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Hubble Space Telescope
INSTRUMENT SCIENCE REPORT FOC-052

INSTRUMENT SCIENCE REPORT  FOC-052		December 19, 1991

"How to get the North-East Orientation of an FOC frame"

By: W. Hack, A. Nota, P. Hodge, R. Jedrzejewski

Abstract excerpted from the report:

This report outlines the basic procedure for determining the North-East
orientation of any FOC frame and provides examples. This paper discusses 
also the header keywords which report the orientation of the image, and then
describes how and when these keywords have been changed. The first section
explains the coordinate systems involved in understanding the orientation of
a frame with respect to North and outlines the basic procedure to transform
from image coordinates to celestial coordinates. The second section discusses
more in details these transformations and all the variables involved. The
third section provides examples illustrating how to determine the North-East
orientation from the keywords in image headers and what to look for in 
assuring the consistency of the results.

The examples are given separately for images taken before 25 January
1991, between 25 January 1991 and 11 November 1991, and then for images 
taken after 11 November 1991 (which best describes the current state of 
understanding of the accuracy of the parameters involved).

Each set of examples should serve as a model for users to determine
the orientation of their own images depending on the time of the observation.

This report is intended to be an exhaustive description of our present
knowledge of the orientation of the FOC apertures on the sky. The information
provided is complete and detailed, but the report can also be used as a quick
guide to "finding your image orientation" by simply checking the date your
image was taken, and the mode, and then reading the relevant example.  In
general, the quickest way is to use the standard header packet keyword
PANGAPER (known as PA_APER after 11 November 1991). For non-ZOOM'ed images,
the value of this keyword gives the angle you would rotate the image in a
counter-clockwise direction to align the image's Y-axis with North. For 
images that use the ZOOM format, you must subtract 90 degrees from PANGAPER.