An Example of a Solar Telescope in Space
Building a solar telescope to collect high energy radiation such as
Extreme UltraViolet (EUV) and X-rays is not trivial. Building them big enough to see the
Sun in as much detail as possible adds to the difficulty. Unlike
Earth-bound telescopes, the space-borne ones have to be designed to fit
on a rocket and they have to be robust enough to survive the launch.
To get a feel for what is involved in building a telescope to observe
EUV radiation from the Sun we will take a look at the Transition
Region And Coronal Explorer (TRACE) instrument which was built at
Lockheed Martin Solar and Astrophysics Laboratory and launched on
April 2, 1998. The picture below shows the cut-away view of the inside of the TRACE telescope.
The front opening of the TRACE telescope is only 30 cm across and
187cm long. This combination, in part, allows TRACE to see an area on
the Sun of approximately 360,000km x 360,000km. A typical TRACE image
is made up of 1.05 million small squares called pixels, each 350km on a side. The main telescope
of TRACE consists of a primary mirror near the back of the instrument
and a smaller secondary mirror near the front which can be moved to
help change focus.
What makes TRACE different from most telescopes that you are used to
is the fact that it controls the kind of light which gets through to
the detector which can be seen behind the primary mirror in the image
above (the CCD camera). The trick used is to coat the two mirrors
with special materials which block out certain bands of light and lets
through others (much like red cellophane can be used to filter out all
other colors but red). Each mirror is divided into four quadrants and
each quadrant has a different coating.
The special coating are devised such that a particular wavelength of
radiation gets through. The four TRACE coatings are chosen to select
three separate EUV bands and one UV band:
T = 1.5 million degrees
T = 2.5 million degrees
T = 4,000-250,000 degrees
T = 1 million degrees
As indicated in the table, the EUV bands
are sensitive to coronal temperatures of about 1 to 3 million Kelvin,
while the UV filters respond primarily to the temperature regimes of
the photosphere, chromosphere, and transition region.
Once the EUV and UV light gets through the telescope it has to be recorded. This is done by using a very high quality digital camera called a CCD camera:
Finally, a very powerful computer is used to handle the large amounts of data taken by TRACE. This computer puts the data in a form for transmission to the ground (see today's activities) and sends it to the radio transmitter for download.
The completed TRACE instrument prior to launch
TRACE was then attached to the spacecraft and launched into an orbit about the Earth which gives it an uninterrupted view of the Sun for most of the year. If you have Quicktime 3.5 or higher installed on your computer you can see a movie of the TRACE launch and an animation showing the deployment of the spacecraft solar panels.
