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 it has 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 ican 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. |
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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:
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T = 1 million degrees T = 1.5 million degrees T = 2.5 million degrees T = 4,000-250,000 degrees
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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 use of 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.
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.
Another type of
telescope which is currently in space is the Soft X-ray Telescope (SXT) on board the Yohkoh
satellite. Yohkoh (the Japanese word for "sunbeam") is a Japanese
mission with instruments built in Japan, the United States and the
United Kingdom. It was launched on August 31 1991 so it may be as old
as you. What makes the SXT so special is that it has the ability make
pictures from X-ray radiation coming from the Sun. X-rays have a lot
more energy than the EUV radiation looked at by TRACE and so it is
much harder to stop the radition and focus it to make a picture. The
SXT does this by using a cleverly designed mirror:
The SXT is designed only to let in X-ray radiation. All other forms of light are blocked by a set of filters at the front of the telescope. In this way, SXT is able to image the hottest parts of the solar atmosphere where the temperatures can be as high as 20 million degrees.
SXT is able to look at different energies of X-rays using a set of filters which ar emade of different thicknesses of metal. The thicker the filter the more it stops the low energy X-rays leaving only the most energetic to make it through to the CCD camera. This allows us to "measure" the temperature of the solar atmosphere. You learned about filters in Monday's activities.
TRACE looks at EUV radiation from the Sun while SXT looks at X-rays. Another important difference is in the the amount of detail that each telescope can see. Like TRACE an SXT image is made of 1.05 million pixels BUT it looks at the whole Sun not just a part of it. One pixel of SXT sees a region of the Sun which is 2000km x 2000km whereas one TRACE pixel covers a solar area of 350km x 350km. This means that TRACE can see something 25 times smaller than SXT can see. To give you a feel for this: a telescope like SXT could spot a penny at a distance of 1 mile but one like TRACE could see the date on the penny!. The Sun in detail section discusses the impact of these differences on the study of the Sun.
One thing to note is that the TRACE and SXT telescopes are roughly the same size: TRACE has a 30cm aperture and is 187cm long while SXT has a 23cm aperture and a 155cm length. So the question arises as to why they have such differnet abilities in observing detail on the Sun. The different kind of mirrors has something to do with it but the main reason is that TRACE pays a price for its higher resolution. It can only see part of the Sun at a time whereas the SXT sees the whole Sun all of the time:
Even though seeing the Sun in more detail leads to a number of new discoveries and a better understanding of some solar phenomena, it is important to remember that only through the combination of information from all of the radiation emitted by the Sun can we reach a full understanding of what is going on.