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SN 2013dy

roe
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Joined: 2010-07-25

I've been observing SN 2013dy for some time now and I notice that my measurements are diverging from what others are reporting.  I seem to get about 0.5 mag (V) brighter than the others.  I never prejudge my measurements against others but a red flag goes up when I see such divergence and I go back to check each measurement.  In this case I can't find anything wrong so would ask for advice on where to look next.

I use VPHOT and have been careful (I think) to set the aperture snugly around the target and to avoid the nebulosity of the galaxy in the sky annulus.  It seems to me that should excess nebulosity get into the sky annulus it would make the sky appear brighter and, hence, the SN fainter?  But I'm reading brighter.

Jim Roe [ROE]

SN 2013dy
SHA
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Joined: 2010-07-29

I've observed sn2013dy a couple times in V during the past few days, getting the
fainter result.  Although both of our observations are untransformed, I would
doubt that adding color terms would resolve the discrepancy.  After reading your
message, I went back and tried different aperture and annulus configurations.
These did make some difference, but only at the several hundredths of a magnitude
level.  This was done with MaxIm DL. However, it is clear that as the supernova
fades, it is getting harder to properly distinguish it from the galaxy and getting
good aperture photometry is becoming more difficult.  I've heard of observers in
some instances waiting until the supernova has completely faded to get final
photometry, modeling the background galaxy brightness to do so. My CCD field of
view is relatively small, so I cannot use the comparison stars you used to see whether
that makes a difference.  Other than the usual routines that you have undoubtedly already
thought of, I'm afraid that I don't have any good suggestions. If you think of
something to try, please let me know.

Horace Smith
 

SN 2013dy
roe
roe's picture
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Joined: 2010-07-25

I, too, have wondered about the background problem.  I have been using a background annulus large enough to exclude any (obvious) galaxy.  If the galaxy contribution is significant, that would make my readings too bright (because I am including the galaxy background in the star measurement).  On the other hand, if others are over estimating the galaxy background then their readings would be too faint.  This may be beyond aperture photometry to handle.  What to do?

Jim Roe

AAVSO 49 Bay State Rd. Cambridge, MA 02138 aavso@aavso.org 617-354-0484