COS Instrument Handbook for Cycle 24

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13.6	The COS User Coordinate System

Figure 13.30 presents a schematic layout of the COS focal plane. In this figure, the x and y axes denote the COS user coordinate system. In this system, x lies along the wavelength (dispersion) axis and increases with increasing wavelength for both the FUV and NUV channels. For the NUV channel, y increases with increasing wavelength in the cross-dispersion (XD) direction. All references to COS (including POS TARG specifications in APT, the Astronomer’s Proposal Tool, detector pixel coordinates, and science header keywords) employ the user coordinate system.

Figure 13.30: Schematic Layout of the COS Detectors

This view is from the front of the telescope looking aft. The dashed arrows show the direction of increasing wavelength for the two detectors, and “XD” indicates the direction of increasing wavelength for the NUV cross-dispersion direction. The x and y axes denote the COS user coordinate system. For both the FUV and NUV channels, wavelength increases in the +x or (+V2, –V3) direction. Note that this diagram is purely schematic and it is intended to show only relative directions. This diagram does not show the locations of apertures. The bottom corner of this square (at V1) corresponds to the center of the WF3 camera (see Figure 13.29).