The Hubble Space Telescope - Analytical Chemistry (ACS Publications)

Jul 15, 1990 - The Hubble Space Telescope. Alan R. Newman. Anal. Chem. , 1990, 62 (14), pp 787A–793A. DOI: 10.1021/ac00213a725. Publication Date: ...
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The Hubble Space Telescope


inally. After nearly 15 years of planning, construction, and delays, the Hubble Space Telescope (HST) has been safely deposited in Earth's orbit. Despite a handful of glitches, HST will soon begin its scheduled 15-year program of astronomical observations. As astronomers prepare to use their new research tool, analytical chemists can appreciate HST's importance by considering this orbiting observatory as a premier analytical instrument. The heart of HST is its remarkable 2.4-m diameter primary mirror. Ground and polished to exacting tolerances, the fused-silica glass mirror does not deviate by more than 25 nm from the ideal concave shape. An aluminum coating protected by a Mg'F2 layer forms the reflective surface. Light collected by the primary mirror is funneled to a secondary 0.34-m diameter mirror which, in turn, sends light back through a central 60-cm hole in the primary mirror and into one or more of the five detector packages on board. This two-mirror design is described as a //24 Ritchey-Chrétien, a variant of the compact Cassegrain arrangement that is common in many contemporary research telescopes (see top of Figure 1). Sitting above Earth's atmosphere,

HST will resolve points of light to a mere 0.05 arc sec—a 10-fold improvement in resolution over even the giant ground-based 5- and 6-m diameter telescopes. (Each arc sec equals 1/3600 of a degree.) Only once before has astronomy benefited from such a large jump in resolution: in 1609, when Galileo first turned his hand-built 5-cm refractor telescope to the heavens. Furthermore, HST is not just limited to visible light; the telescope's optical range extends from the UV to the nearIR (115-1100 nm). Situated behind the primary mirror

FOCUS are HST's instrument packages (Figure 1). Over the next 15 years, shuttle astronauts will visit the space telescope and replace these instruments with packages containing different detectors or better equipment. The five instruments now on board are the wide-field and planetary camera, faint-object camera, high-resolution spectrograph, faint object spectrograph, and high-speed photometer. In addition, three sensitive fine-guidance sensors in this compartment hold the telescope rock-steady in space. "Pick-off mirrors intercept

the light and direct it into each instrument. What follows is a discussion of the capabilities of each instrument package and a brief discussion of primary research objectives. Wide-field and planetary camera. Designed and built by the California Institute of Technology and the Jet Propulsion Laboratory, this instrument will provide HST's high-quality photographic images. In the wide-field mode the camera views 2.67 arc min of space, a field approximately 1/10 the diameter of the moon as viewed from Earth, with a resolution of 0.1 arc sec per pixel. The planetary camera views a narrower 1.15 arc min across, resolving 0.043 arc sec per pixel. In the planetary mode Jupiter just covers the camera's field of view, yielding images that rival the photographs obtained by the Voyager space probes. Planners claim that the camera's images of distant Neptune should be better than ground-based photographs of Jupiter (see Figure 2). The camera will also be the instrument of choice to search for planets orbiting nearby stars and to look at such diverse phenomena as comets, supernovas, galaxies and galactic centers, and quasars. The camera's detector consists of two independent sets of four charge-

ANALYTICAL CHEMISTRY, VOL. 62, NO. 14, JULY 15, 1990 · 787 A

FOCUS coupled devices (CCDs) that respond to photons across the near-IR to the UV range. The detectors produce a 1600 X 1600 pixel image. In the planetary mode Jupiter will cover about 103 pixels. The camera is also equipped with 48 filters, 3 gratings, and a polarizer mounted on 12 wheels that can rotate into the light path. Shutter speeds range from 110 ms to

28 h. A 1-h exposure should resolve objects as faint as 28th magnitude. (Magnitude and brightness are inversely related; every one unit increase in magnitude represents a factor of 2.512 drop in brightness.) A system of heat pipes, thermoelectric coolers, and a radiator keep the CCDs cooled to - 8 0 °C to - 1 0 0 °C. A second-generation instrument with

newer CCDs and a revised filter set is scheduled to replace this camera in 1993. Faint-object camera. As a partner in the HST project, the European Space Administration has contributed several components of the telescope, including the faint-object camera. Both a complement and an extension to the wide-field and planetary camera, the faint-object camera may detect stars as faint as 30th magnitude—more than 20 times fainter than those seen by ground-based telescopes. The faint-object camera is also the only instrument to use the full spatial resolving power of HST and thus will provide many of the highest resolution images. The trade-off is a narrower field of view than the wide-field and planetary camera offers. This camera is more sensitive than the wide-field and planetary camera to wavelengths < 450 nm and therefore can acquire an image faster in UV or blue light, whereas the wide-field and planetary camera records faster in red and near-IR light. These are important considerations because, as detailed later, observation time on HST is tightly controlled.






Figure 1. Schematic of HST's optical assembly and instrumentation. Top: light blue—primary mirror with central hole; dark blue—light baffles (secondary mirror sits inside forward baffle); red—fine-guidance sensors; yellow— wide-field and planetary camera; green—axial instruments: faint-object camera, high-resolution spectrometer, faint-object spectrometer, and high-speed photometer. (Courtesy of Dana Berry, Space Telescope Science Institute.) Bottom: location and schematic of major instruments. (Courtesy of NASA.)

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Figure 2. Resolution with HST. Left: simulated ground-based view of Pluto and its moon Charon. Right: simulated view from space tele­ scope clearly distinguishing the t w o objects. (Courtesy of STScl.)

The faint-object camera has been equipped with two detector systems, both of which employ electronic inten­ sifies to generate an image on a phos­ phor screen that is 105 times brighter than the original image. The phosphor picture is then scanned by an electronbombarded silicon TV camera. Be­ cause of the high sensitivity, objects brighter than 21st magnitude must be filtered to avoid saturating the camera's detectors. The final image is provided in a standard TV format of 512 pixels square. Depending on the detection system used, the resolution per pixel ranges from 0.043 (//48) to 0.0072 (//288) arc sec. For faint objects of interest that are closely associated with a brighter ob­ ject (e.g., imaging gases surrounding a bright quasar), the camera can insert two different-sized "occulting fingers" into the light path to block out light from the brighter object. In addition, there are a grating and a number of filters, prisms, and polarizers that can be rotated into the focal path. Goddard high-resolution spectro­ graph. Designed by National Aeronau­ tics and Space Administration's God­ dard Space Center researchers and built by Ball Aerospace, this UV spec­ trometer can resolve a 120-nm line to ±0.0012 nm. This instrument should generate much of HST's chemical data, thereby providing chemical composi­ tions of planetary atmospheres, detect­ ing interstellar molecules, and measur­ ing elemental abundances. Light passes into the spectrograph through either a 2- or a 0.25-arc sec slit and then through one of six gratings fixed on a rotating carousel. The carou­ sel is designed to hold the grating

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steady, to ±0.13 arc sec or < 1-in. jitter viewed at 25 mi, and maintain precise control over its motions by subdividing its 360° carousel motion into 65 536 steps. Two 512-channel Digicon electronic light detectors count incoming photons across specific spectral ranges: CsTe on MgF2 responding to 110-320 nm and Csl on LiF covering 110-170 nm. Visi­ ble wavelengths are excluded on this instrument because Earth-based de­ tection systems can match the resolu­ tion that the Goddard spectrograph of­ fers. To determine exposure times, the spectrograph is equipped with a cam­ era-like light meter. In addition, light intensity fluctuations (e.g., from vari­ able stars) as rapid as every 100 ms can be detected by the instrument. The spectrograph's high resolution comes at the expense of sensitivity. At its highest resolution the instrument de­ tects objects only brighter than 16th magnitude, whereas at its "low" resolu­ tion setting, capable of measuring 120 ±0.06 nm, the sensitivity extends to 19th magnitude. Faint-object spectrograph. To compensate for the sensitivity limita­ tions of the Goddard high-resolution spectrograph, HST also carries the faint-object spectrograph for viewing images as dim as 26th magnitude. Built by Martin Marietta, this instrument offers sensitivity that, again, comes at the cost of resolution. At the most sen­ sitive setting the faint-object spectro­ graph can resolve a 120-nm peak to ±0.5 nm. With the increased sensitivity it is now worthwhile to study visible light. Thus this spectrograph detects light

from the UV to the near-IR. Light en­ ters the spectrograph through 1 of 11 slits that range in size from 1.0 to 0.1 arc sec. Two 512-element Digicon de­ tectors record spectra: One measures photons from 115 to 550 nm, and the other covers 170-850 nm. Light varia­ tions as rapid as 40 ms are detectable with this device. Like the faint-object camera, the faint-object spectrograph contains two occulting fingers for blocking out bright sources that could overwhelm spectral information from dimmer nearby targets. Included in the faint-object spectro­ graph's mission are studies of the chemical composition of comets before they develop a "tail" and chemical de­ terminations for other galaxies. This spectrograph will also examine far-dis­ tant quasars that could reveal the way in which galaxies first formed and the chemical abundances of the early uni­ verse. High-speed photometer. Billed as the simplest with no moving parts (and, at $11 million, the "cheapest" in­ strument on HST), the high-speed photometer is basically a light meter for measuring light intensity, variation, and polarization. The instrument was designed and constructed at the Uni­ versity of Wisconsin-Madison. High speed refers to the instrument's ability to distinguish light intensity fluctua­ tions as rapid as every 20 μβ. Groundbased observers are limited by air tur­ bulence to periodic variations of 1 s. The photometer contains a GaAs photomultiplier tube and four magnet­ ically focused image dissector tubes—

photomultiplier-like tubes that count only the photoelectrons emitted from the region of the photocathode that is activated by incoming light. The image dissector tubes respond to a range of 120-700 nm, whereas the photomulti­ plier detects only red light. With expo­ sure times of about one-half hour, the high-speed photometer should be able to detect light sources as faint as 24th magnitude. Despite the absence of moving parts, the photometer can be operated in > 100 different modes. To accomplish this, controllers point HST so that light reaches the photometer assembly through one of four entrances (one for each dissector tube). Bolted to 3 of the entrances is a plate with 13 different interference filters arranged in a dou­ ble row (Figure 3). Behind each filter plate is an aperture plate with 50 dif­ ferent-sized holes. The precise aiming of HST sends light through a specific filter-aperture combination. All to­ gether, 27 different filter-aperture combinations are available for each of these three detectors. Light coming in one of the entrances also passes through a beam-splitter that sends photons to the photomultiplier tube. The fourth entrance, for polarimetry measurements, contains four near-UV filters, each backed with a polarizing filter rotated 45° from its neighbor. Fine-guidance sensors

To lock HST on target, the telescope carries three fine-guidance sensors. Only two are needed to point the satel­ lite, leaving the third sensor free to measure star positions (astrometry) 10

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Figure 3. High-speed photometer filter-aperture tube configuration. Depending on HST's aim, light passes through one of 13 filters and one of 50 apertures. (Courtesy of Uni­ versity of Wisconsin-Madison.)

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62, NO. 14, JULY 15, 1990 · 791 A


Figure 4. Simulated HST view showing the tieids for the fine-guidance sensors. Elliptical galaxy M87 sits in the center of this 0.5° star field. Candi­ date guide stars are marked with + ; guidance sensor fields or "pickles" are outlined. (Courtesy of STScl.)

times more precisely than groundbased determinations. Each sensor measures a pickle-shaped swatch of space 4 X 16 arc min, resolving star positions to a few milliarc sec (Figure 4). In 10 min a single guidance sensor can study 10 17th magnitude stars. However, using those stars for posi­ tioning HST presents a new problem. For HST to locate at least two guide stars, about 100 guide stars per square degree of sky need to be catalogued. In 1979 researchers at the Space Tele­ scope Science Institute (STScl) in Bal­ timore initiated a program to collect and determine sufficient star positions with enough accuracy—at least ±0.3 arc sec—for HST's guidance sensors. Their final product, called the Guide Star Catalog, combines other sky cata­ logs and contains 18 819 291 entries. Within five years 10% of the guide stars will be too far out of position to be useful because of the movement or proper motion of stars through space. A program is now under way to deter­ mine the proper motions of guide stars, a project HST will aid. When these mo­ tions are known, it will be possible to calculate and update guide star loca­ tions. HST's guidance sensors will also have other jobs. As the Earth orbits the sun our position in space changes, so repeated astrometric measurements of "nearby" stars will produce apparent shifts in stellar positions because of parallax. This shift is relative to a fixed background of other stars that are too distant to produce a measurable paral­ lax. The parallax provides the best measurement of stellar distances, and, with HST's high precision, stellar par­

Figure 5. Planning an observation with HST. The telescope locks onto guide stars (blue), collects light for studies (green) and for cali­ brating instruments (purple), pivots to new position (yellow), and waits half an orbit to be­ gin again as sunlight and the Earth block its view (red). (Courtesy of STScl.)

allax distances as far as 300 light years will be measurable. The guidance sensors could also de­ tect "wobbling" in stellar motions—an indication of extrasolar planets. Plan­ ets orbit about a solar system's center of mass. That point lies close to the center of the sun in our solar system. However, a system with massive plan­ ets and extensive debris could pull that center far enough away from the home star's center that the star would signifi­ cantly change positions like a planet, adding a wobble to its proper motion. Any measurement HST performs re­ quires that the observatory hold its own position steady and smoothly track its target. A system that includes the fine-guidance sensors, gyroscopes, and star trackers keeps HST's aim on target to within 7 milliarc sec during an observation. Positioning of HST is gov­ erned by four reaction flywheels that move the telescope by transferring mo­ mentum. Thrusters to turn the tele­ scope would contaminate the optics. As a result HST slews slowly, at about the same rate as a clock's minute hand. The slow rotation of the telescope is one factor program managers must jug­ gle when allocating observation time. It can take 30 min to locate guide stars and properly aim HST. HST's orbital position also must be considered (Figure 5). Because of the telescope's relatively low orbit, much of HST's field of view is occulted by the Earth. Like riding a merry-go-round, as HST sweeps around the Earth every 95 min many astronomical targets will regularly appear and disappear (it is possible to view polar regions continu­ ously). Targets must be chosen so as to

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protect HST's optics and take advan­ tage of "dark skies." The telescope must view at least 20° away from the Earth's bright limb and 50° away from the sun. Furthermore, the best obser­ vations will occur at "nighttime," in the Earth's shadow. Thus the telescope may need to be aimed and ready before a target "rises" into its horizon. Finally, because of power restric­ tions only two instruments can be switched on simultaneously. It can take up to 12 h to bring an instrument to thermal equilibrium; thus, HST will ideally use an instrument continuously for several days. Deciding when and where HST should point is such a complex job that it is handled on the ground by an artifi­ cial intelligence computer program with 2 Χ 107 lines of code. Instructions are relayed up to the satellite three times a day and stored on board. Additional research objectives Much has been written about HST's research objectives. By imaging old light, HST will peer farther than ever into the universe's early history, possi­ bly viewing the earliest times of galaxy formation. However, HST won't see back to the Big Bang. As STScl direc­ tor Ricardo Giaconni points out, what remains of the Big Bang is a uniform 3K microwave background and that is "featureless and boring." As seen in Figure 6, astronomers esti­ mate that HST will look back approxi­ mately 14 billion years. A better mea­ sure than time is the redshift, z, which is the measured ratio of the Doppler frequency change in spectral lines (Δλ) to the lines' rest frequency: ζ = Δλ/λ 0 .

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Flgure 6. Cosmological view of the universe. The orange regions show the extent of ground-based observations (bar at ζ = 10 3 represents 3 Κ cosmic background). Yellow regions are not observable. HST could extend UV and visible sections into the era of galaxy formation. (Courtesy of STScl.)

The redshift seen in spectra from extragalactic objects results from light traveling through space that is expand­ ing, and thus it is a measure of both distance and age. (Astronomers com­ pare it with two spots on the skin of a balloon—as the balloon inflates, the dots effectively recede from each other because of the expanding surface.) Currently the most distant known gal­ axy lies at ζ = 3.8 and the most distant quasar at ζ = 4.3. Astronomers hope that HST will find signs of early galaxy formation around ζ = 10. Redshift is related to distance by the Hubble constant. A major goal for HST is to refine the value of this constant by providing better distance measure­ ments of stars that are millions of light years away. The Hubble constant, in turn, measures how fast the universe is expanding and its inverse value pro­ vides the age of the universe. Another key goal of HST is to ob­ serve stellar objects in UV light, look­ ing at spectral lines and measuring the temperature of hot gases. HST will also take occasional snapshots of the sky, taking advantage of its greater sensitiv­

ity to uncover yet-unknown objects. Fi­ nally, the space telescope will be able to peer at transient events such as a su­ pernova or a new comet. As expected, requests for time on HST have been overwhelming. In rec­ ognition of their contribution to the discipline, time has been set aside for amateur astronomers. Proposals from five nonastronomers have been accept­ ed and time has been allotted to them. New proposals from amateurs are be­ ing requested, and for interested chem­ ists it could be a way to experience the excitement of HST firsthand. Informa­ tion and instructions are available from HST Proposal, American Association of Variable Star Observers, 25 Birch St., Cambridge, MA 02138. Deadline is November 1. Alan R. Newman Suggested reading Bahcall, J. N.; Spitzer, L., Jr. Sci. Am. 1982, 247, 40. Fienberg, R. T. Sky & Telescope April 1990, 79, 366. Chaisson, E. J.; Villard, R. Sky & Telescope April 1990, 79,378. Villard, R. Astronomy June 1989, 38.

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