This figure shows the NICMOS coordinate system projected onto the sky for the
POSition
TARGet Special Requirement. The
POS TARG coordinate system will be aligned parallel to rows and columns in each camera as shown in the diagram above. The alignment of each camera is not exact, and the internal coordinate systems attached to each of them will differ by small rotations (probably less than 2 degrees). The FITS format data files generated for NICMOS observers will have a World Coordinate System specified appropriately for each camera. The origin of the coordinate system will be located as shown in the diagram above.
Due to the linear arrangement of the three NICMOS cameras on the sky, it is sometimes advantageous to specify a unique telescope orientation. A simple example is shown above. A binary star with a position angle (PA) of 30 degrees 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 degrees. (The NICMOS offset angle for orientation specifications is 225 degrees; see
Table 7.4: Instrument Orientations with respect to U3-Direction.)
The above graph shows the probability that guide stars will not be available (“failure rate”) as a function of the tolerance in roll (“roll range”) that the visit allows, for targets at high galactic latitude. For visits with
ORIENTation requirements, the roll range would be half the difference between <angle1> and <angle2>. Note however that other special requirements, such as
SAME ORIENT,
ORIENT FROM, and
BETWEEN, can also restrict the orientation at which a visit may be scheduled. Unavailability versus roll range is plotted for the full range of pointings within each of the
NIC1,
NIC2, and
NIC3 apertures. Note that in all three cases the unavailability rises dramatically as the roll range shrinks to zero. The risk of not finding guide stars is considerably higher for patterns that cover the larger
NIC3 aperture. For patterns larger than the
NIC3 aperture, and which may approach the maximum pointing variation of 2 arcmin, unavailability will be still higher. Therefore, observations at high galactic latitude (above 45 degrees) with large patterns and tight
ORIENT restrictions carry a high risk of having to be reworked later for lack of guide stars. At lower galactic latitudes, the risk still exists but is considerably reduced.
