Comet ISON Light Curves by an Amateur
Webmaster: Bruce Gary, Last
updated: 2013.08.15, 00.0 UT
This website is where I show Comet
C/2012 S1 (ISON) light curves by amateur observer Bruce
Gary, using a Celestron 11-inch telescope at the Hereford
Arizona Observatory (G95). My long-term goal is to determine
a rotation period. CometHomePage
Aug 13, added light curves
for the 20-minute period on Aug 12, showing effect of photometry
aperture size changes.
2013.08.12 Recovery Observations
Warning: I've never tried to do photometry for targets at ~ 10
air masses before, and this is my first observation of a
comet, so there probably are systematic errors that I'm not
aware of in the following analysis.
This section investigates the brightness of comet ISON versus
photometry aperture size. It's an attempt to distinguish
between the coma's brightness and that for the nucleus. The
comet's PSF (point-spread function) was greater than nearby
stars in all images (~ 10.6 px vs 6.0 px, FWHM). We therefore
expect brightness to increase with aperture size. Indeed, it
Apparent magnitude versus photometry aperture, using 7
nearby reference stars for calibration (using APASS r'
When the photometry aperture radius was changed, a
corresponding change was made to the gap annulus; when one
increased the other decreased. This assured that the same sky
background annulus was used for all photometry measurements.
The coma appears to extend to at least ~ 16 "arc from the coma
center (i.e., nucleus). Most of the coma's flux is captured
for photometry radii of ~ 18 "arc, where a "total" flux
corresponds to r'-mag = 14.03 ± 0.08 (estimated SE).
Here are the light curves for each photometry size. The
relevant info box is the one in the UR corner, giving
magnitude for the photometry aperture circle.
r'-mag versus UT for 6 images (each image an average of 4
or 5 individual images). The first group average (5.0 degree
was noisy so it wasn't included in determining an average
for the following 5 groups; it is displayed merely to
indicate the level of
systematics that can be introduced by very low elevation
There's a trend for the highest air mass magnitudes (the first
ones) to be fainter. Someone has suggested that this might be
caused by blockage of my telescope aperture by the dome
opening (bottom edge) and the scattering of light that this
might produce. I'm currently working to determine if this is
likely and will update this section later today.
I am making the fits images used to create the above LCs
available to anyone who can convince me that they are
competent and serious about using them for scientific
purposes. Just e-mail me and ask, at:
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WebMaster: B. Gary. Nothing on this web page is copyrighted. This
site opened: August 12, 2013