[vsnet-chat 2182] Re: SN1999cp field photometry

[aah@nofs.navy.mil]

I observed the field of SN1999cp because Berto Monard
felt that the VSNET sequence for this field was about
0.3mag too faint, and requested additional photometry
to confirm this conclusion.  I've reported the photometric
night of data and placed the file on
  http://ftp.nofs.navy.mil/pub/outgoing/aah/sequence/sn1999cp.dat
Subsequent to this, Brian Skiff noted that Kevin Krisciunas
has calibrated this field, and his 6 stars are found on
http://vsnet.astro.washington.edu/kevin/apo.html
Kevin seems to be keeping up with the recent nearby
supernovae, and I recommend bookmarking his page for future
supernovae sequences.  Unless he is not planning on
calibrating a field, I don't see any value in both sites
getting photometry!
  However, Brian also notes that Kevin's photometry has
an offset with respect to mine.  While offsets between
nights is common, and I expect my single night to have
an offset on the order of 0.02-0.03mag for V or (B-V),
the errors that Brian quoted for (B-V) are outside of my
expectations.
  I've gone back and used the Tycho star to calibrate my
first night of non-photometric data, and then averaged both
nights to get slightly better statistics on the fainter stars.
For both nights, I took a very short exposure sequence to
calibrate the 8.39mag Tycho star, and then a deeper exposure
(2min/1min for B/V) to calibrate the fainter stars.  All four
exposure sets (short&long for 2 nights) agree with one another
within statistical error.  Comparing against Kevin, I get for
his 6 stars:
ID   Vk      Vh     diff    BVk    BVh    diff
01  16.372  16.351  0.021   0.594  0.509  0.085
02  15.122  15.106  0.016   0.258  0.127  0.131*
03  14.442  14.474 -0.032   0.601  0.534  0.067
04  15.246  15.255 -0.009   0.669  0.626  0.043
05  16.699  16.715 -0.016   0.678  0.610  0.068
06  14.515  14.617 -0.102*  0.991  0.961  0.030
mean dif            0.006                 0.058

where the means are the average of 5 stars (the starred value each
of V, B-V has been removed).
  Star 6 has a significant V-magnitude difference between Kevin's
observations and mine.  Given its red (B-V) color, this star
could be variable.
  Star 2 has a significant (B-V) difference between Kevin's
observations and mine.  It is the bluest star in the field; I have
no explanation for this difference.
  The V offset between Kevin and me is well within statistical error.
  The B-V offset is outside of statistical error.  I have no explanation
for this difference!  My measures of the Tycho star are close to its
nominal values, so I tend to believe my measures over Kevin (of course;
every observer trusts his/her own data first!).  I'd like to hear
from Kevin as to how his two nights were calibrated, such as the
quality of the nights, telescope used, and standards used, to see
if there is a possible error in his calibration.  Barring that, I
would have to reobserve the field on another photometric night or
two to check my reductions (and I would, except the field is
rapidly disappearing into dusk and we are in the midst of our
summer monsoon).
  Note, however, that the visual sequence is in agreement between
the datasets and can be considered reliable.  I haven't checked
its value with respect to the VSNET sequence; someone (Berto?) should
do so to see if there is an offset in the VSNET sequence.  Only
CCD observers should be concerned about the (B-V) dilemna.
  Sorry about the length of this posting, but Brian started it!
Arne

--------------------------------------------------------------------
Arne Henden                               Instruments/software/CCDs
US Naval Observatory Flagstaff Station    Cepheids/photometry/IR
P.O. Box 1149                             http://ftp.nofs.navy.mil  
Flagstaff, AZ 86002-1149                  Voice: (520)779-5132
aah@nofs.navy.mil                         FAX:   (520)774-3626

VSNET Home Page

Return to Daisaku Nogami

---

vsnet-adm@kusastro.kyoto-u.ac.jp