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Operational Constraints are those which involve routine telescope
operations. They are imposed by the observatory hardware, safe operations
requirements, or constraints inherent to the spacecraft and science
instruments (including instrument calibrations). These cannot be altered
by an observer, unlike the observer-imposed
constraints. For more discussion of operational constraints in general
(including more figures and some examples), see the Spitzer Observer's Manual (SOM),
especially Chapters 3-5.
Spacecraft constraints:
- Visibility constraints:
- Due to several operational requirements such as solar avoidance and
solar panel illumination, Spitzer is constrained at all times to point the
telescope no closer than 82.5 degrees toward and no further than 120 degrees
away from the Sun. This defines the Operational
Pointing Zone (OPZ) of the Observatory.
Experienced Spitzer observers should note
that the OPZ was reduced by 2.5 degrees in January 2004.
This change allowed us to increase the maximum slew rates used by Observatory,
which gave us faster slews and therefore more time to integrate
on the sky.
The shortest period that any object is visible in the OPZ is approximately
80 days per year (in two intervals of 40 days, separated by 6 months) for
targets near the ecliptic. This increases to approximately 120 days per
year at +/- 45 degrees ecliptic latitude, to year-round visibility very
near the ecliptic poles. This region around the ecliptic poles is
referred to as the Constant Viewing Zone (CVZ). For more information, see
section 3.2 of the Spitzer Observer's
Manual (SOM).
- Roll constraints:
- The roll angle of Spitzer is at all times precisely determined by the
spacecraft pointing constraints. The sunshade on Spitzer must always be
kept within 2 degrees of the radial direction to the sun. Because of
this, the orientation of the focal plane on the sky is a function of
calendar date and the ecliptic latitude of the object. This orientation
changes by approximately 0.5 to 1.0 degrees per day, depending upon the
ecliptic latitude of the target. The result is that if spectra of an
object near the ecliptic poles were taken using the same slit separated by
one week, then the orientation of the slit on the object would differ by 7
degrees. In the case of point sources this should not be a concern, but
in the case of extended sources (such as nearby galaxies), observations
which are not taken in sequence will sample different regions of the
object. This gradual roll of the sky can be particularly important for
IRS observations due to the narrow slits, or for mapping observations
where the coverage of an extended object is split over more than one AOR.
Instrument constraints:
Only one science instrument is on at any given time with Spitzer. Because
some instruments require lower operating temperature than others, the
instrument campaign order will be IRAC -> MIPS -> IRS -> IRAC. Early
in the mission, the campaigns were about 9 days long for IRAC, 8 days for
MIPS, and 6 days for IRS. During Cycle-1, the campaigns are 8, 13, and 14
days long (respectively), and during Cycle-2, the campaigns are 10, 12,
and 12 days long (respectively). These durations represent the ratio of
IRAC/MIPS/IRS usage in the science program AORs. Later in the mission,
these campaign lengths are likely to change yet again.
Instrument and spacecraft activities include:
- At the beginning of any instrument campaign, at least 30 minutes will
be used for instrument warm-up and calibration. Thus, no observation with
one instrument can immediately be followed by observations with
another.
- Star tracker alignments must be performed every 8 hours.
- Downlinks (communication between Spitzer and Earth) must occur at
least every 24 hours, sometimes every 12 hours. The length of downlinks
can vary from 10 to 60 minutes depending on the Deep Space Network (DSN)
schedule and can vary from a 12 hour schedule fairly significantly.
- Inertial reference unit (IRU) calibrations must be performed every few
weeks, and star tracker to PCRS calibrations are done about once every 12
hrs (no more than about 16 hrs apart), usually in conjunction with a
downlink.
- MIPS requires an anneal and calibration activities every 3 hours.
Therefore no single observation with MIPS can exceed 3 hours in length.
This limitation is automatically imposed by Spot.
- IRAC and IRS AORs are restricted in length to 6 hours or less in order
to accomodate the above-mentioned spacecraft activities every 8 or 12
hours, as well as instrument calibrations at least every 12 hours. This
limitation is automatically imposed by Spot.
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