SRE-01-13
11/22/2013
VIRS Calibration Changes 
Prepared by: MASCS/VIRS instrument team

1. Introduction

This report describes changes to the calibration procedure that have been 
implemented to remedy an observed shift in the wavelength scale of the VIRS 
visible channel. All data have been updated to include the new wavelength 
scale in the latest calibration and will be included in PDS release 11.

2. Calibration Changes: Wavelength Shift (visible detector only)
- An analysis of the accuracy of the wavelength calibration has been 
conducted using the location of a prominent solar Fraunhofer (absorption) 
line. The Hydrogen alpha absorption occurs at a wavelength of 656.3 nm, and 
the relative position of this feature was tracked in radiance data 
throughout the mission (Fig. 1). 
- There is a systematic shift in the mean position of the absorption feature 
in September of 2011. This shift does not appear to correlate with detector 
temperature, and is beyond the normal periodic variability observed (which 
may be a function of temperature).
- In the early portion of the mission this feature is measured to occur most 
often at a wavelength position of 656.7 nm, shifting to 655.6 nm after 
September 2011 (Fig. 2).
- On the basis of this analysis, CDR processing has been adjusted to import 
a corrected wavelength vector based on the day of year on which the 
observations were obtained.  There are two distinct wavelength scales 
relevant for the data obtained before and after Sept 1, 2011.  The team will 
continue to search for correlations between the wavelength scale and 
environmental factors, and to monitor the data for subsequent events.

Fig. 1. The top row shows the absorption band center (based on the fit of a 
Gaussian profile) as a function of date (left) and detector temperature 
(right) and the bottom row shows the absorption band width as a function of 
date and detector temperature. The colors represent bins of time throughout 
the mission.  As can be seen, there is some inherent scatter in both band 
center and width, but it is not uniquely correlated with temperature.

Fig. 2. This plot shows the distribution of measured wavelength positions 
for the 656 nm solar feature.  The right, smaller distribution represents 
the data obtained before Sept 2011 while the left, larger distribution 
represents the data obtained after.

See PDF version of document for figures.