April 6, 2020

The Great Orbiting Observatories II: The Ultraviolet.

When we last left our installment of this saga, we covered the observatories that target the visible edge of our spectrum. This is a narrow slice; a tiny sliver of what we call the electromagnetic spectrum. This week, we move into the ultraviolet, a span of the spectrum at roughly between 10 to 320 nanometers. UV from space is almost entirely absorbed by our atmosphere, and thus, if you want to observe the universe or do UV astronomy, you have to go into space to do it.

So, what does the universe look like through UV eyes, and why study it? If you could tune your eyes to the UV, familiar bright stars would fade out as younger, hotter objects became more prominent. The same holds true for younger, more energetic galaxies in the throes of star formation. It’s also much easier to grab a spectrum from these objects in the UV, as they produce a larger signature. Inter-solar objects such as comets also merit UV study, and of course, UV gives us a dramatic new window on the Sun.

What makes a telescope UV? To sensitize mirrors to UV, coatings such as silicon carbide or aluminum and lithium fluoride are typically used. Wolter-style mirror configurations also become necessary the closer you get to far-UV.  Diffraction gratings or masks may be placed in front of the mirror, and further optimizations may be enhanced at the instrumentation package.

Of course, many space telescopes are multi-payloads; a few of the telescopes listed below may not be dedicated ultraviolet instruments. What follows is a list of high flying performers, UV scopes that have performed admirably and expanded our knowledge, or were (or will be) notable in some way. The list is by no means exhaustive; we didn’t, for example, include scopes that malfunctioned after launch, or were shot off towards orbit only to tumble into the sea. Such is this business of ours…  If your favorite UV scope didn’t make the list, please write us and tell us about it!


- The 1st UV space telescope: OAO-2 was launched in December 7th, 1968, (a few days before Apollo 9) and was the first successful (OAO-1 “Screwed the Pooch”) orbiting UV telescope. Significant discoveries included a hydrogen halo seen around cometary nuclei and studies of novae in the UV spectrum.

-Copernicus: This was the most successful of the OAO series of observatories, launched in August, 1972. A joint UK-NASA venture, it operated until 1981, and like many UV scopes, was a dual role UV-X-ray scope. The UV payload was an 80cm instrument built by Princeton University, and the telescope was christened to coincide with the 500th anniversary of the birth of Nicholas Copernicus.

-Extreme Ultraviolet Explorer (EUVE) This was perhaps the first true UV space telescope to “blow the doors” off of the field. EUVE launched in June, 1992 and operated for nearly 9 years, producing the first all-sky map of about 801 objects in UV. The first dedicated extreme ultraviolet mission, EUVE sported no less than 4 Wolter-Schwarzchild grazing incidence mirrors. Other highlights of the mission included the first UV detection of extragalactic objects, and a detailed analysis of variables stars in UV.

Astro-1 &2: This series of instrument packages was flown on the Space Shuttle. Astro-1 was first flown on shuttle Columbia on STS-35 in December, 1990. It consisted four telescopes, one X-ray and three UV, including HUT, the Hopkins Ultraviolet Telescope. Operating at a range of 82.5 to 185 nms with a resolution of 0.3 nm, HUT studied galactic nuclei, supernovae remnants, and cataclysmic variables, as well as characterized the planets in UV. This was followed by Astro-2, aboard shuttle Endeavour in March, 1995. On this mission, an improved version of HUT flew for twice as long, building upon the science in Astro-1.


-FUSE: It’s only been in the last decade or so that UV astronomy has really taken off. FUSE, The Far Ultraviolet Spectroscopic Explorer, was launched on June 24, 1999, as part of NASA’s Origins program. FUSE was designed to specifically target the poorly studied region of the far ultra-violet at a window of 90.5-119.5 nm. Of primary interest at that wavelength is primordial deuterium, a remnant of the Big Bang. FUSE lost its final reaction wheel in July 2007 and was declared by mission controllers as dead in space.  

-Swift: Primarily a gamma-ray platform, Swift is a capability that cosmologists have wanted in the hunt for gamma-ray bursts for a long time. But among its array, Swift also utilizes the UV/Optical Telescope, (UVOT), an instrument that can grab the energetic UV afterglow with an accuracy down to 0.5” arc seconds.

-GALEX: The Galaxy Evolution Explorer is the true dedicated flagship of current cutting edge UV astronomy in orbit. Launched in 2003 from the belly of a jet aircraft, GALEX has surpassed its original slated 29-month mission and is now in an extended phase. GALEX has repainted an all-sky survey in the UV with unprecedented resolution and clarity. Alas, GALEX’s days may be numbered, as it is now down to one detector. One outstanding picture it provided was a UV shot of the gas cocoon surrounding the star Mira.

-Hubble: No, this is not an error on our part; it is little appreciated that the storied Hubble Space Telescope (HST) can view the universe in UV and IR, as well as the visible spectrum. In May 2009, astronauts aboard STS-125, the final repair mission placed a very special package aboard Hubble: The Cosmic Origins Spectrograph (COS), a UV spectrograph optimized for point UV sources in the far ultraviolet. COS will not only provide insight into the nature large scale structure of the universe and the galaxies within it, but also look at early dust accretions of budding proto-planetary systems.

-SOHO: I couldn’t write this without at least mentioning solar UV astronomy, and the space telescope that has revolutionized the study as a whole; the Solar Heliospheric Observatory (SOHO). One of SOHO’s lesser known instruments is UVCS, the UltraViolet Coronagraph Spectrometer. This instrument studies the solar corona and the solar wind out to about 10 solar radii. More familiar is SOHO’s Extreme ultraviolet Imaging Telescope (EIT) which studies the solar corona in a range corresponding to ionized iron and helium.


-ATLAST: This is always our favorite part of the research for these posts… what’s coming down the line? ATLAST, the Advanced Technology Large-Aperture Space Telescope, will, if funded, be the true successor to Hubble. ATLAST will be of the 8-(single mirror) or 16- meter (segmented) range and will build on technologies learned from HST and JWST. Like JWST, it will be placed at the Sun-Earth Lagrange point. And yes, there will be a UV and IR component, just like Hubble. Among other things, ATLAST will have the capability to target key spectral signatures for life.  ATLAST is envisioned for launch in the 2020’s; key to getting it off the ground is a new heavy lift rocket proposed by the Obama administration for 2015. The idea of the Ares V gets astronomers mental wheels turning as to the payloads they could place in space!

-Baryon Structure Probe: The BSP is another telescope on astronomers’ wish-list for an EELV (Evolved Expendable Launch Vehicle) payload. BSP would sit at L2 and characterize the large scale structure of the universe in UV as well as study point sources such as quasars.

-THEIA: The Telescope for Habitable Exoplanets and Interstellar/Intergalactic Astronomy would sport a 4-meter optical/UV optimized mirror equipped with an occulting disk to study exo-planets. THEIA would cover the “spectrum gap” after Hubble and complement JSWT until ATLAST comes online.

So there you have it, the wonderful world of orbital UV astronomy. Like so many other sub-fields, it’s amazing to think that astronomy in the ultra-violet realm barely even existed a decade ago! But from brief sounding rockets and shuttle payloads giving us a glimpse of this little understood end of the spectrum, dedicated platforms now conduct routine surveys and give us continual coverage of the UV band. Who knows, the first spectral confirmation of life elsewhere in the universe might just come from the ultraviolet … it is definitely a portion of the spectrum worth watching!


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  2. [...] Last week, we delved into the exciting world of orbital ultraviolet astronomy. Keeping with a theme, this week, we here at Astroguyz will review a favorite new toy of ours; the [...]

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