[News] QC Photometry kick-off email

[Ronald Vermeij]

Dear all,

 I hereby resend the email about the QC Photometry that was send earlier to Mark and 
 Roberto.

Regards,

 Ronald Vermeij

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Dear Roberto and Mark,

 As was agreed in our telecom of 1 July, I send you a small list of QC measures that are 
 in place/defined in the photometric software as it stands today. Also included are some 
 suggestions for any other QC measures. Not all QC measures are visible to the user or 
 have a QC flag associated with them.

 Note that color terms are not mentioned here, because deriving these falls outside the 
 scope of the automatic photometric pipeline. Also note that because the photometric 
 pipeline runs on images that have been (pre-)processed by the image pipeline, that 
 quality issues regarding the flatfield/image pipeline also play a role. 

 I)   Catalog creation:

      1) Correctly identifying standard stars:

         Depends on : proper astrometric calibration, quality of catalogued coordinates
                      of standard stars. 

         Currently, the robustness of identification is improved by removing artifacts 
         from the catalog before association with the standard star catalog (hot pixels).
         Whenever possible the catalogued coordinates of the standard stars are also 
         improved.

      2) Removing sources that are `unfit for purpose':   
                                             
         Before association, saturated sources, blended sources, and sources with clipped
         or otherwise de-formed apertures are removed from the catalog. This way, `rotten'
         standard stars never enter the photometric pipeline. This measure also helps to 
         improve the robustness of the association process.   

      Suggestions for improvements/points of concern:

          How to monitor and influence the quality of the aperture photometry ? Can any
          meaningful QC scheme be defined for this ? Curves-of-growth ? It will involve 
          heavy manipulation of SExtractor. Suggestions are welcome.

 II)  Atmospheric extinction:

      1) In daily routine, the atmospheric extinction will be dealt with by the monitoring
         requirement of the OmegaCAM pipeline (req 562). This requirement produces a report
         containing a measurement of the extinction for every image of the polar field that 
         goes into making it (at least three). This measurement is obtained by fitting a 
         standard extinction curve to the data that has been simultaneously observed in 
         every one of the four key-bands (composite filter). 

         The QC measures/flags in place are:

	    - the measured zeropoints for all the stars observed in one kwadrant of the 
              composite filter are clipped for outliers before a single value for the
              extinction is determined from these. This clipping will also remove any 
              sources from the pipeline that have mistakenly been identified as a standard 
              star. After determining this value, the result is checked against a standard 
              extinction curve. Raising a QC flag at this point is only a first warning 
              that something fishy might be going on with the atmosphere. This QC warning 
              is only send to the log.
            - when for all the four kwadrants of the composite filter an extinction has 
              been derived, an extinction curve is fit to the four points to derive the
              ultimate extinction for that particular moment in the night. The fit 
              parameters are send to the log for perusal.   
            - after having processed all the polar images in the same way, an assesment 
              is made of the quality of the night (QC = SHIFT_OF_CURVE_VARIED), of the 
              stability of the overall transmission properties of the atmosphere 
              (QC = SHAPE_OF_CURVE_VARIED), and of the validity of the assumed shape of
              the standard extinction curve (QC = INVALID_ASSUMPTION_FOR_SHAPE_OF_CURVE).
	      These QC-flags are stored in the DB together with the separate values for
              the extinction through the night, and the fitted shifts of the extinction
              curve.


	 Suggestions for improvements/points of concern:

	     Although QC-flags have been defined, the software that checks on the anomalies
             must be written. A set of criteria on the basis of which the QC-flags are 
             raised has not fully solidified yet.

 III) Zeropoint:

      1) The zeropoint for a given chip/filter combination is derived by combining the 
         extinction results from req 562 with the raw zeropoints contained in the catalog
         for that particular chip/filter combination. Note that these catalogs are derived
         from equatorial standard field data. The error bar on the resulting zeropoint serves
         as a QC parameter; this number should be within the specs as given in the requirements. 
         For further QC, a comparison can be made with the zeropoint derived the day before. 
         (Remember that any color terms that might be needed have already been taken care of 
         in the catalog creation stage.)  

	 Suggestions for improvements/points of concern:
             
             Clipping of the input data should also be done here. It might be interesting 
             to make a residual plot for validation. Systematic off-sets from the zero-line
             could invalidate the photometric calibration. A residual slope is a sign that
             the color-terms determined independently in req 565 need revision. The scatter
             is an indicator for the quality of the illumination correction and the 
             flatfielding. 

 This is it for now. Hope to hear from you.

Regards,

 Ronald Vermeij

Astro-Wise
Kapteyn Astronomical Institute
Groningen, The Netherlands