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

Abstract

For observations of supernovae, kilonovae, tidal disruption events, and other non-repeatable observations, it is important the science data is taken successfully within a specific time window. Part of obtaining that data is often centering objects in the aperture to a higher accuracy than is available from Hubble Space Telescope's (HST's) blind pointing. On HST STIS the sequence of exposures responsible for this centering is the target acquisition or STIS ACQ sequence, and it is most often placed only at the beginning of a visit. Unfortunately, STIS ACQ sequences will fail if the observatory experiences issues locating guide stars in time for the start of the required exposures. If the guide stars are located at a later point in the visit, the remaining science exposures can be taken but the pointing might not be as accurate as is required. This work discusses both the frequency of this issue and the feasibility of placing redundant or "safety" STIS ACQ sequences in a multi-orbit visit to regain the desired pointing accuracy in an affected visit. To do so we select a subset of all 113 STIS ACQ sequences from September 2018 to September 2023 which have experienced this issue. We find that this problem occurs in ~5% of the total STIS ACQ sequences taken during that time period, with a recent increase in the rate to ~9% from March to September 2023. Since the observatory goes through periods of better or worse pointing performance, this recent increased failure rate is not guaranteed to continue. For those failed visits which span multiple orbits, ~39% never obtain a lock on the guide stars and thus take no data. Of the multi-orbit visits that do recover the guide stars, the majority (~78%) do so before the beginning of science exposures in the second orbit. We also provide advice for users on how to make a risk assessment based on the analysis presented here.

Figures (8)

• Figure 1: Amount of total slewing required to move the science target into the slit during the STIS ACQ sequence. We have selected only a subset of observations where one of the three FGSs (FGS 2) was dominant. The vertical line represents an update to the FGS-STIS alignment which improved the blind pointing accuracy. These updates occur periodically. From August 2022 onward FGS 1 and 3 have primarily been used as the dominant sensor and have blind pointing errors slightly less than 0.35 arcseconds. We show the FGS 2 dominant observations here since of the three it most clearly shows the improvement the FGS-STIS realignment made to the blind pointing accuracy.
• Figure 2: Images taken during a successful STIS ACQ sequence for a follow up visit in the same program discussed in Section \ref{['sec:Introduction']} which experienced a failed STIS ACQ sequence in a prior visit. From left to right is the coarse-locate image representing the initial pointing, followed by the fine-locate phase image taken after a slew based on the position of the target calculated in the coarse-locate phase, and finally the HITM exposure taken in order to calculate the true position of the slit. The bottom two images are Gaussian smoothed versions of the first two, titled with the sum of the counts in each image. The X overlaid in the images is at the 2D centroid.
• Figure 3: Image of the orbit planner tool in APT showing the planned progression of events in the first two orbits of the visit discussed in Section \ref{['sec:Introduction']} which experienced a failed STIS ACQ sequence. The initial guide star acquisition is placed at the beginning of the first orbit. The STIS ACQ sequence (labeled Exp. 1) is shown in the dashed blue lines following the guide star acquisition in the first orbit. This is followed by a pointing maneuver to center the target in the science aperture (the last step of the fine-locate phase). The science exposures in are shown with as dotted blue bars (labeled Exp. 2 and 3) and are sets of dithered exposures (hence the pointing maneuvers between each exposure to move the target along the detector). Note that the STIS ACQ is only present in the first orbit. When this visit was executed the initial guide star acquisition failed causing the STIS ACQ sequence and remaining exposures in the first orbit to be blank. The guide star re-acquisition in the second orbit was successful and the following science exposures were taken, but the pointing could not be guaranteed due to the missed STIS ACQ sequence.
• Figure 4: Total HITM counts for each STIS ACQ sequence from September 1, 2018 through September 1, 2023. The dashed line indicates the cutoff below which images were assumed to be of an unilluminated frame.
• Figure 5: Images taken during the unsuccessful STIS ACQ sequence which occurred at the beginning of the visit discussed in Section \ref{['sec:Introduction']}. In this scenario the initial HST guide star acquisition failed at the beginning of the orbit resulting in the TDF remaining down for the STIS ACQ sequence and science exposures during that orbit.