How Hubble Observations Are Scheduled

This is the third in a three-part series.

After observing time is awarded, the Institute creates a long-range plan. This plan ensures that the diverse collection of observations are scheduled as efficiently as possible. This task is complicated because the telescope cannot be pointed too close to bright objects like the Sun, the Moon, and the sunlit side of Earth. Adding to the difficulty, most astronomical targets can only be seen during certain months of the year; some instruments cannot operate in the high space-radiation areas of Hubble ’s orbit; and the instruments regularly need to be calibrated. These diverse constraints on observations make telescope scheduling a complex optimization problem that Institute staff are continually solving, revising, and improving.”

Preparing for an observation also involves selecting guide stars to stabilize.the telescope’s pointing and center the target in the instrument’s field of view. The selection is done automatically by the Institute’s computers, which choose two stars per pointing from a catalog of almost a billion stars. These guide stars will be precisely positioned within the telescope’s fine guidance sensors, ensuring that the target region and orientation of the sky is observed by the desired instrument.”]

A weekly, short-term schedule is created from the long-range plan. This schedule is translated into detailed instructions for both the telescope and its instruments to perform the observations and calibrations for the week. From this information, daily command loads are then sent from the Institute to NASA’s Goddard Space Flight Center to be uplinked to Hubble.

Hubble’s Flight Operations Team resides in the Space Telescope Operations Control Center at NASA’s Goddard Space Flight Center in Greenbelt, Md.  In addition to monitoring the health and safety of the telescope, they also send command loads to the spacecraft, monitor their execution, and arrange for transmission of science and engineering data to the ground.

Hubble’s Flight Operations Team resides in the Space Telescope Operations Control Center at NASA’s Goddard Space Flight Center in Greenbelt, Md. In addition to monitoring the health and safety of the telescope, they also send command loads to the spacecraft, monitor their execution, and arrange for transmission of science and engineering data to the ground.

The journey from proposal through selection and scheduling culminates in the email informing astronomers that their data is ready to be accessed. Usually, the process takes more than a year from idea to data—sometimes even longer. Of course, that’s when the real work begins—the analysis of the data and the hard work of uncovering another breakthrough Hubble discovery!

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.

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.