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This question has been reposted from Space Exploration

Gegenschein is a faint bright spot in the zodiacal light, centered at the antisolar point.

enter image description here By ESO/Y. Beletsky - ESO, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=22975456

It is analogous to Heiligenschein, the bright spot on the ground, at the antisolar point, commonly seen from aircraft.

Gegenschein is backscattered sunlight from interplanetary dust. There is a concentration of these particles centered at the Earth-Sun L2 point. https://en.wikipedia.org/wiki/Gegenschein

The JWSTs field of regard excludes 45* around the antisolar point. It cannot aim at targets in the Gegenschein field.

enter image description here https://jwst-docs.stsci.edu/jwst-observatory-characteristics/jwst-observatory-coordinate-system-and-field-of-regard

Answers to the question "Why the JWST blind spot?" https://space.stackexchange.com/questions/56076/why-does-jwst-have-such-a-big-blind-spot explained JWSRs blind spot in terms of technical challenges.

Yet renderings of the proposed VUVOIR telescope show full articulation between telescope and sunshield. Presumably a steerable sunshield is achievable if desired.

enter image description here https://spaceflightnow.com/2017/07/21/study-teams-comb-through-nasas-wish-list-for-a-new-telescope

Gegenschein backscatter could potentially decrease the quality of JWST observation in the anti-sun direction. Six months later, the Gegenschein would have moved 90* across the celestial sphere, allowing unobstructed observation.

Thus, resources devoted to increasing JWST field of view to include the anti-sun direction, would only provide lower quality observation than what is already available. It would be wasted money and effort. However, this argument depends on the relative intensity of backscattered IR vs target IR, something I could find no information on.

Question: By design, JWST cannot observe in the anti-sun direction. Is this due to Gegenschein backscatter?

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I'm going to say no.

Gegenschein is, as you note, reflected sunlight, and is the brightest part of the (reflected component of the) zodiacal light. It's not very bright in the optical (there are no zodiacal-light-based pointing restrictions for the Hubble Space Telescope), and it rapidly gets fainter the further into the infrared you go. For wavelengths longer than about 3.5 microns, the zodical light is actually dominated by thermal emission from the dust grains. Since this has nothing to do with reflected sunlight, there is no "Gegenschein" at longer wavelengths, which means Gegenschein is irrelevant for most of the wavelength range JWST is intended to observe.

enter image description here Figure 16 from Tsumura et al. (2013), showing the contributions of scattered sunlight (filled circles) and thermal emission (open circles) to the zodiacal light, as a function of wavelength. This shows how rapidly the scattered sunlight component decreases as one moves further into the infrared, as well as the fact that thermal emission from the dust grains (no Gegenschein effect) dominates at longer wavelengths.

Yet renderings of the proposed VUVOIR telescope show full articulation between telescope and sunshield. Presumably a steerable sunshield is achievable if desired.

Those are mockups for a hypothetical telescope to built after JWST, using technology that has not yet been developed (and assuming a simpler sunshield design than JWST uses). This could perhaps have been done for JWST -- if you were willing to tolerate even more years of delay and an even higher cost, along with a reduction in the size of the mirror, the number of instruments, and/or fuel for maneuvering, since adding an articulation mechanism would add mass and volume to the final spacecraft, which would have to be taken away from other components.

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I'm going to say no as well. If you look at the way JWST is pointed in the anti-sun position, you can see that the telescope boresight remains parallel to the plane of the sunshield (see diagram below). In other words, there is no "elevation" axis to the mounting. Similarly, you can see that there is no need for an "azimuth" axis because the whole assembly can be rotated about the sun line using the reaction wheels.

enter image description here

The telescope boresight can only be tilted so far before the telescope is no longer fully in the shadow of the sun shield. That limitation leads to JWST's field of regard (which excludes observing the gegenschein):

enter image description here

The diagram below confirms that there is no gimbal in the telescope's mounting. In fact, the electronics boxes are so close to the sunshield that any "elevation" motion would be problematic.

enter image description here

Why did they design it this way? I can only speculate since I have not found any design documents regarding the decision. However, I did spend 35 years designing and building scientific spacecraft, so I will use my judgement. By eliminating the "elevation" gimbal, the design was lighter and more compact (no need to allow room for the telescope to pivot without hitting the sunshield). Both made it easier to launch. In addition, they eliminated the risk of the gimbal failing and getting stuck, and they simplified operations (fewer commands to send to point the telescope). The cost was a restriction in the field of regard in the anti-sun direction. I am certain that the scientists involved agreed with such a fundamental trade-off.

Sources:

https://jwst-docs.stsci.edu/jwst-observatory-characteristics/jwst-observatory-coordinate-system-and-field-of-regard

https://directory.eoportal.org/web/eoportal/satellite-missions/j/jwst#foot33%29

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