113.617  15.43V  Kis.VSOLJ
> MONV838        20021113.624  9.37Ic  Kis.VSOLJ
> MONV838        20021113.641  13.98Rc  Kis.VSOLJ
> MONV838        20021128.651  9.29Ic  Kis.VSOLJ
> MONV838        20021128.661  14.00Rc  Kis.VSOLJ
> MONV838        20021128.677  15.93V  Kis.VSOLJ
> MONV838        20021215.561  9.40Ic  Kis.VSOLJ
> MONV838        20021215.590  14.02Rc  Kis.VSOLJ
> MONV838        20021225.640  13.92Rc  Kis.VSOLJ
> MONV838        20021225.651  9.29Ic  Kis.VSOLJ
> MONV838        20021225.662  15.89V  Kis.VSOLJ
> MONV838        20021226.542  15.61V  Kis.VSOLJ
> MONV838        20021226.551  14.09Rc  Kis.VSOLJ
> MONV838        20021226.574  9.38Ic  Kis.VSOLJ
> MONV838        20021227.628  15.84V  Kis.VSOLJ
> MONV838        20021227.640  9.38Ic  Kis.VSOLJ
> 
> 
>

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