The RHESSI Imager

Because X-rays and Gamma rays cannot be easily focused (for example, they pass directly through a glass lens without being bent.), other techniques have to be used to form images. It's easy to understand how medical x-rays work: an image of a person's bones are formed on the x-ray sensitive film because the bones block, or are opaque to, the x-rays photons. The RHESSI imager starts out with this basic principle: being able to block x-ray and gamma ray photons as they pass through it. However, unlike a medical x-ray, we are interested in imaging the source of the rays, not the material that blocks them.

The imager instrument and its unique technique to produce an image is one of the most difficult parts of the RHESSI mission to understand. It will take you some time and repetition of the material to understand how it works, and it will also be challenging to teach it to your students. At the very least, secondary students should understand the challenge of imaging in the high energy parts of the electromagnetic spectrum. For the very best explanation of how the imager works please visit our partner website at:  GSFC RHESSI Education Outreach, which includes a graphic summary of the RHESSI imaging technique.


The RHESSI Spectrometer

Traditional astronomical spectrographs work much like a prism. They spread out the light of an object, say the sun, by refracting or bending it into a very dispersed spectrum and then portions of the spectrum are imaged to determine precisely which frequencies of light are coming from the object of study. The power of this technique is that the frequencies of light are a direct result of the atomic or nuclear chemistry in the object where the light originated.  It's analogous to the way people read each other's emotions. If we see a person smiling, or scowling, we can use that information to determine something about their internal emotional state.

Unlike a traditional spectroscope, the RHESSI spectroscope doesn't spread out the spectrum from a solar flare (remember, x-rays and gamma rays don't readily bend when passing through matter). Instead, it's done electronically. The photon detectors for the imager count the number of photons passing through the grids, but they are also able to measure the energy (in electron volts) of each photon with excellent precision. Since the energy level of an electromagnetic photon is directly related to the frequency of the electromagnetic wave, you get spectrographic information about the flare. When this information is combined with the imaging information, you get a high resolution picture of a flare which also shows the wavelengths (i.e. energy) of each part of the flare. (See example (http://hesperia.gsfc.nasa.gov/hessi/images/fd-close.gif).

Putting it all Together
Since solar flares are rapidly changing features, RHESSI can produce high resolution spectrographic MOVIES of flares. This allows scientists to really see what goes on in a solar flare, from birth to death. The improvement in spatial and spectral resolution that RHESSI provides over previous high energy studies of solar flares is revolutionizing the understanding of them in much the same way that the electron microscope allowed biologists to understand cell division. Using their previous knowledge of atomic and nuclear physics, the RHESSI scientists putting it all together to learn more about basic physics at these very high energies, energies that are very difficult to reach in earth-based laboratories.

Want to know more about technology of RHESSI? Go to the RHESSI Challenge (http://hesperia.gsfc.nasa.gov/hessi/challenge.htm) (particularly the section entitled How Are Hard X-ray Images Made?) at GSFC for a more comprehensive explanation of the RHESSI imager.