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| HST Two-Gyro Handbook | |||
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2.2 Scheduling Considerations and Visibility
Periods for Fixed Targets
2.2.1 Overview
The primary observational constraints on the schedulability of most fixed targets in two-gyro mode are the position of the target in the sky, the required orientation or roll angle of the observatory (if any), and the required timing of the observation (if any). Orientation constraints are usually specified with the ORIENT special requirement and often involve a restricted range of allowable roll angles that correspond to a particular time period that HST is able to achieve this orientation. Timing requirements may be specified either implicitly through the ORIENT special requirement or explicitly through timing special requirements (e.g., BETWEEN, AFTER, etc.). In some cases, an observation may not be schedulable in two-gyro mode because of the restrictions imposed by orientation and/or timing special requirements.
The operational definition of orbital visibility period for two-gyro operations is the same as it was for three-gyro operations. Orbital visibility is the unocculted time available during the orbit for guide star acquisitions (6 minutes), target acquisitions, science exposures, calibration exposures, and instrument overheads.
Figure 2.3 provides a graphical description of the general decision process involved in determining the schedulability and orbital visibility periods for fixed targets observed in two-gyro mode. The decision process for unconstrained observations involves minimal effort, whereas constrained observations require more careful consideration of the times of year that an observation can be scheduled. We discuss both types of observations below.
Figure 2.3: Two-Gyro Scheduling and Visibility Decision Tree2.2.2 Unconstrained Fixed-Target Observations
If you do not need to specify the orientation of the observatory or the time of year of the observation, then the impact on scheduling is minimized and the observation will be schedulable at some time during the year. The orbital visibility of an unconstrained fixed-target is determined primarily by its declination. Table 2.1 lists the two-gyro orbital visibility periods as a function of declination. These average values are sufficient for Phase I orbit calculations.
Table 2.1: Standard Two-Gyro Fixed-Target Orbital Visibility Periods
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1The orbital visibility periods in this table are the typical unocculted times available for guide star acquisitions and instrument-related activities.
2LOW visibility refers to low-sky observations specified with the LOW special requirement.
3SHADOW visibility refers to Earth-shadow observations specified with the SHADOW special requirement.
If your observation has no timing or orientation special requirements, the orbital visibility period can be found in Table 2.1, and you do not need to use the scheduling plots found later in this chapter or on the
Two-Gyro Science Mode web site.
2.2.3 Constrained Fixed-Target Observations
If your science goals require specification of the orientation of the observatory and/or the timing of the observation, determinations of the schedulability and orbital visibility period are slightly more complicated. An on-line tool to help you determine when a fixed target can be scheduled during Cycle 16 is available on the Two-Gyro Science web page at:
http://www.stsci.edu/hst/HST_overview/TwoGyroMode/AllSkyInf ormationEnter the coordinates of your target into the web form, and the tool provides several graphical products that can be used to assess when and for how long the target is visible. The calculations used to construct this output were performed on a 5°x5° grid on the sky. The output returned is appropriate for the grid point nearest the input coordinates. Thus, any input position is within 3.5 degrees of a grid point. This sampling is sufficient to provide accurate scheduling and visibility information for any position on the sky for the Phase I proposal process. The models used as input to produce this information rely upon realistic representations of the constraints expected for two-gyro operations. For clarity, Moon avoidance constraints are not included in these results; this does not alter the schedulability of a fixed-target or its orbital visibility significantly. Complete models including all constraints will be available for Phase II proposal processing.
The products returned by the Available Science Time and Orientation web tool include:
- A plot and table of the total number of days per year that each orientation is available.
- A plot and table of when each orientation is available during Cycle 16 and the first half of Cycle 17.
- A plot and table of the target visibility as a function of date during Cycle 16 and the first half of Cycle 17.
The plots and tables contain information about the schedulability of the target and how much time per orbit is available for the observation. Examples of each of these plots and tables are discussed below.
Plot Example: Number of Available Days as a Function of Orientation
The number of days that a particular HST orientation (roll angle) can be achieved in two-gyro mode in Cycle 16 is shown as a function of orientation for a low-latitude location (
= 0°,
= 0°) in Figure 2.4 and for a high-latitude location (
The number of days of availability may be quite limited for some orientations, especially at low declinations, as a result of Sun angle constraints. The range of available orientations expands at higher latitudes, with the number of days of availability increasing for some orientations and decreasing for others. In some cases, the number of days available may be zero.
In the low latitude example in Figure 2.4, there is a restricted range of orientations over which the target is observable. There is minimal availability for orientations from 0°-216° and from 277°-360°. There is a large window available for orientations centered around 245°. Using orbits with less than the nominal (52 minute) visibility does not increase the availability significantly. Note that, unlike three-gyro operations, it is not possible to observe at orientations with a 180° separation.
Figure 2.4: Orientation Availability for a Low-Latitude Target Near
In the high latitude example in Figure 2.5, many orientations are available for ten or more days, with notable exceptions occurring over restricted ranges in orientation where the availability dips to zero. Unlike the low latitude case, there are several orientations (e.g., 180°) for which the use of orbits with less than nominal (55 minute) visibility increases the availability; in some cases, the target is available only with these short orbits. However, scheduling these short orbits results in efficient use of the telescope. Requests for short orbits must be well-justified scientifically and are expected to be available only in exceptional cases.
Table 2.2: A Portion of the Tabular Output for the Data Shown in Figure 2.4Figure 2.5: Orientation Availability for a High-Latitude Target atPlot Example: Availability of Roll Angles in Cycle 16
If a sufficient number of days exists to observe a target, the next step in determining its schedulability is to check when the target could be scheduled. If there are roll angle (ORIENT) constraints, you should check when the particular orientation is available by examining the web tool plot illustrating ORIENT versus time. Example plots are shown in Figure 2.6 and Figure 2.7, and a sample of the tabular output is shown in Table 2.3. The time axis on these plots extends for 18 months from the start of Cycle 16 (01-July-2007 to 31-Dec-2008) so that observers can judge whether observations that begin in Cycle 16 could be concluded in the first half of Cycle 17. The blue regions of the figures indicate what orientations are available for the specified date when there is at least one orbit with 30 minutes of visibility. The gold regions indicate less than 30 minutes of visibility for some ORIENTs on that date. Note that most orientations are available only for limited periods of time.
Consider first the low-latitude pointing in Figure 2.6. The February to mid-May time period is unavailable because of Sun avoidance restrictions. The Sun-leading region of the sky (October-February) is also relatively inaccessible in two-gyro mode (gold regions) because of the need for fixed-head star tracker (FHST) visibility prior to fine-lock. Thus, in 2007, available orientations are limited to the June through September time frame. A somewhat larger range of orientations is accessible for a very restricted range of dates in late September 2007 and 2008 when the target is located near the anti-Sun direction.
Figure 2.6: Low-Latitude Target Roll Angles Available During Cycle 16For the high latitude pointing in Figure 2.7, the availability of roll angles in two-gyro mode is broken into several time intervals separated by periods when the object is unobservable. These breaks in availability occur primarily because precession of the HST orbit causes the Earth to block the FHSTs, which are needed for guide star acquisitions.
Table 2.3: A Portion of the Tabular Output for the Data Shown in Figure 2.6Figure 2.7: High-Latitude Target Roll Angles Available in Cycle 16Plot Example: Target Visibility as a Function of Date
For constrained observations, the orbit visibility may change dramatically throughout the year, unlike unconstrained observations, the visibility cannot be described as a simple function of declination alone (e.g., Table 2.1). The web tool displays the visibility information in graphical form in a plot of target visibility versus time for an 18 month period beginning at the start of Cycle 16 (01-July-2007 to 31-Dec-2008). Plots for the low- and high-latitude sight lines are shown in Figure 2.8 and Figure 2.9. Sample tabular output for Figure 2.8 is shown in Table 2.4. The orbital visibilities in these plots are shown with blue points indicating the maximum visibility available, and by green lines indicating the full range of visibilities for the orientations available. In some instances, the maximum visibility depicted by the blue line may be identical to the minimum visibility in which case there is no green line, just a blue point.
The horizontal black line at 30 minutes in Figure 2.8 and Figure 2.9 indicates the minimum orbital visibility that will be allowed for Phase I proposals in Cycle 16 without special scientific justification. Most observations can be scheduled at times when the orbital visibility exceeds this amount, and those few that cannot will likely have other restrictions that will preclude such observations. For example, in the high-latitude example, the visibility window on 01-October-2007 is only 25 minutes. Therefore, a target at this location in the sky will not be scheduled on this date.
Figure 2.8: Orbital Visibility for a Low-Latitude Target in Cycle 16Table 2.4: A Portion of the Tabular Output for the Data Shown in Figure 2.8Figure 2.9: Orbital Visibility for a High-Latitude Target in Cycle 16In calculating the orbit visibility period in two-gyro mode for Cycle 16, observers should adopt the maximum visibility estimate indicated by the blue points in the visibility plots unless they have both ORIENT and timing restrictions, in which case they need to examine the more detailed visibility plots described in the next section. The HST scheduling system will make every effort to schedule observations when the visibility is optimized.
Detailed Target Visibility Considerations
In some cases it may be necessary to have a more detailed look at the target visibility for various orientations on a particular date to assess whether a highly constrained observation is feasible. The
Detailed Visibility Toolon theTwo-Gyro Science Mode web pagecan be used to determine the target visibility. You can enter the target coordinates and the desired date of the observation, and the tool will return a plot and table of the science time available as a function of orientation for the 11 day interval centered on the input date.Figure 2.10 contains an example of the detailed visibility plot for the high-latitude sight line example on a set of dates centered on 26 September 2007. The resulting visibilities are color coded by date. The information is shown in tabular form in Table 2.5. Observers requiring a specific orientation on a specific date should specify the appropriate visibility indicated by these plots (or tables) in their Phase I proposals.
Figure 2.10: Detailed Visibility Plot for a High-Latitude TargetTable 2.5: A portion of the Tabular Output for the Data Shown in Figure 2.102.2.4 Examples
In this section we provide some examples of how to determine the schedulability and visibility period for different types of observations in two-gyro mode.
Example 1: Time-series observations of the Hubble Deep Field (HDF) and Hubble Ultra Deep Field (HUDF).
Observer #1 wants to search for supernovae in the HDF and HUDF by repeating a set of ACS observations every ~45 days for as many consecutive 45-day intervals as possible. The fields are tiled with multiple pointings, each of which has five 400 second integrations designed to fit in a single orbit. The total time required to tile either field is 15 orbits (~1 day). Orientation is not critical, as the field can be tiled in a manner that allows nearly full coverage of the field regardless of orientation. The HDF and HUDF are located at
Figure 2.11: Time Available per Orbit for the HDF in Cycle 16Let's consider first the HDF. Using the on-line tool available at the Two-Gyro Science web site, we examine the scheduling and visibility plots available for the
This program could be conducted with a set of 4 observing opportunities spaced ~45 days apart (e.g., 15-Oct-2007, 01-Dec-2007, 15-Jan-2008, 03-Mar-2008). The science times available per orbit on these dates are listed in Table 2.6. A larger set of observations may be difficult to schedule since the next opportunity in this set (~April 15) has very poor visibility. When dealing with visibilities that change rapidly over the course of a month, flexibility in time series spacing may improve the schedulability. For example, in this case allowing a 54 day separation from March 3 to April 26, instead of the 43 day spacing between March 3 and April 15, would increase the visibility sufficiently to make another epoch of observations possible.
Table 2.6: Two-Gyro Time for the HDF on Selected Dates in Cycle 16Now consider the HUDF. Using the on-line tool available at the Two-Gyro Science web site, we examine the scheduling and visibility plots available for the
Table 2.7: Two-Gyro Time for the HUDF on Selected Dates in Cycle 16
Example 2: Time-series light curve observations of a supernova found in Example 1.
Observer #1 finds a supernova in the HDF using the experiment outlined in Example 1 and wants to obtain the light curve of the supernova by obtaining photometric images of the supernova and surrounding field once a week for 7 weeks. The supernova was discovered after the fourth set of observations was obtained on 03-March-2008.
Using the visibility versus month plot generated for Example 1, Observer #1 notes that the orbital visibilities for the HDF and surrounding areas are good on march 10 and 17 (54 minutes), as well as March 24 and 31 (54 and 43 min, respectively) and April 7 (41 min). However, by April 14 the visibility has dropped to 21 minutes, and on April 21 it is only 23 minutes. Thus, it is not possible to follow the supernova light curve throughout the entire 7-week period as hoped.
Example 3: An orientation-constrained observation of the Vela supernova remnant.
Observer #3 wants to take an ACS image of a portion of the Vela supernova remnant near the position of the star HD 72089 (
Entering the coordinates of HD 72089 into the scheduling tool on the web yields the following plots of the number of days each orientation is available (Figure 2.13) and orientation versus month (Figure 2.14) for the nearby position at
Realizing that the planned orientations are not viable, Observer #3 checks the availability of orientations 180 degrees from those originally envisioned. The inverse orientations at 250 degrees and 340 degrees are accessible near 01-October-2007 and 01-January-2008, respectively. Therefore, this observation is feasible in two-gyro mode as long as the requested orientations are changed to 250±5 degrees and 340±5 degrees. The availability of allowable orientations must be described appropriately in the Phase I proposal.
Figure 2.13: Cycle 16 Orientation Availability in the Direction of the Vela SNRFigure 2.14: Cycle 16 Orientation Angles Versus Month Toward the Vela SNR
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