[vsnet-chat 2620] Re: Tmz85

[Taichi Kato <tkato>]

Re: [vsnet-chat 2619] Re: Tmz85

> Congratulations to the Kyoto group for getting a 12-hour
> run -- that is 2+ hours longer than Joe Patterson got! :-)

   Actually it was nearly 12.5-hour long, which was probably one of our
longest records of continuous photometry.

>   One of the reasons I offered to get a time series this
> upcoming week was the two observations I obtained last
> year.

   We are very looking forward to hearing result, since your observation
probably falls the stage immediately following superoutburst, when the
disk most shrinks, or some turbulence in the disk may be still present.

>   I respectfully disagree with Kato-san.  Most photometry packages
> have problems when dealing with faint objects near
> sky limit.  What he says holds only if there are no
> systematics in the magnitude determination.  In general,
> most photometry packages have systematic errors when measuring
> faint objects, primarily due to the determination of
> sky within the sky annulus.

   What is necessary (and is a challenge) for a programmer is to write
a convenient software which can automatically do this.  Since TmzV85 is
fairly isolated in the sky (except for the possible faint galaxy), there
is little problem in the background determination.  Our experience has
shown PSF profile fitting works very well (we believe we could enough
accurately measure the object even in the eclipse minimum, at mag ~19).
This, of course, strongly depends on the "linear response" of the
algorithm to the pixel variation.

> I've found results to be far superior by first combining images and then
> measuring rather than the reverse.

   When comparing images of the same net exposure time, a single exposure
is better than stacked images (on devices like CCDs).  This is true.

> If you don't have sufficient
> signal, then you really can't get high time resolution.

   Care should be taken in interpreting this.  What we usually need is
the "averaged" eclipse profile, rather than the individual ones (exceptions
of course exist).  In such a case, phase-stacked eclipse profiles are
usually used.  When you coadd, for example, four eclipses, half photons
in individual eposures are enough to achieve the same S/N (disregarding
other noises).  This means an observer can reasonably shorten exposure
times when he or she is going to cover mutiple periods, or to combine
with other observer's result.  The combination of a long run and a short
exposure time satisfies this condition.  A plenty of information of
short-term variation (like eclipse profiles or QPOs) is permanently lost
even in a long time-series at a long exposure time.

Regards,
Taichi Kato

VSNET Home Page

Return to Daisaku Nogami

---

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