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The Curious Droid YouTube video NASA's Mega Hubble - The Roman Space Telescope illustrates the shape of the telescope's field of view and the outline has a smile or frown shape. For whatever reason it reminds me of a PacMan "ghost".

Why does the field of view have this curved shape, I'd naively expect it to be rectangular. Is there some off-axis aberration correction magic going on? Something to do with its adaptive optics system?

screenshot from Curious Droid YouTube video "NASA's Mega Hubble - The Roman Space Telescope" https://youtu.be/PNO_ofQijbY

From WFIRST Update; Jeffrey Kruk, WFIRST Project Scientist (Archived)

from "WFIRST Update; Jeffrey Kruk, WFIRST Project Scientist" https://science.nasa.gov/science-pink/s3fs-public/atoms/files/Kruk_WFIRST_APAC_April2018.pdf Archived https://web.archive.org/web/20211010211555/https://science.nasa.gov/science-pink/s3fs-public/atoms/files/Kruk_WFIRST_APAC_April2018.pdf

Screenshot from Google Doodle pacman10-i note: it starts playing as soon as you click, adjust your device volume first, then enjoy!

Screenshot from Google Doodle https://www.google.com/logos/2010/pacman10-i.html note: it starts playing as soon as you click, adjust your device volume first, then enjoy!

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  • $\begingroup$ “Something to do with its adaptive optics system?” It’s a space telescope, so it doesn’t need or have adaptive optics… $\endgroup$
    – Peter Erwin
    Oct 13, 2021 at 8:22
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    $\begingroup$ @PeterErwin and yet, it most certainly does! In order for the (really amazing) coronagraph to work sufficiently to allow imaging of exoplanetary systems, the wavefront has to be correct to tens of picometers, in order to allow for deep nulling of the bright light from the primary. See sections starting below figure 37 There's no way to build something like that on the ground, launch it into space and maintain it in a space environment to such optical tolerances passively. $\endgroup$
    – uhoh
    Oct 13, 2021 at 8:32
  • $\begingroup$ DM1 and DM2 in Figure 37 are deformable mirrors. Unlike ground systems that work at close to kHz, the figure shows their actuation bandwidth is about 5 milli-Hertz, consistent with a very slow (sub picometer per second) thermal drift of a well built space telescope optical system. $\endgroup$
    – uhoh
    Oct 13, 2021 at 8:44

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By offsetting the SCA columns, the Field of View layout follows the natural annular curve of an off-axis TMA optimized field. Short answer: Yes, off axis aberration magic.

Longer answer: The following journal article, "Optical design and predicted performance of the WFIRST phase-b imaging optics assembly and wide field instrument" (Bert et al., 2018) includes this statement that answer this question:

By offsetting the SCA columns, the Field of View layout follows the natural annular curve of an off-axis TMA optimized field.

Unpacking the jargon a bit – SCA == sensor chip assembly, these are the detector array assemblies, 4k x 4k pixels 10um each, Near IR TMA == three mirror anastigmat, a telescope design form that minimizes aberrations over a wide field

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    $\begingroup$ Sweet! Thank you for the speedy, concise, clear and well-sourced answer, and welcome to Stack Exchange! $\endgroup$
    – uhoh
    Oct 14, 2021 at 13:27

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