# Hubble Space Telescope (HST) observing modes

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I have been reading some literature on using the HST to take spectra of exoplanet atmospheres when exoplanets transit in front of their stars. The literature mention two modes of observations, 'spatial scanning' and 'staring', in order to obtain spectra of the atmospheres of these exoplanets.

Can anyone explain conceptually what the difference is between these two modes?

"Staring mode" is the traditional way of simply pointing the telescope and exposing the detector.

In "spatial scanning mode", you slowly move the telescope, so that the image is slightly smeared out in the spatial direction.

The figure below shows the principle: On the left is how a regular image is taken; in the middle, a regular spectrum is taken in staring mode, and in the right panel, the detector is moved during the exposure, resulting in the spatial scanning mode. The farther you scan spatially, the brighter and object you may observe before over-exposing, but the downside is that more objects will start to overlap, ruining the spectra.

Left panel: "Imaging"; each observed object shows up on the detector at its corresponding position on the sky.
Middle panel: "Staring mode"; when a grism (sort of like a prism) is inserted, the light from each object is spread out according to its wavelength, producing a spectrum.
Right panel: "Scanning mode"; if the detector is moved slightly during exposure, the spectra are spread out perpendicular to the wavelength direction, effective using a larger number of pixel to collect the photons in. However, if the detector is moved too much, the spectra begin to blend.

This picture, from HST's WFC3 handbook shows the difference in a real observation: It's quite small, but particularly for the "$$0^mathrm{th}$$ object" (on the left) you can see that in the staring mode it's over-exposed, creating diagonal spikes, which are reduced in spatial scanning mode:

In the picture, the objects are located in the vertical direction, while their spectra are dispersed in the horisontal direction. You can see that in the spatial scanning mode, the objects are slightly broader vertically.

The advantage of the scanning mode is that you can expose for a longer time, without the detector getting saturated (which can be a problem for bright stars). When you expose for longer, you collect more photons and hence obtain a higher signal-to-noise ratio (S/N), meaning that your observation becomes more "accurate". The figure below (from these notes) shows the advantage of a high S/N:

Two spectra of the same object, with the right having been exposed 9 times longer than the left, resulting in a $$sqrt{9}=3$$ times higher S/N. Determining for instance the central wavelength of this spectral line becomes much more accurate with higher S/N.

Note that both modes refer to so-caled slitless spectroscopy. A way to avoid the spectra to overlap is to insert a "slit" which allows only the light from a narrow (of the order of an arcsecond) part of the sky to be observed. In that way you will only obtain spectra of objects lying exactly along the slit, but in return you will have a "cleaner" spectrum.

Note also that you can obtain the same effect by dividing your observation into multiple staring mode exposures and then add them together. Scanning mode is a more practical way of not having to do this.

## 41 Hubble Space Telescope

Perhaps the best known unmanned spacecraft is the Hubble Space Telescope, HST, launched in 1991 and still fully operational. Even though the HST was not the first space telescope, it is by far the most famous and productive to date. The images returned by the HST are often beyond words. They give astronomers and stargazers a look at the Universe in detail never seen before.

The idea of a space telescope was first mentioned in writings in 1923 by German rocket pioneer Hermann Oberth. The direct link to what is now known as the Hubble Space telescope can be traced directly to Lyman Spitzer, an astronomer, who wrote in 1946 of the advantages such a space-based telescope would have. Being above Earth’s atmosphere would allow such a telescope to “see” without any atmospheric interference. And, the telescope could observe in the ultraviolet and infrared ranges of the spectrum these wavelengths are mostly blocked by the Earth’s atmosphere a good thing for humans but a bad thing when one is trying to study and understand the Universe.

Progress was made in the 1960’s towards the development and launch of a large space-based telescope. First, NASA launched a space telescope 1962 specifically to study the Sun, the Orbiting Solar Observatory. And in 1966, NASA launched the first telescope, the Orbiting Astronomical Observatory.

Artist’s concept of the Orbiting Astronomical Observatory in orbit. Four OAOs were built and launched as a part of the program. [” Orbiting Astronomical Observatory ” by NASA, in the Public Domain ]

Two technicians work on the Orbiting Solar Observatory, before launch. [” OSO4 ” by Uwe W ., in the Public Domain ]

With the experience and knowledge from the OSO and OAO missions, NASA began planning for a larger, longer term space telescope. The concept for the Large Space Telescope was that it features a 3 meter-diameter primary mirror (about 118 inches) and be placed into orbit by the under-development Space Shuttle as early as 1979.

As is often the case, funding became an issue, so the Large Space Telescope’s primary mirror was reduced in size from 3 meters to 2.4 meters (about 94 1 / 2 inches). Additionally, the European Space Agency was brought on as a partner of the Large Space Telescope. The proposed launch was pushed way back from the original 1979 target, due to both Space Shuttle and Large Space Telescope developmental issues.

In 1983, the Large Space Telescope was renamed the Hubble Space Telescope, to honor Edwin Hubble for his work in astronomy and specifically Hubble’s discovery that the Universe is expanding. One of the key objectives for the HST was to determine the rate at which the Universe is expanding.

Development and construction of the HST took longer than planned, yet finally a launch date was set: October 1986. With the Challenger tragedy January 28, 1986, all shuttle launches were stopped until the Space Shuttle could undergo major modifications. Finally, on April 24, 1990, the HST was flown into orbit aboard Space Shuttle Discovery and successfully deployed on orbit.

American astronomer Edwin Hubble, at the eyepiece of the 100-inch telescope at Mount Wilson (California, USA). [” Edwin Hubble ” by NASA & ESA is licensed under CC BY 4.0 ]

The HST, after a Space Shuttle Discovery servicing mission, with Earth in the background. [” Hubble 01 ” by NASA, in the Public Domain ]

Hubble Space Telescope ( HST ) is an Earth-orbiting telescope with a 2.4-m diameter mirror, named after the astronomer Edwin Hubble. It is operational at ultraviolet, optical and infrared wavebands. When first launched by the Space Shuttle ‘Discovery’ in 1990, it provided unprecedented spatial resolution due to its position above the Earth’s atmosphere, observing through which results in diffraction-limited seeing for non-adaptive optics ground-based telescopes at optical and infrared wavelengths. Cosmic UV radiation is blocked by the Earth’s atmosphere meaning ground-based observing in this band is not possible.

The main instruments onboard HST are the Wide Field Camera 3 (WFC3), the Space Telescope Imaging Spectrograph ( STIS ), the Near Infrared Camera and Multi-Object Spectrometer ( NICMOS ), the Cosmic Origins Spectrograph ( COS ) and the Advanced Camera for Surveys ( ACS ).

HST is a joint programme between ESA (European Space Agency) and NASA (National Aeronautics and Space Administration) and science operations are co-ordinated by the Space Telescope Science Institute (STScI) in Baltimore, Maryland, USA . It has been maintained via ‘on-orbit’ servicing, whereby every 3 years a Space Shuttle mission is sent to repair equipment and install new instruments.

Servicing Mission 4 (SM4), expected to be the last servicing mission to HST , launched on May 11, 2009 and installed two new instruments: Wide Field Camera 3 (WFC3) and the Cosmic Origins Spectrograph ( COS ).

HST will be replaced by the next-generation James Webb Space Telescope, which is primed for observing at long-wavelength optical to mid-IR wavebands and is scheduled to launch in 2014.

## Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report (2005)

### OVERVIEW

The Hubble Space Telescope (HST) is arguably the most powerful single optical astronomical facility ever built. Hubble provides wavelength coverage and capabilities that are unmatched by any other optical telescope currently operating or planned, and there is nothing on the horizon to replace it. Hubble is a uniquely successful NASA science program and is a national asset well worth maintaining in operation.

The Hubble telescope provides four key advantages over most other optical astronomical facilities: unprecedented angular resolution over a large field, spectral coverage from the near infrared to the far ultraviolet, an extremely dark sky, and highly stable images that enable precision photometry. Hubble&rsquos imaging fields of view are also considerable, permitting mapping of extended objects and significant regions of sky.

Unlike standard ground-based telescopes, 1 whose view is blurred by the atmosphere and wholly impeded in the ultraviolet and large portions of the near infrared, Hubble can see sharply and clearly at all wavelengths from the far ultraviolet to the near infrared (Figure 3.1). Hubble images are 5 to 20 times sharper than those obtained from the ground, in effect bringing the universe that much &ldquocloser&rdquo (Figure 3.2). Image sharpness and the extremely dark sky help Hubble to see objects 10 times fainter than those that can be observed with even the largest ground-based telescopes. Moreover, Hubble&rsquos images are extremely stable, in contrast to those of standard ground-based telescopes, which are subject to changing atmospheric clarity and turbulence that continually distort the view. Singly, each of these advantages would represent a significant advance for science. Coupled together they have resulted in the most powerful astronomical facility in history. Hubble is a general-purpose national observatory that enables unique contributions to and insights regarding most of the astronomical problems of greatest current interest.

Ground-based telescopes equipped with adaptive optics are discussed in &ldquoComparison of Hubble with Other Planned Facilities&rdquo below in this chapter.

FIGURE 3.1 An example of the Hubble Space Telescope&rsquos superior resolution compared with that of a standard ground-based telescope: (left) a distant, peculiar interacting galaxy imaged with the Subaru telescope on Mauna Kea (right) the same object imaged with Hubble. Subaru (8 m) telescope image courtesy of National Astronomical Observatory of Japan Hubble (2.4 m) image courtesy of STScI/NASA.

FIGURE 3.2 Two Hubble Space Telescope images illustrate the value of observing at different wavelengths. (left) An image obtained at near-infrared wavelengths, which penetrate the dust, reveals hundreds of stars in the region, as well as a large complex of newly forming stars deep within the dusty column itself. (right) An image obtained at visible wavelengths shows a column of obscuring dust and gas in the famous Eagle nebula (M16). The sculpting away of the dust by an intense rain of radiation from nearby hot stars (off image to top) reveals denser globules of gas inside the column that are seen as protuberances on the surface of the cloud. These protuberances are likely sites of star formation.

Each wavelength imaged by Hubble provides unique information about the sources studied. Images courtesy of STScI/NASA.

TABLE 3.1 Principal Hubble Science Instruments

Cosmic Origin Spectrograph

Of course, Hubble cannot do everything. It is not sensitive to very-high-energy radiation like x rays and gamma rays, or to low-energy radiation in the mid- and far-infrared or radio regions. It cannot collect the sheer quantity of light available to larger ground-based telescopes, a capability that is vital for obtaining high-resolution spectra. To fill these important gaps, Hubble must work synergistically with other telescopes to complete the portraits of celestial objects at all wavelengths.

FINDING: The Hubble Space Telescope is a uniquely powerful observing platform in terms of its high angular optical resolution, broad wavelength coverage from the ultraviolet to the near infrared, low sky background, stable images, exquisite precision in flux determination, and significant field of view.

The Hubble telescope is currently equipped with a selection of cameras operating at different wavelengths, as summarized in Table 3.1. The Space Telescope Imaging Spectrograph (STIS) failed in 2004, but several of its ultraviolet modes would be replaced with the installation of the Cosmic Origins Spectrograph (COS) during a servicing mission. A flexible mix of wavelengths, spectral resolutions, and field-of-view sizes is a key element of Hubble&rsquos power.

### OBSERVING WITH HUBBLE

Hubble observing is open to the worldwide astronomical community, and astronomers compete fiercely to win time on the telescope via their scientific proposals. Independent peer review of the proposals is the basis for selection by the Space Telescope Science Institute (STScI), and chosen programs cover the entire range of astrophysics. Requested time typically exceeds that available by a factor of about seven. This rate of oversubscription has remained essentially constant over the lifetime of the telescope and is about twice that of large U.S. ground-based telescopes.

Selection among the wealth of excellent proposed programs is done by panels of astronomers with significant international representation. In the most recent cycle, some 100 scientists participated in the review process. Two hundred proposals were selected, authored by 955 U.S. astronomers and 358 astronomers from 13 other countries. Many of the successful proposers were graduate students and postdoctoral fellows, making Hubble one of the most important astronomical training resources in the world. Roughly 60 percent of the grant funding in a typical proposal cycle (e.g., Cycle 12) goes to postdoctoral associates, fellows, and graduate students.

Observations are scheduled by the STScI based on detailed instructions from the proposers. The data acquired can be held by the investigators for a 12-month period, after which they become publicly available in the HST archive. Hubble has led the way in making astronomical data archives accessible, and the archived data are nearly as popular for analyses as are new data, given that each Hubble observation can be reused many times by new investigators for new projects. The archive currently boasts 1500 registered users and 19 terabytes of data. Its value continues to grow as new data arrive, and its total impact has increased the productivity of the telescope greatly. The data archive will be one of the most enduring elements of the HST&rsquos legacy.

For successful U.S. proposers, an award of Hubble observing time carries with it a monetary grant to support the scientific research. This money pays for the salaries of researchers, stipends for students and postdoctoral fellows, computers, and publication costs. The annual HST grants program in Cycle 13 (the current cycle) is approximately $20 million, an appreciable fraction of the entire budget (approximately$31.5 million) for university grant programs and fellowships in all disciplines and wavelengths in the Astronomical Sciences Division at the National Science Foundation.

### SCIENCE HIGHLIGHTS

The Space Telescope Science Institute has studied the scientific impact of Hubble observations using two metrics: the number of citations in the professional astronomical literature and references to Hubble discoveries in the popular media. Table 3.2 lists the top 10 Hubble contributions based on astronomical citations, and the following text expands on 5 representative examples from the list.

#### Ultradeep Images of the Universe&mdashGalaxies in Formation

Hubble looks so far out into space that it observes objects whose light has taken many billions of years to reach us. Astronomers therefore see these objects as they were at some distant time in the past in effect, Hubble provides a &ldquotime machine&rdquo that can show us how the universe evolved. The Hubble Ultradeep Field penetrates back more than 12 billion years to within 1 billion years of the Big Bang (Figure 3.3). Infant galaxies can be seen in the process of forming, harbingers of a great wave of star formation that soon afterward bathed the universe in the light of 10 billion trillion stars, and the major stages in the history of galaxy formation are accessible to direct observation.

#### Measurement of the Hubble Constant, the Distance Scale of the Universe

Knowledge of the size and age of the universe had long been uncertain by a factor of two, a level of uncertainty that was a major obstacle to the testing of cosmological theories. Hubble measured the apparent brightness of so-called Cepheid variable stars in nearby galaxies and used them to estimate the distances to those galaxies. This approach provided an accurate value for H0, the Hubble constant, thereby calibrating the distance scale and size of the universe.

TABLE 3.2 Top Ten Hubble Contributions

Ultradeep images of the distant universe

Shows the formation of galaxies and confirms that the universe evolves. Tells the story of how our Milky Way was born.

Accurate measurement of the Hubble constant, H0

Establishes the size and age of the universe.

Discovery of giant black holes at the centers of galaxies

Confirms longstanding theory of the &ldquocentral engines&rdquo of quasars.

Confirmation of accelerated expansion of the universe

Requires the existence of &ldquodark energy.&rdquo

Discovery of spectral lines in active galaxies

Reveals that black holes can trigger massive star formation.

Expansion of the census of the intergalactic medium

Establishes existence of a web of invisible matter filaments linking galaxies over hundreds of millions of light-years and controlling the matter-energy budget of the universe.

Importance of chemistry of the interstellar medium

Probes the formation and distribution of the chemical elements and reveals the physical state of the gas in interstellar space.

Identification of gamma-ray bursts with distant galaxies

Confirms that sources of gamma-ray bursts lie at cosmological distances and that gamma-ray bursts (during their brief flashes) are the brightest objects in the universe.

Resolved images of protoplanetary disks

Reveals flattened, rotating disks of dust and gas that almost certainly resemble our own solar system in its infancy.

Studies of extrasolar planets

Offers a sensitive method for finding planets around other stars, based on partial eclipses when a planet passes in front of a distant star.

#### Giant Black Holes at the Centers of Galaxies

Hubble&rsquos high angular resolution allows astronomers to peer into the hearts of galaxies to measure the orbital speeds of gas and stars close to their centers. The speeds of stars reach 1000 km/s in many objects, thereby indicating the presence of intense gravitational fields caused by massive black holes of up to a billion solar masses. Though mostly invisible today, these black holes shone brilliantly in the past as quasars, fueled by the infall of then-abundant interstellar gas. Key data found by the Hubble telescope reveal a correlation between black hole mass and galaxy properties that may provide crucial clues to how and why these holes formed.

#### Accelerated Expansion of the Universe&mdashDark Energy

Einstein&rsquos theory of general relativity says that gravity should slow the expansion of the universe.

FIGURE 3.3 The Hubble Ultradeep Field, the deepest image of the universe yet taken. Deep images like this one look back in time as well as out in space, revealing the universe as it was billions of years ago. Representative galaxies are shown at the right, along with their ages after the Big Bang (Gyr, 1 billion years). The bottom image in the column is of one of the most distant galaxies yet seen, taking us to within 1 billion years (0.8 Gyr) of the beginning of our universe. Distant galaxies are seen as progressively smaller and dimmer compared with nearby galaxies. Astronomers are using look-back Hubble images like these to chart the course of galaxy evolution. Images courtesy of STScI/NASA.

Hubble data, when coupled with those from other telescopes, show to the contrary that the expansion is accelerating and that galaxies move apart ever faster with time. This observation can be reconciled with general relativity only by invoking a new kind of energy density that remains constant despite the dilution expected from expansion. This so-called dark energy is unlike ordinary matter or energy in that it generates a repulsive gravity that is literally blowing the universe apart. Discovery of this fundamentally new cosmic entity is considered by many physicists to be the most important milestone in physics since the advent of general relativity and quantum mechanics in the early 1900s.

FIGURE 3.4 The Orion nebula, one of the regions of intense star formation nearest to Earth, is a cloud of glowing interstellar gas that has been ionized by the intense ultraviolet radiation coming from five hot, massive stars (the Trapezium) near the center. In this montage of Hubble images, these five very luminous stars can be seen near the center of the main mosaic and in the enlarged image at the bottom left. Energy input from these and other young stars stirs up the gas, giving rise to a network of delicate striations. Despite the chaotic environment, dozens of smaller stars are forming by condensing out of the cloud under their own self-gravity. Some of these stars are surrounded by opaque, dusty disks (&ldquoproplyds&rdquo) that are forming proto-solar systems much like our own. A few young stars are expelling jets of matter perpendicular to their proto-solar system disks (lower right). Fine details of star birth such as these are visible only at the resolution possible with Hubble. Images courtesy of STScI/NASA.

#### Protoplanetary Disks&mdashPlanetary Systems in Formation

Many luminous nebulas are dense regions of interstellar gas lit up by ultraviolet radiation from newly born massive stars. In the nearest such nebulas in our galaxy, Hubble&rsquos high resolving power has uncovered a cornucopia of proto-solar systems seen as dark, flattened disks silhouetted against the glowing background of nebular gas (Figure 3.4). At the centers of such disks, young suns can be seen in the process of formation. Powerful jets of plasma and magnetic fields are spewed out from some of these disks by a magnetic propulsion mechanism not yet fully understood. The discovery of proto-solar systems and energetic phenomena in nearby glowing nebulas has turned them into gold mines for studying the formation of stars and planets&mdashincluding, by analogy, that of our own solar system.

FIGURE 3.5 (left) The number of refereed scientific papers produced annually based on work enabled by major leading telescopes. (right) The number of citations in the scientific literature annually to papers produced from work enabled by major leading telescopes. The criteria used to assign papers to a telescope are parallel for all the telescopes shown here.

### HUBBLE IN THE SCIENTIFIC AND POPULAR PRESS

Nearly 5000 scientific papers have been published based on Hubble observations, and the publication rate in refereed journals is currently about 500 per year. Except possibly for the Chandra X-ray Observatory, which rivaled Hubble in terms of papers published in 2003, Hubble outstrips all other telescopes by more than a factor of two in both the quantity of papers published and the rates at which they are cited (Figure 3.5).

The importance of Hubble science is clear to all&mdashone need not be a trained scientist to know that unveiling the birth of stars and galaxies, finding billion-solar-mass black holes, and helping to discover an entirely new form of energy in the cosmos are ground-breaking milestones in the history of science. But progress in fundamental science is not the only way to judge Hubble&rsquos achievements. To the list of science highlights can be added an even longer list of spectacular images that, though not necessarily in the top 10 scientifically, have had extraordinary public impact by virtue of their sheer beauty or arresting novelty (Figure 3.6). Among these one might list the big &ldquoblack eye&rdquo left by comet Shoemaker-Levy&rsquos direct hit on Jupiter, an image that alerted the public to the dangers of asteroids and comets hitting Earth a panoply of jewel-like planetary nebulas that illustrate the ultimate death of our Sun portraits of planets in our solar system, including auroras on Jupiter and Saturn and, of course, the spectacular &ldquopillars of dust&rdquo in the Eagle nebula that appeared on nearly every front page in America and became iconic for Hubble itself. Intense public interest in Hubble is borne out by many media studies of its impact an example of the results of such an assessment is shown in Figure 3.7. Having garnered sustained public attention over its entire lifetime, the Hubble Space Telescope is clearly one of NASA&rsquos most noticed science projects. In effect, Hubble has become a model that shows how NASA can combine its own unique expertise with that of scientists to educate the public about the natural world.

FIGURE 3.6 Montage of famous Hubble Space Telescope images. From upper left: (1) Eagle nebula (M16), (2) Lagoon nebula (M8), (3) Cat&rsquos Eye planetary nebula, (4) M2-9 planetary nebula, (5) gravitational lens arcs in the Abell 2218 galaxy cluster, (6) colliding galaxies NGC 4038-9 (the Antennae), (7) Eta Carina, (8) &ldquolight-echo&rdquo ring around Supernova 1987a in the Large Magellanic Cloud, (9) the Hubble Deep Field, (10) auroras on Saturn, (11) Mars, and (12) the black-hole galaxy NGC 4261. Images courtesy of STScI/NASA.

FIGURE 3.7 The cumulative impact of various NASA space science programs as indicated by media coverage. &ldquoDiscovery points&rdquo reflect the number and importance of news stories appearing annually in &ldquoScience News.&rdquo Courtesy of STScI/NASA.

FINDING: Astronomical discoveries with Hubble from the solar system to the edge of the universe are among the most significant intellectual achievements of the space science program.

### SCIENCE IMPACT OF HUBBLE SERVICING MISSIONS

Hubble today is not the same telescope that was launched in 1990. A series of servicing missions, summarized in Table 2.1, has repaired many key components, added new observing modes, and increased existing capabilities, typically by factors of 10 to 100. As a result, Hubble now produces much more data per unit time than it did originally. If the total data rate summed over all instruments can be taken as a rough measure of spacecraft productivity, Figure 3.8 shows how science data volume and thus productivity increased as a result of each of the three servicing missions that added science instruments. The total rate of calibrated data has grown by a factor of 33 since launch. A further increase is expected with the installation of Wide-field Camera 3 (WFC3) and COS, each of which would provide more than a 10-fold improvement in scientific efficiency and sensitivity with respect to previous instruments.

FINDING: The scientific power of Hubble has grown enormously as a result of previous servicing missions.

FIGURE 3.8 Growth as a function of time in the volume of data returned by the Hubble Space Telescope, 1990 to 2003, based on the rate of return just after launch. The rate tends to jump after each servicing mission (SM), due mainly to the installation of larger and more efficient detectors. Shown at the right is the volume of data projected as a result of the addition of two new instruments, the Wide-field camera 3 (WFC3) and the Cosmic Origins Spectrograph (COS) in a fifth servicing mission, SM-4.

The efficiency of a science instrument is a measure of the time needed to make a given observation doubling the efficiency, for example, halves the time. Efficiency on Hubble has risen by orders of magnitude with increases in the size of the detectors and improvements in total optical throughput, and would increase further with the installation of the two new instruments planned for servicing mission 4 (SM-4). WFC3 is an imager with two separate arms operating in the ultraviolet (UV)-visible and the near infrared. With more sensitive detectors and larger fields of view, it affords a gain of 10 in efficiency at 0.17 to 0.30 micron, and a gain of 50 at 0.80 to 1.7 microns. These numbers are huge for astronomy: for example, doubling the diameter of a ground-based telescope gives an efficiency gain of only 4, yet even this much improvement is highly sought after. Science programs that would be able to exploit the gain to be provided by WFC3 are indicated in Figure 3.9.

The second instrument planned for installation by SM-4 is COS. COS is a moderate-resolution ultraviolet spectrograph that achieves large efficiency gains of 10 or more over STIS by virtue of a more sensitive, larger detector, a reduction in background noise, and an improved optical design with much higher throughput. This last feature is possible because COS is optimized for a small but very important group of cosmological problems (see below). Installation of COS is even more important if STIS, the

FIGURE 3.9 (left) Anticipated increase in Hubble&rsquos imaging efficiency in the ultraviolet (UV) and near infrared (IR) with the addition of the Wide-field Camera 3 (WFC3). Curves indicate efficiency (explained in the text) for WFC3 versus other cameras on Hubble as a function of wavelength. Note the large gains in the ultraviolet below 0.3 micron and in the near infrared beyond 0.6 micron. (right) Anticipated increase in the field of view within the ultraviolet arm of WFC3 and in the near-infrared arm compared with the field of view of existing cameras on Hubble. Courtesy of STScI/NASA.

other moderate-resolution spectrograph, cannot be repaired&mdashbecause COS can substitute to some degree for the UV arm of STIS.

FINDING: The growth in the scientific power of Hubble would continue with the installation of the two new instruments, WFC3 and COS, planned for the SM-4 shuttle astronaut mission.

### DETERIORATING CAPABILITIES THAT AFFECT HUBBLE&rsquoS SCIENCE PERFORMANCE

Several Hubble subsystems have limited life and were to be serviced on SM-4. Chapter 4 presents a comprehensive review of these systems and establishes overall norms for spacecraft performance. This section discusses two of these systems in particular&mdashthe gyroscopes and the fine-guidance sensors (FGSs)&mdashbecause their status bears particularly on the quality of the data that Hubble can return. The status and projected lifetimes of the science instruments are also reviewed.

Gyroscopes. Rate-sensing units and their associated electronics (collectively known as &ldquogyros&rdquo) are used to slew the telescope and to maintain highly accurate pointing during exposures. Normal observing requires three working gyros. Currently there are three gyros operating, with a fourth working gyro held in reserve. Based on the gyro reliability assessment discussed in &ldquoRate Sensor Unit (Gyroscope) Assessment&rdquo in Chapter 4, it is expected that Hubble will enter a two-gyro mode in 2006 (see Chapter 4), and plans are being made to operate the telescope that way. This transition will not greatly affect overall science productivity: increased pointing jitter will smear images somewhat in the highest-resolution modes, but the workhorse, wide-field modes will be affected only slightly. It will be more difficult to schedule observations because a much smaller portion of the sky will be accessible to the telescope at any one time, and a few targets may become totally inaccessible. However, the impact on science of Hubble&rsquos operating in a two-gyro mode is mainly inconvenience rather than loss.

The effects of dropping down to a one-gyro mode are not well understood but could be severe. Given this uncertainty, the committee believes that it is prudent to assume that Hubble&rsquos drop to a onegyro mode will result in a considerable drop in scientific output. This status is likely to occur in mid-2007 (see Chapter 4), providing a natural time frame for any servicing mission.

Fine-guidance sensors. Fine-guidance sensors are necessary to maintain accurate pointing during an observation they line up on bright guide stars near the target. The telescope has three of these sensors, any two of which are normally used for each observation. Three are needed to ensure that at least two can find guide stars at any time. If one FGS fails, two options are available. The first would force the spacecraft to roll at each pointing so that the two remaining FGSs could find guide stars this option would place restrictions on scheduling and would probably render some astronomy targets permanently unobservable. Nevertheless, Hubble would still be very productive. The second option would be to devise a way to observe some targets with only one FGS currently this option would cause a degradation of image quality, but workarounds are under study to reduce image smear by using either an idle science detector or a fixed head star tracker as a substitute FGS. These studies have not progressed very far to date, and success is not assured. If two of the three FGSs were to fail, it would be necessary to observe in single-FGS mode all the time, a very risky prospect at the present time.

The conclusion is that it is necessary to maintain a minimum of two out of three FGS units operational through the end of the Hubble mission. Currently, two of the three FGS units have deteriorated, and the time at which they will fail can be estimated. Thus, as discussed in Chapter 4, one FGS unit should be included in a Hubble servicing mission. The shuttle version of SM-4 includes such a unit, but the baseline robotic mission does not.

Science instruments and related systems. Information on deteriorating systems that potentially affect science instrument performance is summarized in Table 3.3. As noted, Side B of the STIS electronics failed in August 2004, and its repair is currently under study. The failure of STIS illustrates why redundancy is so important to spacecraft health&mdashat the time of its failure, STIS was one of only two non-redundant science instruments on the telescope, Side A having failed 2 years earlier. STIS&rsquos failure was therefore in some sense foreseeable. The NICMOS cooler is also non-redundant, but the lowest-resolution, workhorse mode of NICMOS (NIC3) would be replaced by the WFC3 IR channel (although the two higher-resolution NICMOS modes would still be used). No other instruments exhibit serious problems or non-redundancies that imperil their functioning through 2011, although radiation damage to the Advanced Camera for Surveys (ACS) and WFPC2 is causing the charge-transfer efficiency of their detectors to decrease and the number of &ldquohot&rdquo pixels to increase, leading to uncertainties of a few percent in their photometry by the end of the period. WFPC2 would be removed in SM-4 to make room for WFC3, and so its condition would then become irrelevant, but

TABLE 3.3 Deteriorating Capabilities of Hubble Systems That Affect Scientific Operations

Current Status and Planned Fix

Side A electronics failed in 2002 Side B electronics failed in August 2004 feasibility of Side B repair under study.

With loss of redundancy, Hubble now has no moderate-resolution spectrograph.

Charge capacity is decreasing SM-4 would replace.

All science operations will cease when batteries fail.

Reduction to two functioning gyros likely by early 2006, one gyro by mid-2007 new gyros to be installed during SM-4.

Nominal operations require three gyros. Two-gyro mode will degrade highest-resolution images slightly and reduce target visibility no proven workaround for one-gyro mode.

Some degradation in two of the three currently available FGSs one is predicted to fail between 2007 and 2009, leaving two without redundancy.

Two-FGS mode will reduce target visibility and scheduling efficiency no proven workaround for one-FGS mode.

Charge-transfer efficiency is gradually degrading, and &ldquohot&rdquo pixels are increasing no plan to service during SM-4.

Degradation significant but not expected to be serious until after 2011.

Cooling unit is non-redundant mechanically no plan to service during SM-4.

NIC3 becomes backup when WFC3 is installed. High-resolution NIC1 and NIC2 modes will be lost if cooler fails.

Charge-transfer efficiency is degrading to be replaced by WFC3 during SM-4.

Degradation not important if WFPC2 is replaced by WFC3.

ACS, which is a workhorse camera with the largest field of view, would continue to operate. For this reason, early servicing is desirable to minimize the accumulating radiation damage. No servicing of ACS or NICMOS is planned for SM-4.

Two other systems potentially affect the thermal health of HST&rsquos science instruments. These are the Aft Shroud Cooling System and the New Outer Blanket Layer, an outer insulation layer. Both of these are included in the shuttle version of SM-4 but not in the baseline robotic mission. These systems are discussed in Chapter 4, which indicates that they are desirable but not essential for instrument functioning.

To summarize, with the exception of STIS, all important items needed to keep Hubble functioning well through 2011 are included in the shuttle SM-4 servicing plan. Replacement of batteries and gyros and one FGS is deemed essential. Any spacecraft is subject to unanticipated failures, but if the repairs envisioned for SM-4 are carried out promptly, there is every prospect that Hubble can operate effectively for another 4 to 5 years after servicing.

### THE PROMISE OF FUTURE DISCOVERIES

What important science programs would be enabled if Hubble&rsquos life were extended? This essential question is examined here, starting with programs that could be done with the existing instruments and proceeding to those depending on the two new instruments, WFC3 and COS. It is important to note that typically only about half of all major discoveries made with new astronomical facilities are foreseen, while the other half are serendipitous. Hubble has been no exception in this regard&mdashonly five of the contributions listed in Table 3.2 were anticipated. Space here also permits listing only a small faction of the science projects likely to be undertaken. For both reasons, the following list provides a lower limit to the future discovery potential of Hubble.

One of the most active and exciting frontiers in astronomy in coming decades will be the discovery and study of planets in solar systems beyond our own. Finding planets, especially down to Earth-like size, has become an official goal of NASA. More than 100 extrasolar planetary systems have been discovered (by ground-based telescopes), and they are very different from those in our own solar system. Planets similar in mass to Jupiter have been found, but they are very close to their parent stars and often in highly elliptical orbits&mdashnot at all like the giant planets Jupiter, Saturn, Uranus, and Neptune that all orbit far from the Sun in nearly circular orbits. Given an example of exactly one solar system&mdashours&mdashtheorists had invented tidy theories that predicted that its structure was inevitable. The new discoveries have overturned these ideas, and the field of solar-system formation is now in ferment.

A rapidly developing technique for finding planets detects them as they transit across the face of their parent star and block a small part of the light. The great advantage of Hubble for transit photometry is its extraordinary photometric stability, which allows it to detect much smaller decreases in light than can be measured through Earth&rsquos fluctuating atmosphere. This is evident in Figure 3.10, which shows a scatter in the measurements of only 0.02 percent, some 50 times smaller than is possible with typical ground-based photometry. This scatter is only a factor of two larger than the dip caused by Earth as it passes in front of the Sun, as seen by a hypothetical distant observer. HST&rsquos high accuracy is important to this effort in three ways. The first is illustrated in Figure 3.10, where HST actually resolves the time needed for ingress and egress. This is the only known way to measure planet radii. The second is that Hubble can provide rapid confirmation for NASA&rsquos Kepler mission, 2 which is planned for launch in late 2007 and is specifically designed to search for transiting extrasolar planets, including Earth-like planets. The Kepler technique will produce many false positives that will have to be screened out by other methods. Kepler can do much of this itself, but the process will take years for Earth-size candidates high-resolution Hubble photometry could provide much more rapid feedback and possible optimization of further Kepler observations. For maximum benefit, Hubble operations should overlap the Kepler mission from 2008 to beyond 2010. Finally, Hubble can take exceptionally accurate spectra of planetary systems during eclipse, yielding measurements of water and other species in jovian-sized planetary atmospheres. 3

Photometry with the James Webb Space Telescope (JWST) will also have higher accuracy than that possible from ground-based telescopes and will also play an important role in planet detection. However, JWST&rsquos system is not as well understood at this time, and its launch is still several years away.

David Charbonneau, &ldquoHubble&rsquos View of Transiting Planets,&rdquo in From Planets to Cosmology: Essential Science in Hubble&rsquos Final Years, STScI 2004 May Symposium, Space Telescope Science Institute, Baltimore, Md., in press.

FIGURE 3.10 The presence of an otherwise invisible planet can be detected by the small drop in light caused as the planet travels in front of its parent star. The &ldquolight curve&rdquo of such a transit is shown here, with the drop in light at slightly more than 1.5 percent, as would occur with a giant Jupiter-like planet passing in front of the Sun. However, the scatter in the Hubble measurements is so small that even smaller planets could be detected. Hubble has begun to monitor rich star fields like that shown in the background, which is a region near the center of the Milky Way Galaxy. In this manner, several hundred thousand stars can be searched for Jupiter-size and smaller planets in roughly 1 week of Hubble Space Telescope observing time. Courtesy of STScI/NASA.

Similarly, most of the stars targeted by the Kepler mission are too faint for effective imaging with ground-based adaptive optics systems. For proven high accuracy and overlap/coordination with the Kepler mission, Hubble is preferred.

Besides detection of extrasolar planets, a great variety of other important work will be able to continue if Hubble remains operational. A large number of new supernovas could be found for the study of dark energy, reducing uncertainties in its properties by a factor of two. A wealth of data would be taken to explore the nature of stars in the Milky Way Galaxy and in neighboring galaxies. Hubble is just beginning to image objects being found by sister NASA missions such as the Chandra X-ray Observatory, GALEX (an ultraviolet imager), and Spitzer (an infrared imager and spectrograph), which are currently in orbit. These satellites are relatively wide-field survey telescopes, one of whose expressed purposes is to detect objects for Hubble follow-up observations. The chance for these follow-ups would be severely limited if Hubble&rsquos life were curtailed, because the areas of the sky surveyed by Hubble for any one observation are much smaller than those observed at other wavelengths, and thus it requires more time to cover a field.

In the closing years of the Hubble telescope&rsquos active life, emphasis is turning toward the gathering of large, homogeneous data sets&mdashincluding spectral libraries and imaging surveys of large areas within the Milky Way, nearby galaxies, and the distant universe. These data sets, called Treasury Programs, will go into the data archive they are Hubble&rsquos lay-away plan for the future. These programs are extremely important because there are no plans in the foreseeable future to replace Hubble with a telescope of comparable size and wavelength coverage. The servicing mission SM-4 is needed to allow an orderly completion of this important aspect of Hubble&rsquos mission.

Forefront programs would be enabled by the two new instruments to be installed by SM-4&mdashstarting with the near-infrared arm of WFC3. Long-wavelength imaging has been a popular mode on Hubble, but the relatively small field of view of the NICMOS camera has been a serious handicap. Important new vistas would be opened by the near-IR arm of WFC3. A major goal is observing the most distant galaxies, whose light is highly red-shifted by the expansion of the universe. Light from the most distant galaxies detectable by Hubble is red-shifted so much that it is &ldquotoo red&rdquo for ACS, whose sensitivity ends at about 1 micron. Critical spectral features needed to measure age and distance are red-shifted entirely out of ACS&rsquos range. WFC3 will reach these objects and enable Hubble at last to see the full distance to which its mirror is capable of giving access.

The deepest image taken yet with Hubble is its Ultradeep Field, in which a handful of objects have been identified beyond a redshift of 6 (see Figure 3.3). The age of the universe at this redshift is already 1 billion years WFC3 images of the same field should reach back to redshift 10, nearly twice as close to the Big Bang. This capability is critical because the universe evolved rapidly at these epochs, and even a small increase in look-back time can reveal new phenomena. This is the era of the first galaxies, when stars began shining and black holes began to evolve toward quasars, when the featureless cosmic void began to condense and lay the foundations for planets and life. WFC3 looks through a window that will shed light on our own distant past.

How and when galaxies form stars is another great astronomical mystery. Much of the early star formation seems to have occurred in bursts triggered by collisions of massive galaxies. Such bursts are hidden within dark clouds of gas and dust and cannot be seen at visible wavelengths. WFC3&rsquos near-infrared detector can penetrate the dust to reveal underlying properties of the starburst (see Figure 3.11). In this quest, WFC3 would work synergistically with the Spitzer infrared satellite, which will detect dust-enshrouded starbursts in great numbers but will rely on Hubble for high-resolution follow-up work.

A third important task of WFC3 is to pursue and extend the supernova discovery program. These objects have provided the best evidence that the universe is expanding faster with time, requiring dark energy to drive the acceleration. WFC3 could establish whether the amount of dark energy is evolving with time or has remained constant&mdashpotentially an extremely important question for fundamental physics. Even without WFC3, Hubble would make progress by likely discovering some 30 new supernovas in 4 years. WFC3 would increase this detection rate by a factor of 2.5, and should also detect some extremely important supernovas at much larger distances. Such distant supernovas are invisible now but should be detected in significant numbers by WFC3. The result would be much tighter constraints on the properties of dark matter.

Other programs for the WFC3-IR camera include a hunt for water-bearing rocks on Mars and ices on outer satellites in the solar system. In each case, capabilities provided by Hubble will be unique among existing astronomical facilities.

Because Earth&rsquos atmosphere is opaque to wavelengths of less than 0.30 micron, the Hubble telescope offers unique opportunities at ultraviolet wavelengths. This potential has been only partly realized to date, because of the difficulty of making space-qualified ultraviolet detectors. High UV efficiency will be achieved on Hubble for the first time when both WFC3 and COS are installed. WFC3&rsquos short-

FIGURE 3.11 An illustration of the power of near-infrared light to penetrate dust clouds and reveal embedded, newly formed stars. (left) A Wide-field Planetary Camera 2 (WFPC2) view of the center of the Orion nebula with the five Trapezium stars. (right) The same region imaged in the near infrared with the NICMOS camera, which makes many previously hidden stars visible. This pair of images illustrates why observing at many different wavelengths is required. Wide-field Camera 3 will be 50 times more efficient than NICMOS for this work. Courtesy of STScI/NASA.

wavelength detector would provide sensitive ultraviolet imaging below 0.30 micron. Stellar populations redden as they age, as hot, blue, massive stars die away. Slicing the spectrum into colors thus slices the stellar population into age cohorts, with the youngest, most recently formed stars visible in the ultraviolet. It will be exciting to turn WFC3&rsquos UV capability onto distant galaxies, whose star-formation histories can be captured at previous epochs and merged to synthesize the history of cosmic star formation.

While detecting radiation is usually the goal, sometimes not detecting it is even more important. Imaging at ultraviolet wavelengths can reveal the presence of distant proto-galaxies because light at wavelengths below 0.12 micron is absorbed by intervening clouds of intergalactic hydrogen gas, thereby creating a &ldquohole&rdquo in the spectrum where it appears black. In distant objects, this hole is redshifted to longer wavelengths, so that objects disappear or &ldquodrop out&rdquo in certain colors. WFC3&rsquos greater UV sensitivity will allow it to discover UV dropouts nearly 10 times fainter than those currently known, deepening our knowledge of distant galaxies beyond the brightest ones currently known.

The other gap in instrumentation in the ultraviolet&mdashspectroscopy&mdashwill be significantly filled by the Cosmic Origins Spectrograph. COS is an instrument optimized for a number of highly important programs in cosmology. The first of these is study of the &ldquocosmic web&rdquo consisting of diffuse matter not yet coalesced into galaxies (Figure 3.12). The cosmic web forms a huge network in space around our galaxy but is largely invisible because no stars or galaxies have yet formed in it. It contains many vital

FIGURE 3.12 Theoretical models of galaxy formation predict that the universe is threaded by filaments of matter between the galaxies. It is at the intersection points of this so-called cosmic web that galaxies, and then clusters of galaxies, form. Because it contains only dark matter and gas that has not yet condensed into stars, the web is invisible. However, gas inside it is capable of absorbing light that passes through it on the way to Earth from background objects. Evidence of this absorption can be seen in the spectrum of a background object, which has dips where light is removed by web-gas atoms. A sample spectrum is shown at the lower right. The much higher efficiency of the Cosmic Origins Spectrograph would enable it to take spectra of many more background quasars, creating a dense network of sight lines with which to probe the cosmic web.

clues to cosmogenesis. The density and geometry of the web reflect the original density ripples in the universe that gave rise to all the structure seen today. Galaxies form at &ldquonodes&rdquo in the web, where filaments intersect and grow via the pull of gravity, which drags matter along web-lines into the nodes. How and when does this happen, and how do galaxies &ldquoturn on&rdquo? If it were visible to the eye, the web would reveal the distribution of matter that has not yet fallen into galaxies&mdashwhich is most of the matter in the universe! The web is thus the dominant player in the cosmic-matter energy budget.

With COS it would be possible to study the cosmic web in detail for the first time. Though not radiating much by itself, the web absorbs light from bright, background sources such as quasars, leaving

dips at particular wavelengths in the spectrum. Each quasar line-of-sight is thus a &ldquocore-drilling&rdquo through space that reveals pieces of the cosmic web. The big advantage of COS is higher sensitivity, some 10 to 30 times that of STIS. As a consequence, many more faint quasars can be studied, making a much denser pattern of core-drillings through space. The dense coverage should reveal the geometry of the web and its evolution with time.

The total observing program of COS would be rich because the same spectral features that delineate the web are also found in interstellar gas and in stellar atmospheres. The tracer elements involved include nitrogen, silicon, aluminum, oxygen, carbon, and iron&mdashelements basic to the formation of Earth and life. COS spectra can be used to explore the chemical evolution of galaxies and the intergalactic medium via nucleosynthesis of these elements. Velocities of gas clouds can be measured to show how hot stars and quasars feed back their energy into surrounding gas, driving massive &ldquowinds&rdquo from galaxies. These UV spectral features are also important for studying the chemistry and physics of planetary atmospheres in the solar system. In total, the large efficiency gains enabled by COS would open for the first time a wide window for UV spectroscopy.

Of the two instruments slated for SM-4, WFC3 is the more powerful because of its wide wavelength range and its sensitivity in the near infrared, which is particularly important for studying the highly redshifted distant universe. WFC3 is thus essential for any servicing mission, and the installation of COS is highly desirable.

FINDING: A minimum scientifically acceptable servicing mission would install batteries, gyroscopes, WFC3, and one FGS. The installation of COS is highly desirable.

### FUTURE SCIENCE POTENTIAL RELATIVE TO PAST ACHIEVEMENTS

Hubble&rsquos oversubscription by a factor of about 7 indicates that scientific productivity with the present instruments is already high the new instruments WFC3 and COS would extend the power of the observatory significantly further. In an attempt to quantify this statement, selected objectives from the above list of future science programs have been identified that, in the opinion of the committee, are comparable in importance to the top 10 Hubble contributions listed in Table 3.2. The result is five objectives listed in Table 3.4. Allowing for the overwhelming likelihood of important unforeseen discoveries in addition to those listed in Table 3.4, the committee concludes that the promise for future Hubble discoveries following a servicing mission is comparable to the telescope&rsquos promise when first launched. The programs listed in Table 3.4 are also very well aligned with the list of key problems highlighted by the most recent decadal survey report for astronomy and astrophysics, Astronomy and Astrophysics in the New Millennium. 4

### COMPARISON OF HUBBLE WITH OTHER PLANNED FACILITIES

The unique advantage of HST with respect to other astronomical tools is its exquisite angular resolution extending from the ultraviolet to the near infrared. Observations in the ultraviolet and part of the near IR (IR) are impossible from the ground at any resolution. Even at wavelengths accessible from the ground, HST still has a big advantage for imaging and low-resolution spectroscopy because of its

## NASA HUBBLE FELLOWSHIP PROGRAM

The NASA Hubble Fellowship Program (NHFP) supports outstanding postdoctoral scientists to pursue independent research which contributes to NASA Astrophysics, using theory, observation, experimentation, or instrument development.

The NHFP preserves the legacy of NASA&rsquos previous postdoctoral fellowship programs.

Once selected, fellows are assigned to one of three sub-categories corresponding to NASA&rsquos &ldquobig questions&rdquo:

###### Are We Alone? - Sagan Fellows

The Space Telescope Science Institute administers the NHFP on behalf of NASA, in collaboration with the NASA Exoplanet Science Institute (NExScI) at the California Institute of Technology and the Chandra X-ray Center at the Smithsonian Astrophysical Observatory.

Applications are now closed for fellowships to begin in fall 2021. We anticipate offering up to 24 fellowships this year, contingent upon funding. The NHFP is open to applicants of any nationality who have earned their doctoral degrees in astronomy, physics, or related disciplines on or after January 1, 2018, or who will receive their degree before September 2021. Under certain circumstances, this date may be extended back to January 1, 2017. See the Announcement of Opportunity for further information.

NHFP fellowships are tenable at U.S. host institutions of the fellow's choice, subject to a maximum of two new fellows per host institution per year, and no more than five fellows at any single host institution, except for short periods of overlap. The duration of the fellowship is up to three years: an initial one-year appointment, and two annual renewals, contingent on satisfactory performance and availability of NASA funds.

Detailed program policies and application instructions may be found in the official Announcement of Opportunity.

#### 2021 NHFP Fellows Announced

NASA has selected 24 new fellows for its prestigious NASA Hubble Fellowship Program (NHFP).

#### NHFP Host Institution Employment Policy Change

Starting with the academic year 2022-2023, NHFP Host Institutions will be required to offer their NHFP Fellows employee status in order to be eligible to host additional NHFP Fellows.

### Synergy with HST, JWST, and Observatories in the 2020s

Roman will produce large-scale maps of the night sky at the resolution and sensitivity of the Hubble Space Telescope (HST), but with a field of view 100 times larger. The James Webb Space Telescope (JWST) will have higher resolution, and sensitivity 100 times as powerful as Hubble, but will have about the same sized field of view as Hubble. With its large field of view and high resolution, Roman surveys will discover rare astronomical objects that can be followed up by JWST and other powerful telescopes being built in the 2020s. Roman&rsquos Hubble-like resolution and sensitivity will allow astronomers to test and calibrate the photometric precision, astrometry, classification systems, and the effect of source blending for large ground-based surveys performed in the 2020s, such as the Rubin Observatory's Legacy Survey of Space and Time (LSST). Roman will add valuable near-infrared colors to the LSST&rsquos visible imaging catalogs, improving measurements of the photometric redshift of galaxies and the properties of their stellar populations.

### Partners

The Roman mission involves work by various institutions. The Roman Space Telescope Project Office is at NASA's Goddard Space Flight Center (GSFC), which also oversees the work on the Wide Field Instrument (WFI), the Spacecraft Bus, and System Integration. The NASA's Jet Propulsion Laboratory (JPL) oversees the work on the coronagraphic instrument. The Space Telescope Science Institute (STScI) is the Science Operations Center for Roman, and shares science support responsibilities with IPAC, foreign partners, and GSFC. STScI leads the work on the mission's observation scheduling system, wide field instrument data processing system for the direct-imaging mode, and the mission's entire data archive.

## Current HST Proposal Opportunities

Release Notice: January 13, 2021

We are pleased to announce the Cycle 29 Call for Proposals for Hubble Space Telescope (HST) observations and funding for archival research programs.

Participation in this program is open to all categories of organizations, both domestic and foreign, including educational institutions, profit and nonprofit organizations, NASA Centers, and other Government agencies.

Cycle 29 will extend from October 1, 2021 to September 30, 2022. We will accept proposals for the following instruments: ACS, COS, FGS, STIS, and WFC3.

We anticipate allocating up to 2700 orbits in this cycle. See Hubble Space Telescope Call for Proposals for Cycle 29 for further details.

This solicitation for proposals will be open through April 9, 2021 at 8:00pm EDT. The Astronomer's Proposal Tool (APT), which is required for Phase I Proposal Submission, was made available for Cycle 29 Phase I use on December 18, 2020. Results of the selection will be announced by early July.

Questions can be addressed to the STScI HST Help Desk (https://hsthelp.stsci.edu or email [email protected]). We encourage the use of the new website where you can submit questions directly to the appropriate team of experts.

## HST Telescope Allocation Committee (TAC)

HST proposals are selected through competitive peer review. Panelists are chosen based on their scientific expertise in the areas under review by the topical panels. Each topical panel will be managed by a panel Chair and a vice-Chair (except Solar System) , and the Meeting Chair overseeing the review process.

Small GO proposals requesting fewer than 16 orbits, Archival (including Theory), and Snapshot proposals are reviewed by External panelists. All other proposals are reviewed by Virtual panelists.

To assist in the review process, each panel will also be assigned a Panel Support Scientist (PSS). The role of the PSS is to ensure the process runs smoothly and act as liaison between the panel and STScI. Virtual panels will also be assigned a Leveler whose role is to ensure the discussion remains focused on the scientific merits of the proposals. Proposals will be graded on an absolute scale against the primary criteria:

• Scientific impact within the sub-field,
• Broader importance for astronomy,
• Suitability of HST’s unique capabilities for achieving the scientific goals.

The TAC Chair, At-Large TAC members, and panel Chairs and vice Chairs will form the Executive Committee. The Executive Committee will review the Large GO, Treasury and AR Legacy proposals. The Executive Committee will also rank the Pure Parallel programs, and will adjudicate any cross-panel scientific issues, as needed. All recommendations for the Cycle 29 science program are advisory to the STScI Director, who is responsible for the final allocation of HST observing time and funding.

## The Lake County Astronomical Society

primary mirror led many to claim that powerful new ground-based telescopes will equal or surpass HST's performance. On the other hand, when optically corrected after the Servicing Mission in late 1993, the sensitivity of HST to very faint objects increased by about 10 times. Each type of telescope offers astronomers unique strengths. Astronomy will make its mark on history with a combination of advanced ground-based systems and the Hubble Space Telescope.

Two features of telescope design are crucial in comparing the capability of ground-based telescopes: resolving power and light gathering power. Location also is a key factor which in many cases outweighs the differences in resolving power and light gathering power of telescopes.

One measure of a telescope's capability is light gathering power. The bigger the area of a lens or mirror, the more light from an object that can be captured and focused to make a brighter image. For cameras, it's the f-stop which controls how much area of the lens is available: The more area (lower f-numbers), the shorter the exposure needs to be to form an image. Because astronomers study very faint objects in the sky, they need telescopes with as big an area as possible to collect and concentrate light into an image.

The most light-hungry instruments are the spectroscopes which take the incoming light and split it into an array, like colors of the rainbow, called a spectrum. With these spectra scientists can tell what kinds of atoms and molecules are found at very great distances or far back in time and how hot and how fast they are moving.

The world's largest telescope, the W.M. Keck telescope in Mauna Kea, Hawaii, instead of one mirror, is made of many individually controlled, hexagonal mirror segments. Keck's multi-mirror array has a 10-meter (33-foot) aperture. This colossus has 17 times more light collecting area than HST's 2.4-meter (7-foot) mirror. Consequently, Keck's "light bucket" is significantly faster at collecting faint starlight. This makes the Keck telescope a powerful instrument for performing spectroscopic studies of faint objects it could gather spectral data from astronomical sources much more quickly than Hubble.

Resolving power or resolution is the ability to yield sharp, detailed images. An optometrist calls this "visual acuity". In theory, a telescope's resolving power improves as the diameter of the telescope's mirror or lens increases. However, the blurring of starlight by Earth's atmosphere prevents telescopes from realizing their theoretical potential.

Ground-based telescopes can resolve detail about 60 times better than the human eye. Keck's larger mirror array is limited by the atmosphere to a resolving power of about .5 arc second. HST, located high above the Earth's atmosphere, has a resolving power that is 5-10 times better. This means that the HST can concentrate starlight into much smaller spots and separate objects that are much closer together than the Keck can.

Although HST will remain the premier observatory for high resolution studies of astronomical objects, new techniques are being used with ground-based telescopes that will allow them to compete with HST even in resolving power. However, these techniques only work for bright star-like objects. They also work only for small patches of sky, a few arc seconds across, near the center of the telescope's field of view. This means that these techniques will not work for studies of extended targets such as star-forming nebulae.

Active Optics, Adaptive Optics

If you look at a star on any clear night, it appears to twinkle. Light from the star passing through the turbulent atmosphere is shifted slightly by parcels of air moving to and fro thousands of times per second. By the time the light reaches the eye, or the telescope, the dancing of the light causes the image to blur into a spot about one arc second wide.

New technologies can sharpen these stellar images by distorting a telescope's mirror to compensate for blurring, or by extracting the blurring later using image processing. In active optics, the shape of the telescope mirror is adjusted hundreds of times per second to cancel the distortions caused by the atmosphere. In adaptive optics, the telescope mirror is moved thousands more times per second to follow the dancing image of the star and keep it focused into a small spot. This technique requires a reference source, either a bright star located in the vicinity of the celestial target or an "artificial star" created by reflecting a laser off selected layers in the atmosphere.

By contrast, Hubble's images and other data are optically stable so astronomers can revisit a target at any time of year and expect the same quality data. This assures a repeatability that is not possible with adaptive optics on ground-based telescopes. An enormously complex ground-based system would be required to match HST performance. Such a system does not currently exist, and could be a decade away - beyond the working lifetime of HST.

By taking many short exposures in succession rather than one long-time exposure, the effects of atmospheric turbulence can be "frozen." A computer combines each image by shifting it to a common center. This subtracts the blurring effects and recovers the telescope's best resolving power. For ground-based Mt. Palomar's 200-inch mirror this is an improvement from 0.4- at best to 0.02-arc seconds.

Because HST is a space-based observatory, it is the only telescope that can view any celestial target located anywhere in the sky unhindered by either the Earth, Moon or Sun. Keck is located at an ideal observing spot near Earth's equator, allowing it to observe most of the celestial sphere. However, stars and galaxies located near the north and south celestial poles (below 30 degrees altitude from Hawaii) are inaccessible.

Space-based Ultraviolet Astronomy

The ultraviolet region is a gold mine. The most common elements in the universe - hydrogen, helium, carbon, nitrogen, oxygen and silicon - all leave spectral signatures in the ultraviolet. Ultraviolet light typically radiates from extremely hot, dynamic phenomena, such as cores of active galaxies, quasars, energetic stars, and vast disks of dust around black holes. For those looking through HST's ultraviolet window, a wealth of science about many mysterious sources of energy is open.

A broad range and variety of objects in the universe radiate energy at ultraviolet wavelengths, including the atmospheres of most stars, the surfaces of stars far more massive than our Sun, white dwarfs and hot regions of interstellar gas. However, Earth's atmosphere absorbs almost all the ultraviolet light arriving from other celestial bodies. A telescope in space, equipped for ultraviolet observations, is therefore necessary for the study of objects detectable in this important region of the spectrum. In particular, HST is the most powerful telescope ever launched for ultraviolet astronomy.