Space Telescopes in the Ultraviolet, Optical, and Infrared (UV/O/IR)

Abstract

Space telescopes are essential for advancing our understanding of the physics ofthe cosmos. The vacuum environment of space means the light entering theaperture of a space telescope has not suffered any atmospheric extinction,enabling observations in ultraviolet, far-infrared, and X-ray wavelengths. Atwavelengths that are accessible from both space and ground-based facilities,the sky background levels in space are considerably lower than those atground-based sites, particularly in the near and mid-infrared. The lack ofatmospheric turbulence allows space telescopes to obtain diffraction-limitedimage quality without the need for adaptive optics. Their inherently highstability, and ability to observe for long continuous periods of time, allowsobservations to be performed from space that are difficult or impossible toaccomplish from the ground. In Sect. 1, comparisons are made between thesensitivities, spatial resolutions, and exposure times for ground- and space-basedobservatories.

Fundamentally, the science objectives drive the design of a space telescope, but thesehave to be balanced by the technological maturity of the space systems needed,the availability of suitable launch vehicles, and ultimately the overall mission cost.Today, it is normal practice to use cost models to attempt to capture the physicalparameters that drive cost. If the early estimates of mission cost become too large,science objectives may have to be reduced, or new technologies developed and themission redesigned accordingly. In Sect. 3, the types of high-level tradesthat have to be made in designing and building a space observatory arediscussed.

To illustrate the kind of trades that are undertaken, three space telescopes thathave different science objectives and span nearly 30 years in launch dates aredescribed: the Hubble Space Telescope (HST), Spitzer Space Telescope (SST), andthe James Webb Space Telescope (JWST). Overviews of the three missions aregiven in Sect. 2. HST is a warm observatory with a 2.4 m primary mirror diameterin low Earth orbit launched in 1990. It observes in ultraviolet, optical, andnear-infrared (NIR) wavelengths. SST is a cold infrared observatory with a 0.85 mprimary mirror in a drift-away orbit launched in 2003. JWST will be a cold,infrared observatory with a 6.6 m primary mirror in orbit at the second Lagrangepoint of the Earth-Sun system (L2). JWST is currently scheduled to launch in2018.

Different orbits were selected for HST, SST, and JWST, as discussed in Sect. 4. Alow Earth orbit was chosen for HST to allow for human servicing that changedand improved the observatory over its multi-decade lifetime. SST’s heliocentricdrift-away orbit was selected because it minimized the mission mass and allowedthe observatory to continually radiate heat into deep space, but contact with themission will eventually be lost. JWST’s orbit about the metastable Sun-EarthLagrange Point 2 (L2) keeps the Sun, Earth, and Moon constantly behind itssophisticated sunshield, enabling the telescope assembly to passively cool to 40 K,operate in a highly stable thermal environment, and stay in constantcommunication with the ground system for its 10-year mission using NASA’s DeepSpace Network (DSN).

The ratios of telescope aperture diameter to launch volume and mass haveincreased over time, as discussed in Sect. 5, driven by the need for missions to fitinto available launch vehicles. For HST and JWST, some components are folded tofit into the launch vehicles and are deployed after launch. JWST, in addition,requires that optical alignment be carried out on orbit because the opticsthemselves deploy after launch (Sect. 6.4).

Optical trades are discussed in Sect. 6. HST and SST are compact, two-mirrorCassegrain telescopes. HST minimizes the number of mirrors in order to achievereasonable throughput at ultraviolet wavelengths. JWST has a three-mirror designthat achieves a reasonable field of view to accommodate multiple instrumentsdespite the very large primary mirror diameter. The three-mirror design alsoproduces an accessible pupil, and a planar mirror and mask at that location areused for fine-pointing adjustments and stray light control. Because it operates inthe IR, JWST’s throughput remains high despite the additional reflections in thetelescope.

HST uses a glass primary with a low thermal expansion, which enabled it to bepolished to reach the lower surface errors required for observing at visible andultraviolet wavelengths. JWST and SST, operating in infrared wavelengths, wereable to make use of a beryllium primary mirror technology with much lowerareal density than that of HST’s light-weighted glass primary mirror. Theberyllium technology has excellent cryogenic properties as discussed in Sect. 6.3.

HST’s telescope and instruments operate near room temperature, with theexception of its actively cooled IR and CCD detector systems. HST experiences asomewhat unstable thermal environment because of its low Earth orbit, asdiscussed in Sect. 8. SST and JWST must operate cold to ensure that thesensitivity of the infrared detectors is limited by the shot noise from thedeep-space background rather than by infrared emission from the telescopestructure. SST uses a combination of passive cooling for the optics and activecooling for the detectors and instruments in order to reach their respectiveoperational temperatures in its drift-away orbit. JWST relies almost solely onpassive cooling in its orbit about the Sun-Earth L2 point, with the exception ofthe detectors on the mid-infrared instrument which are actively cooled by amechanical cooler.

Many of the science objectives for future space telescopes will build on thescience legacies of the telescopes covered in this chapter and will requirelarger primary mirror diameters. With a combination of greater angularresolution and higher sensitivity, future observatories will be capable ofdetecting signs of life on extrasolar planets, will reveal the mechanisms thatcontrol early galaxy formation, and will provide windows into new andunexplored regimes of the universe from ultraviolet to infrared wavelengths.Such missions are likely to make use of new, larger launch vehicles, butwill still need to fold more compactly for launch and have lower arealdensities than current observatories. Consequently, automatic deployments orin-space assembly will be required, as will on orbit optical alignment.Future large telescopes will likely also incorporate real-time active andadaptive optical and alignment control; such technology has been successfullydeveloped for the recent generation of very large ground-based telescopes.With its thermal stability and high observing efficiencies, L2 will continueto be an attractive orbit for these future missions. Other orbits mightbe selected if there is renewed interest in servicing these future “GreatObservatories.”