How do you measure and analyze a planet that's one-and-a-half times the size of Earth and 300 light years away? How do we know the elements from which it is composed? The answer: With the incredible technology of a cryogenic telescope like the Spitzer and Kepler Space Telescopes.
Let's take a look at the Spitzer telescope and how it works. It's simple basics.
Cryogenic telescope assembly (CTA)
This amazing device has a liquid helium tank known as a cryostat. It cools the telescope down to 5K (-459°F). The telescope detects faint heat radiating from objects in space. Instruments must be kept extremely cold, so their own heat will not interfere with the measurements.
(Source: Caltech NASA/JPL)
In Figure 1, the telescope is the tubular-looking device vertically mounted atop the multiple instrument chamber, which sits on top of the cryostat assembly (which looks like your barbecue propane tank), surrounded by the outer shell.
Instrument chamber electronics
Here is where the electronics comes in. The instrument chamber contains three key electronic systems: an infrared array camera (IRAC), an infrared spectrograph (IRS), and a multiband imaging photometer (MIPS).
The IRAC detects light at near- and mid-infrared wavelengths. This is a four-channel camera (four different detectors measuring light of a particular wavelength). Simultaneous images are taken at the wavelengths sensed by each of the four arrays, which are 256 x 256 pixels in size.
a young star captured by Spitzer's IRS.
(Source: Caltech NASA/JPL)
The IRS functions like a prism and breaks up light in a rainbow of colors. Since each chemical element has a unique fingerprint in its light signature, we can tell what elements and molecules make up the object we are viewing. (Water was found on another distant planet with this method.) The detectors are 128 x 128 pixels in size, and the long and short wavelength modules each have their own entry slit to allow infrared light into the spectrograph.
Figure 3 shows a spectrometer using an array detector.
(Source: Texas Instruments)
Processing the CCD signal
Analog Devices' Erik Barnes wrote in an article on his company's website:
Accurately recovering and digitizing the CCD signal requires several operations, including correlated double sampling and dc restoration (clamping), gain, offset, and A/D conversion.
Correlated double sampling (CDS) serves two important purposes: it calculates the difference between the reference and data levels of the CCD signal, and it reduces some of the noise components in the CCD signal. Conceptually, the CDS is a differential-in-time amplifier: it takes separate samples of the input signal and outputs the difference between them.
(Source: Analog Devices)
The MIPS is also an imaging camera. It detects light in the far-infrared wavelengths and can perform some simple spectroscopy. The detectors are different sizes for the different wavelengths: 128 x 128 pixels for 24 microns, 32 x 32 pixels for 70 microns, and 2 x 20 pixels for 160 microns.
Measuring a planet light years away
The Spitzer and Kepler Space telescopes have made the most precise measurements of a planet's radius outside our solar system. An uncertainty of just 74 miles on either side of the planet was unheard of until now. This is like measuring the height of a six-foot-tall person to within three-fourths of an inch on Jupiter.
To make this accurate measurement, both the Kepler and Spitzer telescopes watched the planet Kepler-93b move across the face of its star. This minute eclipse changed the infrared (Spitzer) and visible (Kepler) light received. This measurement was corroborated by both telescope methods and eliminated a false-positive result.
NASA discussed Spitzer’s accuracy last week on the agency's website:
Spitzer racked up a total of seven transits of Kepler-93b between 2010 and 2011. Three of the transits were snapped using a “peak-up” observational technique. In 2011, Spitzer engineers repurposed the spacecraft's peak-up camera, originally used to point the telescope precisely, to control where light lands on individual pixels within Spitzer's infrared camera.
The upshot of this rejiggering: [NASA scientists] were able to cut in half the range of uncertainty of the Spitzer measurements of the exoplanet radius, improving the agreement between the Spitzer and Kepler measurements.
In 2013, NASA discussed the repurposing of the Spitzer camera:
This camera was used during the original cryo mission to put gathered infrared light precisely into a spectrometer and to perform routine calibrations of the telescope's star-trackers, which help point the observatory. The telescope naturally wobbles back and forth a bit as it stares at a particular target star or object. Given this unavoidable jitter, being able to control where light goes within the infrared camera is critical for obtaining precise measurements. The engineers applied the Peak-Up to the infrared camera observations, thus allowing astronomers to place stars precisely on the center of a camera pixel.
Since repurposing the Peak-Up Camera, astronomers have taken this process even further, by carefully “mapping” the quirks of a single pixel within the camera. They have essentially found a “sweet spot” that returns the most stable observations. About 90 percent of Spitzer's exoplanet observations are finely targeted to a sub-pixel level, down to a particular quarter of a pixel.
So it was as simple as that.