13.13 COS Apertures

Valid COS apertures are listed in Table 13.2 below. Science observations should use the PSA or BOA apertures. The BOA aperture is the same size as the PSA but contains a neutral-density filter that attenuates the source by a factor of ~200. The WCA is intended for wavelength calibrations using the WAVE target.

Table 13.2: COS Apertures

Aperture	Definition	Size	Usage
PSA	Primary Science Aperture	2.5 arc-seconds	Default science aperture
BOA	Bright Object Aperture	2.5 arc-seconds	Contains neutral density filter (ND2) for bright objects
WCA	Wavelength Calibration Aperture	20 microns (dispersion) x 100 microns (cross-dispersion)	Wavelength Calibrations