
8.2 Definitions
8.2.2 Pointing origin coordinate system
- The pointing origin XY coordinate system is a 2D system fixed in the focal plane, with its origin on the rotator and elevation axes (assumed coincident). Note that it should not in fact be necessary to align these two axes since the telescope has a pointing term which defines the (vector) offset between the two axes and will thus correct for the misalignment.
- Points in this coordinate system are conventionally referred to as (XIM,YIM), where "IM" refers to "IMage plane". This coordinate system is used for specifying locations in the focal plane; coordinates are specified in meters[24].
- The coordinate system orientation is defined by reference to the acquisition camera: XIM and YIM are aligned with its rows and columns. There are no rules stating whether XIM is aligned with rows or columns or which way up the detector should be (Keck I instruments already use three of the four possibilities) and we can mount the acquisition camera in whatever orientation we choose.
- We can take Figure 8-1 on page 8-2 and transform it so that it now represents the view from the acquisition camera towards the sky. Assuming that there are no extra reflections between the acquisition mirror and the acquisition camera, the acquisition camera beam encounters eleven reflections (3 in the rotator, tip/tilt mirror, OAP #1, DM, OAP #2, IR dichroic, sodium dichroic, fixed mirror, acquisition mirror). Thus the handedness is flipped (to give conventional sky orientation!) and we get Figure 8-2 on page 8-4.
FIGURE 8-2. Telescope coordinate systems as viewed from AO acquisition detector
- The original picture has not only been flipped but it has also been rotated so that (XIM,YIM) look like conventional cartesian coordinates.
- As can be seen from the diagram, zero rotator angle is defined so that, at zero elevation, the (XIM,YIM) axes are aligned with the (Az,El) directions respectively. A calibration procedure to define the zero will have to be specified.
[24] They can also be specified in millimeters for user convenience. Sometimes arc seconds are convenient too (the conversion factor is simply the focal length of the telescope). However meters (or millimeters) are preferred since they emphasize the fact that this coordinate system describes actual locations in the focal plane.
AO Software Design Book - 25 FEB 1997

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