Introduction

Most general special requirements directly or indirectly restrict the times when observations (and/or their component visits) can be scheduled. These should be used to provide the schedulers at STScI with enough constraints to ensure that the observations are properly scheduled. Special requirements should not be used unless necessary to accomplish the scientific objectives of the proposal.

Visit Special Requirements

VISIBILITY INTERVAL <time>

This Special Requirement will override the visibility interval normally computed for the visit. No pads will be added, nor will any adjustment be performed based on the target declination. This should not be used in conjunction with the CVZ or SCHEDulability <percentage> Special Requirements.

This Special Requirement may also have the value CORON.

Exposure Special Requirements

MAXimum DURation (time or %) <value>

Allows the Program Coordinator to set limits on the expansion of primary exposures to take advantage of unused orbital visibility. The TRANS tunable constant (enable--adjust t) must be set in a proposal override file to activate MAX DUR. If <time> is specified, the exposure will not be lengthened beyond <time>.If <percentage> is specified, the exposure will not be lengthened beyond <percentage> of the specified exposure time. If <time or percentage> is omitted, the exposure will not be lengthened at all.

MINimum DURation (time or %) <value>

Allows the Program Coordinator to limit shortening of exposures to allow them to fit in a orbital visibility period. The TRANS tunable constant (enable--adjust t) must be set in a proposal override file to activate MIN DUR. If <time> is specified, the exposure may be shortened until its duration is <time>, but not further. If  <percentage> is used, the exposure will be shortened until its duration is decreased to, at most, <percentage> of the specified exposure time.If <time or percentage> is omitted, the exposure will not be shortened at all.

EXPAND (pure parallel exposures only)

Allows the proposer to choose which exposures within an orbit will be expanded to take advantage of unused time at the end of the orbit (note that the exposures will be expanded to fill the orbit unless the MAXimum DURation (time or %) value Special Requirement is also listed). This special requirement can only be used in pure parallel observations; it has no effect on primary exposures. If there is no EXPAND within the orbit, the last exposure in the orbit will be considered for expansion (up to an additional 20%) first, followed by the previous one and the one before that, etc. Exposures containing the EXPAND requirement are considered first for expansion. 

NEW OBSET [FULL ACQ]

NEW ALIGNMENT

These Special Requirements will override the normal decision of whether an exposure will be merged into the same alignment or OBSET as the previous. Using it will force the exposure to start a new OBSET or alignment regardless of the existence or the criticality of any merging rules to the contrary (but error checking will be performed).

In addition, it is possible to force a full guide star acquisition at the beginning of an obset, even if TRANS would not insert one there. The syntax for this capability is NEW OBSET FULL ACQ.

SAME OBSET <exposure-list>

This Special Requirement will override the normal decision of whether the exposures in the same list will be merged into the same OBSET. Using it will force the exposures in <exposure-list> to be included in the same OBSET regardless of the existence or the criticality of any breaking rules to the contrary (but error checking will be performed).

Note that this differs from the SAME GUIDE STARS Special Requirement, which does not necessarily place any requirements on how exposures are to be arranged into obsets.

SAME ALIGNMENT <exposure-list> 

Note that this differs from the SEQ NON-INT Special Requirement, which is purely a timing requirement (it requires exposures not to be separated by earth occultation), and does not necessarily place any requirements on how exposures are to be arranged into alignments. This Special Requirement will override the normal decision of whether the exposures in the same list will be merged into the same alignment. Using it will force the exposures in <exposure-list> to be included in the same alignment regardless of the existence or criticality of any breaking rules to the contrary (but error checking will be performed). The exception is that the alignment may be split if it exceeds the time constraints of the orbital visibility period.

SAME GUIDE STARS <exposure-list> 

This Special Requirement will override the normal decision of whether two obsets must share the same set of guide stars. If two members of <exposure-list> are placed in different obsets, those two obsets will be forced to share the same guide stars regardless of the existence or criticality of any rules which would otherwise assign them separate guide star sets.

GSPAIR <pair-id->

This Special Requirement will tell NGSS to use the specific guide star pair indicated. For pairs, <pair-id> is the 24-character NGSS guide star pair id and is of the form nnnnnnnnnnFdmmmmmmmmmmFe, where nnnnnnnnnn and mmmmmmmmmm are the 10-character guide star ids of the two stars. The Fs are required; d and e are the FGS numbers that the stars are to appear in (that is, 1, 2, or 3). Note that the ordering of the stars is as NGSS will construct the pair.

For singles, <pair-id> is as above except mmmmmmmmmm = ‘0000000000’ and e = ‘0.’

OBSET ID <id>

This allows a non-default OBSET id to be chosen. <id> must be a two-character alphanumeric string and may be specified only once per obset.

EXP PCS MODE [FINE | GYRO]

These Special Requirements allow the PCS requirements for a particular exposure to be overridden. The PCS mode for an OBSET is still computed based on the most restrictive PCS mode in that OBSET. Therefore, in order to override the PCS mode for an entire OBSET with a less restrictive value, it is necessary to place this Special requirement on every exposure within the OBSET, or it should be used at the Visit level.

It is possible for an alignment to be executed on GYRO in an OBSET that uses FGS guiding. If EXP PCS MODE GYRO is included on every exposure within an alignment, it will be executed using gyro guiding, even if it is included in an OBSET which uses FGS guiding.

These may also be entered as EXP PCS MODE F and EXP PCS MODE G.

GS ACQ SCENARIO [PAIR | SINGLE | GYRO | <scenario>]

This Special Requirement allows the guide star acquisition scenario for an obset to be overridden. This may be specified only once per obset.

The uncertainty associated with single guide star acquisition poses a danger to the ACS/SBC, STIS/MAMA and COS detectors. Use of any single guide star acquisition scenario requires approval after an appropriate additional bright object check.

Tables 1 and 2 summarize the legal values for <scenario>.

Table 1: FGS Guiding Scenarios

Table 2: Gyro Guiding Scenarios

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