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Comment by nabla9 | Hacker News Reader

Hershel operated 4 years and one month, but this was because it needed liquid helium coolant and it eventually ran out. Hershel was planned to operate 3.5 years.

btw. Hershel wast the first optical telescope placed in L2. It was also it was also the largest optical space telescope before JWST.

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Wait if Hershel was the largest optical telescope in orbit, how come it didn't catch the public imagination like Hubble did?

Because it must be American to catch attention in American media.

Hershel took awesome images. For example:

Galactic outflow with the Suzaku X-ray astronomy satellite https://www.esa.int/var/esa/storage/images/esa_multimedia/im...

Herschel’s view of new stars and molecular clouds https://www.esa.int/var/esa/storage/images/esa_multimedia/im...

The Little Fox and the Giant Stars https://www.esa.int/var/esa/storage/images/esa_multimedia/im...

Star formation on filaments in RCW106 https://www.esa.int/var/esa/storage/images/esa_multimedia/im...

Feathery filaments in Mon R2 https://www.esa.int/var/esa/storage/images/esa_multimedia/im...

Herschel was more modest in size, 3.5 meters vs JWST:s 6.5 meters. Also ESA PR is not as great as NASA. NASA is the gold standard in this regard.

Also because it was imaging far infrared with wavelengths 100-1000 times that of visible light, the resolution of the optics was much lower and yielded kind of bla images (with a few notable exceptions):

https://www.herschel.caltech.edu/images

I think a big reason Hubble managed to do that was that it captures in the visible spectrum due to being a repurposed spy satellite. That spectrum just isn't that interesting these days afaict, especially since ground based observatories have gotten a lot better at seeing through the earth's atmosphere. The false color images of most of today's missions are just a lot harder for the public to understand. It was also launched in a time where NASA desperately needed good publicity.

"... due to being a repurposed spy satellite."

What? Where did this come from?

KH-11 ("Keyhole-class") spy satellites. It's generally believed that Hubble's optical technology came directly from that.

> KH-11s are believed to resemble the Hubble Space Telescope in size and shape, as the satellites were shipped in similar containers. Their length is believed to be 19.5 meters, with a diameter of up to 3 meters (120 in). A NASA history of the Hubble, in discussing the reasons for switching from a 3-meter main mirror to a 2.4-meter (94 in) design, states: "In addition, changing to a 2.4-meter mirror would lessen fabrication costs by using manufacturing technologies developed for military spy satellites".

https://en.wikipedia.org/wiki/KH-11_KENNEN

https://space.stackexchange.com/questions/3448/was-hubble-re...

I doubt general public even knows about false color difference, they are nice looking images either way.

In the late 1990s you'd walk into Walden Books or the equivalent store and this would be staring at you:

https://i.imgur.com/Go5VDi1.jpg

And it seemed truly astounding. Like we were getting to journey across the universe. The visuals that Hubble presented, the variety and scale of that style of imagery (great for PR), and the way it cultivated the imagination of the mass public, is still unrivaled.

"though many factors could ultimately affect Webb’s duration of operation." JWST also needs helium coolant with no possibility to recharge therefore limiting its lifespan to ~10 years, I believe. Interesting about Herschel and L2.

IIRC, the helium coolant is only used on one of JWST's 4 instruments. So if they run out of helium the other three can still be used.

It does use helium only as refrigeration fluid in closed cycle, it does not get depleted. Building a cryocooler capable of achieving so low temperatures without boiling helium off was a major hurdle in JWST development.

Isn’t helium notoriously hard to contain without leakage even in a closed system?

that's hydrogen. IIRC from my chemistry days, Helium is hard to contain in plastic closed systems, but "generally OK" in metal tubing.

It actually uses a closed system:

https://webb.nasa.gov/content/about/innovations/cryocooler.h...

Doesn't liquid helium always evaporate?

I don't believe Hershel was optical at all - pretty certain it was far infra-red

Infra-red is optical. Just not visible light but it uses lenses and mirrors like every other visible light telescope. The main difference is that those lenses are designed to be as transparent to IR as possible instead of accidentally filtering it out.

The big trick for Webb is that it will allow for extremely red shifted IR to be received, which translates into 'light from very long ago and very far away'. To be able to do that the sensors are cooled to extremely low temperatures to ensure that the instrument noise level is brought down as low as possible, allowing for extreme sensitivity.

Optical refers to principles that work in visible, ultraviolet, and infrared light.

JWST is also primarily infrared telescope. It as some capability in red and the yellow part of the visible spectrum.

> Optical refers to principles that work in visible, ultraviolet, and infrared light.

Is there something qualitatively different about infrared and ultraviolet that they share with visible, that they are all grouped into 'optical'?

Yes, you can use normal mirrors and lenses for these wavelengths

Mirrors, mostly yes. The situation for lenses is more complicated.

Fused silica is falls off in the mid IR and is opaque for long wave IR, and optimizing for UV costs you IR response. ( https://escooptics.com/pages/materials-fused-silica-quartz )

The main optical path of JWST is all reflective for this reason.

There are refractive optics in some the instruments, but those are more wavelength specific.

E.g. NIRcam uses refractive elements made from LiF, BaF2, and ZnSe ( http://ircamera.as.arizona.edu/nircam/pdfs/5904-9_Ryder.pdf ) while I don't think MIRI ( https://arxiv.org/pdf/1508.02488.pdf ) uses any refractive components at all except a ZnS-GE double prism ( https://orbi.uliege.be/bitstream/2268/95768/1/DPA%20v8.pdf ) which is opaque to visible light.

Thanks. How is 'normal' defined?

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