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The Phys.org news item Hubble is the ultimate multitasker: Discovering asteroids while it's doing other observations mentions the Hubble Space Telescope's use of parallel fields:

A couple years ago, the NASA/ESA Hubble Space Telescope was observing very distant galaxies, some of them billions of light years away. It was part of the Frontier Fields Project (FFP,) an ambitious observing program which aimed to observe six massive galaxy clusters. The FFP pushed Hubble to its limits.

[...]

This image is a couple years old now, and it shows what's called the parallel field for the Abell 370 galaxy cluster, about 4 billion light years away. It's called a parallel field because of the technique used to capture it.

A press release explains parallel fields this way:

"While observing each cluster with one of the cameras on Hubble, the team also used a different camera, pointing in a slightly different direction, to photograph six so-called 'parallel fields." This maximised Hubble's observational efficiency in doing deep space exposures, imaging a myriad of far away galaxies."

Question: How are "parallel fields" implemented on the Hubble Space Telescope? How are two cameras able to take different exposures in different directions at the same time? What optical components points them in different directions?

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As the question Instrument aperture sizes on Hubble Telescope shows, the focal plane area is large enough to focus on several instruments at the same time (but with each capturing a different area).
If two objects of interest are separated by a certain angle (the instruments are fixed within the focal plane), the telescope can be rotated so that two instruments are exposed, one to each object. If desired, the telescope can later be rotated to swap the instruments on each object, so that both objects get a wider spectral coverage, for example.

No optical components point the instruments (cameras/spectrometers) at different objects; rather, the whole telescope orients on one direction in space, and by rotating around the main mirror axis to a particular angle (matching the positions of the fixed instruments) can choose to expose multiple instruments to multiple objects in that Field-Of-View. All instruments (except the WFC UV/IR sensors) are always exposed to something, but data collection/transmission stresses the systems, so 'parallel' viewing is used selectively.

Only the Wide Field Camera uses a pick-off mirror (steerable for UV or IR sensors). The other instruments are aligned with their patches of the focal plane and used COSTAR lenses for correction until Service Mission 4 removed COSTAR and replaced the actual instruments (which now have built in corrections). The Wide Field Camera was replaced several times and had its own corrective mirrors instead of a lens via COSTAR.

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  • $\begingroup$ Thanks! +1 but the question I've written goes a bit deeper though. How are "parallel fields" implemented... What optical components points them in different directions? $\endgroup$
    – uhoh
    Commented Jun 27, 2019 at 8:38
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    $\begingroup$ They don't point in different directions. The telescope captures an area large enough to contain several stars or galaxies, and each instruments only captures part of that area. $\endgroup$
    – Hobbes
    Commented Jun 27, 2019 at 10:05
  • $\begingroup$ The diagram in the linked-to question shows the layout of all the instruments in the field of view produced by the primary and secondary mirrors (the OTA). In principle, all instruments could be used simultaneously but data handling and computer constraints limit the combinations: documents.stsci.edu/hst/acs/documents/handbooks/cycle20/… $\endgroup$
    – astrosnapper
    Commented Jun 27, 2019 at 15:07
  • $\begingroup$ @astrosnapper thanks, your link then links to this page which has explicit labels for WFC1, WFC2, and WFC3 so now I can see that there are cameras that really pick off different parts of the focal plane. I had no idea! The image in the linked question is less clear to me. So these must be small fixed mirrors that pick off these different areas and send the light to the different instruments? Something a bit like that shown here? $\endgroup$
    – uhoh
    Commented Jun 29, 2019 at 11:35
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    $\begingroup$ @uhoh - only the Wide Field Camera uses a pick-off mirror (steerable for UV or IR sensors). The other instruments are aligned with their patches of the focal plane and used COSTAR lenses for correction until Service Mission 4 removed COSTAR and replaced the actual instruments (which now have built in corrections). The Wide Field Camera was replaced several times and had its own corrective mirrors instead of a lens via COSTAR. $\endgroup$
    – amI
    Commented Jun 30, 2019 at 5:30

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