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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 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 its 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 design choice made because because of Gegenschein backscatter?

Edit: this question was cross-posted and answered on the Astronomy site. Accepted answer contains helpful data in IR reflectance which shows Gegenschein is not an issue.

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  • $\begingroup$ +1 We need to know the size distribution of the dust to know if it's expected to be any less reflective at JWST wavelengths than it is in visible wavelengths i.e. if $\lambda^{-4}$ applies or not. Some quick poking around the internet suggests that the effect (brighter scattering near 180°) is due to "shadow hiding" or "opposition surge" rather than Mie or Rayleigh scattering. $\endgroup$
    – uhoh
    Commented Jan 3, 2022 at 5:48
  • $\begingroup$ But we need some good scientific sources before any conclusions can be drawn from my ramblings. $\endgroup$
    – uhoh
    Commented Jan 3, 2022 at 5:50
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    $\begingroup$ I (hopefully) clarified the question a bit, to make clear that this is about the design choice, not about limitations of the realization. (Assuming I understood the question correctly) $\endgroup$
    – Ludo
    Commented Jan 3, 2022 at 8:36
  • $\begingroup$ 6 months later or 3 months later? $\endgroup$
    – AJN
    Commented Jan 3, 2022 at 12:44
  • $\begingroup$ @AJN You are correct. 3 month would rotate the Gegenschien 90* on the celestial sphere.,, 6 months would put the target behind the Sun. $\endgroup$
    – Woody
    Commented Jan 3, 2022 at 16:38

2 Answers 2

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I worked on the Canadian portion of JWST. To look in the direction directly opposite the Sun the primary mirror would have to be facing normal to the plane of the sun shield. The diameter of the primary mirror is 6.5m; the diameter of the fairing on the Arianespace launch vehicle is 5.4m. The launch origami is a lot less complicated when the primary mirror points in a direct parallel to the sun shield, not normal too it. Keep in mind that whatever is currently in deep space in the anti-sun direction can still be imaged by JWST 3 months (25% of a year) later. As with most things JWST related, patience pays dividends.

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  • $\begingroup$ ----- Welcome, Steve. If the sunshade-to-optic axis angle is fixed (as in the current design), the current 80* angle offers the most aiming flexibility. If JWST was designed with a sunshade which could pivot 45* around the V-2 axis, the field of regard would be enlarged to include the anti-sun direction. Gegenschein does not preclude observation in that direction. What is the reason for lack of pivoting? Was it just not seen as a design priority? $\endgroup$
    – Woody
    Commented Jan 15, 2022 at 17:45
  • $\begingroup$ @Woody this answer links to How can the proposed LUVOIR space telescope slew to different directions while keeping the sunshade in a fixed orientation? What compensates? which covers some aspects of how much more complicated that would be to implement in reality $\endgroup$
    – uhoh
    Commented Jul 29, 2022 at 12:05
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Question: By design, JWST cannot observe in the anti-sun direction. Is this due to Gegenschein backscatter?

"Nothing to do with Gegenschein, merely a consequence of..."

not making the design so complex and advanced that it couldn't be launched by 2007, though even still didn't work out so well.

tl;dr: I can't say that Gegenschein wouldn't have been visible or even a problem, just that there was no way that direction would have been possible for this generation of thermal IR space telescope. Maybe LUVIOR...

The primary goal was to look at things billions of years in the past, at the edge of the visible universe, fairly near the big bang in time. It doesn't matter if you looked at them now, or three months from now, those scenes are not going to change.

And neither will the exoplanets some scientists want to look at in order to make the public think they're getting something they can better understand for their \$10,000,000,000.

In its present configuration, the smaller hole in the observatory's Field Of Regard or FOR is directly due to the limits of how the sunshield could extend behind the back side of the mirror.

You can not substantially extend it and wrap it around the backside without causing big problems with the photon pressures involved, and it would probably cause problems keeping the primary at a constant temperature.

It's not impossible, it's just a huge jump in complexity; the sunshield is already super-complicated and the source of a substantial delay and budget increase, making it now 3D or at least much much bigger would be prohibitive.

The other way you can do that is by finding a way to point the telescope independent of the current sunshield attitude. That is planned for a future as-yet-to-be-funded mission, the Large Ultraviolet Optical Infrared Explorer (LUVOIR).

For more on that see


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