SRE-05-12
12/5/2012
VIRS Calibration Changes 
Prepared by: Rachel Klima & the MASCS team

1. Introduction

This report describes calibration changes implemented to simplify the 
handling of dark subtractions for VIRS, and an adjustment to the 
DATA_QUALITY_INDEX provided in the data records. All data have been updated 
to use the latest calibration and will be included in PDS release 9.

2. Calibration Changes: 
2.1. Dark Subtraction Algorithm
* The default dark current fitting algorithm now models the dark current as 
a function of temperature instead of time for all cases except for extremely 
short (less than 1.5 minute) data records. Previously, only data records 
with 25 or more dark observations were fit as a function of temperature. The 
rationale for fitting the short data records as a function of time is that 
the temperature variations over such short timescales are not captured with 
the precision necessary to model the dark current as a smooth function of 
temperature.

* The dark polynomial fitting algorithm has been changed from POLY_FIT to 
SVDFIT and now incorporates the thermal noise estimation, as derived from 
dark quick-cal measurements, as measurement errors during fitting. This 
provides a smoother fit to the dark solution that is less sensitive to 
noise, particularly on short data records.

2.2. DATA_QUALITY_INDEX
* An index term was added to the DATA_QUALITY_INDEX to warn users when the 
exclusion of saturated dark measurements has resulted in three or fewer dark 
observations being used in the polynomial fit for one or more pixels. The 
DATA_QUALITY_INDEX is of the form ABCD-EFGH-IJKL-MNOP. Digit N now displays 
a '0' when all dark fits in the CDR were calculated using at least four 
shuttered dark observations, and a '1' when one or more pixels were modeled 
using three or fewer shuttered dark observations.