About this Article

Marc Rafelski (mrafelski[at)stsci.edu)

Published April 29, 2026

The Hubble Space Telescope (HST) relies on its Fine Guidance Sensors (FGSs) to achieve the milliarcsecond-level pointing stability that underpins virtually all its science. Typically, two out of three available FGSs are used simultaneously during science exposures: one locks onto a primary guide star, while a second locks onto an additional star to maintain the telescope’s roll orientation. Starting in Cycle 34, qualifying imaging observations with the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) and Wide Field Camera 3 (WFC3) UVIS and IR will execute with single guide star (1GS) guiding by default with the telescope roll under gyro control — and no impact to image quality.

The transition to 1GS as the default guiding mode is motivated by three complementary goals: extending the life of the FGSs by reducing their usage, maximizing the observing success rate, and maximizing scheduling flexibility for science programs. First, each guide star acquisition causes mechanical wear on the FGS, so using one FGS rather than two reduces cumulative usage and prolongs their lifetime. Next, using only one FGS reduces acquisition failures and the risk of a loss of lock during observations, which has recently become more common when including FGS2. Finally, when HST transitioned to reduced gyro mode (RGM) in June 2024, the orientations available for a given target became more constrained, and there are times when only one suitable guide star can be found within the accessible FGS fields. By using only a single guide star, those targets are more likely to be observable during their visibility windows.

Image Quality with Single Guide Star Observations

During execution of observations with 1GS guiding, roll control relies on the gyro rather than a second guide star. Gyro drift in 1GS can cause a slow rotation around the primary guide star, which translates into a pointing drift at the science detector. The magnitude of this drift depends on the roll rate, the angular distance between the guide star and the detector, and the exposure time. Since fall 2024, all Snapshot (SNAP) program exposures have been acquired under 1GS guiding, improving the SNAP success rate while simultaneously providing a growing dataset to evaluate 1GS performance.

The SNAP data, in conjunction with a dedicated calibration program for longer exposures, was used for a comprehensive evaluation of 1GS image quality (WFC3 ISR 2025-07). For exposures shorter than 500 seconds, the point spread function (PSF) quality under 1GS is indistinguishable from that under two guide stars (2GS). For longer exposures, up to the duration of an orbit (~2,500s), the drift-induced PSF variation is mostly comparable to the PSF variations normally seen due to focus breathing and position on the detector. We note that for science requiring precise astrometric information, the Instrument Science Report (ISR) also showed that using a perturbed PSF model in post-acquisition analysis with the hst1pass software effectively removes all astrometric residuals and most photometric residuals, even for drifts as large as 0.5 pixels.

Qualifying Observations for a Single Guide Star in Cycles 34 and 35

Beginning in Cycle 34, 1GS guiding will be applied as the default for observations meeting all the following criteria:

• The observation uses ACS/WFC or WFC3 (UVIS or IR) in imaging mode;
• All exposures in the visit have exposure times less than 1,001 seconds; and
• The observation is not a moving-target or spatial-scan observation.

This covers the majority of ACS and WFC3 observations. We will evaluate increasing the maximum exposure time requirement for default usage of 1GS as more data become available.

Requesting to Opt out of 1GS

While 1GS observations are of very high quality, an opt-out mechanism is available for PIs who believe their science requires 2GS guiding, such as programs sensitive to sub-pixel pointing drifts in ways that cannot be corrected in post-processing.

• In Cycle 34, PIs may request an exception by submitting a written scientific justification to their Contact Scientist after Phase II notifications are sent out. If approved, the PI can select the "No Single Star Guiding" flag in APT at the visit level.
• Starting in Cycle 35, opting out of 1GS will require selecting a 2GS special requirement in APT during Phase I and providing a scientific justification in the proposal.

We expect that moving to 1GS will have no perceptible impact on image quality. The benefits are significant: reduced mechanical wear on the FGS system extends the lifetime of hardware that cannot be replaced; reduced acquisition failures and loss of lock on the guide stars increases the observing success rate; and the system gains additional flexibility in scheduling observations. All of these contribute to keeping HST as productive as possible as it continues operations into the 2030s.

This article was updated June 12, 2026.