The Spitzer Science Center (SSC) conducted the science operations for Spitzer, and was charged with 1) acting as an interface and advocate for users, 2) capturing and conducting the science program efficiently, 3) producing and securing the Spitzer science legacy, and 4) conducting public and scientific outreach for the Spitzer program. The SSC was located on the campus of the California Institute of Technology in Pasadena, California, USA.
In carrying out its charter, the SSC issued annual Calls for Proposals, organized science and technical reviews, selected observing programs based on a Time Allocation Committee review, and administered data analysis funding awards. The SSC provided tools for detailed planning of Spitzer observations and proposal submission, and offered Science User Support services. In addition, the SSC scheduled observations on the telescope, provided basic (pipeline) science data processing and data quality assessment, and created a public data archive of Spitzer observations.
4.2 Spitzer Science Operations Overview
4.2.1 The Life Cycle of a Spitzer Observation
Figure 4.1: The life cycle of a Spitzer observation. The detailed definition of observations will occur at the time of each annual proposal solicitation.
4.2.1.1 Proposal Planning and Preparation
Figure 4.1 shows the basic stages through which a Spitzer observation passed. The process of defining a Spitzer observation starts with the choice of an astronomical target and the selection of one of the ten observing modes (Astronomical Observation Templates - AOTs). An SSC-supplied software tool called Spot was used to enter the target information and the observation details. After the AOT was chosen, Spot was used to enter all the parameters (e.g., integration time, choice of modules, etc.) needed to fully specify the observation. This process is called “filling out the AOT front-end.” Spot also provided wall-clock time estimates for the total duration of observations. An observation which has been fully defined by supplying parameter values for an AOT is known as an Astronomical Observation Request (AOR), and is the basic scheduling unit for Spitzer.
4.2.1.2 Proposal Review and Selection
During the cryogenic mission General Observer (GO), Legacy Science, Archival Research, Theoretical Research, and Guaranteed Time Observer (GTO) proposals were solicited through yearly Calls for Proposals issued by the SSC. During the warm mission three more program types were added: Exploration Science, Frontier Legacy and Snapshot. The proposals were evaluated by topical science panels and Time Allocation Committees (TACs) consisting of members of the international astronomical community. The topical panels were grouped into six broad categories:
Extragalactic – distant universe
Extragalactic – nearby universe, local group (except stellar studies in nearby galaxies)
Galactic – brown dwarfs, circumstellar disks
Galactic – ISM, galactic structure, star formation, (plus stellar studies in nearby galaxies)
Galactic – evolved stars, compact objects
Planetary systems – extra-solar planets and our Solar System
Evaluation criteria for the proposals in order of descending importance were:
The overall scientific merit of the proposed investigation and its potential contribution to the advancement of scientific knowledge.
The extent to which the proposed investigation requires the unique capabilities of the Spitzer Space Telescope.
The technical feasibility and robustness of the proposed observations.
The extent to which the observations can be accommodated within routine Spitzer operations and the extent to which the overall science program enables an efficient use of the observatory.
The long-term archival value of the proposed observations.
The demonstrated competence and relevant experience of the Principal Investigator and any Co-Investigators as an indication of their ability to carry out the proposed research to a successful conclusion.
The TACs recommend lists of programs to the SSC Director, who was the ultimate selection official for all Spitzer research programs.
4.2.1.3 Scheduling
After the Time Allocation process has been completed, the programmatic information (Principle Investigator, title, abstract, etc.) and AORs of the approved proposals were loaded into a database known as the Science Operations Database (SODB), and were stored there in the form of a set of specific parameters and values. The AOR parameters were used by the software AOR/IER Resource Estimator (AIRE) to expand the set of parameters into a set of instrument and spacecraft commands which execute the observation on board Spitzer, as well as to provide extremely high fidelity estimates of execution time for the scheduling process. Observers also could access AIRE through Spot for their time estimates. The Spitzer planning and scheduling process produced observing schedules based on these resource estimates. After the content of the schedules had been approved and finalized, AIRE takes the AOR parameters and produces command product files which were then processed by the JPL Mission Sequence Team and finally uplinked and executed by Spitzer. It is important to realize that the process of creating commands to carry out an observation based on the AOR parameters was done by software, not by support astronomers at the SSC.
4.2.1.4 Scheduling Methodology
Observations were grouped into instrument campaigns, which are periods of several days during which only one instrument is used. Instrument campaigns vary in length as needed to accommodate the science program, typically running about 7 to 21 days. The planning and scheduling process included determining the optimum structure of instrument campaigns – called the Baseline Instrument Campaign Plan (BIC), and included tentative long-range planning of large, coherent programs or observations. This type of campaign planning occurred shortly after a new set of observations become available following solicitation, review, and selection (i.e., a Call for Proposals). The BIC was published on the SSC website once it was considered stable, but it was subject to change without notice, due to the need to respond to actual events on-orbit.
Spitzer scheduling was done using the LISP-based software SIRPASS which includes the traveling salesman algorithm GREEDY. After the BIC was set and long-range planning of highly constrained observations for the next three to six months was completed, the scheduling process then focused on the short-term scheduling of each week. At any one time there were about six weeks in the process – one week was being scheduled by the scheduler; a second week was being reviewed for schedule approval; a third week was in Pass 1, having had any adjustments applied after schedule approval; a fourth week was in Pass 2, having fixed any issues that came up in the Pass 1 review (many weeks did not need a Pass 2 review – the Pass 1 review was clean); a fifth week was having its commands uplinked to the spacecraft while a sixth week was executing on the spacecraft. Early in the mission a Pass 3 review was often required (occurring later in the same week as the Pass 2 review) but after several months of actually scheduling (and reviewing) most weeks were good to go at Pass 1 or 2. Only very rarely were Pass 3’s needed. During the cryogenic mission there was one scheduler per week (6 people), by the end of the warm mission there were 3 schedulers. In general, Spitzer “weeks” run from Thursday to Wednesday and, during the cryogenic mission, could contain observations using more than one instrument.
Scheduling for a given week started about five weeks ahead of the actual start of execution of the week with the layout of the science instrument(s) calibrations and any spacecraft maintenance observations or activities. Then the science observations were pooled and inserted into the timeline, with various iterations/adjustments needed to optimize the slewing and remove or minimize violations of observer requested scheduling constraints. This was the longest part of the process usually taking three to four days to complete. If an observation could not be scheduled within its observing constraint(s), due to a conflict with another science observation or with a spacecraft activity (including downlinks), the SSC would contact the observer to see if the constraint can be relaxed. If the constraint could not be relaxed the observation was removed from the current schedule and an attempt was made later to schedule it when the target was again visible to Spitzer.
Approximately three weeks before a schedule was been uplinked to the spacecraft, the list of scheduled AORs and their nominal execution times was published on the SSC website. An email was sent to the observer, notifying him/her that his/her observation had been scheduled. Once an AOR has been scheduled, only a significant anomaly (e.g., a missed downlink opportunity or a safing event) or a rapid-turnaround ToO could cause the schedule to change.
For five years (2015-2019) during the summer months Spitzer conducted a microlensing campaign to provide parallax measurements to microlensing events in the galactic bulge. During this time additional events were added to the usual build schedule. See the end of section 3.4.3 of this document for more details.
4.2.1.5 Scheduling Constraints for Science Reasons
The scheduling software was able to handle common types of constraints and logical linkages, such as those needed for periodic monitoring of a target. If a scheduling constraint was required, it was strongly recommended that the minimum constraint necessary to preserve the scientific goal of the observation(s) be applied. In general, larger timing windows and/or loose groupings were preferred to non-interruptible sequences of observations. For very long observations, strategies which permit independent scheduling of the component AORs were much preferred; this not only enhanced scheduling efficiency, but also made the observation as a whole much more robust against the failure of component AORs. As a general rule of thumb, groups of constrained AORs that occupied more than about half of the time period during which they can be scheduled were difficult to accommodate. For example, 100 hours of observations that must be done within a 100-hour period cannot be scheduled, whereas the same 100 hours of AORs may have been quite feasible if they can be scheduled anytime within a 200 hour period. The ability to be scheduled is further enhanced by making the observations shorter; 100 hours of 2-hour AORs constrained to a 120-hour period may be barely feasible, whereas 100 hours of 6-hour AORs cannot be scheduled in a time period shorter than 200 hours.
The following types of scheduling constraints are supported for both inertial and moving targets:
Timing Constraints Timing constraints consist of defining a window or series of windows for the start time of an AOR. Absolute-time observations that will be executed at a specific time, or no more than 3 seconds later, can only be supported for moving targets. To specify an absolute time observation in a moving target AOR, set the open and close times for the timing window to be identical. Spitzer’s scheduling architecture generally operates on relative time, so for inertial targets, the (inertial target) AORs will simply run in order. Timing constraints for inertial target AORs should be macroscopic (days, weeks, months), not microscopic (seconds, minutes, hours).
Relational Constraints Relational constraints are ordering or grouping constraints that are applied to a group of AORs. There are four basic types of relational constraint supported by Spitzer:
Chain = Ordered Non-interruptible Group A chain can be thought of as a list of AORs that must be executed consecutively in the order specified and without any other kind of activity intervening. Note that the total time for the entire ordered non-interruptible group cannot exceed the maximum time for an individual AOR; for longer observing sequences, an interruptible group must be used. This type of constraint might be used for an on/off source pair of observations.
Sequence = Ordered Interruptible Group A sequence constraint is similar to, but less stringent than, a chain constraint. The AORs will be executed in the order specified and a duration in which they should be completed is specified. The sequence constraint should only be used when the science requires sequential ordering of the AORs. For AORs in which the order of observation is not important, a “group within” constraint (see below) should be used, instead of a sequence constraint.
Group-within A group-within constraint specifies that a group of AORs will be executed within a specific length of time, but with no particular starting date/time constraint. Once the first AOR has been executed, the rest of the AORs in the group will begin within the specified time interval. They may be executed in any order within the time interval. This is similar to a sequence constraint, but the observations may be executed in any order.
Follow-on This constraint forces a follow-on AOR to be scheduled within a given time after a precursor AOR. It can be thought of as a statement that Follow-On-AOR must be scheduled within Time-Window after the end of Precursor-AOR. The follow-on constraint can be used to prevent early execution of an observation when the success or content of the follow-on is dependent upon the successful execution of a precursor observation. This constraint could also be used for periodic observation of a target where the interval between observations is relatively short (hours to a small number of days). One AOR may serve as the precursor to more than one follow-on, but a follow-on may have only one precursor (e.g. one follow-on constraint can tie together only two AORs).
Shadow The shadow constraint is a special case of the follow-on constraint, and is used to obtain background measurements for moving targets. The primary AOR is executed as specified. The shadow AOR will be executed to repeat the track of the primary observations. The selected AOR parameters must be identical in the two AORs. The shadow may be executed before or after the primary AOR. Note that the shadow does not re-observe the target at a later date, but rather the background of the primary observation. (As with all constraints, shadow observations must be strongly scientifically justified in the observing proposal.)
Timing and Relational Constraints can be combined. For example, a series of AORs used to obtain spectra of a comet over a long track which needed to be broken up into segments due to curvature, might be constrained as a chain with an associated timing constraint related to the acceptable range of solar elongations.
4.2.1.6 Pipeline Processing
After an observation had been scheduled and carried out, the resulting data were pipeline-processed, undergo a brief quality-checking process, and were placed in the Spitzer science archive and made available to the observer, usually within a week or two after the end of the campaign.
4.2.2 Data Products
Three levels of processed data were created: Level 0 (raw data), Level 1 (Basic Calibrated Data; BCD) and Level 2 (Ensemble data, also known as “post-BCD data”). Level 0 data are the unprocessed data, which have been packaged into FITS format. Level 1 data are single-frame FITS format data products, which have been processed to remove instrumental signatures, and which are calibrated in scientifically useful units. Level 2 data are higher-level products, and may include mosaicking, co-addition, or spectrum extraction. The data products are described in more detail in the Instrument Handbooks.
In general, the final calibration for an instrument campaign, which is typically about one to three weeks in duration, is based upon calibration observations taken at the beginning and end of the campaign, so pipeline data products were typically available one to several weeks after the end of an instrument campaign. The goal was to have the data available to the observer two weeks after the end of the campaign. This was usually the case for IRAC and MIPS; IRS took longer.
The SSC validated the pipeline processing for each observing mode prior to the first release of data. A basic quality assessment was performed on all Spitzer data before they are delivered to the archive. Quality assessment information is also available to the user through the Spitzer Heritage Archive interface.
4.3 Astronomical Observation Templates – AOTs
The observer’s interface to the observatory, including the science instruments, was the Astronomical Observation Template (AOT). Spitzer’s science instruments are relatively simple in the sense of having few modes and even fewer moving parts (only two - the MIPS scan mirror and the IRAC shutter (see the Instrument Handbooks for more information ). The use of relatively simple parameterized observing modes enhanced the reliability of observations and calibration, improved the archival value of Spitzer data, and reduced cost. An AOT was a specific observing mode. In the cryogenic mission there were nine AOTs for the three science instruments. Five of the AOTs (IRAC Mapping, IRS Staring, IRS Spectral Mapping, MIPS Photometry, and MIPS Scan) were commissioned during the first three months following launch. Another AOT (MIPS Spectral Energy Distribution [SED] mode) was available for the Cycle-1 Call for Proposals, though observations were not scheduled until the AOT was commissioned in October 2004. Two AOTs became available, beginning in Cycle-2: MIPS Total Power [TP] mode, and IRS Peak-Up Imaging (PUI). For the warm mission one AOT was commissioned in July 2009: the IRAC Post-Cryo Mapping. The observing modes are:
InfraRed Array Camera (IRAC) Mapping/Photometry The IRAC AOT is used for simultaneous imaging at 3.6, 4.5, 5.8 and 8.0 microns, over the two ~5.2’ x ~5.2’ fields of view. This AOT was used for IRAC observations in the cryogenic mission.
InfraRed Array Camera (IRAC) Post-Cryo Mapping/Photometry The IRAC AOT is used for warm mission observations. It has simultaneous imaging at 3.6 and 4.5 microns, over the two ~5.2’ x ~5.2’ fields of view. The 5.8 and 8.0 micron arrays were not operational in the warm mission. In September of 2011 peak-up functionality was added to this AOT to assist with high precision time series photometry.
InfraRed Spectrograph (IRS) Staring-Mode Spectroscopy The IRS staring mode is used for low-resolution long-slit spectroscopy (R= 64–128) from 5.2 to 38.0 microns and high-resolution spectroscopy (R~600) from 9.9 to 37.2 microns. The IRS Staring mode also supports raster mapping. It also returns images from the IRS Peak-Up array, which has a field-of-view of 1’ x 1.2’ and two filters covering 13.5–18.5 microns and 18.5–26 microns.
IRS Spectral Mapping The IRS Spectral Mapping AOT is used to perform slit scanning spectroscopy for fields up to a few arcminutes in extent.
IRS Peak-Up Imaging The IRS Peak-Up Imaging AOT provides imaging only using the Peak-Up array, which has a field-of-view of 1’ x 1.2’ and two filters covering 13.5–18.5 microns and 18.5–26 microns.
Multiband Imaging Photometer for Spitzer (MIPS) Photometry and Super Resolution Imaging The MIPS Photometry and Super Resolution AOT is used for imaging photometry and high-resolution imaging at 24, 70 and 160 microns. An “enhanced” small-field mode for 160 microns observations is also available.
MIPS Scan Mapping The MIPS Scan Map AOT is used for large field maps at 24, 70 and 160 microns. The maps are constructed using slow telescope scanning, combined with motion compensation using a cryogenic scan mirror. Maps are built up of ~5 arcminutes (2.5 arcminutes for full coverage at 70 microns) wide strips between 0.5° and 6° in length.
MIPS Spectral Energy Distribution (SED) The MIPS Spectral Energy Distribution AOT is used for very low-resolution (R=15–25) spectroscopy covering 52-97 microns using the MIPS 70 micron Ge:Ga array.
MIPS Total Power Measurement (TP) The MIPS Total Power Mode AOT provides zero-level-reference observations for absolute brightness of extended sources.
Each AOT and its usage are discussed in detail within the respective Instrument Handbooks.
4.4 Astronomical Observation Request – AOR
When all the relevant parameters for an AOT are specified and linked to a description of the target, the resulting fully specified observation is called an AOR, which is the fundamental unit of Spitzer observing. An AOR can be thought of as a list of parameters that, when properly interpreted, completely describe an observation. In fact, an AOR, as represented in the SODB, contained a series of keywords and values that were used to create the sequence of commands that are sent to the observatory to carry out the observation.
An AOR could not be subdivided, could not be interrupted for other activities (such as downlinks), and was handled as a unit by the observatory. Because of this non-interruptible nature and the need to perform certain activities periodically (e.g., detector anneals, pointing system calibrations, and downlinks) a maximum duration existed for an AOR. Originally all AORs had a maximum duration of three hours. As the prime mission progressed the excellent performance of the instruments and the observatory allowed the maximum duration to be extended to 8 hours for IRS and 24 hours for IRAC AORs. The maximum duration of MIPS AORs was extended to six hours but it was reduced back to three hours when it was determined that germanium anneals every three hours really did produce the best calibrated data. Longer observations could be specified using multiple AORs and relational constraints to identify these AORs as members of a related group.
An AOR contains three categories of information:
Astronomical Target The target of an AOR can be a single pointing or a cluster of pointings within a 1º radius, at which the specified observation is repeated identically. The single pointing or cluster may be either of an inertial target or a moving target.
AOT-Specific Parameters As the name implies, these vary from AOT to AOT. They include instrument configuration, exposure time and dedicated mapping parameters.
Timing and Relational Constraints These constraints represent scheduling directives for an AOR or for a related group of AORs. The details of the kind of constraints that are supported are in section 4.2.1.5. Timing constraints are used to specify a window when an AOR should be executed (e.g., to observe a comet at maximum solar elongation). Relational constraints are used to specify how AORs within a group are related to one another (e.g., a series of AORs that define a very deep map and must be executed consecutively).
4.5 Science User Tools
Science User Tools are software packages and other materials (such as tables and graphs) that are provided by SSC to help the astronomical community plan, prepare, submit, monitor, and interpret the results of their Spitzer observations. They are all available from the IRSA Web site https://irsa.ipac.caltech.edu/data/SPITZER/docs/. We highlight a few tools here for illustrative purposes.
4.5.1 Spot
Spot was a multi-platform Java-based, client-server, GUI-driven software tool intended to assist potential and approved observers in planning and modifying their observations. It allowed investigators to construct and edit detailed AORs by entering targets and selecting from a variety of preset instrument-specific functions (e.g., exposure times, instrument modes, dither patterns, and observing constraints). Spot also included useful visualization tools to permit an investigator to see how proposed observations and the Spitzer focal plane is laid out on the celestial sky. These capabilities allowed observers to retrieve relevant images from other astronomical surveys (in any of a number of wavelengths) and archives. It calculated estimates of Spitzer observing time (including telescope overheads) for each AOR in a proposed program, along with target visibility information, focal plane position angle for a selected observation date, and estimates of the zodiacal and cosmic infrared background at the target. Spot also allowed investigators to view AORs from all previously approved programs.
Spot required a network connection to the Spitzer Science Center servers to obtain observing time estimates, visibility, orientation, or background estimate information, and to submit and update proposals. A network connection was not necessary for selecting observation parameters. Spot is no longer available for download and is no longer supported.
4.5.2 Performance Estimation Tool (PET)
The most important website-based SSC tool is the Performance Estimation Tool (PET). The PET is actually a set of JavaScript tools to aid in designing Spitzer observations. There are four PET components: The Sensitivity PET, the Spectroscopy PET, the Extragalactic PET, and the Stellar PET. Users of the PET should pay close attention to the various notes and warnings on both the main PET pages and the help pages.
The Sensitivity PET (SENS-PET) is an imaging sensitivity estimator. It takes as input IRAC, IRS PUI, and/or MIPS imaging instrument configurations, and a background level. It produces as output the instrument sensitivities (both for point source and extended objects), and the total exposure depth per pixel.
The Spectroscopy PET (SPEC-PET) is designed specifically for spectroscopic sensitivity estimates. Users configure IRS and/or MIPS SED observing parameters, and the SPEC-PET returns an estimate of the instrument sensitivity.
The Extragalactic PET (EX-PET) makes predictions for imaging of extragalactic sources. As input, users may choose an SED model, background level, and IRAC + MIPS instrument configurations. The output includes flux in the instrument passbands, instrument sensitivities, S/N, and total exposure depth per pixel.
Finally, the Stellar PET (STAR-PET) predicts fluxes for imaging of stars. For input, users select a stellar spectral type + MK class, K-band magnitude or K-band flux density. The tool calculates the expected flux density in the IRAC 3.6, 4.5, 5.8, 8.0 microns passbands, and at 15 microns (IRS peak-up blue channel and short wavelength module), and 24 microns (IRS peak-up red channel and long wavelength module, and MIPS Si:As array).
The first observer’s interface to the archive was called Leopard (delivered within the Spitzer-Pride package of Observation Planning software), and was very similar to Spot (see above). In November 2009 the Spitzer Heritage Archive (SHA), a web-based interface, began to also serve proprietary Spitzer data and became the Spitzer Archive. Information about how to retrieve public and/or proprietary data using the SHA can be found at the SHA URL: http://sha.ipac.caltech.edu/applications/Spitzer/SHA .
4.6 Solar System Objects - SSOs
Spitzer supported observations of Solar System objects, tracking in linear segments at rates up to 1 arcsecond/second. All instruments and all observing modes could be used while tracking. During IOC/SV, IRS peak-up for IRS spectroscopy was successful for all moving targets attempted, which included objects with rates between <1.0 arcseconds/hour and just over 200 arcseconds/hour, and fluxes between 40 mJy and 1 Jy at 15 microns. Peak-up on both point source and extended targets were supported; during IOC/SV, both moving point sources and extended sources were acquired. However, note that peak-ups were not restricted to be performed only on the moving target being tracked, or a point source that is co-moving with it; instead, observers could peak up on an inertial target and then offset to a moving target.
4.6.1 Tracking Performance
Spitzer’s SSO tracking capability is similar to its scanning capability. One significant consequence of this for the Solar System observer involves sources whose tracks, on an equatorial map, have significant curvature during an AOR. Such AORs may have needed to be broken up into a series of short (linear) AORs. The spacecraft does not carry any target ephemerides on board, so the track was defined at the time of scheduling and formulated as a vector rate in an equatorial frame. A start time and equatorial J2000 start point and time were also provided for use by the PCS. Once the track command has been issued, the on-board system maintained knowledge of where the telescope should be at what time, and “catches up” with the specified track and maintains it.
4.6.2 Ephemeris Management
Spitzer used a database of ephemerides for known SSOs derived from the Horizons database maintained by the Solar System Dynamics group at the Jet Propulsion Laboratory. For proposal planning purposes, Spot could retrieve the ephemerides for a specified target by resolving the NAIF ID. These ephemerides were used to calculate visibility windows and resource estimate calculations for SSOs through Spot.
4.6.2.1 Shadow Observations
The infrared flux from background sources, and particularly small-scale structure in that background, frequently limits the sensitivity of Spitzer, particularly in the wavelength range 24–160 microns. To assist in background subtraction, Solar System observers could specify “shadow” background observations for all instruments and observing modes. A shadow observation allowed the track across the sky taken during observation of a moving target to be replayed or pre-played when the target is not there. Shadow observations allow the Solar System observer to remove background small-scale structure, thereby improving moving target sensitivity. To reduce potential errors due to time-dependent changes in the zodiacal light, instrument characteristics, and calibration, a shadow observation is generally most effective when taken as close in time to the primary observation as is scientifically possible. Execution of the shadow observation after observation of the science target was the default (much easier to schedule both observations), although there are situations where this is not the optimal executing order.
4.7 Targets of Opportunity - ToOs
Targets of Opportunity are transient events whose timing is unpredictable. Predictable phenomena whose precise timing is not known a priori (e.g., novae, newly discovered comets, gamma-ray bursts) may have been requested in a General Observer proposal (i.e. within a normal Call for Proposals). Observations of completely unanticipated phenomena could be requested through Director’s Discretionary Time proposals.
ToOs were classified based solely on their impact on the observatory scheduling process, which depends on the time elapsed between the activation of a ToO observing request and the desired date of execution of the corresponding observation; see Table 4.1 for ToO classification criteria.
Table 4.1: Classification of ToOs
High impact
<1 week (48 hour minimum turnaround)
Medium impact
1–5 weeks (1-8 weeks in warm mission)
Low impact
>5 weeks (>8 weeks in warm mission)
4.8 Generic Targets
Generic Targets can be scientifically described, but do not have exact celestial coordinates or brightness estimates at the time of proposal submission. An example of this would be objects discovered in a survey (by Spitzer or any other telescope) for which IRS spectroscopy is sought. Integration time estimates within a factor of 1.5 must have been provided in the generic target AOR and a position within 2° of the expected final position.
4.9 Second-Look Observations
Second-look observations (SLOs) were deemed to be a predictable element of an integrated Spitzer observing program, even if the specific targets cannot be provided at the time of proposal submission. For example, an investigator could propose for 80 hours of time to conduct one-hour observations of 30 targets and then ten-hour observations of the five most interesting of these based on criteria spelled out in the proposal. Because of the expected end of cryogen in Cycle-5 SLOs were not allowed as part of Cycle-5 programs. Second look observations were allowed in the warm mission.
4.10 List of Safings or Standbys in the Mission
There were 13 safe modes or standbys during normal science observations in the mission. Several were due to single events (i.e. a proton hit) that caused a piece of hardware (i.e. the CE or the IRU) to produce an error and the fault protection requested a standby or safe mode as it was designed to do. The events are listed in the order in which they occurred. The “Days Back to Science” column lists the number of days (rounded to the nearest whole day) before science observations were resumed. Within the 18 August 2006 safe mode there was another event on 28 August (false OPZ violation by the WASS) before science observations had been re-started.
Table 4.2: List of Safe/Standby modes during normal science operations
