Getting HST located and oriented properly lies at the heart of successful observations, especially when a small aperture is being used, and there are a number of ways to do that. The remarks here apply specifically to fixed targets, and mostly apply to the use of small apertures, although many of them can be applied to moving targets as well. For more information, see
Section 3.2.2 on page 34.
First, you have to acquire an object successfully that is at or near the position at which the science observation will be made. The object to be acquired should meet these conditions:
Second, once the acquisition has been made, the telescope must be repositioned to the precise point desired. This step is unnecessary, of course, if the object acquired in the first instance is the object to be observed. Repositioning can be implicit or explicit.
An offset is
implicit when a target such as “XX-OFFSET” is acquired with some ACQ mode, and then “XX” is observed via a science exposure. This often leads to confusion because no specific motion of the telescope has been provided, but that motion is implied by specifying the separate targets with different coordinates. “XX-OFFSET” is specified for the acquisition because it is bright enough and point-like enough to be acquired successfully, but the coordinates specified by “XX” are what is to be observed.
An offset is
explicit when you use a Special Requirement such as
POS TARG (see
POSition TARGet <X-value>,<Y-value>) to move the telescope away from the position acquired. In this scheme, the position specified in the acquisition is placed at the fiducial point for the aperture requested (in general the geometric center of the aperture). The
POS TARG then moves the telescope relative to that nominal position. Thus
POS TARGs are not cumulative, and always refer back to the original acquired position.
and do a search for a selection of proposals from the most recent cycle. For any program that appears as though it could serve as a useful example, the APT file can be obtained by typing the proposal ID number into the search form at
When you are entering text in a field the formats described in this document must be followed
exactly, since the information in the forms is interpreted by computer software. Some items that warrant repetition are:
See the discussion of required position accuracies in Table 3.11: Required Coordinate Accuracies. The requirements are much more stringent than is typically the case for ground-based observations.
Proposers should be aware that several of the Special Requirements impose serious constraints on the scheduling system because they require the use of limited resources; for example,
RT ANALYSIS requires real-time use of the TDRSS that is only available some of the time. Hence these Special Requirements should be requested only if they are absolutely necessary to achieve the scientific goals of a project. It is quite possible that some proposals will be impossible to schedule because of their resource requirements, rather than a lack of scientific merit. The limited-resource Special Requirements can force the planning system to schedule the observations at a less than optimal time. The use of limited-resource Special Requirements by many observers can reduce the overall efficiency with which the planning system can schedule the science program. For these reasons, these Special Requirements should only be used when necessary to achieve the science objectives of the program. The STScI will review the necessity for the Special Requirements and in some cases may suggest removing them, or using alternate methods to obtain the same goal.
The need for many of these Special Requirements must be justified in the Proposal Description. Note that several of these Special Requirements must have been justified in the Phase I Proposal in order to be used legitimately in Phase II; those are
CVZ,
SHADOW,
LOW-SKY, and
ON HOLD for Targets of Opportunity.