1.4 COS Coordinate System

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.

The three coordinate systems of interest are the:

• 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 (X_user, Y_user): the frame associated with a pipeline science image. It is aligned with the detector, where the wavelength increases along +X_user.
• Detector Frame (X_detector, Y_detector): 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 (X_POSTARG, Y_POSTARG): 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.

The angles associated with these frames that appear in the headers of COS data files are:

• 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 Y_POSTARG measured at the aperture reference (in the science header).
• ORIENTAT: The angle from North through East to Y_user measured at the aperture reference (in science header). For COS, PA_APER and ORIENTAT are equal, i.e., Y_POSTARG = Y_USER. 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.

Refer to ISR TEL 2008-02 for a complete discussion of the COS reference frame geometry.

Detector coordinates are related to user coordinates as follows:
 
NUV channel: X_user = 1023 −Y_detector; Y_user = 1023 − X_detector
FUV channel: X_user = 16383 − X_detector; Y_user = Y_detector