How Hubble Observations Are Planned

This is the second in a three-part series.

Researchers awarded telescope time based on the scientific merit of their Phase I proposal must submit a Phase II proposal that specifies the many details necessary for implementing and scheduling of the observations. These details include such items as precise target locations and the wavelengths of any filters required.

Once an observation has occurred, the data becomes part of the Hubble archive, where astronomers can access it over the Internet. Most data is marked as proprietary within the Institute computer systems for 12 months. This protocol allows observers time to analyze the data and publish their results. At the end of this proprietary-data-rights period, the data is made available to the rest of the astronomical community. (Most of the very large programs, such as Frontier Fields, have given up proprietary time as part of their proposal.)

This is a view of the many computers that are part of the Barbara A. Mikulski Archive for Space Telescopes (MAST), located at the Space Telescope Science Institute (STScI) in Baltimore, Md. The archive is named in honor of the United States Senator from Maryland for her career-long achievements and becoming the longest-serving woman in U.S. Congressional history. MAST is NASA’s repository for all of its optical and ultraviolet-light observations, some of which date to the early 1970s. The archive holds data from 16 NASA telescopes, including current missions such as the Hubble Space Telescope and Kepler. Senator Mikulski is in the center, STScI Director Matt Mountain at her right, and STScI Deputy Director Kathryn Flanagan at her left. The plaque to image right is a photo of Supernova Milkuski, an exploding star that the Hubble Space Telescope spotted on Jan. 25, 2012. It was named in honor of the Senator by Nobel Laureate Adam Riess and the supernova search team with which he is currently working. The supernova, which lies 7.4 billion light-years away, is the titanic detonation of a star more than eight times as massive as our Sun.

This is a view of the many computers that are part of the Barbara A. Mikulski Archive for Space Telescopes (MAST), located at the Space Telescope Science Institute (STScI) in Baltimore, Md. The archive is named in honor of the United States Senator from Maryland for her career-long achievements and becoming the longest-serving woman in U.S. Congressional history. MAST is NASA’s repository for all of its optical and ultraviolet-light observations, some of which date to the early 1970s. The archive holds data from 16 NASA telescopes, including current missions such as the Hubble Space Telescope and Kepler. Senator Mikulski is in the center, STScI Director Matt Mountain at her right, and STScI Deputy Director Kathryn Flanagan at her left. The plaque to image right is a photo of Supernova Milkuski, an exploding star that the Hubble Space Telescope spotted on Jan. 25, 2012. It was named in honor of the Senator by Nobel Laureate Adam Riess and the supernova search team with which he is currently working. The supernova, which lies 7.4 billion light-years away, is the titanic detonation of a star more than eight times as massive as our Sun.

Along with their Phase II proposal, observers can also apply for a financial grant to help them process and analyze the observations. These grant requests are reviewed by an independent financial review committee, which then makes recommendations to the Institute director for funding. Grant funds are also available for researchers who submit Phase I proposals to analyze non-proprietary Hubble data already archived. The financial committee evaluates these requests as well.

Up to 10 percent of Hubble ’s time is reserved as director’s discretionary time and is allocated by the Institute director. Astronomers can apply to use these orbits any time during the course of the year. Discretionary time is typically awarded for the study of unpredictable phenomena such as new supernovae or the appearance of a new comet. Historically, directors have allocated large percentages of this time to special programs that are too big to be approved for any one astronomy team. For example, the observations of the Frontier Fields use director’s discretionary time.

In my last post, I talked about how observations are proposed.  In my next post, I will talk about how observations are scheduled.

How Hubble Observations Are Proposed

This is the first in a three-part series. 

Time on the Hubble Space Telescope is a precious commodity. As a space telescope, Hubble can observe 24 hours a day, but its advantageous perch also attracts a large number of astronomers who want to use it. The current oversubscription rate—the amount of time requested versus time awarded—is six to one.

The process of observing with Hubble begins with the annual Call for Proposals issued by the Space Telescope Science Institute to the astronomical community. Astronomers worldwide are given approximately two months to submit a Phase I proposal that makes a scientific case for using the telescope. Scientists typically request the amount of telescope time they desire in orbits. It takes 96 minutes for the telescope to make one trip around the Earth, but because the Earth usually blocks the target for part of the orbit, typical observing time is only about 55 minutes per orbit.

Longer observations require a more compelling justification since only a limited number of orbits are available. Winning proposals must be well reasoned and address a significant astronomical question or issue. Potential users must also show that they can only accomplish their observations with Hubble ’s unique capabilities and cannot achieve similar results with a ground-based observatory.

The Institute assembles a time allocation committee (TAC), comprising experts from the astronomical community, to determine which proposals will receive observing time. The committee is subdivided into panels that review the proposals submitted within a particular astronomical category. Sample categories include stellar populations, solar system objects, and cosmology. The committee organizers take care to safeguard the process from conflicts of interest, as many of the panel members are likely to have submitted, or to be a co-investigator, on their own proposals.

The time allocation committee (TAC) discusses which proposals will receive observing time on Hubble.

The time allocation committee (TAC) discusses which proposals will receive observing time on Hubble.

Proposals are further identified as general observer (GO), which range in size from a single orbit to several hundred, or snapshot, which require only 45 minutes or less of telescope time. Snapshots are used to fill in gaps within Hubble ’s observing schedule that cannot be filled by general observer programs. Once the committee has reviewed the proposals and voted on them, it provides a recommended list to the Institute director for final approval.

In my next post, I will discuss how observations are planned.

Frontier Fields Q&A: Redshift and Looking Back in Time

Q: What do you mean when you say you’re “seeing some of the earliest galaxies in the universe?” How does looking into deep space allow you to look back in time?

The simple answer is that light travels and the universe is huge. Light travels very fast – 186,000 miles (300,000 km) per second, but it still has to move across the vast distances of space. Remember that for us to see anything – from the flash of a camera to the glow of a really distant galaxy, we have to wait for its light to strike our eyes.

That camera flash shows in our vision instantaneously because it doesn’t have far to go. But distances in the cosmos are so vast that it takes light a long time to reach us. The light from our closest companion, the Moon, takes about 1.3 seconds to cross the 239,000 miles (390,000 km) between us. So when you look up at the sky, you don’t see the Moon as it currently is. You see it as it appeared 1.3 seconds ago.

This is so 1.3 seconds ago. Credit: Luc Viatour, Wikimedia Commons

This is so 1.3 seconds ago.
Credit: Luc Viatour, Wikimedia Commons

The greater the distances, the greater the time difference. Light from the Sun needs about 500 seconds, or about eight minutes, to reach us from 93,200 miles (150 million km) away. Light from Neptune needs about four hours to cross the solar system.

We refer to these distances by the time it takes light to cross them. So Neptune is four light-hours away, and the Sun is 500 light-seconds away. Light from the next nearest star, however, needs four years to reach us across space. We say that star is four light-years away. The light we see from that star in today’s sky is also four years old. For galaxies, we’re talking millions to billions of light years. So we see the farthest galaxies as they appeared in the early universe, because the light that left them way back then is finally reaching us just now.

Q: What does it mean when you talk about a galaxy’s redshift?

When we’re discussing the Frontier Fields project, we’re talking about something more precisely called “cosmological redshift.” The space light is traveling through is expanding. That means that the light wave gets stretched as it travels, like a spring being pulled into a different shape. This stretching shifts light into longer wavelengths.

Since red light has a longer wavelength than blue light, the light is said to be "red-shifted." Credit: NASA

Since red light has a longer wavelength than blue light, the light is said to be “redshifted.” Credit: NASA

The farthest galaxies in the universe would have originally emitted visible and ultraviolet light, but since that light has been stretched as it travels, those galaxies appear to us instead in the form of infrared light. Cosmological redshift refers to that change and the measure of that change.

Q: Why do we hear the Frontier Fields galaxies described in terms of redshift and light-years? Which is right?

They tell us different things. Light-years are a measurement of distance defined by the time it takes light to travel in a year. But distance is notoriously difficult to measure in astronomy.

Cosmological redshift is a direct measurement of the expansion of space. Astronomers describe galaxies in terms of their redshift because unlike distance, it’s a clear and definite value that’s relatively easy to measure without many errors.

Astronomers have different models of how the universe works, and they can plug the redshift into those models to get the distance to a galaxy – but the distance will differ depending on which model of the universe they use. The variations in those models include things like the shape of the universe, the rate at which it’s expanding, the amount of normal matter it contains, etc.

Astronomy is about figuring out how the universe works and narrowing down all those models to the best one, and we still have a long way to go. Projects like Frontier Fields will help us rule out those models that don’t fit the incoming data.

Q: Everywhere we look with the Frontier Fields project, galaxies appear to be moving away from us. Does this mean we’re in the center of the universe?

No. It’s evidence that space is expanding. The easiest way to visualize this is to imagine a balloon. If you cover the balloon with dots, and then inflate it, no matter which dot you pick to represent your position, all the other dots will appear to be moving away from it as the balloon expands. Imagine this happening in three dimensions instead of on a flat surface, and you can understand why it looks like other galaxies are rushing away.

Q: So space is expanding and the light from the earliest galaxies has traveled over 13 billion years to reach us. If space is expanding, are those galaxies even farther away now?

Yes. For nearby galaxies, the expansion doesn’t make much of a difference. But for galaxies extremely far away, the distance is significant. That’s because the farther away an object is, the more space there is between us and the object. That in turn means there’s more space to undergo expansion, so the objects appear to be moving away from us much faster. Light from the earliest galaxies may have traveled 13 billion years to reach us, but those galaxies could be around 45 billion light-years distant by now.

Q: Does this mean the galaxies are moving faster than the speed of light?

No. No object can travel through space faster than the speed of light. But the expansion of space itself is not so constrained – in fact, theories of the beginning of the universe visualize the initial expansion of the Big Bang happening with unthinkable speed. But because the speed of light is only so fast, there are galaxies in the distance whose light we cannot yet see. We call this the edge of the visible universe.

Q: What’s out there, past the edge?

Space dragons! Ok, probably not. Credit: Uranometria

DRAGONS! SPACE DRAGONS! GIANT, COSMIC FIRE-BREATHING SPACE DRA– Ok, fine, probably not. Credit: Uranometria, Wikimedia Commons

We expect more of the same, though this is still an open question that astronomers are researching and theorizing about. We’ve found we tend to see the same distribution of galaxies no matter which direction we look in the universe. If we were somehow transported to a galaxy on what, for Earth, is the edge of the visible universe, the border of the visible universe would move, but the universe would neither change nor look very different to us.

Q: Do you have a question about the Frontier Fields project?

Leave it in comments, and we’ll see if we can answer it.

How Were the Galaxy Clusters Chosen?

The 12 Frontier Fields will greatly expand upon our knowledge of the earliest galaxies to form in the universe. These images of the distant universe (in space and time), will provide us with a sneak peek at the first billion years of the universe. So how were these fields chosen?

The Frontier Fields program was sketched out by the Frontier Fields team in the earliest phases of a recommendation process. Much can change in the process of going from an initial recommendation to a final program. The final program hinged upon finding the best galaxy clusters to anchor the Frontier Fields program. Team members deliberated between several different galaxy clusters, nominated by both those directly involved in the program and the broader astronomical community, before settling on the final candidates.

Special consideration was given to galaxy clusters that

  1. maximize magnification and fit within Hubble’s view;
  2. were located in “clean” locations on the sky;
  3. were observable by ground-based observatories in the Northern and Southern hemispheres.

The Frontier Fields team, with input from the broader astronomical community, was able to narrow down the galaxy cluster candidates to the six chosen for the program. Although it was not possible to select six clusters that met all of the criteria, most of the clusters satisfied most of the criteria. Let us explore the three criteria in a little bit more depth.

 

Maximize Magnification

Astronomers focused on massive galaxy clusters as candidates because the gravitational lenses they create are likely to provide the greatest magnification of background galaxies, but there were other considerations as well.

Hubble is observing the Frontier Fields with a visible-light instrument and an infrared-light instrument. The fields of view of these instruments, defined to be the area of the sky they can image in one pointing, are relatively small – a box with sides about 1/15 the width of the full moon. Because of the small fields of view, the galaxy clusters need to be relatively compact so that any magnified background galaxy remains within the fields of view.

There is another reason why the galaxy clusters must  be relatively compact in size. For each of the galaxy clusters, Hubble is also imaging an adjacent parallel field. For the goals of the program, the parallel fields need to contain unobstructed views of the early universe, devoid of the metropolis of galaxies that make up the galaxy clusters. Astronomers lose the magnifying power of the galaxy clusters, but gain simplicity. For the parallel fields, astronomers do not require detailed models of how the light from the distant galaxies are lensed by the foreground clusters.

 

Clean Locations on the Sky

Below is a map of the sky showing the locations of the six pointings required for Hubble to acquire the 12 Frontier Fields, labeled in order of when Hubble plans to observe them. The green labels are previous deep-field programs. The map is in right ascension and declination coordinates.

For a map of the Frontier Fields on the sky, with respect to the constellations, see this previous post.  Note: The right ascension of the map in the previous post is flipped with respect to the map below in order to portray the constellations as they appear to us on Earth.

 

Locations of the Frontier Fields on the sky. The colors denote the amount of extinction of background light due to dust - red is greatest dust extinction, blue is least dust extinction. The wavy dust band across the sky is our Milky Way galaxy. Credit: D. Coe (STScI),  D. Schlegel (LBNL), D. P. Finkbeiner (Harvard), M. Davis (Berkeley)

This map shows the locations of the Frontier Fields on the sky using right ascension and declination coordinates. The Frontier Fields are numbered in the order of their observations. The colors denote the amount of extinction, or dimming, of light from distant galaxies due to foreground dust. Dark red denotes the greatest dust extinction. Dark blue denotes the least dust extinction. The wavy dust band across the sky is our Milky Way galaxy. The thick purple line is the ecliptic, which is the plane of our solar system. The two thinner parallel purple lines mark 30 degrees north and 30 degrees south of the ecliptic. Previous deep-field programs are labeled on the map in green: HDF-N (Hubble Deep Field North), HDF-S (Hubble Deep Field South), UDF (Ultra Deep Field), UDS (Ultra Deep Survey), COSMOS (the Cosmic Evolution Survey), and EGS (the Extended Groth Strip). Sgr A* denotes the position of the center of our Milky Way galaxy.
Credit: D. Coe (STScI), D. Schlegel (LBNL), D. P. Finkbeiner (Harvard), M. Davis (Berkeley)

 

The two main features to note on the above map are the colors that signify dust that can lessen the light from distant galaxies reaching Hubble’s mirror and the thick purple line that marks the plane of our solar system, known as the ecliptic. The locations of the Frontier Fields’ galaxy clusters were chosen to be in relatively “clean” parts of the sky.  By that we mean that the galaxy clusters are not located where there is a large quantity of foreground dust.

Dust Extinction

The galaxy clusters in the Frontier Fields were chosen to avoid areas of greatest dust extinction. Dust extinction is the scattering or absorption of light by dust. It is problematic because it lessens the light we receive from distant objects. On the above map, dark red denotes areas of greatest dust extinction. Dark blue denotes little dust extinction. The red, high-extinction band in the all-sky map is due to the dusty disk of our own Milky Way galaxy. It appears wavy due to the projection of the sky onto the right ascension and declination coordinate system.

Zodiacal Light

The thick purple line denotes the plane of our solar system, called the ecliptic. Dust within our solar system is clustered around the ecliptic. This dust scatters the light from our Sun and produces a bright haze. It can be very difficult to observe faint objects through the zodiacal light. For this reason, the galaxy clusters were chosen to avoid the ecliptic.

 

Observable from Telescopes across the Earth

Much of what we learn from the Frontier Fields will come from follow-up observations using ground-based telescopes. Most of the galaxy clusters in the Frontier Fields are observable by state-of-the-art astronomical telescopes in both the Northern and Southern hemispheres. These include the new radio telescope in Chile, named ALMA, and the suite of telescopes on Mauna Kea in Hawaii.

For more info, the Frontier Fields galaxy cluster selection was also recently described in a Google+ Hubble Hangout.

 

 

Frontier Fields: Locations on the Sky

The galaxies in Hubble’s Frontier Fields project are so far away that they cannot be seen with either your eyes or a backyard telescope. It takes a state-of-the-art telescope like Hubble, Spitzer, or Chandra to collect enough of the scant photons streaming in from the most distant galaxies to produce a scientifically valuable image. In fact, Hubble’s views of the Frontier Fields, coupled with the natural lensing power of the galaxy clusters, allow astronomers to potentially detect objects that are 40 billion – yes, billion – times fainter than your eyes can see.

The galaxies in the Frontier Fields are so far away that they appear absolutely tiny in the night sky, even to Hubble. Hubble has the exquisite ability to resolve tremendously small features on the sky and discern details that would otherwise be blurred beyond recognition. If prior deep field observations are any indication, Hubble will observe thousands of galaxies in an area approximately the size of a pin-prick in a piece of paper held up at arm’s length.

The 12 Frontier Fields are located at six positions in the sky. You may not be able to see the Frontier Fields galaxies, but you can still find the area of the sky where they are located using the graphic below.  

The location of the Frontier Fields on the sky, using Right Ascension and Declination coordinates.  The Milky Way in this coordinate system is shown as a wavy band of diffuse light across the sky.

The location of the Frontier Fields on the sky, using Right Ascension and Declination coordinates. The Frontier Fields are numbered in the order that Hubble plans to observe them over the three-year program. The names refer to the galaxy clusters targeted in each pointing. Each pointing also has an adjacent parallel field. A few of the previous Hubble deep-field observations are labeled as well – Hubble Deep Field North (HDF-N), Hubble Deep Field South (HDF-S), and the Hubble Ultra Deep Field (HUDF). The Milky Way in this coordinate system is shown as a wavy band of diffuse light across the sky.
SOURCES: Frontier Fields locations: STScI; All-sky star chart: J. Cornmell and
IAU

The map above uses a coordinate system familiar to astronomers. Right Ascension is similar to longitude in that it measures the position of an object east or west of a reference position. Right Ascension is measured in hours, from 0 to 24 hours, with the reference position set at 0 hours. Declination is similar to latitude. It measures the position of an object, in degrees from 0 to 90, north or south of a reference position. The reference position (0 degrees) for declination is the celestial equator, which is the projection of Earth’s equator onto the sky.  In this particular map we have truncated declination at 70 degrees north and south.

The Frontier Field’s site map (above) is a representation of the sky on a rectangular grid. When we view the sky from the surface of the Earth, it appears as the interior surface of a hemisphere, or dome — half of what people in ancient times referred to as the “celestial sphere” surrounding the Earth. Just as there are distortions when map-makers make a rectangular map of the spherical Earth, there are distortions in projecting the celestial sphere onto a rectangular grid. Constellations located near the northern and southern celestial poles (90 degrees north and south in declination) are represented on the map as spanning more of the sky than they actually do.

To help find the locations of the Frontier Fields, zoomed-in regions of the six pointings are shown below:

1) Abell 2744

Location of the Abell 2744 galaxy cluster field and its parallel field in the Sculptor constellation.SOURCES: Frontier Field location: STScI; Enlarged constellation map: International Astronomical Union (IAU)

Location of the Abell 2744 galaxy cluster field and its parallel field in the Sculptor constellation.
SOURCES: Frontier Field location: STScI; Enlarged constellation map: International Astronomical Union (IAU)

2) MACS J0416

Location of the MACS J0416 galaxy cluster field and its parallel field in the Eridanus constellation.SOURCES: Frontier Field location: STScI; Enlarged constellation map: International Astronomical Union (IAU)

Location of the MACS J0416 galaxy cluster field and its parallel field in the Eridanus constellation.
SOURCES: Frontier Field location: STScI; Enlarged constellation map: International Astronomical Union (IAU)

3) MACS J0717

Location of the MACS J0717 galaxy cluster field and its parallel field in the Eridanus constellation.SOURCES: Frontier Field location: STScI; Enlarged constellation map: International Astronomical Union (IAU)

Location of the MACS J0717 galaxy cluster field and its parallel field in the Auriga constellation.
SOURCES: Frontier Field location: STScI; Enlarged constellation map: International Astronomical Union (IAU)

4) MACS J1149

Location of the MACS J1149 galaxy cluster field and its parallel field in the Eridanus constellation.SOURCES: Frontier Field location: STScI; Enlarged constellation map: International Astronomical Union (IAU)

Location of the MACS J1149 galaxy cluster field and its parallel field in the Leo constellation.
SOURCES: Frontier Field location: STScI; Enlarged constellation map: International Astronomical Union (IAU)

5) Abell S1063

Location of the Abell S1063 galaxy cluster field and its parallel field in the Eridanus constellation.SOURCES: Frontier Field location: STScI; Enlarged constellation map: International Astronomical Union (IAU)

Location of the Abell S1063 galaxy cluster field and its parallel field in the Grus constellation.
SOURCES: Frontier Field location: STScI; Enlarged constellation map: International Astronomical Union (IAU)

6) Abell 370

Location of the Abell 370 galaxy cluster field and its parallel field in the Eridanus constellation.SOURCES: Frontier Field location: STScI; Enlarged constellation map: International Astronomical Union (IAU)

Location of the Abell 370 galaxy cluster field and its parallel field in the Cetus constellation.
SOURCES: Frontier Field location: STScI; Enlarged constellation map: International Astronomical Union (IAU)

For more tips and information about observing the night sky, including access to free monthly sky charts, visit the NASA Night Sky Network. For monthly highlights of interesting objects to observe in the night sky, visit Hubblesite’s Tonight’s Sky.