HST Cycle 28 Primer: Orbital Constraints

An overview of the HST positional constraints.


HST is in a relatively low orbit (less than 600 km above Earth), which imposes a number of constraints upon its observations. As seen from HST, most targets are occulted by the Earth for varying lengths of time during each 96-minute orbit. Targets lying in the orbital plane are occulted for the longest interval, about 44 minutes per orbit. These orbital occultations, analogous to the diurnal cycle for ground-based observing, impose the most serious constraint on HST observations. (In practice, the amount of available exposure time in an orbit is further limited by Earth limb avoidance limits, the time required for guide star acquisitions or reacquisitions, and instrument overheads.)

Continuous Viewing Zone (CVZ)

The duration of target occultation decreases with a target’s increasing angle from the spacecraft’s orbital plane. Targets lying within 24° of the orbital poles are not geometrically occulted at any time during the HST orbit. This gives rise to so-called Continuous Viewing Zones (CVZs). But note that the actual size of these zones is less than 24° due to the fact that HST cannot observe close to the Earth limb (see HST Cycle 28 Primer: Pointing, Orientation, and Roll Constraints).

Since the orbital inclination is 28.5°, any target located in two declination bands near ≈ 61.5° may be in the CVZ at some time during the 56-day HST orbital precession cycle. Some regions in these declination bands are unusable during the part of the year when the sun is too close to those regions. The number and duration of CVZ passages depend on the telescope orbit and target position and may differ significantly from previous cycles. Please refer to the Orbital Visibility and Scheduling webpage for information on determining the number and duration of CVZ opportunities for a given target location. Also note that the South Atlantic Anomaly (SAA; see HST Cycle 28 Primer: Pointing, Orientation, and Roll Constraints) limits any uninterrupted observation to no more than five to six orbits per day.

The brightness of scattered earthshine background during CVZ observations is not greater than during non-CVZ observations since the same bright earth limb avoidance angle is used. However, the duration of relatively high background can be much longer for CVZ observations than for non-CVZ observations, because the line of sight may continuously graze the bright earth limb avoidance zone during CVZ observations.

In general, CVZ observations should not be requested under normal conditions if observations are limited by sky background. The increased earthshine means that CVZ observations offer virtually no efficiency gain for background-limited broad-band imaging in the optical or infrared. There have been cases in the past (e.g., Hubble Deep Field observations) where optical imaging has been interleaved with other kinds of observations. However, such observations are difficult to schedule and require strong scientific justification. Observers contemplating using CVZ observations are encouraged to contact the STScI Help Desk. CVZ observations are also generally incompatible with special timing requirements (e.g., timing links, special spacecraft orientations, or targets of opportunity; see HST Cycle 28 Observation Types and Special Requirements more details).

South Atlantic Anomaly (SAA)

The South Atlantic Anomaly, a lower extension of the Van Allen radiation belts, lies above South America and the South Atlantic Ocean. No astronomical or calibration observations are possible (except for some specialized and infrequently executed WFC3 observations) during spacecraft passage through the SAA because of high background induced in the scientific instruments and FGSs. 

As the HST orbit precesses and the Earth rotates, the southern part of the HST orbit intersects the SAA each day for seven to nine orbits in a row (so-called “SAA-impacted” orbits). These SAA-impacted orbits are followed by five to six orbits (eight to ten hours) without SAA intersections. During SAA orbit intersections, HST observing activities must be halted for approximately 20 to 25 minutes, except for the previously mentioned WFC3 observations. This cycle of SAA-impacted orbits and SAA-free orbits lasts approximately 24 hours, so is repeated every day. It affects all observations, including CVZ observations.

Typically, uninterrupted observations can execute in five or six orbits before they have to be halted due to HST passage through the SAA. STIS MAMAs and the ACS/SBC have tighter operating constraints than other instruments in that they cannot be operated in an orbit that is even partially impacted by the SAA. These detectors can only be used during the five to six orbits each day that are completely free of SAA intersections. (This restriction does not apply to COS.) Some WFC3 science observations are allowed during SAA passage (e.g., planetary occultations and transits). Please refer to WFC3 ISR 2009-40 and WFC3 ISR 2009-47 for more information about detailed studies of WFC3 camera operations during SAA passage; those documents remain current for Cycle 28.

Predicted HST Position

Atmospheric drag on the spacecraft is a significant issue because HST is in a low Earth orbit. Moreover, the amount of drag varies depending on the orientation of the telescope and the density of the atmosphere (which, in turn, depends on the level of solar activity). Consequently, it is difficult to predict in advance where HST will be in its orbit at a given time. For instance, the predicted position of the telescope made two days in advance can be off by as much as 30 km from its actual position. An estimated position 44 days in the future may be off by ~4000 km (95% confidence level). 

Positional uncertainty can affect observations of time-critical phenomena and near-Earth solar system bodies. In the former case, a target could be behind Earth during the event so it may not be known if a given event will be observable until a few days before the observation. In the latter case, positional uncertainty could introduce uncertainties in the parallax correction.

Next: HST Cycle 28 Primer: Pointing, Orientation, and Roll Constraints


Barker, E. A., McCullough, P., and Martel, A. R. 2009, "WFC3 IR SAA Passage Behavior"

Martel, A. R., Borders, T., and Barker, E.A. 2009, "WFC3 SMOV Proposal 11448: UVIS SAA Passage Behavior"

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