Higher Resolution

Astronomy was at first a visual science. Scientists peered at the skies, making catalogs of what they saw, describing forms and colors of unknown objects. Modern age astronomers also look at the sky, but they do so with much better instruments - instruments that have extended their quest to parts of the electromagnetic spectrum that our eyes can't see.

In everyday life, your eyes act as lenses and focus light onto your retina. Your brain reconstructs an image, allowing you to see. Just like a photo taken with a camera out of focus, if your eyes are not working as perfect lenses, you will see a blurred picture. One example of how imperfect optics can hinder the effectiveness of an instrument is the Hubble Space Telescope. After the telescope was placed in orbit, it became apparent that the primary mirror in Hubble's Wide Field and Planetary Camera 1 had a flaw. It was just barely the wrong shape, and the light reflected from the edges of the mirror was focused differently than the light at the center of the mirror. This spherical aberration produced a blurry image. However, engineers were able to fix this flaw by installing corrective optics when the Wide Field and Planetary Camera 2 was installed, bringing the telescope back into proper focus.

At left: Hubble's Wide Field Planetary Camera 1 image of the M100 galactic nucleus, demonstrating the blur resulting from WFPC1's flawed primary mirror. At right: A much clearer image of the same object, after Wide Field and Planetary Camera 2 was installed.

One challenge of very high-energy astronomy – from ultraviolet and above – is the ability to build a "lens" to focus the incoming photons on a detector. Once the light is collected by detectors at the focus of the lens, the image is reconstructed using computers to process the recorded signal. High-energy optics often consist of pairs of paraboloid and hyperboloid mirrors that are nested and coated. The incoming photons bounce off the mirrors' surfaces and are directed toward the collecting area of the detector.

Chandra ACIS-I image of the Crab nebula
(click on the image for a larger view).

There are many examples of the power of high-resolution imaging to solve long-standing mysteries. The image to the right shows an image of the Crab Nebula obtained with the ACIS-I onboard the Chandra Observatory. The image shows the large ring and jets that had been seen previously by ROSAT. But the image also revealed the existence of a bright, small ring surrounding the center of the nebula that had never been seen. That ring is probably where the electrons accelerated by the pulsar get splattered into the nebula. The transfer was, until that point, a complete mystery because the previous images were too blurred to see the details of what was happening in the region closest to the pulsar. The other spectacular features of the Chandra image (the large ring and the jets) had been detected during observations with the ROSAT satellite's HRI instrument.

Chandra has since monitored the Crab nebula frequently, and the video below shows a time lapse of Crab images over a span of seven months. Over even such a small period of time, the Crab can be seen to change.

The images below show another object (the supernova remnant Cassiopeia A) as observed by two generations of X-ray detectors. The image of Cassiopeia A shown at the lower right was also obtained using ACIS-I. The details present in this picture allowed scientists to see for the first time that there was probably a pulsar (a spinning neutron star) hidden in the middle of the remnant. It appears as a small point source almost exactly at the center of the image. It was not visible in previous images of Cassiopeia A.

Reconstructed X-ray images from the supernova remnant Cassiopeia A, the remains of a supernova explosion which probably occurred in the middle of the seventeenth century. The images are from different instruments on-board of three different satellites. From left: Einstein IPC, ROSAT PSPC, ROSAT HRI, and Chandra ACIS-I.

One of the big improvements between the Einstein HRI, ROSAT HRI and Chandra ACIS-I is the spectral response of the latter, unavailable for the first two detectors.

(Credits: Einstein image from NASA, ROSAT images are from Max-Planck-Institut für extraterrestrische Physik (MPE), Chandra image from NASA and the CXO.)

Higher resolution does not only mean better images. As the need for higher resolution images increased, so did the need for higher resolution spectroscopy. The ability to distinguish between emission lines from different elements turned out to be a powerful diagnostic tool for the characterization of objects probed. Every object emits radiation that reveals its interaction with its environment. For example, a plasma spectrum will show emission lines for all the elements present. The strength, the width and the energy of those emission lines provide crucial information about the plasma itself (its temperature, density,and composition). These studies hinge on the ability to disentangle the contributions from different elements and identify their signature line emission. The images below show the spectrum of a young supernova remnant as it was observed by different detectors spanning two decades of X-ray missions. It becomes apparent how much easier it is to identify the contributions of all the elements present in the remnant when the energy resolution is better.

Graphs representing the emission spectrum of an average supernova remnant as detected by four X-ray instruments. The detectors are from top to bottom, left to right: ROSAT PSPC, Einstein SSS, ASCA SIS, and XXM MOS.

There were two main problems that faced scientists tackling high-resolution spectroscopy in its early days: one was the minuscule size of the detectors and the small portion of the sky they covered; the second was the difficulty in interpreting and identifying the measured emission lines. This required reliable theoretical computations for line emission strength, or precise measurement of those lines in a laboratory. Ideally, both would be put into use. Now, thanks to improvements in the optics used in recently launched observatories, scientists can obtain much more detailed readings of spectra – essentially, the lines are sharper, allowing for more precise assessment of the chemical composition and physical condition of an object located thousands of light-years from Earth.

Updated: June 2011