In-orbit operation of the soft x-ray spectrometer onboard the Hitomi satellite

Abstract. We summarize all of the in-orbit operations of the soft x-ray spectrometer (SXS) onboard the ASTRO-H (Hitomi) satellite. The satellite was launched on February 17, 2016, and the communication with the satellite ceased on March 26, 2016. The SXS was still in the commissioning phase, in which the set-ups were progressively changed. This paper is intended to serve as a concise reference of the events in orbit in order to properly interpret the SXS data taken during its short lifetime and as a test case for planning the in-orbit operation for future microcalorimeter missions.


Introduction
The ASTRO-H observatory, 1 which was renamed "Hitomi" after the successful launch on February 17, 2016 (Y þ 0; hereafter, the elapsed days from the launch are labeled as Yþ), was decommissioned due to the loss of the attitude control and resultant breakup of the spacecraft on Y þ 38. The soft x-ray spectrometer (SXS) 2 onboard the spacecraft was powered on Y þ 0, started 50 mK control on Y þ 5, achieved first light on Y þ 7, and continued observations with superb performance 3 and without any major issues until the breakup event. The commissioning of the instrument was planned to be completed in April 2016. The major remaining activities included opening of the Dewar gate valve (GV), the use and tuning of the modulated x-ray source (MXS), 4 and adjustment of the event threshold. Cryogen-free operation 5 was not tested in orbit, since its use was planned for years into the mission after the liquid helium was expended. During the short lifetime, we acquired data with the SXS that have both scientific and engineering values. Many science papers have already been published 6,7 or are in preparation. These data were taken with changing instrumental set-ups that were different from the planned normal configurations, which we need to consider for interpreting these data properly.
Here, we describe the SXS operations made in orbit until the end of the mission. The purpose of this paper is twofold: (1) we summarize all of the major changes made for the SXS components in orbit (Sec. 2), which serves as a concise reference for all publications based on the SXS in-orbit data. (2) We record the in-orbit operations planned in the normal phase of the SXS (Sec. 3) and summarize some lessons learned from the in-orbit operations (Sec. 4) in order to aid in planning for future microcalorimeter missions. Details are found in other articles of the same special section for the spacecraft, 1 the overall mission instrument, 2 the sensor and its performance, [8][9][10][11] the mirror and the aperture assembly, 12,13 the cooling chain, 5,14-17 the vibration isolation system, 18 the filter wheel (FW), 4 and the signal processing. 19,20 All data presented here are available in Ref. 21. Acronyms used in this manuscript are summarized in Table 1. Table 2 summarizes all of the major operations in orbit, whereas Table 3 gives a summary of different set-ups for all scientific observations in comparison to the planned final set-up. The house-keeping (HK) telemetry plots are shown for the contact passes on Y þ 0 ( Fig. 1), the first two days (Fig. 2), and the remaining days (Fig. 3). From the top, the panels show (a) the telescope pointing by the right ascension, the declination, and the roll angle, (b) the total primary power of all the SXS components [SXS power distributor (SXS-DIST), SXS-SWR-A/B, shield cooler driver (SCD), precooler driver (PCD), Joule-Thomson cooler driver (JTD), adiabatic demagnetization refrigerator controller (ADRC), x-ray (amplifier) BOX (XBOX), pulse shape processor (PSP)-A/B, and filter wheel electronics (FWE)], (c) the secondary power input to the mechanical cryocoolers, (d) Joule-Thomson (JT) circuit pressures or the filter wheel position, (e) magnet (MAG) current of the first-and second-stage adiabatic demagnetization refrigerators (ADRs), (f) 50-mK control temperature, (g) He tank and porous plug temperatures, (h) inner vapor-cooled shield (IVCS) temperature, and (i) the temperature difference between the saddle of the evaporator of the loop heat pipes (LHP) and the shield coolers for the four units of the SXS-LHP. These telemetry items show a wide range of activities in the complex system of the SXS. All events in Table 2 are shown with gray-dashed vertical lines, whereas the noise spectra operations (Sec. 3.2) are shown with orange-dashed vertical lines.

Power Distributor, Cooler Drivers, SpaceWire Routers
All of the SXS components except for the SXS-DIST were not powered at launch. The most critical operation immediately after the launch was to reestablish the low pressure of the liquid He by opening the vent valve using a stored command sequence, which required a very tight condition of the rocket being accelerated and the ambient air pressure being <130 Pa. 15 The evaporated He is pumped out through the porous plug. In order to start it in a controlled manner, the liquid He needs some gravity while the outside environment needs to be in a low pressure. 15 The resultant valve status change was confirmed in real time, as the satellite was still in the visibility range from the launch site, which was beyond our expectation.
The start of He pumping should be confirmed independently by the He tank temperature and the temperature difference between up-and down-streams of the porous plug due to the start of the vapor cooling [ Fig. 1(g)]. 15 To have these telemetry items available at the first contact, we powered on two space wire routers (SXS-SWR-A and B) and the SCD by stored commands. The He fill line was also opened. All of these were confirmed successful at the first contact ∼45 min after the launch.
The other two cooler drivers (PCD and JTD) were powered on during the second contact. The SXS-DIST, SXS-SWR-A and   Thermal equilibrium of the instrument is reached ("y") or not ("n").  B, and the three cooler drivers (SCD, PCD, and JTD) were continuously operated until the breakup.

Cryocoolers and Loop Heat Pipes
The next urgent operation was to start the cryocooling to minimize the temperature increase of the He tank. A delay could cause a significant, or even entire, loss of the liquid He and thus a shortened lifetime of the mission. 16 This was the most difficult part of the operation planning, as we needed to find time in limited contact passes, which were filled with many critical operations when the spacecraft was most vulnerable to accidents. On Y þ 0, we started two shield coolers (SC-A and B) . We soon figured out that this was not a malfunction, but an expected behavior when the working He gas in the circuit was quickly liquefied at a certain temperature. We never experienced this during ground testing with different cooling speeds. This AT function was necessary only for the cryogen-free mode, so we restarted JTC on Y þ 2 after disabling this AT function. Since then, the cryocoolers showed a very stable and high performance. 17 In fact, the cooling power was higher than expected presumably because the Dewar temperatures were closer to the equilibrium than ground testing. This allowed us to lower and rebalance the input powers to the coolers.
We operated JTC at 14.2 W, which was much lower than the nominal power (25.2 W) on the ground until Y þ 26∕27. By utilizing this extra margin, we increased the power to 14.8 W for JTC and decreased it from 51 to 43 W for PC-A [Figs. 3(c)]. This was because the compressor temperature of (i) PC-A was higher than the others, and the decreased wattage would help to prolong the lifetime with a lower out-gassing rate at a lower temperature. The SXS had four sets of LHP (SXS-LHP) to support heat dissipation from the shield-cooler compressors (SC-A/B CMP) and their cold heads (SC-A/B CHD). All four units were started successfully with a use of preconditioning and start-up heaters before the ramp-up of SC on Y þ 0. The two units for CMP worked until the end, but the other two finished working 1.0 and 19.5 h later for SC-A CHD and SC-B CHD, respectively [Figs. 2(i)]. This was expected as the heat input was too small for the LHP to keep working. The LHP was not necessary with a low-heat input at the beginning of the mission, and we left the units as they were.

Adiabatic Demagnetization Refrigerator Controller
The ADRC was powered on Y þ 0 and the thermometers were calibrated for precise temperature monitor and control of the low-temperature stages. The function test was performed for the heat switches (HS) and the ADR magnet current control on Y þ 4, and the 50-mK control was started on Y þ 5. Since then, a total of 18 ADR recycles were performed successfully, 14 which are visible in Figs. 3(e)-3(h). The function and the control of the third-stage ADR for the cryogen-free operation were not tested in orbit. The ADRC also controls the heaters on the optical blocking filters. 8 The heaters for both the IVCS and outer vapor-cooled shields (OVCS) filters were checked for their aliveness on Y þ 13. The temperature control of the Dewar main shell (DMS) filter heater was tested for 3.5 h on Y þ 13. The DMS filter heater control was planned to be restarted permanently a few days before the GV open to protect the filter from contamination.

Signal Processing Electronics
On Y þ 2, we powered on both the analog and digital signal processing electronics (XBOX 2,9 and PSP, 19 respectively). We started the temperature control of the JFET and the bias input to the sensors with XBOX. The CPU program of the PSP was started and the initial parameters were set for the signal processing. Noise measurement (Sec. 3.2) and bias test were performed on the same day with the detector at ∼1 K, confirming no problem. On Y þ 5, we measured the noise of the detector at 50 mK for the first time in orbit. The detector I − V curve was also measured. No change was found in comparison to the ground results. 3,13 Some PSP parameters needed adjustment in orbit because we did not know the precise noise level and the consumption rate of the data recorder (DR) during the South Atlantic Anomaly (SAA) passages. After the successful first light observation, we lowered the event threshold from 120 to 75 (equivalent to 166 to 104 eV) on Y þ 14 as the noise level was low enough. The final threshold was planned to be fixed after the GV was open.
On Y þ 4 to 6, we adjusted the rate of pulse and noise record dumps. The default settings were too high, which resulted in the loss of noise record data between around February 19 19:50 to February 21 3:45 with a full usage of the DR. We also changed the priorities of data when writing into the DR (data with a lower priority are overwritten by those with a higher priority) so that we could record the diagnostic and event data at a higher priority than the others.

Filter Wheel
The FWE was the last SXS component to be powered on Y þ 26. FWE controls the FW and the MXS. For the commissioning of the FW, we executed a rotation by a small angle (AE2 deg) for the first time on Y þ 27 and started the weekly maintenance operation on Y þ 29 and 36 (Sec. 3.2). A large angle (60 deg or 120 deg) rotation to change the filters was not originally planned until it was required during the performance verification phase. However, in order to calibrate the absolute gain of individual pixels before opening the GV and using the MXS, 10,11 we changed the position to illuminate the detector array with x-rays using the 55 Fe sources on the wheel for 22.3 h on Y þ 30 and 31 [ Fig. 3(d)]. This data set became the only reference for the absolute gain of the array pixels in orbit.
On Y þ 37, we started a two-week campaign of commissioning the MXS. The high voltage of 11.3 kV was much higher than other high voltages employed in the spacecraft, and we carefully designed a plan to ramp-up with a step-by-step verification. The first day was successful with the lowest voltage 0.5 kV ramp-up. The second day was not started due to the communication loss by the breakup.

In-Orbit Operation Plan
The operation of the SXS was planned to be as simple as possible with the assumption that any misoperation could cause damage and that any malfunctions may not be responded to until real-time contact is available as dictated by the ground station schedule. Thus, (i) we implemented automated responses (Sec. 3.1) necessary to protect flight hardware by themselves, (ii) we designed the hardware, so that they continue observations safely without ground commands in various orbital environmental conditions, and (iii) we minimized user-selectable observational parameters (only the selection of filters was open to choose). As a result, the list of routine operations (Sec. 3.2) is short considering the complexity of the instrument.

Automated Responses
Many automated responses for safety are designed and implemented within a component, and the details can be found in papers describing the individual SXS components. Some components assume that others are working properly, especially those responsible for the multistage cooling chain, thus system-wide protections were needed in addition to componentlevel protections. We thus implemented and tested such automated responses using the autonomous (AT) and request (RQ) functions of the satellite management unit (SMU), or the data handling system.
Using the AT functions, SMU can send a command sequence based on the telemetry it collects from all onboard components. Using the RQ functions, any flight hardware can request the SMU to send a command sequence to itself and the others based on its own judgment. We used AT to protect the SXS hardware based on their own telemetry and RQ to protect the SXS hardware using the spacecraft information (low battery condition and loss of spacecraft attitude control) at a sampling rate of 1 Hz. Table 4 tabulates the AT and RQ functions that were actually implemented. The JFET voltage AT is intended to protect the hardware from He leakage in the guard vacuum, which can be recognized as an excessively high voltage of the heaters that control the JFET temperature at 130 K. The wheel safe hold (WSH) and reaction control system (RCS) safe hold (RSH) RQ is intended to protect from the loss of the Sun, which could come in the line of sight to bring excessive heat input to the system.

Routine Operation
As the mission ended before we completed the commissioning of the SXS, only a part of the routine operations was started. Here, we describe the planned routine operations for reference.

Filter wheel rotations
For the SXS, the only user selectable parameter was the choice of a filter among an open position, an open position with a 55 Fe source, a neutral density filter with ∼25% transmission, and a Be filter. It was planned to execute these rotations in the performance verification phase when observing very bright sources.
After the turn-on of the FWE, the FW was planned to be rotated AE2 deg once every week during a ground contact to verify its proper functioning. By the time of the spacecraft breakup, this was performed three times.

Adiabatic demagnetization refrigerator recycling
ADR should be recycled before the 50-mK hold time expires. The hold time is a function of the He tank temperature, 14 which kept decreasing to an equilibrium 16,17 in the first few weeks. The hold time became longer and longer at every recycle.
ADRC has a function to start ADR recycling automatically when the first-stage magnet current is close to 0 A. This function was not enabled in its entire lifetime. The 50-mK control is lost during the recycling for ∼1 h. In the initial operation, we executed all the ADR recycles by command at a specified time to minimize the impact to observations. Also, in the normal phase, we planned to decide the recycling times based on the observing efficiency.

Noise collections
A set of noise data was obtained by PSP once per day during night Earth (NTE) occlusions. When NTE was not available, the data were taken in other conditions. The times of SAA passages, ADR recycles, and the spacecraft maneuvers were avoided. The noise data set consists of the following, which usually took a few minutes for collection. The noise was collected 34 times in the orbit (Fig. 3). • Short (1024 sample length) noise spectra from 100 noise records for all 36 pixels. Here, a sample length is 80 μs. A noise record is continuous samples without events.
• Long (8 × 1024 sample length) noise spectra from 50 noise records for all pixels except for pixels 11, 12, and 13. For the three pixels, 10 records were used due to the constantly high count rate by the 55 Fe source on the calibration pixel (pixel 12) and its cross-talk on the neighboring pixels (11 and 13).
• 50 sample dumps for all 36 pixels and 2 anti-co channels at the same time. Here, one sample dump is a dump of continuous data of 1024 samples regardless of whether it includes events or not.
• Twice of 5 s continuous data for all 36 pixels (p0 to p35) and 2 anti-co (ac-a and ac-b) channels. Two channels per CPU are dumped at the same time.

Parameter and memory dumps
Some onboard memory contents were not included in the regular HK telemetry. We performed weekly dumps from such memory areas from ADRC and FWE. Also, we dumped the parameters, templates, and average pulses from PSP for each observation so that users can track the set-up used for onboard pulse processing.

South Atlantic Anomaly passages
During the SAA, the temperature reading becomes noisy and the rate of false events increases. However, all the SXS hardware were designed to require no operations to cope with these changes, which worked as expected. For additional safety, we decided to ramp down the high voltage of MXS during SAA passages. The MXS commissioning was started only one day before the breakup, and the operation of ramping down at SAA ingress and up at SAA egress was tested only very partially. The extent of the SAA used for operations is defined based on the anti-co count rate of >3 s −1 above 30 keV for the SXS in the pipeline processing. The map (Fig. 4) is slightly different from the one conventionally used for Suzaku.

Lessons Learned
After the mission ended, the team summarized a long list of lessons learned. The majority of the lessons learned from the in-orbit operation of Hitomi were at the mission level or related to programmatic limitations, which are beyond the scope of this paper. Overall, the in-orbit operation of the SXS instrument was quite successful. Some of the SXS lessons learned from the in-orbit operations are related to specific subsystems, and these are addressed in accompanying instrument papers, which include calibration 10 and CSI performance 3 and in-orbit calibration papers to be published in a special edition of the publications of the Astronomical Society of Japan. There were a few minor SXS operation issues that were unexpected and have been addressed in Sec. 2; we review these below, in addition to other SXS operations lessons learned.
• Unexpected automated power down of the JTC (Sec. 2.2).
While this behavior was easily explained and the automated shut down was in fact a behavior consistent with the AT command logic, this can serve as a general caution that since the thermal environment will be different in orbit, all AT commands should be verified by simulation for a variety of operating conditions, beyond those encountered during ground testing.
• Loss of noise records for ∼40 h on the third day of flight when the detector was still warm (Sec. 2.4). There were only monitoring contacts (no commanding and data downlink) on February 19 to give a rest for the operation team, which delayed our identifying this issue. This loss of data could have been prevented by measuring the actual data rate and implementing a more appropriate setting before we started a long monitor-only period. This was not considered a high priority when the detector was warm but should have been anticipated and corrected prior to launch. In general, the prioritization scheme of the DR for SXS HK, science data, and diagnostic data should have been better-simulated and reviewed prior to launch.
• Conduct more extensive and detailed rehearsals of the initial flight operations, including unexpected behavior or data, prior to launch. These rehearsals should include the instrument teams and would have been useful for identifying tools and communication protocols needed for flight.