Repeatability of the FOS G130H Grating External Wavelength Zeropoint





Anuradha Koratkar and Ian Evans

Space Telescope Science Institute





Instrument Science Report CAL/FOS-142

May 1995







Abstract



Measurements of the position of the geo-coronal Lya are used to determine the repeatability of 
the G130H grating and the external wavelength zeropoint for the first time since launch. From 10 
observations with the 0.3 aperture we find the offset from the rest wavelength (vacuum) of the 
geo-coronal Lya to be 1.43  0.66 pixels (4 pixels = 1 diode), which translates to 0.356Å  
0.165Å or 88  41 kms-1. The 1s repeatability of the wavelength calibration is 0.159Å or 0.64 
pixels or 39 kms-1. Pre-flight measurements for the FOS blue detector showed that the filter-grat-
ing wheel repeatability was ~0.8 pixels which is similar to that observed in the present analysis 
(~0.6 pixels).