Chapter 37
GHRS Error
Sources

A GHRS spectrum has several dimensions, each with associated error and uncertainty. The two most obvious dimensions are flux and wavelength, which we plot to show the spectrum. But time is also a dimension of the spectrum, in the sense of when the spectrum was obtained, how long the exposure lasted, and the extent of interruptions of that exposure. Spatial dimensions-the positioning of the object within the aperture-are also relevant. Finally, there are instrumental and spacecraft errors that influence the data.

In Table 37.1, we provide a summary of the accuracies you can expect from your data when you have recalibrated it with the latest calhrs software and the best reference files (as recommended by StarView's Best Reference File screen-see Chapter 1 in Volume 1), assuming your target acquisition was successful and appropriate for your science slit, and, for photometry, assuming a wide aperture was used. In some cases improvements can be made further by using the calibration information and post observation refinement tasks described in this Data Handbook-these cases are noted in the table.

Summary of GHRS Errors and Uncertainties¹

Attribute	Accuracy	Limiting Factors
Flux, absolute	5%	Confidence in absolute flux scale
Flux, relative: same star, same wavelength	2-4%2	Grating at same position for all observations
Flux, relative: within an exposure	2 to 4%	2% typical, 4% worst case (G140L, 100 Å away)
Flux, relative: different setups	4%	2% typical; vignetting, sensitivity determinations
Flux, relative: pre-COSTAR	4%	Centering in LSA, poor PSF
Wavelengths, relative: within an exposure	<0.1 diode	Dispersion highly reliable
Wavelengths, absolute: default wavelength calibration	1.1 diodes	Assumes worst case from Table 37.8
Wavelengths, absolute: use of SPYBAL	0.4 diode	GIM effects and carrousel uncertainty
Wavelengths, absolute: use of wavecal	0.3 diode	GIM effects

1 See page 37-17 for applicability to G270M. 2 Can vary to 4% at shortest wavelength. For side 2, can be improved by correction for time-dependent -variations, as in Figure 37.3.

In this chapter we will discuss each of the dimensions affecting the accuracy of your data in turn and examine what is known about them from the experience gained since the GHRS started operation.