|COS Data Handbook v.4|
References to multiple coordinate systems appear in the headers of COS data. These are tied to the OTA frame, the User frame, and the POS-TARG frame. The following is a brief explanation of how these systems (shown in Figure 1.11) are related, and a more thorough explanation can be found in the Phase II Instructions.
• OTA or “V” Frame (V1, V2, V3): the common coordinate system for Scientific Instruments and the FGSs. It is a distortion-free frame whose unit is arc seconds.
• User Frame (Xuser, Yuser): the frame associated with a pipeline science image. It is aligned with the detector, where the wavelength increases along +Xuser.
• Detector Frame (Xdetector, Ydetector): the frame associated with the detector, which is related to the User Frame and the size of the detector. The relation between the two is provided below.
• POS-TARG Frame (XPOSTARG, YPOSTARG): a distortion-free frame with units of arc seconds. Its origin coincides with the science aperture and its axes are closely aligned with the user frame.
• PA_V3: The position angle of the V3 axis; the angle from North, towards East, to V3, measured at the center of the HST focal plane (in the spt header).
• ROLL_AVG: The average angle from North towards East to V3, measured at the position of the COS field in the HST focal plane (in the jit header, computed).
• PA_APER: The angle from North through East to YPOSTARG measured at the aperture reference (in the science header).
• ORIENTAT: The angle from North through East to Yuser measured at the aperture reference (in science header). For COS, PA_APER and ORIENTAT are equal, i.e., YPOSTARG = YUSER. Note that this is not the same angle as the ORIENT specified in Phase II, which gives the position angle of the U3 axis, where U3 = –V3.NUV channel: Xuser = 1023 −Ydetector ; Yuser = 1023 − XdetectorFUV channel: Xuser = 16383 − Xdetector ; Yuser = YdetectorFigure 1.11: COS Coordinate Systems