Spitzer is unique in many ways; the cryostat and mission design allowed a very long cryogenic lifetime (5.7 years) with a relatively small amount of cryogen (~360 liters of superfluid helium). [For comparison, IRAS had a 10-month lifetime using 560 liters of liquid helium, and ISO had a 28-month lifetime using 2140 liters of cryogen. During the prime mission the helium bath temperature was maintained at 1.24 K, the telescope temperature could be ≤ 6 K, and the outer shell temperature is approximately 34 K. Some of Spitzer’s unique features were its Earth-trailing solar orbit and the fact that the telescope was launched at ambient temperature. This “warm launch” architecture and earth trailing helocentric orbit (which separated the telescope from the heat of the Earth) allowed Spitzer to achieve its lifetime goal with a smaller, lighter cryostat than would otherwise be required.
A basic external view of Spitzer is shown in Figure 3.2. The spacecraft provided structural support, pointing control, part of the instrument electronics, and telecommunications and command/data handling for the entire observatory. The spacecraft, including the spacecraft bus, the solar panel and Pointing and Control System (including the Pointing Control Reference Sensors, or PCRSs, which are located in the focal plane) were provided by Lockheed Martin Space Systems Company.
The CTA, shown in Figure 3.3, consists of the telescope, the superfluid helium cryostat, the outer shell group and the Multi-Instrument Chamber (MIC), which hosts the cold portions of the IRAC, IRS, and MIPS science instruments and the PCRS. The telescope assembly, including the primary and secondary mirrors, metering tower, mounting bulkhead (all made of beryllium) and the focus mechanism, was mounted and thermally connected to the cryostat vacuum shell. The barrel baffle of the telescope assembly was separately attached to the vacuum shell at its flange. The MIC, an aluminum enclosure containing the instrument cold assemblies, was mounted on top of the helium tank. The vapor-cooled shields, the vacuum shell and the outer shields are connected to the cryostat by a series of low connectivity struts. The CTA was provided by Ball Aerospace and Technologies Corporation.
The spacecraft and CTA (and science instruments) together comprise the observatory, which is ~4 m tall, ~2 m in diameter, and has a mass of ~900 kg.
Figure 3.2: Basic external view of Spitzer. The observatory coordinate system XYZ (shown in Figure 3.4) is an orthogonal right-hand body-fixed frame of reference. The X-axis passes through the geometric center of the top surface of the spacecraft, is parallel to the CTA optical axis (which passes through the primary and secondary mirror vertices), and is positive looking out of the telescope. The Z-axis intersects the line forming the apex of the two surfaces of the solar panel. The Y-axis completes the right hand orthogonal frame. The X-axis origin is defined such that the on-axis point between the CTA support truss and the spacecraft bus mounting surface is located at X = +200 cm, in order to maintain positive X values throughout the observatory. The Sun always lies within 2º of the XZ plane (i.e., the roll angle is constrained to ±2º).
Spitzer executed autonomously a pre-planned, typically week-long, schedule of science observations, calibrations and routine engineering activities, which had been uploaded in advance and stored on board. During the cryogenic mission this “master sequence” might have up to 14 different 12–24 hour Periods of Autonomous Operation (PAOs), containing observations and calibration activities, each followed by a 30-60 minute period spent re-orienting the spacecraft for downlink and transmitting the data to the ground. IRS campaigns routinely had a 24-hour PAO with one downlink lasting longer than 40 minutes. At the end of the mission the data volumes were lower and a master sequence typically had 3-5 PAOs of 24-72 hours each and 3-4 downlinks to Earth of 120-150 minutes each. After the downlinks, Spitzer returned to the pre-planned sequence of observations and calibrations. Because efficient communication with the ground requires use of the high-gain antenna mounted on the bottom of the spacecraft (Figure 3.2), executing a downlink of the collected data required slewing the spacecraft to orient the high gain antenna toward one of the Deep Space Network (DSN) stations on Earth. Any of the three DSN sites (Canberra, Madrid and Goldstone) could be used when visible. During the time that data were being transmitted to the ground, no science data could be collected. In extreme circumstances, Spitzer was designed to survive for up to a week with no ground contact at all.
Figure 3.3: Cryogenic Telescope Assembly.
Figure 3.4: Observatory Coordinate System.
3.4.1 The Warm Launch and Telescope Temperature Management
One of the unique aspects of Spitzer is the warm launch concept. The telescope was at ambient temperature at the time of launch and gradually reached its lowest operating temperature (≤ 6K) approximately 45 days after launch, through passive cooling and by vapor vented from the cryostat. The science instruments were in contact with the helium bath at all times and were thus cold at the time of launch. The on-orbit performance confirms the soundness of the Spitzer thermal design principles.
Because the telescope was cooled by vapor vented from the cryostat, the telescope temperature depends upon how much power is dissipated in the cryostat. Because the cryogenic design and performance result in near-zero parasitic heat loads into the cryostat, the power entering the helium bath was essentially that of the instrument in use. Due to variations in the power dissipation of the instruments, the telescope temperature would naturally vary from ≤ 6 K to ~12 K. A “make-up” heater was available to add additional heat when needed to lower the telescope temperature. Since very low background is required for 160 µm observations, make-up heater operations were always planned to cool the telescope to ≤ 6 K and maintain it at that temperature during MIPS campaigns that are obtaining 160 microns observations. This is accomplished using a heat pulse that has been optimized to use the least amount of cryogen, while still guaranteeing a cold telescope for MIPS; it uses considerably less cryogen than holding the temperature constant all the time. The heat pulse is typically ~10 hours in duration and takes place about 10 days prior to the start of the MIPS campaign. The details of pulse size and duration were optimized to minimize cryogen usage and maximize scheduling efficiency.
In order to maximize Spitzer’s cryogenic lifetime, starting in Cycle 2, MIPS campaigns were organized into “warm” (telescope cooled to ~8.5 K) or “cold” (~5.5 K) campaigns. AORs requiring 160 microns were scheduled only in cold (~5.5 K) MIPS campaigns. MIPS observations not requiring 160 microns were scheduled in warm (~8.5 K) or cold (~5.5 K) campaigns.
In the warm mission, both IRAC arrays were actively thermally controlled to 28.7K. The programmable voltages for each array were optimized. In addition to the operating temperature, the primary difference in array operation was a lower applied bias of 500 mV for the 3.6 micron array compared to 750 mV of applied bias in the cryogenic mission.
3.4.2 Data Storage
The Spitzer spacecraft includes the command and data handling subsystem (C&DH) which shares the flight computer (a RAD 6000) with the pointing control subsystem (PCS). The C&DH validates and executes either previously stored or real time commands, receives and compresses data, and writes the compressed data into the mass memory. The C&DH also provides a stable clock to correlate data and events.
During normal science operations, the observatory collects, compresses, and stores 12 to 72 hours’ worth of science data prior to downlinking it. Spitzer has 16 Gbits of solid state memory (for both science and engineering data). During each 12 to 72-hour PAO, up to 6 Gbits of the memory are filled. Enough additional storage capacity is left unused during that 12 to 24-hour PAO to permit missing a downlink opportunity (e.g., due to a problem at the ground station) and continuing to observe without overwriting previously collected science data. Since the full mass memory (16 Gbits) is available to whichever of the two redundant flight computers is in use, in general multiple passes would have to be missed before the risk of losing science data that had not been transmitted to the ground becomes significant. However, e.g., MIPS sometimes generated enough data that a single missed pass could become important. During the warm mission the downlink rate decreased as the distance to the spacecraft increased, and data volume management became a driving force behind the scheduling process.
3.4.3 Data Transmission
Spitzer had two antenna systems for data uplink and downlink. The high gain antenna (HGA) supported a maximum downlink rate of 2.2 Mbit/s for the first 2.6 years of the Spitzer cryogenic mission. Since Spitzer drifts away from the Earth at about 0.12 AU per year, the data transmission rate on the HGA decreased as a function of time, and the downlink strategy had to change to accommodate this by lowering the downlink rate and adding arrayed DSN dishes. The two low gain antennae (LGA) gave wide angle coverage but they only supported downlink at 44 kb/s at the beginning of the mission and at the end of the mission they were only capable of carrier-only signals. The LGA were used in safe and standby mode, but were not normally used to transmit science data during routine science operations. It was also possible to command Spitzer “in the blind” by sending commands to the spacecraft though the LGA if necessary.
Because of the location of the HGA, it was necessary to stop observing and point the telescope in the anti-Earth direction to downlink the science data though the antenna fixed to the bottom of the spacecraft. Depending upon the details of the observations that are scheduled, Spitzer produced ~1 to 6 Gbits of (compressed) science data during 12–72 hours of observing. During the cryogenic mission, Spitzer collected data for about 12–24 hours and then spent 0.5–1 hour downlinking the data. The scheduling system predicts the compressed data volume that is generated during each downlink and schedules the downlink contact time accordingly. The downlink periods are also used for some spacecraft maintenance activities (such as dumping angular momentum) and for uplinking commands. If low volumes of data are being generated, longer periods of time between downlinks may be used.
At each downlink opportunity, Spitzer attempted to transmit all the data in the mass memory. All data was stored onboard the spacecraft until it was confirmed to be on the ground safely. Any data that the ground had not confirmed as received was retransmitted at the next downlink pass. It was anticipated that some data could be missed at any pass, and it sometimes took several passes before the ground received all the science data for a given PAO. At the beginning of the mission downlinks were planned so that all data on board could be transmitted during the next downlink and the retransmit feature was used during anomalous situations. After October 2013 the distance to the spacecraft was so great (up to 1.77 AU) that the downlink rate had to be lowered to 550 kB/s and this retransmit feature was used on a regular basis during normal operations. The schedulers would plan for the telescope to take more data then could be downlinked during the next pass and store it onboard the spacecraft until a subsequent downlink.
Whenever the HGA was used for downlink, it was also possible to uplink commands and files to Spitzer. Normally ~3 to 5 communication periods per week were used to send up all the sequences and information needed to support the next upcoming week of observations. No communications with Spitzer were planned outside the scheduled downlink sessions.
The downlink strategy had little impact on observation planning with Spitzer in the cryogenic mission. It was one component in determining the maximum length of an Astronomical Observing Request (AOR)(a filled in AOT, defining a Spitzer observation), but the necessity for instrument maintenance operations and pointing system calibrations on shorter than 12 hour time scales constrained the maximum AOR length more tightly than the downlink schedule1. A 12 to 24 hour interval between downlinks does imply that it can be a day or longer before science data are available on the ground after an observation is complete.
During the warm mission only 2 of 4 channels of the IRAC camera were operational, which cut the data volumes generated by IRAC in half. In addition the IRAC sub array mode, which was used for high precision photometry light curves (mostly of exoplanets), did not store the readout from the entire chip and could had extremely low data volumes, depending on the duration of the observation and the frametimes used. This allowed for much longer PAOs (24-72 hours) and made longer observations possible. Waivers were granted for these larger gaps in the instrument maintenance activities, and the pointing system calibrations were deferred until after the long observations, and then executed in bulk (e.g. after a 36 hour observation, three were executed back to back).
The uplink strategy did not directly affect observation planning in the cryogenic mission, but it drove the time scale on which changes to the stored science schedule and its contents can be made (e.g., for a ToO). In the warm mission, a minimum number of downlinks per week had to be scheduled to provide enough uplink opportunities.
During the warm mission a microlensing campaign was executed each summer for 6 weeks in the years 2015-2019 that required coordinated observations between ground-based observatories and Spitzer. During this time the uplink strategy drove the observation planning and the schedule for the entire Spitzer project. For a normal week the scheduling team built the week starting approximately 5 weeks in advance of execution. Two weeks were budgeted for reviews and fixes to the week, and it was uploaded to Spitzer in the week before it was set to execute (see section 4.2.1.4 “Scheduling Methodology” for more information). Microlensing events are on the order of a couple of weeks long, but Spitzer’s distance from Earth meant that it alone could obtain a parallax measurement for the ground-based events, if the usual scheduling process could be expedited. In the end, each week was built according to the usual process with a placeholder AOR placed in the schedule once every 12 hours for the six weeks each summer that the galactic bulge (where the number of microlensing events is highest) was visible to Spitzer and Earth simultaneously. Special downlinks with backup dishes at highly prescribed times were negotiated with the DSN, with uplinks on Wednesday and Thursday afternoons. The microlensing team sent their targets, based on the ground-base observations, to the Spitzer Science Center by 8am on Monday morning, and the placeholder AORs were replaced with the microlensing AORs, and then the week was rebuilt, reviewed, modeled and approved by late Tuesday afternoon, uplinked to the spacecraft during the downlinks on Wednesday and Thursday, and started executing onboard Thursday afternoon. The process was successfully executed each time, and none of the weeks with the placeholder AORs were ever flown.
