Safety Acquisitions: Redundancy for non-repeatable multi-orbit STIS visits

1. Problem Statement

The Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) requires high pointing accuracy to center targets within scientific apertures (slits). While HST's "blind pointing" accuracy is approximately 0.35 arcseconds, this is often insufficient for precise spectroscopy. To achieve the required 0.01 arcsecond (2σ) accuracy, STIS performs a Target Acquisition (ACQ) sequence at the beginning of a visit.

The Core Issue:
The ACQ sequence relies on the successful acquisition of guide stars by the Fine Guidance Sensors (FGS) at the start of the first orbit. If the observatory fails to locate guide stars in time:

• The ACQ sequence is skipped.
• Science exposures in the first orbit are lost (shutter remains closed).
• If the visit spans multiple orbits and guide stars are reacquired later, science data can be taken, but without the ACQ correction, the pointing accuracy remains at the inferior blind level (~0.35").
• For non-repeatable observations (e.g., supernovae, kilonovae, tidal disruption events), losing the ACQ correction can result in permanent scientific data loss that cannot be recovered via a standard Hubble Observation Problem Report (HOPR) due to time constraints.

2. Methodology

The authors conducted a statistical analysis to quantify the frequency of these failures and evaluate the feasibility of "Safety ACQ" sequences (redundant ACQs placed in subsequent orbits).

• Data Selection: The team analyzed all STIS ACQ sequences from September 2018 to September 2023.
• Failure Identification:
  • They identified failed ACQs by filtering for sequences where the Hole in the Mirror (HITM) lamp exposure (the final step of the ACQ sequence) was unilluminated.
  • A cutoff of < 45,000 total counts in the HITM exposure was used to flag unilluminated frames.
  • This yielded 113 candidate visits.
  • Verification: Each candidate was cross-referenced with HST Exception Reports and the STIS Target Data Flag (TDF) monitor to confirm the failure was specifically due to guide star acquisition issues (ruling out other causes like lamp delays).
• Orbit Analysis: For multi-orbit visits within this catalog, the team tracked:
  • The total number of orbits.
  • Whether guide stars were eventually reacquired.
  • The specific orbit number where reacquisition occurred.

3. Key Contributions

• Quantification of Failure Rates: The report provides the first comprehensive statistical breakdown of STIS ACQ failures specifically caused by guide star issues over a 5-year period.
• Risk Assessment Framework: It offers a data-driven methodology for Principal Investigators (PIs) to assess the risk of pointing failure for time-sensitive targets.
• Proposal of "Safety ACQ" Strategy: The paper advocates for and analyzes the feasibility of inserting redundant ACQ sequences in later orbits (post-guide star reacquisition) to restore pointing accuracy if the initial ACQ fails.

4. Key Results

A. Failure Frequency

• Overall Rate: Guide star issues caused ACQ failures in approximately 5% of all STIS ACQ sequences between 2018 and 2023.
• Recent Trend: The failure rate has increased significantly recently. From March to September 2023, the rate rose to ~9%.
  • Note: The authors attribute this to pointing control system issues (specifically Gyro-3) and FGS-2 issues in 2021, noting that these rates fluctuate and are not guaranteed to remain high.

B. Multi-Orbit Recovery Statistics
Of the 59 multi-orbit visits in the failure catalog:

• Total Loss: ~39% (23 visits) never reacquired guide stars, resulting in zero science data for the entire visit.
• Partial Recovery: ~61% (36 visits) successfully reacquired guide stars.
  • Timing of Recovery: Of the recovered visits, ~78% (28 visits) reacquired guide stars by the second orbit.
  • Only ~22% recovered after the second orbit.

C. Impact on Science

• In the 2023 supernova case study (Section 1), the initial guide star failure caused the loss of the first orbit's data. Although data was taken in subsequent orbits, the lack of an ACQ sequence meant the target was not centered with the required precision, potentially compromising the science goals.

5. Significance and Recommendations

Significance:
This report highlights a critical vulnerability in observing strategies for transient events. For targets that cannot be re-observed (e.g., a supernova fading after a few weeks), a single guide star failure can render the entire observation scientifically useless if the target is not centered in the slit.

Recommendations for PIs:
The authors suggest that PIs consider implementing "Safety ACQ" sequences for high-priority, non-repeatable, or highly constrained observations.

• Strategy: Place an additional ACQ sequence at the beginning of subsequent orbits (after guide star reacquisition) to correct pointing if the first ACQ failed.
• Trade-offs: This sacrifices some science exposure time for "insurance" against total data loss.
• Decision Factors:
  1. Repeatability: Is the event observable again within 21 days? (If no, Safety ACQ is highly recommended).
  2. Aperture Size: Smaller slits (<0.2") are more sensitive to pointing errors; wider slits may mitigate the risk.
  3. Target Brightness: Bright targets allow for shorter ACQ times, minimizing the overhead cost of adding a redundant sequence.
  4. Current Failure Rate: Given the recent ~9% failure rate, the risk is currently elevated.

Conclusion:
For time-critical, non-repeatable STIS observations, the cost of a redundant "Safety ACQ" sequence is often justified to prevent the permanent loss of science data due to guide star acquisition failures. The authors advise users to consult with Program Coordinators to integrate these sequences into their Phase II proposals.