[News] Potential Data Problems

[Dr. Mark Neeser]

On Thu, 5 Jun 2003, Koen Kuijken wrote:

>
> Dear Mark,
>
>  excellent initiative. I have added some comments below.
>
> Greetings
>
> Konrad
>
>
> > 1/ Science Flats:
> >
> >    When creating flat field frames from the science images (master science
> >    flats) there must be a mechanism for eliminating unsuitable frames (a quality
> >    determination).  Of importance here, is the elimination of all frames containing
> >    large objects (bright, saturated stars, nearby galaxies, etc) whose extent exceeds
> >    the dither pattern size.  Otherwise, there will be sufficient overlap between
> >    successive frames that object residuals will be in the master flat and resulting
> >    in "holes" in the flatfielded images.
> >    This is relatively easy to test for since we have a number of SExtractor output
> >    parameters that we can use.  For example, in a high S/N SExtractor pass remove
> >    all frames from the master science flat creation with:
> >    A_IMAGE >= dither/jitter [in pixels]  [A_IMAGE is the 2nd order moment along
> >                                           the image major axis at the chosen
> >                                           detection threshold]
> >    (Other relevant SEx parameters could be ISOAREA_IMAGE, FWHM_IMAGE,
> >     XMIN_IMAGE, YMIN_IMAGE, XMAX_IMAGE, YMAX_IMAGE . . . Emmanuel suggestions?)
> >
>
>
>
> In case of jitter data, this will remove all frames - we expect several
> m=8 stars per field. So this will only work if
>  - this procedure is applied per CCD, not per field
>  - many OBs at different pointings are incorporated. In that case a more
> traditional masking of bright outliers may be sufficient.


Yes, agreed.  The few pixel (~5) shifts of jitter will make super flat
creation difficult without extensive masking.  The other solution, of
course, would be to allow larger jitters of about 2.5 x fwhm seeing. You
will still get a very homogeneous context map, yet have a much easier time
creating high quality super flats.