Phase II Roadmap for Cycle 25
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HST Cycle 25 Phase II Proposal Instructions > Chapter 7: Special Requirements[Visit and Exposure Special_Requirements] > 7.3 Exposure-level Special Requirements

7.3 Exposure-level Special Requirements
The following Special Requirements apply to individual exposures within a visit. All instances of <exposure-list> or <exposure> refer only to exposures in the same visit as the exposure carrying the requirement.
7.3.1 Target Acquisition
Separate target acquisition exposures must be specified at the beginning of most visits, depending on the target and the science instrument used. Target acquisition exposures are used to remove coordinate uncertainties and to identify targets. Once a target acquisition has been performed, HST can move the target from aperture to aperture or move the aperture around the target (with slew accuracies of about 10 milliarcseconds) as long as the same guide stars can be used. Onboard acquisitions are automatically identified by the software. Acquisition exposures must still be specified, but no Special Requirement is needed or appropriate.
Acquisition of a target using an offset target requires that both be defined in the Target List(s). The first exposure will be an onboard or interactive acquisition of a target from which the offset will be made. This target must be designated as an offset acquisition target by appending -OFFSET to the end of the target name (see Table 3.1, Designations of Special Targets). The appropriate offsets will automatically be made from this position to slew to the target of interest.
INTeractive ACQuisition (obsolete)
Interactive acquisitions may no longer be performed with HST. However, the same results can be achieved in other ways, and you should consult your Program Coordinator for more information.
7.3.2 Target Position
Patterns used with instruments to create dithers or mosaics fall within “target position,” but the details have been incorporated into a separate chapter because of the scope of the subject. See Section Chapter 8:
POSition TARGet <X-value>,<Y-value>
Specifies a non-default placement of the target, relative to the aperture fiducial point in the instrument field of view, for the current exposure, which must be on an external target. The X and Y positions are in units of arcseconds (i.e. do not enter “arcsec” after each value). The X-Y coordinate system and the default positioning for each scientific instrument are defined in Part II of these Instructions. An aperture’s fiducial point is ordinarily close to the geometric center of that aperture. Details may be found in the Instrument Handbooks.
Proposers using this Special Requirement should be aware of the following caveats:
Changing the pointing in this way can cause overheads to be repeated at each POS TARG pointing. If a large number of pointings need to be obtained within one orbital visibility period, it may be more efficient to use a pattern designed for this purpose; see Section Chapter 8:.
The primary intent of a POS TARG is to move the specified target to a non-standard position in the “aperture.”
The POS TARG offsets are defined in detector coordinates and do not correspond to a particular motion on the sky unless a specific ORIENT is defined (e.g ORIENT 85 TO 85). Restricting the ORIENT to a specific value will imply a very tight timing constraint. In almost all situations, the preferred method of offset target acquisition is to create an additional target specification for the object of interest (separate RA/DEC or relative RA-OFF/DEC-OFF offset parameters, as described in Section 3.2.4).
On the other hand, if you are using the POS TARG to move an object of concern “out” of the aperture, failure to specify an ORIENT could result in the target getting pushed further onto the detector (e.g., if your POS TARG assumed an ORIENT of 0, but the observation occurred at an ORIENT of 180, you would move the star further onto the detector).
SAME POSition AS <exposure>
SAME POS AS <exposure> requests that the current exposure be done at the same position on the sky and with the same telescope roll as <exposure>. Note that <exposure> must be in the same visit as the current exposure. This requirement is implicit for exposures within a visit with the same aperture, target, and POS TARG combination.
This requirement is used in many astrometric observations, so that the telescope doesn’t try to center successive targets in the astrometer pickle before observing it.
For other instruments, SAME POS AS should be used sparingly and with caution. For example, SAME POS AS 1 will cause the spacecraft to return to the pointing of exposure 1. Thus if the current exposure has a different (non concentric) aperture from 1 and specifies SAME POS AS 1, the target will be placed in the aperture used by exposure 1, not the aperture currently requested. Further, specifying SAME POS AS an onboard acquisition exposure will undo the offsets determined in the acquisition process.
Do not use SAME POS AS with dithering patterns (Chapter 8) because it will negate them. SAME POS AS means exactly the same position as another exposure.
7.3.3 Special Observation Requirements
SPATIAL SCAN <Scan_Rate>, <Scan_Orient>, <Scan_Direction>, <Scan_Line_Separation>, <Scan_Number_Lines>
HST has the capability of performing a single and multi line continuous (boustrophedonic or serpentine) scans relative to the target. Such scans may be useful for observing very bright targets without sacrificing visibility to serial buffer dumps, for example.
Spatial Scans are obtained by moving the spacecraft in a way that moves the target in the instrument aperture along a path in the POS TARG coordinate frame (see POSition TARGet <X-value>,<Y-value>) as designated by the observer. The scan is defined relative to the aperture fiducial point within the SI aperture.
Only one exposure may be executed during a spatial scan, and only one spatial scan may be executed during an exposure. STScI will schedule a spatial scan so that the constant rate portion of the first scan begins at the start of the exposure and the constant rate portion of the last scan ends at the end of the exposure.
Table 7.6: Spatial Scan Parameters 
"Forward", "Reverse" and "RoundTrip"2
Only "Forward" multi-line scans are supported.
0.0 arcsec for all single line scans.

For multi-line scans:
0.0 to 10.0
1.5 arcsec
if Scan_Rate > 0.25 arcsec/sec
1 to 63 lines
if Scan_Rate
0.999999 arcsec/sec and under FGS control
or if Scan_Rate 7.84 arcsec/sec and under Gyro control
if the Scan_Rate > 0.999999 arcsec/sec and under FGS control
See also General Considerations for Spatial Scans. Note that only multi-line "Forward" spatial scans are supported.
A "Forward" scan will start at the aperture fiducial point (nominal target position; see point a in Figure 7.7) and proceed to the end of the scan (point b in Figure 7.7). A "Reverse" scan will run opposite to the corresponding "Forward" one, with it ending at the aperture fiducial point (i.e., from point b to point a in Figure 7.8) and is also subject to consideration 9 below. A "RoundTrip" scan will be performed as two scans: a "Forward" scan followed by a "Reverse" scan with identical setup parameters.

General Considerations for Spatial Scans
The maximum Scan_Rate is 4.999999 arcsec/sec under FGS control and 7.84 arcsec/sec under Gyro control. Spatial Scans will be performed under FGS control by default. If you want faster rate than permitted under FGS control, you must use either the visit level special requirement PCS MODE Gyro or exposure level special requirement EXP PCS MODE Gyro. Please contact your PC about the advantages and disadvantages of Gyro control with Spatial Scans, and how to use of these special requirements.
The SI Optional Parameter CR-SPLIT must be NO via default or an explicit specification.
The angular length of a scan will be determined from the total exposure time (including some overheads in the case of WFC3/IR), the Scan_Rate, the Scan_Line_Separation and the Scan_Number_Lines. For multi-line scans, the total exposure time encompasses all of the scan lines plus the turn around maneuvers between the lines. It is your responsibility to ensure that the scan does not extend off the detector unless necessary to accomplish the goals of the observation.
The turnaround maneuver time is computed in two parts and then combined.
a) The time to ramp-up/ramp-down each scan line:
Ramp-up_Time = max(sqrt(100 * Scan_Rate), 5) seconds
b) The time for the transverse move from one scan line to the next:
If Scan_Line_Separation
10 arcsec then
Transverse_Time = max((800 * Scan_Line_Separation)**(1/3),
10) seconds
Transverse_Time = (Scan_Line_Separation + 10) seconds

Then we combine the Ramp-up_Time and the Transverse_Time to determine the time for each turnaround maneuver:
Turnaround_Time = max(2 * Ramp-up_Time, Transverse_Time) seconds
Be sure to round to the nest higher integer before using it to determine your exposure time.
The scan specifications in Table 7.6 apply to the aperture fiducial point in the SI aperture for the exposure unless you use a POSition TARGet <X-value>,<Y-value> or PATTERN special requirement. When you use POS TARG, the scan is performed relative to the point defined by the POS TARG. When you use PATTERN, the scan will be performed for each exposure of the PATTERN relative to its pattern point.
Special Requirement SAME POSition AS <exposure> is not permitted on and can not refer to a Spatial Scan exposure.
Spatial Scan exposures are not permitted in Coordinated Parallel containers (Special Requirement PARallel WITH is not permitted on and can not refer to a Spatial Scan exposure).
The Spatial Scan parameters describe the desired scan in the POSition TARGet <X-value>,<Y-value> coordinate frame. You may need to adjust the Scan_Orient appropriately to account for the geometric distortion of the detector. You may also have to adjust the Scan_Rate, which is arcsec/sec in the POS TARG frame, to account for the geometric distortion of the detector if you need a particular pixel traverse rate.
Since a multi-line scan is executed as a single exposure, the turnaround part of the scan may be imaged on the detector. This turnaround will not occur at constant velocity or at constant rate. If this is not desired, then a series of single-line scans should be used instead. See Figure 7.9, Multi-line "Forward" Spatial Scan.
Figure 7.7: "Forward" Spatial Scan
Figure 7.7 shows a single-line "Forward" scan from point a to point b at a Scan_Orient angle α with the target at the aperture fiducial point at the start of the scan.
Figure 7.8: "Reverse" Spatial Scan
Figure 7.8 shows a "Reverse" scan starting with the target at point b, moving at Scan_Orient angle α, and ending with the target at the aperture fiducial point (point a).
Figure 7.9: Multi-line "Forward" Spatial Scan
Figure 7.9 shows a multi-line "Forward" scan where the exposure will start with the target at point a and ends with the target at point e. Note that the turnaround portions take place during the exposure and may be imaged by the detector.  If this is not desired, then a series of single-line scans should be used instead.
PARallel <parallel-exp-list> WITH <primary-exp-list>
(replaced by Coordinated Parallel Containers in the APT User Interface)
This special requirement specifies that the exposures in <parallel-exp-list> will execute in parallel with a sequence of exposures in <primary-exp-list>. In the Text Proposal File, both <primary-exp-list> and <parallel-exp-list> must be replaced by either a single exposure number, or a range of exposure numbers separated by a hyphen. See Section 6.3 for more details.
7.3.4 Special Communications Requirements
Specifies that the current science exposure must be made available to the observer for analysis in real time. (See the Call for Proposals for a discussion of real-time observing.) Any science exposures whose execution depends upon a decision based on the real-time analysis should have RT ANALYSIS specified. The REQuires UPLINK Special Requirement may also be used with RT ANALYSIS to establish the ground-to-spacecraft link. The current exposure will be available for analysis at least 16 minutes (for fixed targets) prior to that uplink; for moving targets the time is 24 minutes.
This Special Requirement is a limited resource and should only be used when necessary. Justification for its necessity should be included in the Real_Time_Justification text.
Note that the exposures in the <exposure-list> must be in the same visit as the current exposure, and RT ANALYSIS may not be used if the exposure uses patterns or if Number_of_Iterations > 1.
Indicates that a real-time command uplink is needed to execute this exposure. An uplink will be scheduled prior to the current exposure. This Special Requirement should be used with RT ANALYSIS to replace the capability formerly available with INTeractive ACQuisition (obsolete) and it should specifically identify which exposures need an uplink, assuming that the uplink already provided is not sufficient. This Special Requirement can also be used without RT ANALYSIS if the information which needs to be uplinked is not dependent on real-time analysis of HST data. Usage of this Special Requirement is considered a limited resource and should only be used when necessary. Justification for its necessity should be included in the Real_Time_Justification text.
REQuires EPHEMeris CORRection <id>
Indicates that a correction for position errors due to moving-target and/or HST ephemeris uncertainty may be needed to execute the exposure. This Special Requirement is only valid for exposures with moving targets. The offset will be uplinked during an available (“generic”) uplink prior to the earliest exposure that uses it. The pointing correction may require a minute or two of orbital visibility time.
An ephemeris correction is needed in two cases:
With moving targets, the maximum target ephemeris uncertainty must be specified in the solar-system target list (see Section 4.3.6). STScI will be unable to schedule corrections larger than the maximum offset derived from this uncertainty, so too small an uncertainty may limit the usefulness of the REQ EPHEM CORR procedure. However, offsets larger than about 1 arcminute may make scheduling difficult. If your observation requires a correction this large, contact your Program Coordinator.
The <id> is an alphanumeric string of up to six characters in length. Exposures with the same REQ EPHEM CORR ID (whether in the same visit or in different visits) will use the same offset and must be taken at the same orientation. If exposures in different visits use the same ID, the visits involved are all subject to the same scheduling restrictions as SAME ORIENTation AS <visit>. The SAME ORIENT Special Requirement is implied across such a set of visits, and need not be specified directly.
7.3.5 Timing Requirements
Requests that the current exposure be taken when the total background light is no more than 30% greater than the yearly minimum value of the zodiacal background for that target. To minimize Earth shine, the exposure will also be taken when the target is at least 40 from the bright Earth. This limits orbital visibility time to about 48 minutes per orbit. Efficiency and schedulability are reduced with this Special Requirement, but to a much lesser degree than with SHADOW. This Special Requirement may not be combined with SHADOW or used in a CVZ visit
PHASE <number1> TO <number2>
Requests that the exposure start in the specified phase range (<number1> to <number2>) of a periodic variation. <number1> and <number2> should be between 0.0 and 1.0. The PERIOD <time> ZERO-PHASE (HJD) <date> used in the calculation should have been already entered at the visit level. With short periods, the phase range (difference between <number1> and <number2>) should be made as wide as possible to make scheduling easier.
Please note that there is no need to repeat the PHASE Special Requirement for multiple exposures within the same Visit (unless you are attempting to specify different phases for those exposures). Instead, specify PHASE for only the first exposure in the Visit. If the phase window you specify is short, you may need to adjust PHASE for the first exposure so that the subsequent ones start before the phase window ends.
Note that Number_of_Iterations must be 1 if PHASE is specified
SEQuence <exposure-list> NON-INTerruptible
(replaced by Exposure Group Containers in the APT User Interface)
Specifies that exposures defined in the <exposure-list> be observed without gaps due to Earth occultation or SAA passages. Gaps between exposures may still be necessary to allow time for activities which are necessary to set up for the next exposure, such as pointing changes, readouts of the SI buffers, and SI reconfigurations. This requirement must be included for a series of FGS exposures which are to be executed within the same orbit.
In the Text Proposal File, the exposure list must be a range of numbers separated by a hyphen, such as SEQ NON-INT 55-60.
Note: If this requirement is placed on one exposure (e.g., SEQ 20 NON-INT) and Number_of_Iterations > 1, the sequence of subexposures will not be split.
Requests that the current exposure be taken while HST is in the umbral shadow of the Earth. It is primarily useful when contamination by geocoronal emission (Lyman alpha, OI 1304, etc.) must be minimized. However, it does not minimize zodiacal light, which is the principal source of background at wavelengths longer than 3500 (see the LOW-SKY Special Requirement).
Exposures using this Special Requirement are limited to about 32 minutes per orbit (including overheads). Scheduling may only be feasible for a small percentage of the year. This Special Requirement is a limited resource and should only be used when necessary; most usages of SHADOW will have been approved by the TAC during the Phase I review process. This Special Requirement may not be combined with LOW-SKY or used in a CVZ visit.

HST Cycle 25 Phase II Proposal Instructions > Chapter 7: Special Requirements[Visit and Exposure Special_Requirements] > 7.3 Exposure-level Special Requirements

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