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Almost all the photos I see of the HST show the two small dish antennas at the ends of the two long, opposing booms with the "top" one pointed straight ahead at 12-o'clock, and the "bottom" one at 3-o'clock.

The HST is in LEO, and probably spends most of its time at an attitude either fixed (or nearly fixed) with respect to the stars so it can do science, or efficiently slewing to the next target. That means that these two antenna booms will spend most of their time without a fixed orientation with respect to ground stations on the Earth, or communications linking satellites in GEO.

Do these dishes actually articulate and independently track targets so they can downlink this constant stream of data and uplink their next instructions? If not, does the telescope stop making measurements, stop tracking the heavens, and slew around to point one or the other at communications links?

Either way (stopping science or extra work for ADCS turning the entire telescope) sounds less optimal than just adding articulation to the little dishes.

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above: "An STS-125 crew member aboard the Space Shuttle Atlantis captured this still image of the Hubble Space Telescope as the two spacecraft continue their relative separation on May 19, after having been linked together for the better part of a week." Cropped, from here.

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above: "The Hubble Space Telescope as seen from the departing Space Shuttle Atlantis, flying STS-125, HST Servicing Mission 4." Cropped, from here.

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above: "The Hubble Space Telescope (HST) begins its separation from Space Shuttle Discovery following its release on mission STS-82." Cropped and lightened to better show upper antenna. From here

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above: "IMAX Cargo Bay Camera view of the Hubble Space Telescope at the moment of release, mission STS-31." Cropped, from here.

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Those are the High Gain Antennas, and according to this systems overview, they are steerable:

High Gain Antennas. Each HGA is a parabolic reflector (dish) mounted on a mast with a two- axis gimbal mechanism and electronics to rotate it 100 degrees in either direction (see Fig. 5-9).

I believe they're mounted pointing in opposite directions (12 o'clock and 6 o'clock), so I'm not sure why they're parked at 12 and 3 specifically, but it makes sense that they're parked:

The HGAs achieve a much higher RF signal gain, which is required, for example, when transmitting high-data-rate scientific data. These antennas require pointing at the TDRSs because of their characteristically narrow beam widths. On the other hand, the LGAs provide spherical coverage (omnidirectional) but have a much lower signal gain. The LGAs are used for low-rate-data transmission and all commanding of the Telescope.

As any in-situ pictures of HST would have been taken from the space shuttle on a servicing mission, the HGAs likely wouldn't have been in use.

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  • $\begingroup$ OK so they can be steered. I guess they are bigger than I realized, and those little "hinges" are actually motorized. But I'm still wondering - are they actually continuously moving to keep an Earth station or GEO satellite in view at the same time the telescope is trying to make a long exposure? Is there a problem with vibrations. $\endgroup$
    – uhoh
    May 13, 2017 at 16:30
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    $\begingroup$ @uhoh -- They point to a TDRS, which are geostationary satellites. $\endgroup$
    – David Hammen
    May 13, 2017 at 17:03
  • $\begingroup$ @DavidHammen that certainly makes sense, probably not many alternatives. I wonder if they are actually tracking TDRS satellites at the same time the HST is taking data. I can imagine that it doesn't actually cause any serious problem for at least some classes of measurements. $\endgroup$
    – uhoh
    May 13, 2017 at 17:53
  • $\begingroup$ @DavidHammen Ah, from Wikipedia: "About twice daily, the Hubble Space Telescope radios data to a satellite in the geosynchronous Tracking and Data Relay Satellite System (TDRSS)...Each week, HST downlinks approximately 140 gigabits of data." So at 10 Mbit/s just for example, that's 15 minutes twice a day, not really an inordinate impact if it did interrupt observations, which it might not. $\endgroup$
    – uhoh
    May 13, 2017 at 18:10

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