2.4 Examples and General Advice

2.4.1 Acquisitions and Pointings

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 3.4 .

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:

1. It should be a point source or nearly enough to point-like that the centering algorithms can determine a precise centroid.
2. The object's coordinates must be both precise and accurate and any proper motion must be known. This requirement boils down to the need for the object to fall within the search region at the time of the acquisition. For this to happen the coordinates must also be consistent with the Guide Star Catalog or they must fall within another system that can be related to the GSC. This is why the source of the acquired object's coordinates are required.
3. The object must be neither too bright nor too faint for the instrument mode used. These conditions are described in the various Instrument Handbooks.

The coordinates for the acquired object can be specified in several ways:

• As explicit absolute celestial coordinates, i.e., RA and DEC. See Section 3.4.2 .
• As celestial coordinates relative to another nearby object, using offsets in RA and DEC. See Section 3.4.3 .
• As a REGION; see Section 3.4.4 .

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.

2.4.2 Examples

People who are looking for examples of APT files are encouraged to go to

http://archive.stsci.edu/hst/abstract.html

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

http://www.stsci.edu/hst/scheduling/program_information

Additionally, several examples of ACS dither patterns using POS TARGs which may be used in an APT proposal can be found on the ACS Web site at:

http://www.stsci.edu/hst/acs/proposing/dither

2.4.3 Common Problems

Incorrect Proposal Format

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:

• Visit numbers must be unique.
• Target names must be spelled exactly the same throughout the proposal.
• The format for target positions must be followed to the letter. For more information on coordinates, see Section 3.13 .
• The format for flux data must be followed. Only those defined in these Instructions are acceptable.
• The format for Optional Parameters and Special Requirements must be followed to the letter.
• Observations which cannot be defined using the syntax in these Instructions may be described in Comments fields, but such comments should be used very sparingly, if at all, and their use may impede execution of a program.

Imprecise Target Positions

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.

Lack of Acquisition Exposures with Small Apertures

When exposures are requested in very small apertures or fields of view, a separate acquisition exposure is generally required. Please consult the Instrument Handbooks for the instrument you are using.

2.4.4 Consideration of Limited Resources

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 following table summarizes the Special Requirements that involve seriously limited resources.

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.

Table 2.2: Limited-Resource Special Requirements 

Limited Resource	Reason for constraint
ON HOLD [FOR <visit-list>]	Requires special handling (e.g., Targets of Opportunity).
RT ANALYSIS, REQuires UPLINK	Requires real-time TDRSS links, which are difficult to schedule and may be withdrawn at last moment.
ORIENTation <angle1> TO <angle2>, SAME ORIENTation AS <visit>	A specific orientation can be available for as little as a one-week period every six months.
SHADOW, LOW-SKY, CVZ	Available for only a fraction of orbits.
AFTER <date>, BETWEEN <date1> AND <date2>, BEFORE <date>, SEQuence <visits-checked> WITHIN <time>, SEQuence <exposure-list> NON-INTerruptible (replaced by Exposure Group Containers in the APT User Interface); see Section 5.19, "Exposure Containers: Exposure Groups, Coordinated Parallels and Patterns" , PHASE <number1> TO <number2>,	Constrain scheduling opportunities. Can be mutually incompatible.