[vsnet-chat 2139] Re: Supernovae estimates using USNO SA2.0

[Taichi Kato <tkato>]

Re: [vsnet-chat 2137] Re: Supernovae estimates using USNO SA2.0

Dr. Henden wrote:

>   (1) most supernovae occur in external galaxies, which tend
>       to 'avoid' the Milky Way.  Once away from the galactic
>       plane, USNO-SA looks very much like USNO-A; very few
>       stars are missed.

   Yes, I confirmed this.  At first I considered whether USNO SA CD-ROMs
are usable for making finding charts.  The result was very disappointing
for Mikky Way fields, but looks acceptable for high-galactic lattitude
objects.  I don't know whether faint stars in sparcely populated fields
give an acceptable representation of magnitudes than in crowded fields,
which looks severely affected by confusion as magnitudes go fainter.

>   (2) supernovae are usually relatively faint (say, 15th mag)
>       and therefore the 17-19mag stars should be reasonably
>       exposed if the supernova is exposed properly.

   There is some doubt whether softwares can extract only "proper" matching.
Spurious star detections (noises) in low S/N, or underexposed, images can
often be incorrectly identifed with faint USNO stars.  Special caution
should be payed for short focal-length instruments.

>   (3) there is more error in proper background subtraction
>       (e.g. galaxy contamination) than in zero point setting
>       for most amateur measurements.

   Yes, this is a problem.  I wonder whether there is a good written
prescription for SN phorometry other than elaborated profile fitting
routines.

>   (4) since there is roughly a 0.25mag photometric error for
>       most of the PMM stars, the more stars you include in
>       your solution, the better.  That usually means using
>       many fainter stars to form an ensemble mean.  My software
>       handles this situation properly, and I'm sure Kato-san's
>       software does as well.  However, using something like
>       CCDSoft with its single zero-point star photometry will
>       give poorer results.

   Some softwares apparently don't explicitly use -2.5log(count) formula,
but look to experimentarily determine the term k in -k log(count) from
a matching with catalogs.  This is probably incorrect in the usual
photometric sense, but I don't know whether this prescription can be more
reliable particularly in amateur SN photometry.  This prescription has been
apparently brought into reality by the availability of massive faint star
photometry like USNO A and USNO SA catalogs.  I'd like to know your opinion
on the introduction of the variable -k log(count) term.

Regards,
Taichi Kato

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