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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 Ly 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 Ly to be 1.43 ± 0.66 pixels (4 pixels = 1 diode), which translates to 0.356е ± 0.165е or 88 ± 41 kms-1. The 1 repeatability of the wavelength calibration is 0.159е or 0.64 pixels or 39 kms-1. Pre-flight measurements for the FOS blue detector showed that the filter-grating wheel repeatability was ~0.8 pixels which is similar to that observed in the present analysis (~0.6 pixels).

1. Introduction Pre-flight measurements of the FOS filter-grating wheel (FGW) repeatability showed that the FGW repeatability was of the order of 10 microns (10 y-bases units in Y and 0.2 diodes in X) in both the X and Y directions of the FOS blue detector (CAL/FOS-012, CAL/FOS-017, CAL/FOS049). This non-repeatability of the FGW leads to some photometric and wavelength calibration errors. The FGW repeatability has not been determined accurately since launch (a test is scheduled during cycle 4). An independent technique to determine the FGW non-repeatability at least in the dispersion direction (FOS X-direction) is to use the geo-coronal Ly. In this instrument science report 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 (35 observations of 17 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 geo-coronal Ly feature in each spectrum is determined by fitting a Gaussian to the line between 1214е and 1218е. The results of the fits are shown in Table 1. The measurement error in the position of the geo-coronal Ly is 0.02е (determined by fitting the line many times at different

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continuum levels). For objects with multiple exposures, the individual spectra are co-added and once again the location of the geo-coronal Ly determined as before. Table 2 shows the position of the geo-coronal Ly for each individual object used in the analysis. To calculate the zero-point wavelength offset, the vacuum wavelength for the geo-coronal Ly (1215.6701е) is used.

3. Results 3.1 Repeatability of the Wavelength Calibration from Consecutive Orbits Since some of the objects in our sample have multi-orbit data, we can investigate the repeatability of the wavelength calibration from orbit to orbit. Any scatter in the position of the geo-coronal Ly from orbit to orbit can be associated to measuring errors (of the order of 0.02е), and external effects associated with the changes in the magnetic field environment of the FOS detector. The scatter is independent of the FGW, since there was no motion of the FGW. From Table 1 column 6 we see that the repeatability of the position of the geo-coronal Ly has a 1 uncertainty of 0.243е and a range of -0.2 to +0.6е. Target reacquisition uncertainties do not affect the observed scatter since the geo-coronal Ly fills the aperture. Most of the scatter that we see in the wavelength calibration from consecutive orbits is likely due to changes in the magnetic environment of the FOS, although some of the scatter could be due to changes in the geo-coronal Ly velocity field. 3.2 Repeatability of the Wavelength Calibration The uncertainty of the wavelength calibration can be determined by investigating the repeatability of the position of the geo-coronal Ly. This 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 geo-coronal Ly is determined by comparing the location of the geocoronal Ly in each observation relative to a given observation. From Table 2 column 5 we see that the repeatability of the position of the geo-coronal Ly has a 1 uncertainty of 0.159е and a range of ±0.3е. This uncertainty translates to a 1 uncertainty 0.64 pixels (4 pixels = 1 diode) and a range of ±1.2 pixels for the G130H grating. This uncertainty is smaller than the uncertainty in consecutive orbits because of the averaging of the spectra. The FGW repeatability seen in this analysis is of the same order as 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 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 0.159е (0.64 pixels) with a range of ±0.3е (±1.2 pixels). 3.3 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 geo-coronal Ly 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 correc-

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tions applied in the pipeline. There is no contribution to the zeropoint offset from the external effects associated with target acquisition, because the geo-coronal Ly fills the aperture. The measurements of the geo-coronal Ly from column 3 of Table 2 show that the line center is offset from the vacuum rest wavelength position by -0.358е ± 0.165е. This offset translates to 1.43 ± 0.66 pixels, or 88 ± 41 kms-1. This uncertainty is a measure of the absolute accuracy of the FOS wavelength calibration for the G130H grating.

References Hartig, G. 1984, FOS Filter-Grating Wheel Repeatability, CAL/FOS-12, Hartig, G. 1985, Improvements in the filter-grating Wheel Repeatability, CAL/FOS-17 Hartig, G. 1988, FOS Filter-Grating Wheel Repeatability, CAL/FOS-49

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Table 1: Position of the geo-coronal Ly Prop ID 5339 5339 5346 5346 5346 5460 5460 5460 5460 5460 5379 5379 5379 5664 5664 5664 5664 5664 5664 5664 5664 5664 5723 5723 Object PK316+8D1 PK316+8D1 SK-69D43 SK-67D14 SK-69D228 BR93 BR93 BR93 HD32125 HD32125 3C279 3C279 3C279 OJ287 OJ287 OJ287 OJ287 OJ287 PKS1302-102 PKS1302-102 PKS1302-102 PKS1302-102 HD32402 HD32402 Dataset y25p0105t y25p0106t y25t0105t y25t0305t y25t0405t y2a10104t y2a10105t y2a10106t y2a10204t y2a10205t y2ey0603t y2ey0604t y2ey0605t y2ie0705t y2ie0706t y2ie0707t y2ie0708t y2ie0709t y2ie0f04t y2ie0f05t y2ie0f06t y2ie0f07t y2je0105t y2je0106t Line Center (е) 1216.023 1216.036 1216.100 1216.755 1216.047 1216.410 1216.148 1216.044 1215.691 1215.875 1215.820 1215.915 1215.787 1216.428 1216.059 1215.961 1215.981 1216.068 1216.089 1216.184 1216.122 1216.173 1215.920 1216.340 Line Width (е) 0.935 0.899 1.098 0.145 0.911 1.074 1.067 1.293 0.375 1.088 0.736 1.065 1.121 0.314 1.121 1.279 0.816 1.010 0.900 1.037 0.974 0.694 1.123 0.515 Line Shift (е) 0.000 0.013 N/A N/A N/A 0.262 0.000 -0.104 -0.184 0.000 0.000 0.095 -0.033 0.369 0.000 -0.098 -0.078 0.009 -0.095 0.000 -0.062 -0.011 0.000 0.420 definite grating shift observed faint line observed in y2a10204t no line observed Notes

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Table 1: Position of the geo-coronal Ly Prop ID 5723 5723 5723 5723 5723 5723 5723 Object HD37026 HD37026 HD37680 HD37680 HD32257 HD32257 HD32402 Dataset y2je0205t y2je0206t y2je0305t y2je0306t y2je0404t y2je0405t y2je0505t Line Center (е) 1216.001 1216.274 1216.251 1216.056 1215.245 1215.874 1216.185 Line Width (е) 0.845 1.054 0.524 0.627 0.372 0.402 1.218 Line Shift (е) 0.000 0.273 0.000 -0.195 0.000 0.629 N/A 0.243 Notes definite grating shift observed faint line observed in both observations very faint line observed in each observation, definite grating shift observed

Standard Deviation

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Table 2: Position of the geo-coronal Ly Prop ID 5339 5346 5346 5460 5460 5379 5664 5664 5723 5723 5723 5723 5723 Object PK316+8D1 SK-69D43 SK-69D228 BR93 HD32125 3C279 OJ287 PKS1302-102 HD32402 HD37026 HD37680 HD32257 HD32402 Average Standard Deviation Line Center (е) 1216.030 1216.100 1216.047 1216.184 1215.835 1215.824 1216.060 1216.121 1216.271 1216.118 1215.853 1215.732 1216.185 1216.028 0.165 0.159 Line Width (е) 0.910 1.098 0.911 1.102 1.188 0.975 0.985 0.905 0.834 0.837 1.175 1.077 1.218 Internal Repeatability (е) 0.000 0.070 0.017 0.154 -0.195 -0.206 0.030 0.091 0.240 0.088 -0.177 0.298 0.155

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