Why does ultraviolet astronomy require space telescopes?

Ozone and other molecules in Earth's atmosphere absorb ultraviolet radiation almost completely, so it never reaches the ground. Observing the ultraviolet sky therefore requires lifting instruments above the atmosphere on rockets or satellites.

What does it mean for a detector to be solar-blind?

A solar-blind detector responds to ultraviolet light but is essentially insensitive to the far more abundant visible light. This prevents the bright optical background from swamping faint ultraviolet signals, which is crucial because most astronomical sources emit much more visible than ultraviolet light.

Ultraviolet Astronomy Instruments

Ultraviolet astronomy instruments observe the part of the spectrum just shortward of visible light, accessible only from space and requiring special coatings, optics, and detectors that work where ordinary glass and materials absorb.

Definition

Ultraviolet astronomy instruments are space-based telescopes and detectors optimised for roughly 10 to 400 nanometre radiation, employing reflective optics, special coatings, and solar-blind detectors suited to wavelengths absorbed by the atmosphere and by common optical materials.

Scope

This topic covers the division into near, far, and extreme ultraviolet, reflective optics with specialised coatings such as aluminium with magnesium fluoride, solar-blind detectors including microchannel-plate and photon-counting devices, ultraviolet spectrographs, and the contamination and degradation issues that make ultraviolet instruments demanding to build and operate.

Core questions

Why must ultraviolet astronomy be conducted from space?
What optics and coatings work in the ultraviolet?
What detectors are used and what does solar-blind mean?
Why are ultraviolet instruments especially sensitive to contamination?

Key theories

Ultraviolet optics and coatings: Most glasses absorb ultraviolet, so instruments rely on reflective optics with carefully chosen coatings, such as aluminium overcoated with magnesium fluoride, to maintain reflectivity into the far ultraviolet.
Solar-blind photon-counting detectors: Microchannel-plate and other photon-counting detectors with ultraviolet-sensitive photocathodes can be made insensitive to the far brighter optical sky, an essential property called being solar-blind.
Contamination sensitivity: Thin films of contaminants and even molecular outgassing strongly absorb ultraviolet light, so these instruments demand scrupulous cleanliness in build and operation.

Clinical relevance

Ultraviolet observation probes hot young stars, the interstellar and intergalactic medium through absorption lines, accretion in compact binaries, and star formation in galaxies, accessing diagnostics of hot gas and energetic processes not available at longer wavelengths.

History

Ultraviolet astronomy began with sounding rockets and early satellites, and the long-lived International Ultraviolet Explorer of 1978 made it routine. Instruments aboard the Hubble Space Telescope and dedicated missions extended ultraviolet imaging and spectroscopy across stellar, galactic, and intergalactic studies.

Key figures

Lyman Spitzer
George Carruthers

Seminal works

kitchin2013
rieke2003

Frequently asked questions

Why does ultraviolet astronomy require space telescopes?: Ozone and other molecules in Earth's atmosphere absorb ultraviolet radiation almost completely, so it never reaches the ground. Observing the ultraviolet sky therefore requires lifting instruments above the atmosphere on rockets or satellites.
What does it mean for a detector to be solar-blind?: A solar-blind detector responds to ultraviolet light but is essentially insensitive to the far more abundant visible light. This prevents the bright optical background from swamping faint ultraviolet signals, which is crucial because most astronomical sources emit much more visible than ultraviolet light.