[vsnet-chat 5910] Re: (fwd) Re: ResSkySurvey and PIXY

[Taichi Kato <tkato@kusastro.kyoto-u.ac.jp>]

Re: [vsnet-chat 5906] Re: (fwd) Re: ResSkySurvey and PIXY

> Yes, it was surprising to find a relation between V magnitude measured and
> actual V magnitude of the star.
> And also interesting that there is no color-dependence in this relation.
> I won't try to explain it (maybe some of you can do it), but only show the
> numbers:

   One of my guesses is that this systematic error may be a result of
non-linear measurement of the PIXY implementation.  As I wrote before,
PIXY uses a variable (in effective sense) aperture size.  Bright stars
are measures with larger apertures, and faint stars are with smaller
apertures.  This could partly explain the trend.  As I suggested to
Yoshida-san many times, PIXY clearly needs to implement a more standard
and analytical photometric function, rather then relying on a heuristic
parameter seletion.

   Another possibility would be the instrinsic non-linearity of the CCD
which Maciej used.  If this is the case, the CCD needs to be repaired or
replaced.

Regards,
Taichi Kato

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