Sloan Digital Sky Survey Telescope Technical Note 19981113
It is necessary to specify the coordinate system for the drilling of plug-plates so that the drilling file (currently named plPlugMap), plate drilling, cartridge, rotator wrap and the rotator itself are compatible.
This discussion is based on a phone meeting on Nov 12, 1998 that included Pat Waddell, Walt Siegmund, Russ Owen, Bob Nichol, Aronne Merrelli, and Steve Kent. A ruling guideline is that we wanted to use existing conventions as much as possible. We also discussed the conventions used to define the rotator angle.
The plate design is specified by orthogonal x and y coordinates defined such that +y is the north direction (increasing declination) and +x is the east direction (increasing right ascension). [NOTE: In the presence of refraction, RA and Dec are not necessarily orthogonal when projected on the plate. In this case, the +y direction is defined to be exactly north, and the +x direction is oriented to be easterly trending.] This system is referred to as the TCC Focal Plane system (a.k.a. the milling machine convention) and is indicated on Figure 1.
The TCC rotator angle (object system) is defined to be 0 degrees for this orientation. [NOTE: The TCC instrument block mechanism allows one to deviate from these conventions; however, we shall assume that the instrument block is set up so that these conventions are followed.]
The x,y system is right-handed when one views the sky side of the plate. This is the side from which the milling machine drills. The y axis is the OPPOSITE of the FNAL/Princeton y axis convention for the CCD camera. The handedness of x and y is also opposite. The handedness of the plates is also flipped from the x,y system used to define the optical support system. Caveat emptor.
When the TCC rotator angle increases, the direction of increasing RA rotates from the x axis to the y axis. At 90 degree angle, the y axis points east. Thus, the rotator angle gives the conventional astronomical position angle on the sky of the y axis (although Russell won't acknowledge this necessarily).
The plate identification number is engraved onto the plate along the -y axis. At the instrument exchange position, the rotator is at the midpoint of its range of travel: it can rotate +/- 270 degrees from that point. [THIS CAN BE CHANGED]
At the instrument exchange position, the camera is mounted and oriented such that if one points the telescope to the horizon without moving the rotator, then the camera leading array (and r' CCD rank) is DOWN. This means that the +y axis for the camera is also pointing down. When viewing the telescope from the back, the rotator needs to rotate 90 degrees counterclockwise to reach the proper orientation for drift scanning. This corresponds to increasing the position angle of the y axis as projected on the sky. Thus, for the camera, we would like to define the instrument exchange position as having a position angle of 0 degrees.
At the instrument exchange position, the cartridges are mounted and oriented such that if one points the telescope to the horizon without moving the rotator, then the engraved plate id number is down, or the +y axis is pointing up. Thus, cartridges and the camera have the OPPOSITE convention r.e. +y axis when mounted on the telescope. For the spectrographs, the instrument exchange position has a position angle of 180 degrees.
PROBLEM: The instrument wrap is now implemented such that the midpoint of the rotator wrap occurs when the rotator is at a position angle of 180 degrees. We would like the midpoint to be centered at a position angle of 90 degrees. This means that if we view the telescope from the back with the telescope pointed at the horizon and the rotator oriented at its current midpoint position (which also coincides with its orientation at the instrument exchange position), then we would like to turn the rotator counterclockwise by 90 degrees and reinstall the wrap so that its center of travel is at this new orientation.
Figure 1: Plug-plate drilling sky side view. The x and y axes are shown. With the rotator angle set to zero, +y is in the direction of increasing declination and +x is approximately in the direction of increasing right ascension. A CAD file, plugplate33.dxf, is available.
Date created: 11/13/98 Last modified: 11/18/98 Copyright © 1998, Walter A. Siegmund Walter A. Siegmund
