HST Data Handbook for COS

1.4 COS Coordinate System

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References to multiple coordinate systems appear in the headers of COS data. These are tied to the OTA frame, the User frame, and the POSTARG frame. The following is a brief explanation of how these systems (shown in Figure 1.10) are related.

The three coordinate systems of interest are the:

• 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 metric is arc seconds.
• User (or IRAF) Frame (X_user, Y_user): The frame associated with an OPUS undrizzled science image. It is aligned with the detector.
• POS-TARG Frame (X_POSTARG, Y_POSTARG): This is 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 TEL2008-02 for a complete discussion of the COS reference frame geometry.

Figure 1.10: COS Coordinate Systems