HST @ STScI UpdateR. A. Osten (osten[at]stsci.edu)
In its 30th year of operation, Hubble Space Telescope science operations have achieved "new normal" levels, both amid remote work under pandemic conditions, as well as dealing with the unpredictable (but not pathological) behavior of Hubble's quirkiest gyroscope. Innovative work by the COS team will extend the usable lifetime for sensitive far-ultraviolet spectroscopy well into this decade.
Science Operations in the Time of a Pandemic
Science operations for Hubble have been fully remote since mid-March of this year in response to a work-from-home posture at the Institute. This switch should be transparent to users, as there has been no significant impact on the ability to perform peer-reviewed proposal evaluations, schedule the observatory, calibrate and archive data, provide user support to the community, and disseminate results to the public. The observatory and its instruments are healthy, as well.
The situation required that the annual Telescope Allocation Committee (TAC) activities to select the successful proposals for Cycle 28 be conducted entirely remotely this year. There was one anticipated change in Cycle 28 proposal selection compared to previous cycles, and that was to solicit external panelists to assess and grade the subset of small General Observer (GO) proposals requesting fewer than 15 orbits on nearly all topics. Virtual panels evaluated proposals in the remaining categories. The one exception to this was the Solar System proposal category, which was entirely reviewed by the corresponding virtual panel due to the overall small proposal pool. The outsourcing of the smallest bin of orbit requests will continue into subsequent cycles, with some tweaks based on the experience in Cycle 28.
The proposal pressure for Cycle 28 was similar to that in recent cycles, with more than 22,000 orbits requested. The resulting oversubscription is 8:1 by orbit request, and 6:1 by proposal number. This was the third year in which the proposal preparation and review were dual anonymous, and resulted in roughly one-third of accepted proposals being first time Principal Investigators.
The Ups and Downs of Gyroscopes
A near-constant theme of these newsletter articles of late (Volume 35, Issue 1; Volume 35, Issue 3; Volume 36, Issue 1; and Volume 36, Issue 3) has been describing the evolving nature of Hubble's gyroscopes. This underscores the "new normal" of science operations with Gyro 3 since it was re-activated in the fall of 2018 after the last of the standard flex-lead gyroscopes failed. While the cause of this particular gyroscope's behavior is not well understood, science operations has been able to respond to and accommodate it to ensure continuity of observations with Hubble.
As reported in Volume 36, Issue 3, the rate bias of Gyro 3 had been climbing to values that would not have been sustainable under normal science operations. Twice now, the bias level decreased shortly before reaching values that would have required intervention. This is a closely watched performance indicator and measures are already in place to ameliorate this behavior.
The primary purpose of the gyroscope is to determine where Hubble is pointing, enable it to slew across the sky to its next target, and re-establish the target after the telescope emerges from an Earth occultation. As reported in Volume 36, Issue 1, there are occasions where there is a temporary increase in the number of acquisition and re-acquisition failures, and the frequency of these has increased with the use of Gyro 3. The historical level of failures was in the range of 1–2% of observations, and recent performance has increased to an average of 4–6% of acquisitions and re-acquisitions experiencing a failure. The performance, while not ideal, is not yet at worrisome levels to warrant intervention or switch to a different observing mode. Should a problem occur during one of your observations, filing a Hubble Observation Problem Report (HOPR) will ensure that the failure is reviewed and a request for repeat observations can be made.
New Lifetime Positions for COS
The Cosmic Origins Spectrograph (COS) has been the workhorse for sensitive far-ultraviolet (FUV) spectroscopy on Hubble since its installation in May 2009. The COS FUV detector experiences gain sag, such that observing sources with the instrument reduces the efficiency of further observations by decreasing the detector's ability to convert photons into detectable signals. The solution to refresh the observing capabilities has been to perform regular changes to the location on the detector where spectra are accumulated. This has resulted in a few lifetime position moves within the last 11 years of on-orbit science operations.
As described in an earlier newsletter article (Volume 34, Issue 2), a previous COS2025 initiative identified a strategy to extend the useful lifetime of COS in the limited number of foreseen lifetime positions by changing the rules for how spectra could be obtained. Instead of spreading out the gain sag holes (due to Lyman alpha geocoronal emission) across the detector, the philosophy was changed to concentrate them in a small number of locations. These regions would quickly degrade but then leave other regions of the detector less affected. Implementing this change has indeed extended the lifetime of COS within the currently available detector locations.
Recent innovative changes in thinking about how science observations and wavelength calibration occur have led to opening up a previously closed-off region of the detector. One barrier limiting the available real estate on the detector is the existence of a light leak above the fifth and previously considered final lifetime position, where light from the wavelength calibration lamp passes through a flatfield calibration aperture, which is blocked for lower positions on the detector. This is described in more detail in Oliveira et al. (2013). Normal science operations obtain a wavelength calibration spectrum at the same time (and in a fixed offset position) from the science spectrum; since the location of the science spectrum changes with lifetime position, so does the location of the wavelength calibration spectrum. Decoupling these two increases the usable area on the detector for science exposures, and enables a hybrid mode that optimizes operations for each grating separately. There is a small amount of overhead to performing the wavelength calibration exposure prior to and immediately after the science observation, and at a different lifetime position, but there is little effect on the resultant wavelength calibration (Fox et al. 2020). However, current expectations are that this new approach will extend further the capability of sensitive FUV spectroscopy with COS to perhaps 2030 or beyond. Work is beginning on the initial implementation of this plan.
In 2020, the 30th anniversary of the Hubble Space Telescope was celebrated. This remarkable achievement for one of the most productive scientific instruments ever commissioned was characterized by new heights in scientific result productivity and innovative observing and analysis techniques. The research conducted by the science community and the support of the telescope by NASA and STScI continued despite the considerable challenges posed by the COVID-19 pandemic. Again, the community can anticipate a strong future for HST as we enter a new decade of science.