[AZ-Observing] Re: 9/P Tempel Impact light Curve
- From: Andrew Cooper <acooper@xxxxxxxxx>
- To: az-observing@xxxxxxxxxxxxx, TAAA Forum <taaaforum@xxxxxxxxxxxxxxx>
- Date: Thu, 07 Jul 2005 08:50:25 -0700
Stan, Terri,
I used the magnitude estimation tool in MaxIm 3. It has a set of
programmable apertures in which it integrates the intensity. I narrowed
the inner aperture to include just the inner coma point and widened the
outer aperture to beyond most of the coma for the sky background. I
then loaded and read out data from around 150 images typing the results
into Excel as I went. That was one of the main reasons for using this
tool, it could quickly handle large numbers of images unlike another
software package I tried.
The aperture just follows the mouse and I was a little concerned about
registration, but leaving the inner aperture a few pixels wider than the
inner coma (radius of 5 pixels) reduced the positional sensitivity, I
could be off by a pixel or two and the reading would not shift
significantly.
The next problem I had was calibration. The camera I used was FAST, a
20MHz pixel readout rate with around 7e- noise, great camera! (OK, I
did design large parts of it) It does suffer from very limited full
well as it uses a Sony ICX285, thus the brighter (12th mag) stars in the
image were saturated. I tried to calibrate on the 14-15 mag stars I had
left and was less that satisfied by the results. So I just set the
first image to 10th mag., a even number near the reported magnitude and
read out the relative change.
See the ICQ recent estimates page
http://cfa-www.harvard.edu/icq/CometMags.html
On the ICQ page a bunch of very experienced comet observers publish
their estimates, mostly visual, and their numbers are less impressive,
quite modest actually. If you follow the numbers from the same observer
(like Linnolt) from before to after the results are usually around half
a magnitude change. I suspect the larger numbers are from higher
magnification instruments and were only integrating the very inner coma,
whereas if you considered a slightly larger field the net change was
much less.
Currently the curve has no correction for extinction, but during the run
the air mass would have increased from 2.7 to 3.6, fairly substantial.
As a result the curve is more representative of what an observer at a
telescope eyepiece would have seen. I don't currently expect to correct
for this issue.
Light curves from the event have been difficult to locate, amateurs
haven't been producing them, lots of pictures and animations but few
numbers. The pros seem to be holding back on data as usual, probably
until they can publish. I did find a single curve on the ESA-OSIRIS
webpage the agrees with mine in that the coma brightened linearly for
around 20-25 minutes then leveled off. It was gratifying to see a curve
that matched, but the magnitude was of course much different as this was
a space-based observation unhampered by three airmasses.
http://www.esrin.esa.it/export/esaCP/SEMSJ06DIAE_index_1.html
An interesting exercise!
Andrew
Andrew Cooper
----------------------------------------------------
http://www.siowl.com
Stanley A. Gorodenski wrote:
>Very nice. For my own education, how did you measure the magnitude?
>E.g., did you total pixal readings over a square area? If so, how large
>a square, and how were you able to keep it in the same location from
>frame to frame?
>Stan
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