8.8 Centering Accuracy and Data Quality

A centering accuracy of 0.3 arcsec in the cross-dispersion (XD) direction is required to achieve optimum photometric accuracy and spectral resolution. In the along-dispersion (AD) direction, the minimum accuracy is set by velocity requirements: ±15 km/s for the medium-resolution modes, ±150 km/s for G140L, and ±175 km/s for G230L. Since the AD requirements are in units of km/s, they are grating and wavelength dependent. Assuming that the wavelength error budget is split evenly between the COS TA and wavelength scale accuracy, the strictest pointing requirements are ±0.041 arcsec for the NUV channel and ±0.106 arcsec for the FUV channel.

Note that a poorly-centered object will result in a distorted image. This is a consequence of multiple factors including the effects of vignetting, and that aberration correction is done after the aperture. An additional possibility is that the wavelength scale may be distorted when a target is close to the edge of the aperture, however this has not been quantified.

8.8.1 Centering Accuracy and Wavelength Accuracy

To achieve a wavelength accuracy of ±15 km/s, the target should be centered to within about 0.04−0.07 arcsec for NUV observations and 0.1−0.2 arcsec for FUV observations. The throughput of COS is not affected by centering errors of less than 0.4 arcsec, so high centering precision is not strictly necessary if science goals do not require that the wavelength zero point be well constrained. For example, the spectra of some objects may include foreground interstellar or inter-galactic absorption lines that can be used to establish the zero point of the wavelength scale.

8.8.2 Centering Accuracy and Photometric Precision

Figure 8.5 shows the relative transmission of the PSA as a function of the displacement of a point source from the aperture center, as measured using each of the four NUV gratings. These and the corresponding FUV curves are nearly identical and show that the transmission of the COS apertures is essentially flat within the central ±0.4 arcsec, then tails off in a non-linear but approximately symmetrical profile (COS ISR 2010-09).

8.8.3 Centering Accuracy and Spectroscopic Resolution

Targets placed within 0.3 arcsec of the aperture center will achieve maximum spectral resolution. Centering errors larger than 0.3 arcsec will lead to progressively poorer resolution. Targets at the edge of the aperture have approximately half the spectral resolution of well-centered targets.