HST Cycle 28 Primer: Optical Performance, Guiding Performance, and Observing Efficiency

An overview of HST's optical performance, pointing control, guiding with the FGSs, and observing efficiency.

Optical Performance

HST’s Optical Telescope Assembly (OTA) did not perform as designed because the primary mirror was manufactured with about one-half wave of spherical aberration. This was remedied by corrective optics installed for the science instruments (SIs) during the first servicing mission in December 1993. Since then, all SI detectors (with the exception of the FGSs) have viewed a corrected beam, either via external corrective optics (COSTAR) or via internal optics (for second and third-generation instruments). COSTAR was removed from the telescope during Servicing Mission 4 in May 2009; currently, all onboard instruments have internal corrective optics. The table below gives a summary of general OTA characteristics and performance.


Ritchey-Chretien Cassegrain 


2.4 m

Wavelength Coverage 

From 90 nm (MgF2 limited)
to ~3 μm (self-emission limited) 

Focal Ratio 

f /24

Plate Scale (on axis)

3.58 arcsec/mm

PSF FWHM at 5000Å

0.043 arcsec

Encircled Energy within 0.1" at 5000Å

87% (60% - 80% at the detectors)

Because each SI has unique characteristics, the actual encircled energy is instrument-dependent and may vary with observing strategy. The HST Instrument Handbooks should be consulted for instrument-specific point spread function (PSF) characteristics over various wavelength ranges. The Tiny Tim software tool, developed at STScI by John Krist with support from Richard Hook at the ST-ECF, can be used to simulate the PSFs of several HST instruments. It is available for download from the. PSFs generated by Tiny Tim are coarse matches to observed PSFs because of transient focus changes in HST and other effects. As a result, for many types of data analysis, PSF subtraction using Tiny Tim models is inadequate.

HST Guiding Performance

HST’s Pointing Control System (PCS) has two guiding modes available. The default guide mode uses Fine Guidance Sensors (FGSs) that maintain high precision pointing control by using guide stars to actively control the telescope pointing. However, the telescope pointing can also be controlled using the rate-sensing gyroscopes.

FGS - Dual Guide Star Acquisitions

The default operational practice is to schedule observations using the Dual Guide Star mode. In a Dual Guide Star Acquisition, two FGSs are used, each locked on a different guide star. The combined pointing information is used to control the pitch, yaw, and roll axes of the telescope. Dual Guide Star Acquisition times are typically six minutes. Reacquisitions following interruptions due to Earth occultations take about five minutes. This pointing control method was designed to keep telescope jitter below 0.007 arcsec rms, but the current performance has jitter of 0.008 arcsec rms.  A drift of up to 0.05" may occur over a timescale of 12 hours and more–this is attributed to thermal effects as the spacecraft and FGSs are heated or cooled. As a result, observers planning extended observations in 0.1" or smaller STIS slits should execute a target peak-up maneuver every four orbits (see HST Cycle 28 Primer: Orbital Visibility, Acquisition Times, and Overheads).

FGS - Single Guide Star Acquisitions

In cases where two suitable guide stars are not available, a single guide star acquisition can be used. In this scenario, HST’s translational motion is controlled by a guide star in one of the FGSs, while roll motion is controlled by gyros. Therefore, with the current gyro performance, a gyro drift around the guide star will be present that can be as large as 17 milliarcsec (mas) of roll angle per second. This introduces a translational drift across the target; the exact size of that drift depends on the roll drift rate, as well as the distance between the single guide star and the instrument aperture. Note, however, that the gyro drift builds up through occultations, typically limiting a visit duration to a few orbits.  There are also occasions when a dual guide star acquisition is planned, but one of the guide stars cannot be acquired. In this case, the Pointing Control System (PCS) will usually carry out the observations using single FGS guiding.

HST Observing Efficiency

HST’s “observing efficiency” is defined as the fraction of total time devoted to acquiring guide stars, acquiring astronomical targets, and obtaining the exposure for a target. The main factors that limit the observing efficiency are

  • low spacecraft orbit, resulting in frequent Earth occultation of most targets,
  • interruptions by passages throughout the South Atlantic Anomaly,
  • the number of user-constrained visits,
  • a relatively slow slew rate.

About 90% of the usable observing time is allocated to science observations, with the remainder devoted to calibration and engineering observations (≤10%), and repeats of failed observations (~2%).  Note that the fraction of failures has increased gradually in recent years and is now approximately 5%.

Next: HST Cycle 28 Primer: Scientific Instruments ACS, WFC3, and FGS

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