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SPACE
TELESCOPE
SCIENCE
INSTITUTE
MEMORANDUM
TO:
FROM:
DATE:
SUBJECT:
Rodger Doxsey,
Denise Taylor, Bill Workman, Ian Jordan
August 18, 2004
HST orbit availability after the STIS Suspend
1. Supplementary Orbit Recommendation Summary
Following the STIS suspend on August 3, 2004, the anticipated unavailability of STIS in the
remainder of Cycle 13 implies a dearth of orbits in the remainder of the Cycle 13 HST long range
plan. This document provides an accounting of the orbits allocated to HST and an updated estimate of the number of orbits which would need to be filled by supplementary means. Issues
related to a transition to Two-Gyro mode or the adequacy of the orbit allocation “tail” are not discussed in this memo.
Table 1: Summary Recommedation for post-STIS TAC Orbit Re-Allocation
Recommendation Considerations
80 orbits/wk
85 orbits/wk
Total est_orbits until C13/C14 boundary (44 weeks)
+3520
+3740
“Tail” to maintain efficiency (10 weeks)
+800
+850
Subtotal orbits to plan for (Table 2)
4320
4590
Assumed HOPRs (2.5%)
-108
-115
Remaining planned and anticipated orbits (Table 3)
-2946
-2946
Subtotal orbits available for re-allocation:
1266
1529
STIS --> non-STIS transfer
-176 +/- 105
-176 +/- 105
Total available for re-allocation with STIS transfers:
1090 +/- 105
1353 +/- 105
Operated by the Association of Universities for Research in Astronomy, Inc., for the National Aeronautics and Space Administration
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2. Total Orbits to Plan for in the Remainder of Cycle 13.
The total number of orbits which remain to be planned for is based upon the number of weeks
which are planned to be filled. A “tail” is used to allow smooth and efficient transition between
Cycles and adjust for inability to schedule all visits within Cycle boundaries due to constraints on
some programs. The tail used here is the same tail length specified in the February 24 pre-TAC
ODM orbit recommendation memo. Two different conceivable execution rates are specified.
Table 2: Weeks available within Cycle 13 Boundary ( < 2005.185)
80 est_orbits/
wk
85 est_orbits/
wk
Weeks remaining in current cycle (44 weeks)
3520
3740
“Tail” weeks at end of cycle (10 weeks):
800
850
Subtotal
4320
4590
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3. Total Anticipated Orbits Remaining in Cycle 13:.
Table 3: Unexecuted/anticipated Orbits
Type
Orbits
Planned (with plan windows)
2300
Unplanned (without plan windows)
123
Calibration orbits (Phase 2s not in)
126
non-STIS Chandra allocated orbits (Phase 2s not in)
38
Engineering (2-Gyro Tests)
40
Directors Discretionary (100 orbits minus 7 orbits in-house)
93
Target of Opportunities
226
STIS --> non-STIS transfer
176 +/- 105
subtotal
3122 +/- 105
This section details the orbit totals which we know or which may need to be reserved for the
remainder of Cycle 13. A number of uncertainties exist in recommendations on the number of
orbits which should be substituted for the loss of STIS from the Cycle 13 TAC runners-up. Explicitly, those uncertainties are:
• HST est_orbit execution rate may or may not increase with the loss of STIS.
• Target of Opportunity activations are uncertain.
• Number of STIS orbits transferrable to other instruments has not been finalized. This number represents
a best guess of the possible lower limit on the number that will be actually transferred. The uncertainty
value is based on the fact that a decision is pending on 131 orbits of the Filippenko ToO program (10182)
as well as other programs identified in Paul G.’s aperture and spectal element triage.
• DD allocation rate may change in a post-STIS environment.
This document assumes that all DD time will be used. Also assumed is that all activations in the
PANS ToO program will be used (10339 & 10340) and a significant fraction of the remaining
non-STIS ToOs will be used (60%-100% of remainder used). A guestimate range for the number
of STIS-to-non-STIS transfers is loosely based on a triage performed by the STIS team.
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4. STIS Orbits Lost
Approximately 1270 orbits of prime STIS GO visits will be lost, plus 224 orbits of unexecuted
STIS Target of Opportunity visits, for an approximate total of 1494 orbits. Note that this number
is comparable to the number of orbits computed available in the summary section based upon the
higher execution rate.