Not Humanity’s Last View …

This is being described as “humanity’s last view of the JWST.”

6:50 AM CST, Christmas Morning

I expect imaging, and even direct viewing, of the James Webb Space Telescope from terrestrial telescopes to become a popular amateur astronomical activity in the summer of 2022. Here is why:

A full Moon has apparent magnitude -12.7. This is the result of its distance of ≈380,000 kilometers, its effective area (a circle of radius ≈1,700 kilometers) of ≈9.1 million km², and its albedo of ≈0.12.

The JWST will be at ~4 LD, the effective area of its sunshield will be ≈830 m², and its albedo will be very close to 1.

Its distance makes it 16 times as faint, its effective area makes it 11 billion times as faint, and its albedo makes it 8.3 times as bright. Multiplying all these together yields a factor of 21 billion.

The magnitude scale is measured in increments of ⁵√100 ≈ 2.5, such that each 5 steps downward is 100 times brighter. Venus, which can reach an apparent magnitude of -4.7, is nearly 100 times brighter than Arcturus (α Boötis), at -0.05. The stars in the Big Dipper and in Orion’s Belt are around magnitude +2.

The limits of my experience are the Sun, apparent magnitude -26.7, and some of the fainter Pleiades, magnitude +6.5 or even fainter—note that this takes not only very clear, dark, moonless skies, but also an hour and a half or more of no artificial light whatsoever for excellent dark adaptation, and probably eyes younger than mine are now (I am recalling an incident from my 30s). That’s a factor of almost 20 trillion.

Anyway, doing the math, something 21 billion times fainter than a full Moon has an apparent magnitude of +13.1.

Every amateur astronomer reading this just went huh. Easy.

Taking the usual limiting magnitude of the unaided eye to be exactly +6 and the effective aperture of the human pupil to be 7mm, less than 200mm of primary lens or mirror diameter would be enough. In the real world, it’s going to be harder than that … but I found Pluto in my 333mm f/4.5 Newtonian at magnitude +13.8 or thereabouts during a Texas Star Party in the 1990s.

The challenge will be figuring out which thirteenth-magnitude speck in the field of view is actually the JWST, but one thing’s going to make it a lot easier: it won’t be moving with the starry background. Its motion will essentially be at the solar rate, ~1°/day. That’s 2½ arc-minutes per hour, or 2½ arc-seconds per minute. A pair of images taken even a few minutes apart will pop it out, much like the discovery images of Pluto in 1930.


UPDATE (12/31): en route

6 thoughts on “Not Humanity’s Last View …”

  1. You beat me to it, Jay.

    J.K. That’s an awesome amount of knowledge and experience. I’m considered the sage of the neighborhood because I can point out planets and some stars on really clear nights.

    I just hope I’m around to see what the JWST finds out.

  2. 1. Fingers crossed there’s no problems like Hubble had because it’s completely unrepairable.
    2. It can’t ever get upgraded instruments so its lifetime is going to be much shorter than Hubble’s.
    3. Given the time and cost overruns it’s a miracle it wasn’t cancelled years ago.
    4. Naming it after some bureaucrat is a travesty.
    5. One wonders what NASA can do after this, there’s really no obvious next leap in capability that’s feasible, and budgets are about to run into serious squeezes.

  3. Brian: “One wonders what NASA can do after this”

    NASA could get back to the main mission Barry Soetero set for it — Outreach to Islam.

    Biden*’s handlers would probably add to that — Get trans-sexuals into space! Lesbians and homosexuals will just have to wait. And now the taxpayer is paying for a maternity suit for fighter pilots, can a space suit for pregnant persons be far behind? (Just as soon as they can work out the proper terminology for pregnant persons, since “women” is now seen as triggering).

  4. Cousin Eddie — hahaha, if you can correctly identify any celestial object other than the Sun and the Moon you’re pretty much a 1%er

    Brian — yep, SEL2 is (if I’m doing the math right) at least five weeks away on a Hohmann transfer ellipse, and until Elon gets Starship working regularly, we can’t send anybody out there;
    I frankly expected it would never get off the ground;
    the naming was rather unimaginative … strictly speaking, the Hubble should have been named the Spitzer (first to propose a space telescope), the Spitzer should have been named the Herschel (discoverer of infrared light), and the JWST should have been named the Hubble;
    from a purely technical standpoint, the next step is to put a similar, or larger, instrument, perhaps even an optical interferometer with a very long baseline (100m or more) out beyond the asteroid belt, thereby avoiding the zodiacal light, which is a significant source of interference when observing objects within at least 4° of the ecliptic … which I would expect to require, among other things, laser rather than radio communications for sufficient bandwidth

    Gavin — we could do worse, and probably will, at least briefly

  5. Wow. Yeah you will be able to see the dot, where it is. Very strong scopes may reveal a little more, but it is the last view of the actual craft before it gets to its L2 point.

    I at least appreciate the effort, and applaud NASA and the European Space Agency for this beautiful launch and look forward to the science and images it will produce. I got into photography because of my scopes and had a wonderful time with them. My Astro-Physics 5″ Refactor I waited 6 months for, was amazing and I sold it for considerably more than I paid for it.

    What a bunch of wankers you lot are. ;)

  6. Jay,
    As I’m sure you know, they can do radio frequency interferometry with baselines of thousands of KM. I’m always a little disappointed when I see an especially striking image to find out it’s a rendering of radio astronomy data rather than something “visible”. Of course, optical astronomy has progressed far past the point where someone can “see” what the instrument is recording as well, so the distinction is mostly arbitrary.

    I wonder how long a baseline could be extended for space based instruments before relativistic effect intervened. I wonder if we’ll ever be able to raise the accuracy of synthetic aperture interferometry to where it becomes possible to do it in the visible light band.

    The real challenge of a space based Interferometer would be building a structure that maintained geometric precision to nanometers in space, not that it’s that easy to do on the ground.

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