Hubble in trouble

Excellent news. I have learned that Hubble does UV observations not planned for JWST or any other space scope. Not possible for scopes under atmosphere. Perhaps possible for balloon-carried scopes but they have very limited coverage.

It is too late to modify JWST hardware, but I hope it allows for for swaps such as seem to have saved Hubble.

 

UV observations require a more finely figured mirror system than do optical and IR observations, which would have meant a substantial cost adder.

As for equipment swaps, JWST will be in solar orbit, not earth orbit, near the Sun-Earth L2 point. That is about 4X the distance of the moon. That is quite a ways beyond current crewed craft.

 

Hope they get it lofted.....this year or next.

 

If I am reading the article right, the 'swap' that has restored Hubble was a swap of responsibility between modules already on board.

 

Backup power control unit substituted for primary.

 

I thought he was referring to the long-ago Shuttle missions that swapped in upgraded replacement hardware.

If he was referring to the very recent soft-swap from primary to backup hardware already on board, that sort of built-in redundancy is pretty much the norm for such missions. Stuff happens, things break, and having a big menu of backups built in is far cheaper than sending up a replacement satellite every time something goes wrong.

 

I take @fuzzy1 point #42 that a 'smoother' mirror is needed for UV/Vis than for IR. But I find not evidence that JWST cheaped out in this department. It has segmented mirror with individual segments wiggled to make very nice prime focus. While adaptive optics have become common on 'below-atmosphere' telescopes, JWST is the first flyaway telescope with anything like that.

All the action is now down to small fractions of arc-second resolution, which does require better surfaces for shorter wavelengths. To play this game on earth, use adaptive optics within atmospheric 'windows', or do inferometry with much longer-wavelength radio telescopes. It is my understanding that Hubble (with 'glasses') and JWST both resolve to about 40 milli arc seconds. Further, as space telescopes are not bolted to anything solid, pointing accuracy contributes (just as much?) to this as mirror-surface quality. This involves 'reaction wheels' (like gyroscopes) and all space telescopes address that problem in some way.

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Interestingly different are (LIGO) gravity-wave observatories. They are bolted to the ground but for their game, the ground itself jiggles far too much. What they do to isolate from that motion is insane. It is my understanding that until the first gravity-wave signal was detected by multiple LIGOs, builders doubted that they had adequately 'decoupled'.

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Hubble telescope is named for the fella who convinced others that 'everything' was a vast number of galaxies, like our galaxy, but spanning a spatial volume that will probably be forever beyond the capacity of humans (who deal with kilometers and years) to understand. Hubble did that with early 20th century mirror shaping, photographs, and detailed work by (you may have guessed I'd get to this) by mostly female analysts who don't get things named after them.

Not by himself, Edwin Hubble more correctly revised humans' perception of everything and we should be very happy that his namesake '2.4 meters' can take more pictures.

 

A few checks around the web found mirror surface 'roughness' claims of 24 or 25 nm rms for JWST, and 8 nm rms or "accuracy of 10 nanometers" for Hubble. So JWST just can't be diffraction limited down to the same short wavelengths as Hubble.

But once a telescope is diffraction limited to a desired wavelength, its possible resolution is essentially just the ratio between wavelength and aperture.

 

Hubble recommenced operations:

See the first photos from the Hubble Space Telescope after a major computer malfunction | Space

 

Related to smoothness of mirrors in post #47, Hubble images wavelengths from 300 to 700 nanometers. JWST 'will' ( ) image wavelengths from 600 to 5000 nanometers.

JWST mirror is the first made from beryllium instead of (some recipe of) silica, and maybe it's more challenging to smooth at 20 nm scale. I do not know.

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Having enthused here about JWST being a great replacement for Hubble, it's fair to also mention JWST has a 5-year service life and 10-year 'goal'. Far exceeded by Hubble even before its reboot.

 

Doing some more searching ...

Hubble's mirror has a reflective coating of aluminum, topped with a protective coating of magnesium fluoride. It is kept near room temperature.

JWST's primary mirror is beryllium, which is lighter, and is cut much thinner, so its total mirror assembly mass is lighter than Hubble's much smaller mirror. Beryllium is also holds its shape to the cryogenic temperatures where it will be operated. And it is coated with gold, which has excellent reflectivity in JWST's IR and near-IR sensing range.

Edmunds Optics has a page describing various reflective coatings, see additional graphs and charts there:

 

"Hubble's mirror...is kept near room temperature."

Well. I don't know about that. Define room and kept.

"JWST's primary mirror is beryllium, which ... holds its shape to the cryogenic temperatures where it will be operated."

JWST maintains optical 'cryo temperatures' with electrical heat pumps (as I would describe them), not liquid helium as earlier space IR telescopes have used. This seems among its 'we really do need a billion dollars' advancements. But with only 5 to 10 years of service, it had better not take weekends off

 

I think one source said about 15C, another about 70F, thanks to heaters.

I was seeing stuff indicating that JWST's optics are passively cooled, on the shady side of a large sun shield, but another item indicates it also has a cryocooler for at least one instrument. They are aiming for 50K, not nearly as cold as those liquid helium missions (4K), but cold enough to need the L2 positioning to keep the IR emissions of the Earth and moon away from the cold side of the shield.

Didn't one of those liquid helium missions suffer an ice bridge that wasted its helium supply extremely rapidly?

 

A telescope in low earth orbit, while human-visitable, is peculiar because about every 90 minutes it has wide swings in external radiant energy fluxes. So I'm not surprised that efforts were made towards isothermal, but 15 oC 298 oK seems like an odd target. Guess they had their reasons.

Trying to learn Hubble, I learned that original optical corrector COSTAR had specific outputs for each downstream instrument. And that it was later removed and is now in Smithsonian or whatever. Later servicing missions replaced it with specific correctors for each downstream instrument.

Hubble may qualify as the most-repaired space thing. On earth we have Fiats and General Motors

 

My first guess: that is about the temperature at which it was ground and polished and attached to its frame, so is a good target to minimize mechanical distortions from mismatching of thermal coefficients. Not being an IR system, chilling the optical surfaces wasn't particularly important. (Though electro-optical sensors are a different matter.)

Any other guesses, or better yet, actual design descriptions?

 

Meanwhile here is a long list of space telescopes:

Space Telescopes | Historic Spacecraft

 

Speaking of milli-arc seconds See a nice ring here:

Astronomers Detect Potential Moon-forming Disk around an Exoplanet - Sky & Telescope - Sky & Telescope

By my calculations, the ring is 8 milli-arc seconds across. Somebody could confirm that. Visible now because radio telescopes can be time aligned for inferometry with baselines as long as earth is wide.

The whole thing would be like 1 pixel in visible or near-visible spectra telescopes we discuss here. Maybe next generation (like ELT) will get 'er done, but for now its still radio.

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Was there a pop song called 'video killed the radio star'? Quite the opposite persists in astronomy.

 

My figures are suggesting that the big pretty ring around PDS 70 is closer to a full arc-second. The little tiny disk around PDS 70c, in the inset, is about 10 milli-arc-seconds.

Derivation: 1 a.u. is about 500 light seconds. The target is 400 light-years away, or about (400 years * 365 days/yr * 86400 seconds/day) = 1.26E+10 seconds away. At that distance, 1 a.u. subtends an angle of 500 / 1.26E+10 = 4.0E-9 radian = 2.3E-6 degrees = 8 milli-arc-seconds.

The little tiny inset disk is 1.2 a.u. across, or 10 milli-arc-seconds. The big pretty ring doesn't have a listed diameter, but is farther out from the center than is 70c, which 34 a.u. away. I'll guess the outside of the ring is double that, or ~70 a.u. radius, or 140 a.u. diameter, with computes out as a bit over 1000 milli-arc-seconds.

Now, somebody needs to check my work too. I can also flub it.

I remember VLBI baselines reaching across earth's diameter being achieved very long ago. Wikipedia says 1976. But that was done with a much longer wavelength, over a cm, maybe even 18 cm, and the resulting images were much less detailed.

Now, with greatly improved clocks and hardware, they are doing it at much shorter wavelengths, producing far better resolution. Someday, maybe they'll even advance to achieving it optically, though I probably won't live long enough to see that. That level would probably be based in space, not on land.

 

Ya I did the math based on resolving a 1 AU thing -> 1 MAS. It was poor reading comprehension that led me to suppose that the big disk was 1 AU. Long axis of big disk looks about 100 times larger, so there's the arc second.

Additional optional reading:

Bad Astronomy | Moons may be forming around a nearby exoplanet

https://iopscience.iop.org/article/10.3847/2041-8213/ac0f83