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REPEATABILITY OF THE FOS G130H GRATING
EXTERNAL WAVELENGTH ZEROPOINT
ANURADHA KORATKAR and IAN EVANS
Space Telescope Science Institute,3700 San Martin Drive, Baltimore, MD 21218
Abstract. Measurements of the position of the geo­coronal Lyff are used to determine the repeata­
bility of the G130H grating and the external wavelength zeropoint for the first time since launch.
From 13 observations with the 0.3 aperture we find the offset from the rest wavelength (vacuum) of
the geo­coronal Lyff to be 1.43\Sigma0.66 pixels (4 pixels = 1 diode), which translates to 0.356\Sigma0.165 š A or
88\Sigma41 kms \Gamma1 . The 1oe repeatability of the wavelength calibration is 0.159 š A (0.64 pixels or 39 kms \Gamma1 ).
Pre­flight measurements for the FOS blue detector showed that the filter­grating wheel repeatability
was ¸0.8 pixels which is similar to that observed in the present analysis (¸0.6 pixels).
Key words: FOS -- Wavelength Calibration
1. Introduction
Pre­flight measurements of the FOS filter­grating wheel (FGW) repeatability showed
that the FGW repeatability was of the order of 10 microns (12.8 y­base units in
Y and 0.2 diodes in X) in both the X and Y directions of the FOS blue detector
(CAL/FOS­12, CAL/FOS­17, CAL/FOS­49). This non­repeatability of the FGW
leads to photometric and wavelength calibration errors. The FGW repeatability has
not been determined accurately since launch. An independent technique to determine
the FGW non­repeatability at least in the dispersion direction (FOS X­direction) is
to use the geo­coronal Lyff. We have analyzed all G130H grating observations using
the 0.3 aperture obtained so far in cycle 4 to estimate the FGW repeatability in the
dispersion direction and the errors in the wavelength calibration.
2. Data Analysis
All cycle 4 observations (31 observations of 13 independent objects) obtained so
far which used the G130H grating and the 0.3 aperture were used in the following
analysis. The location of the geo­coronal Lyff feature in each spectrum is determined
by fitting a Gaussian to the line between 1214 š A and 1218 š A. The measurement
error in the position of the geo­coronal Lyff is 0.02 š A (determined by fitting the
line many times at different continuum levels). For objects with multiple exposures,
the individual spectra are co­added before the location of the geo­coronal Lyff is
determined. Table 1 shows the position of the geo­coronal Lyff for each individual
object used in the analysis. To calculate the zero­point wavelength offset, the vacuum
wavelength for the geo­coronal Lyff (1215.6701 š A) is used.

A. KORATKAR AND I. EVANS
TABLE I
Position of the geo­coronal Lyff
Prop ID Object Line Center Line Width Internal
Repeatability
( š A) ( š A) ( š A)
5339 PK316+8D1 1216.030 0.910 0.000
5346 SK­69D43 1216.100 1.098 0.070
5346 SK­69D228 1216.047 0.911 0.017
5460 BR93 1216.184 1.102 0.154
5460 HD32125 1215.835 1.188 ­0.195
5379 3C279 1215.824 0.975 ­0.206
5664 OJ287 1216.060 0.985 0.030
5664 PKS1302­102 1216.121 0.905 0.091
5723 HD32402 1216.271 0.834 0.240
5723 HD37026 1216.118 0.837 0.088
5723 HD37680 1215.853 1.175 ­0.177
5723 HD32257 1215.732 1.077 0.298
5723 HD32402 1216.185 1.218 0.155
Average 1216.028
Standard Deviation 0.165 0.159
3. Results
3.1 Repeatability of the Wavelength Calibration
The uncertainty of the wavelength calibration can be determined by investigating the
repeatability of the position of the geo­coronal Lyff. The uncertainty of the wavelength
calibration is mostly due to internal effects associated with grating wheel movement
and residual magnetic field errors. It is very difficult to separate the effects of the
grating wheel from the effects of the residual magnetic field to understand the exact
source of the wavelength calibration uncertainties. The repeatability of the position
of the geo­coronal Lyff is determined by comparing the location of the geo­coronal
Lyff in each observation relative to a given observation. From Table 1 column 5 we
see that the repeatability of the position of the geo­coronal Lyff has a 1oe uncertainty
of 0.159 š A and a range of \Sigma0.3 š A. This uncertainty translates to a 1oe uncertainty of
0.64 pixels (4 pixels = 1 diode) and a range of \Sigma1.2 pixels for the G130H grating.
The FGW repeatability seen in this analysis is of the same order as was observed
during the preflight measurements. Since all the gratings are on the same grating
wheel, they are likely to have similar repeatability in pixel units. However, to check
this a proper FGW repeatability test has to be done, since the individual mechanical
Calibrating HST: Post Servicing Mission

REPEATABILITY OF THE FOS G130H GRATING
indents associated with the gratings may have different wear histories. Thus, the
relative uncertainty of the FOS wavelength calibration from the present analysis of
the G130H grating observations is 0.159 š A (0.64 pixels) with a range of \Sigma0.3 š A (1.2
pixels).
3.2 The Zeropoint Offset of the G130H Grating
The zeropoint offset of the G130H grating can be determined by comparing the
observed average position of the geo­coronal Lyff with the vacuum rest wavelength of
the line. This offset is mostly due to the errors in the wavelength dispersion solution
and the internal and external offset corrections applied in the pipeline. There is no
contribution to the zeropoint offset from the external effects associated with target
acquisition, because the geo­coronal Lyff fills the aperture. The measurements of the
geo­coronal Lyff from column 3 of Table 1 show that the line center is offset from
the vacuum rest wavelength position by ­0.358 š A\Sigma 0.165 š A. This offset translates to
1.43\Sigma0.66 pixels, or 88\Sigma41 kms \Gamma1 . This uncertainty is a measure of the absolute
accuracy of the FOS wavelength calibration for the G130H grating.
References
Hartig, G.: 1984, `FOS Filter­Grating Wheel Repeatability', CAL/FOS 12,
Hartig, G.: 1985, `Improvements in the filter­grating Wheel Repeatability', CAL/FOS 17,
Hartig, G.: 1988, `FOS Filter­Grating Wheel Repeatability', CAL/FOS 49,
Calibrating HST: Post Servicing Mission