[News] Potential Data Problems

[Dr. Mark Neeser]

Hi All,

In light of Roeland's request, during last Monday's telecon, for
input regarding possible issues and problems that may affect
OmegaCam data, I have summarized some potential issues that
have, perhaps, not yet been considered.

We had a short meeting at the USM among our "observing" experts and
contribute the following:




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?)


2/ Masking of Spurious/Unwanted Features via:

   (i) Automatic detection and masking of prominent features such as:

       (a) Stellar spikes (NOTE:  not entirely trivial as stellar spikes
                           can vary in length/profile/relative_position as
                           a function of seeing, and with camera rotation
			   with respect to telescope spider.
                           Under conditions of very good seeing, one can
            		   get spikes that go across the entire CCD).

       (b) Saturated stars (NOTE:  the masking of saturated stars needs
                            to be sufficiently conservative to also mask
 			    potential offset halos due to reflections in the
                            optics around very bright stars).

       (c) Satellite trails (NOTE:  these are not always uniform.  Satellite
      			     tumbling may result in large brightness variations
			     along the track.  How well does the Terapix wavelet
 			     transform software work at detecting these?).

       (d) Reflections/ghosts/halos

   (ii) Manual masking (is this going to be a Panorapix utility?)

3/ Fringe Pattern Correction:

   This can be relatively straight-forward when the observing conditions are
   stable and photometric.  It can be a nightmare when they are not.
   For light of a given wavelength, the observed fringe pattern can be very stable,
   being determined by the thickness of the chip at each pixel.  The effect is not
   very strong for continuum sources, since the fringe pattern is smeared out.
   However, the fringe pattern seen in flattened images is the result of a few strong
   atmospheric emission lines (mostly OH and O). Since it is caused by the sky, and
   not inherent in the sensitivity of the detector to the stellar flux, the fringe
   pattern should be treated as an additive term remaining after the flat-fielding.
   The problem is that under variable sky conditions the OH and O emission varies in
   intensity apparently independently from one another.  Since they emit at different
   wavelengths, the strength of the fringe pattern will vary substantially relative to
   the sky brightness from exposure to exposure.  I have heard the claim that this
   can be modelled and corrected (WFI users/Meisenheimer?) but I have not seen the
   results and am not familiar with the exact technique.

4/ Illumination Correction:

   Undoubtably, OmegaCam will show significant large-scale spatial gradients in
   the photometry across its field of view and across its individual CCD's.  The
   primary reason for this is the non-uniform illumination of the field.  These
   systematic variations can be removed using superflats (Clowe & Schneider 2001;
   Alcala et al. 2002), or, perhaps better, by observing densely distributed
   secondary standard star fields (globular clusters . . .).  This was done for
   WFI by Koch et al. (see http://www.mpia-hd.mpg.de/SDSS/data) who found strong
   spatial gradients of about +/- 0.05 magnitudes across the field of view.  By
   doing photometry on a globular cluster with SDSS calibration, they fit the
   residuals, and were able to subtract these from the measured instrumental
   aperture magnitudes to remove the spatial dependence of the photometry.  As
   a side effect, observations of these dense star fields produce a good 1st
   order astrometric correction.

5/ Oblique Scattering:

   Since the VST is a fast telescope (f/5.5) and we will be obsering quite
   a bit in grey/bright time, we will undoubtably see the effects of
   oblique scattering.  Here, dust and optical defects get illuminated
   obliquely by a gradient in sky brightness (eg. moon).  This will not be
   a major issue, but it can produce strange scatter patterns, reflections,
   ghosts, etc.  Though definitely not repairable, perhaps these effects can
   be detectable.

6/ Radial Wavelength Dependence:

   Another small effect will occur in the Ha, Stromgren v, and night sky gap filters.
   The narrow-band interference filters (eg. night sky gap filters: delta(lambda)~100A,
   or even the Stromgren v: delta(lambda)~250A) will essentially act as a Fabry-Perot
   between the front and back strata of their interference coatings.  In other words,
   there will be a wavelength dependence with radius.  We will measure this when
   Sagem delivers the filters.  They have promised a 0.2% relative wavelength accuracy
   across the field.  So, for one of the night sky gap filters centered, say, at
   8500A, we can expect a wavelength offset of about 20A from center to edge.  (This
   is relevant to line-emission object searches etc., and needs to be considered by
   these projects, but it could be argued whether this needs to be a pipeline issue).



Hope this is a start,

Cheers,

Mark



=====================================================
Dr. Mark J. Neeser

Institute for Astronomy and Astrophysics,       _/       _/     _/ _/_/  _/
Ludwig-Maximilians-Universitaet Muenchen       _/ _/  _/_/     _/ _/ _/	_/
                                              _/   _/  _/ _/  _/ _/  _/_/
Scheinerstrasse 1,                           _/       _/   _/_/ _/    _/
D-81679 Muenchen, Germany

Tel: +49-89-2180-5994
Fax: +49-89-2180-6003

email:  neeser@usm.uni-muenchen.de