NASA’s “Great Observatories”: Hubble, Compton, Chandra, and Spitzer

 

MULTIWAVELENGTH ASTRONOMY

 

Virtually every wavelength of light carries some important piece of information from astronomical objects, so in order to best understand these objects, it is generally desirable to observe them at many different wavelengths. Atoms and molecules can absorb and emit at wavelengths from radio waves to γ rays. Infrared radiation penetrates the obscuration of interstellar dust, allowing us to see the heart of our own galaxy. X rays probe the energetic emission of neutron stars and possibly black holes.

 

Each part of the spectrum requires somewhat different detector technology and observing techniques. Even the telescopes used may look quite different. Furthermore, because many wavelengths do not penetrate the Earth's atmosphere, satellites must be used to reach those wavelengths. In 1946 Lyman Spitzer recommended that telescopes be launched into Earth orbit to get around these problems. The main reasons for going to space are two:

 

1. Image Size - "Seeing" - The diffraction limit is ~ 6 arc sec/D(inches). That is, the optics of the 200 inch telescope at Palomar could resolve details as small as 6/200 arcsec = 0.03 arcsec. But atmospheric turbulence generally blurs the image size to 1 arcsec, 30 times larger.

 

2. Atmospheric Transmission - The Earth's atmosphere is opaque except for a small band in the visible (0.3 μm to 1.0 μm), a few selected bands in the infrared, and part of the radio region of the electromagnetic spectrum. Yet most atoms have spectral lines in the ultraviolet, and most hot stars emit most of their radiation there as well.

 

Many initial attempts to get at those wavelengths took the form of instruments launched on suborbital rockets and high-altitude balloons.

 

Ultraviolet

 

An example of a balloon telescope for UV astronomy was Stratoscope II, operated by Princeton University. This 36-inch telescope made some of the first UV observations of astronomical.

 

This was followed by a number of specialized UV satellite observatories, such as OAO-2 (OAO being the acronym for Orbiting Astronomical Observatory), OAO-3 (renamed Copernicus), and the IUE (International Ultraviolet Explorer  most of my Ph.D. thesis was based on data from this instrument). A few Shuttle flights carried specialized payloads for doing UV astronomy. The ASTRO-1 and ASTRO-2 flights carried the Ultraviolet Imaging Telescope (UIT), the Hopkins Ultraviolet Telescope (HUT), and the Wisconsin Ultraviolet Photopolarimeter Experiment (a mouthful -  WUPPE for short  pronounced “Wuppee!”).

 

X-rays

 

Because the atmosphere is even more opaque to X-rays than it is to UV photons, X-ray astronomy had to wait for the use of rockets. The first astronomical X-ray instrument flew in 1962 by a group of scientists & engineer that include Riccardo Giacconi and Bruno Rossi. This detected the first astronomical X-ray sources, including Sco X-1.

 

By the 1970’s, orbiting X-ray telescopes were being launched, such as HEAO-1 (HEAO is the acronym for High Energy Astrophysical Observatory), HEAO-2 (renamed the Einstein Observatory), and HEAO-3.

 

γ-rays

 

Going to higher energy yet, a number of gamma-ray facilities were launched. Some of these, such as the Vela satellites, were DoD facilities looking for nuclear tests on the Earth, but made numerous discoveries of astronomical sources.

 

Finally, at much lower energies, below that of visible light, is the infrared. There are a few “windows” in the electromagnetic spectrum that can be used from the ground, but most wavelengths are opaque to this radiation.

 

Infrared

 

In the IR, a number of balloon and rocket instruments culminated in the Kuiper Airborne Observatory, with its 36-inch telescope looking out through a hole in the side of an airplane! You can even take a virtual tour of the KAO.

 

Hubble Space Telescope  Great Observatory #1

 

Planned HST defined to be 94.5 inches - limited by the size of the Shuttle bay and vacuum test facilities.

Spacecraft at Launch:

 

Primary Mirror            2.4m (94")

Focal Ratio                  f/24

Length                         43.5 feet

Weight                         25,000 lbs.

 

A list of the instruments, past and present, can be found here. However, the “future” instruments may be cancelled due to NASA’s decision not to do the last servicing mission. Not in the list are the Fine Guidance Sensors, which keep the telescope pointed at the target object. They are also used to detect close binary stars.

 

Expected performance - "point spread function", how broad a perfect point of light would be smeared.

 

HST originally scheduled for 1986 launch - delayed after Challenger blew up during launch.

 

Extra time used to prepare software, guide star catalog, some instrumentation, but mostly the instrument just sat and aged.

 

Finally launched April 24, 1990 via Discovery in 611 km orbit.

 

"First Light" May 20, 1990.

 

By June 1990 it was realized that the optics were severely flawed by spherical aberration, with 80% of the light spread far away from the image center.

 

Some of the image sharpness could be recovered using computer image processing software, but a complete recovery would require a major optical fix. Since it was designed to be serviced, and other repairs and improvements would be needed, a plan was devised to correct for the aberration. First, the WFPC would be replaced with WFPC2, which had its own corrective optics (much of WFPC2 was built from spares from WFPC). The other instruments would have corrective optics placed in front of them by installing COSTAR (swapping out the HSP to do so).

 

The improvement was dramatic!

 

 

This instrument has not only made some of the most important discoveries in astronomy, but provided some of the most breathtaking images known. Of course, much of the Hubble’s capabilities were in doing spectroscopy (as well as polarimetry), but it is the images that have captured the imagination of the public (as well as the professionals).

 

 

 

Compton Gamma-Ray Observatory

 

The CGRO, was launched April 5, 1991 almost exactly a year after the HST.

 

This facility (not really a telescope at all) was designed to detect the most energetic photons from astronomical sources. This includes photons emitted by particle-particle annihilation, radioactive decay, and the collision of cosmic rays with interstellar matter.

 

The photon energies are measured in eV  13.6 eV is the ionization potential of the hydrogen atom (λ = 0.0912 μm = 91.2 nm = 912 Å). Electron-positron annihilation gives 0.5 MeV photons, while proton-antiproton collisions produce 1000 MeV (1 GeV) photons, since the proton is about 2000 times more massive than the electron.

 

 

Chandra X-ray Observatory

 

Launched July 23, 1999, Chandra was originally supposed to include both imaging and spectroscopic instruments. Due to budget cuts and difficulty of getting it in the Shuttle cargo bay, was “broken into” two separate payloads, one for imaging and another for spectroscopy. Then the budget ax fell, and only the imager was built & launched.

 

Designed primarily to detect hot gas, Chandra was used to study hoot interstellar and intergalactic gas, emission from gas accreting onto black holes, etc. Photon energies are typically in the keV range.

 

 

 

 

 

Spitzer Space Telescope

 

The SST (formerly known as SIRTF, the Space InfraRed Telescope Facility), is the last of the 4 “Great Observatories” to be launched (August 25, 2003). It’s conception dates back to the early 1980’s, when call for proposals for instrument designs were announced. De-scoped in size many times, it is the successor to IRAS. It has three instruments: IRAC (InfraRed Array Camera), IRS (InfraRed Spectrograph), and MIPS (Multiband Imaging Photometer for Spitzer).