This document describes the coordinate systems of interest to
LRIS users.
Pointing Origin Coordinate System (XIM, YIM)
This pointing origin coordinate system is the fundamental
coordinate system for the telescope focal plane and forms the
basis of the KSD 40 description of the isntrument focal planes.
The origin of the coordinate system lies on the optical axis of
the telescope. At zero rotator angle, the pointing origin Y
axis cooresponds to the telescope elevation axis. As viewed by
an observer sitting behind the focal plane and looking toward
the secondary mirror, the X axis is to the "right" and the Y
axis is "up". The symbols used to denote this axis are XIM and
YIM, both are expressed in mm by convention.
INST Coordinate System (INSTX, INSTY)
The LRIS coordinate system shares the same origin with the
pointing origin coordinate system, but is rotated by 90° so
that the INSTX axis corresponds to the direction of increasing
column number as seen on the LRIS image display, and INSTY to
direction of increasing row number. The conversion from
(XIM,YIM) to (INSTX,INSTY) is:
INSTX = YIM
INSTY = -XIM
LRIS Coordinate System (XLRIS, YLRIS)
The LRIS coordinate sytem is primarily used by the AUTOSLIT
software for designing slitmasks. This coordinate system shares
the same pointing origin with the pointing origin coordinate
system, but is rotated and flipped:
XLRIS = -YIM
YLRIS = XIM
The X axis goes through the center of the LRIS entrance
field. In this coordinate system, the slitmask covers the
region X=174 to 436 mm, and Y=-174 to 174 mm. Thus in this
coordinate system, the center of the slitmask is at X = 305.0
and Y=0.0 mm.
Mill Coordinate System (XMILL, YMILL)
The slitmask milling machine has a coordinate system which is
shifted and rotated relative to the LRIS coordinate system. The
origin is at XLRIS=172.7, YLRIS=177.8. The XMILL axis
corresponds to the -YLRIS direction (-XIM) and the YMILL axis
corresponds to the XLRIS direction (-YIM). Hence, the
conversion between the LRIS and MILL coordinate systems is:
XMILL = 177.8 - YLRIS
YMILL = XLRIS - 172.7
The conversion from XIM,YIM is:
XMILL = 177.8 - XIM
YMILL = -YIM - 172.7
Pixel Coordinate System (XPIX, YPIX)
This is the coordinate system of the LRIS image display. The
coordinate axes are in the same sense as the INSTX/Y coordinate
system, but an offset and scale factor apply in order to convert
focal plane measurements into pixel units:
XPIX = (YIM+308.10)/pscale + 1024
YPIX = (3.40-XIM)/pscale + 1024
where pscale = 0.1556 mm/pix (Note: mm is focal
plane mm). The origin of this coordinate system (pixel 0,0)
lies at XIM=162.7, YIM=-467.4.
Slitmask Center in Various Coordinate Systems
The following table provides the coordinates of the slitmask
center/detector center in various LRIS coordinate systems:
| Coordinate System |
Symbol |
X |
Y |
| Pointing Origin |
(XIM,YIM) |
3.40 mm |
-308.10 mm |
| LRIS |
(XLRIS,YLRIS) |
305 mm |
0 mm |
| Mill |
(XMILL,YMILL) |
177.8 mm |
132.3 mm |
| Detector |
(XPIX,YPIX) |
1024 px |
1024 px |