NICMOS orientations are specified relative to the +y axis shown in Figure 6.1. Eastward rotations are counterclockwise (in the usual astronomical convention). Spacecraft orientations are specified relative to the U2-U3 telescope axis (
Figure 6.2). The NICMOS coordinate system is rotated by approximately 225 degrees from U3 axis. The exact angles for NIC1, NIC2, and NIC3 are 225.32, 224.57, and 224.91
degrees, respectively.
Due to the linear arrangement of the three NICMOS cameras on the sky, it may be advantageous to consider the specification of a unique telescope orientation. However, observers should be aware that such constraints may decrease the duration and number of scheduling opportunities for their observations and, under some circumstances, may make the identification of suitable guide stars impossible.
While the Phase II proposal instructions contain the definitive instructions and examples for specifying the desired orientation for HST, we provide a simple example in
Figure 6.2. A binary star with a position angle (PA) 30
° measured east from north is to be positioned with the southern star in Camera 3 and the northern star in Camera 2. That is, we want the line connecting the two stars to lie along the NICMOS + y axis. The resulting HST orientation is 225
° + 30
° = 255
°. (
HST ORIENT = PA + 225
° for NICMOS).
