SRE-02-12
1/13/2012
UVVS Calibration Changes for PDS Delivery 7
Prepared by: Aimee Merkel (LASP) Ron Vervack, Noam Izenberg and the 
             MASCS team


   Removal of solar background and dark counts in the MASCS UVVS data 
retrieval algorithm was refined in the latest PDS delivery. The UVVS data 
have both a dark component and a solar scattering component to the 
background signal. In daylight conditions, these two terms add together. In 
previous deliveries the sum of these two terms was estimated by examining 
the counts in the first and last elements of an observation, which contain 
no atmospheric emission. By isolating the effect of each separately with a 
new approach we are able to more effectively remove these sources of 
background. 
1.  An empirical relationship between the temperature of the photomultiplier 
tube (PMT) and the dark counts was determined using high-solar-zenith-angle 
observations that contain only dark counts. This relationship is used to 
provide a dark count estimate for all observations, regardless of sunlight 
conditions.  
2.  An improvement was made to the removal of solar scattering contamination 
that enters the field of view of the UVVS directly or from solar scattering 
off Mercury's surface. We fit the shape of each spectral scan (per observed 
species) to a solar spectrum in the specified wavelength region. The fit 
effectively scales the magnitude of the solar contamination at the pertinent 
emission lines. This is an improvement over the previous solar background 
subtraction. However, low signal-to-noise values appear in some of the 
subtracted spectra due to thermally induced shifts in the UVVS wavelength 
scale.  This will be mitigated in an upcoming release.

   The solar background subtraction routine is not applied to the cruise 
phase data. It is currently only applicable in the VIS channel. A correction 
scheme is in development for the MUV channel. A number of VIS macros are 
excluded from passing through the solar routine. Macros excluded are:

Instrument/maintenance macros:  1, 2, 3, 4, 8 
Calibration macros:  5, 6, 7, 12, 20 
Surface scans: 24, 48, 49, 50, 51 
Long "Discovery" scans.   76, 77, 78, 79, 80, 81, 82, 83