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Also available: SOFIA Research Program FAQs





What is the material for the reflector and the temperature of the focal plane?

The optics temperature is 240K. The material for the reflector is Zerodur, a unique glass ceramic material that effectively has zero thermal expansion characteristics. See details in the paper, "Stratospheric Observatory for Infrared Astronomy", E.E. Becklin; 1997.

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What are the exact wavelengths that SOFIA will be studying?

The wavelength covered by the seven first light instruments are shown in this chart.

Spectral Resolution

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How does SOFIA get clear pictures? Even at 12km there would still be a lot of [airplane] turbulence and the pictures wouldn't be very clear.

At visible wavelengths, it is neither atmospheric turbulence, the refractive action of mobile air cells which push light rays around, overhead (actually there is not much air left overhead) that causes the blurring problem, nor the aircraft and telescope shaking that causes the problem, but rather the "shear layer" stream of air shooting past the open airplane cavity where the telescope sits, at 500 mph. This air motion worsens the resolution (the opposite of blurring) to 3 arc secs at visible wavelengths.

But the problem at the long wavelengths is different - it's diffraction. Basically, the far-infrared light observed by SOFIA passes through the shear stream of air unperturbed. But this light has such a long wavelength, 100x to 1000 times the wavelength of visible light, that the SOFIA telescope is of insufficient size to focus it sharply, and blurriness results. At wavelengths in the far-infrared, like 60 micrometers, there is significant blurring due to this effect. The telescope is actually held extremely steady while observing occurs, even in turbulence. It's held about as stable as a mountaintop telescope sitting on a 10 meter cement foundation, but diffraction still blurs the image.

So how do you do this? First, you isolate the telescope from the airplane by mounting it on a spherical pressurized oil bearing. The plane shakes and quakes, but the telescope doesn't feel it. Second, you direct the wind away from the telescope by shaping the side of the airplane so as to deflect it, and install a little deflector fence on the edge of the telescope cavity as well. Third, you stabilize the telescope against sudden motion (wind does get through) by spinning three orthogonal gyroscopes which are rigidly attached to the structure, and fourth, you steer the telescope so as to keep it steady, by tracking a distant star and giving the telescope magnetical nudges to point it toward a fixed direction.

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What is the maximum time the telescope can be pointed at an object? The object would eventually be out of the field of view, even if you could adjust the telescope.

A few hours is generally the maximum. Objects make arcs through the sky due to the Earth's rotation. The motion of the airplane also adds or subtracts a bit from this apparent object motion. So, while observing a target, the telescope appears to be following an arc in the sky, although it is actually staying fixed dead-on to a distant point in deep space that just appears to be moving. All that time, the airplane body slowly swings in its own arc relative to the ground to keep the target at a right angle to the airplane, so that the telescope can keep peering out. Typically, after a few hours, the object has either set too near the horizon, or risen too near the zenith to track, or else the airplane has been flying so long in one direction, it is getting too far from home. At this point, another object, located in a different part of the sky is turned to. These flights are carefully planned in advanced.

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What is the anticipated normal operating temperature of the telescope? The information on the website suggests a range of ambient temperatures in the "cavity" from 210-330 K. Depending on what is meant by this, it could have a *huge* impact on the sensitivity!

The telescope temperature is typically at the 41,000 ft ambient which is about -50 C. Note that during the South Pole winter, temperatures are about -80 C because of the inversion layer near the ice. On the ground, the telescope is pre-cooled before the flight such that the ambient 'at altitude' temperature is achieved hours before the actual flight. The thermal time constant for the telescope is expected to be about 2 hours. An aircraft system is used to warm the cavity and dry the air during decent from 41,000 ft. This is to prevent water from moist lower altitude air from condensing on the telescope optics. Water condensation probably contributed to the deposition of small (high emissivity) particles on the KAO (Kuiper Airborne Obs.) telescope. To help with mirror cleanliness, we are planning to use the CO2 snow cleaning process on a regular basis. Another aspect of telescope sensitivity is scattered light from the warm lower atmosphere.

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Can SOFIA see the Lunar Module crash sites on the surface of the Moon and get a record of them for history?

You asked if SOFIA can see very detailed features on the surface of the Moon. The short answer is "No - such features are too small."

Here is the long answer: The best resolution (ability to see fine detail) of any of the world's telescopes is about a tenth of an arc second (explained below). This is achieved by the Hubble Space Telescope. (This statement applies only to telescopes that use visible light and make images or photographs.) The next best telescopes are the Keck Telescopes in Hawaii and some telescopes in Chile. These can see details about three tenths of an arc second. SOFIA does not do as well as these telescopes, seeing details of one or two arc seconds at the very best.

So the score card is: Hubble 0.1 Arc Sec (best); Keck 0.3 Arc Sec many other telescopes are doing as well as the Keck; SOFIA greater than 2.0 arc sec The reasons SOFIA does not do as well is that it is a smaller telescope, it looks through a current of turbulent air rushing by the jet at 450 miles per hour, which blurs the light, and it looks at longer wavelength light; infrared light. (SOFIA does see more detail in infrared light and far-infrared light than most telescopes, but it still can not see "better" than 2 arc seconds.

What does "resolution of a tenth of an arc second" mean? It means that details that are 2 million times smaller than the distance of the object can be seen. Therefore, a telescope with a resolution of a tenth of an arc second (like the Hubble) can pick out a 1 meter (3 foot) object that is 2 million meters away. A telescope with a resolution of two arc seconds (like the SOFIA) can see a one meter object that is 100,000 meters away.

Unfortunately the moon is 384 million meters away (even for Hubble), so the Hubble can only make out objects that are about 190 meters across. The Lunar modules are much smaller than this. SOFIA only can see objects (craters) 20 time larger larger than this.

The best way to see sites on the Moon is to orbit the Moon or visit it. The Moon landing sites (the terrain) were photographed from the orbiting Command Modules on these Apollo Missions. See these many photographs, for example:

Fortunately, the Solar System and the Universe are full of some very large objects, so there will be plenty to see with SOFIA!

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Could you explain the means by which the air around the exposed part of the airframe is kept smoothly flowing? I looked at the CFD calculations that compared the two airframes but was unable to see the broader picture. Is it a case of diverting the air away from the cutout? Nucleating the turbulence somewhere else so it stays away from the cutout?

No, the air is not diverted away from the cavity. The smooth air flow over the fuselage is maintained by a passive aft ramp concept incorporated in the cavity that causes the free stream shear layer to reattach properly downstream of the cavity on the passive ramp which maintains this flow attachment in a stable manner. Wind tunnel tests have shown that the flow/turbulence on the fuselage downstream of the cavity is only slightly different than the original unmodified aircraft.

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Could a hypersonic aircraft capable of fly at Mach 6 and at an altitude of 100K - 200k feet also be useful for infrared astronomy?

While better astronomy can be done at higher altitudes - the platform of choice would likely use balloons rather than aircraft due to cost. Examples of plans are NASA's Ultra long duration balloon mission project and the current success of experiments such as within Japan's ballooning program, and in the US, Boomerang.

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What is the difference between SOFIA research and what the Hubble can do? Is it that the Hubble is all booked up? Or are they two different types of research or applications?

The SOFIA and Hubble Missions share several similarities: both have very large "world class" telescopes (SOFIA 2.7 meter diameter, Hubble 2.4 meter diameter), long mission lifetimes (SOFIA 20 years, Hubble 15 years) and each flies above most of the Earth's atmosphere (SOFIA 12 kilometers, Hubble 600 kilometers). The difference in research is that Hubble concentrates on optical and ultraviolet light, while SOFIA concentrates on infrared and far-infrared light. The reason for this is that ultraviolet light does not get through our Earth atmosphere, not even at the altitudes an airplane flies. But SOFIA can see the infrared light from flight altitudes in the stratosphere.

Ultraviolet light and optical light are given-off by energetic and violent processes in space that happen to stars and galaxies; like nuclear burning, collisions, explosions, falling into black holes. The Hubble has made many discoveries in these areas of astronomy. At optical wavelengths, the Hubble has exquisitely good eyesight (or "resolution") since it not burdened at all by blurring caused by looking through the Earth atmosphere. The Hubble has been able to peer into deep space and see back into time, viewing the ancient universe as it was billions of years ago.

Infrared light is given off by less energetic processes in space in general. Warms clouds of gas and dust reveal their properties at these wavelengths, and these clouds will be studied by SOFIA. The surfaces and atmospheres of planets and moons can be seen in the infrared too. Also clouds of gas will be seen which will give birth to new stars, and it will see the winds that will be springing up from stars just born. The chemical elements and molecules, like H2, CO, CO2, H2O, and the more exotic molecules will be seen floating in space clumped together into clouds by gravity. The composition of gas in space gives clues as to what sort of stars Milky Way Galaxy has been forming in the past and what it will be churning-out in the future. Sometimes these clouds hide interesting objects within. With far-infrared light, there is a bonus; far-infrared light can pass through clouds of gas and dust, and so we can see inside objects with SOFIA that are opaque to the Hubble.

The other difference between SOFIA and the Hubble is that a great variety of instruments can be used by SOFIA, since it lands daily and different instruments can be mounted on the telescope. Astronomers go up into the stratosphere and make sure their instruments work right. Hubble can only be visited each 2-3 years and at great cost. In general, the Hubble observatory costs billions of dollars; SOFIA costs ten times less.

Hubble is all booked up and more. Hubble gets 10 times more requests for observing use from astronomers than it can accommodate. But this is typical for observatories. Good observatories are over-requested by factors of 2 to 10. Of the dozens of "world class" observatories on Earth and in space, all are used all of the time it is possible to use them by astronomers, and there are many more astronomers waiting in line. SOFIA will be no exception. This has to do with the amazing discoveries in space that are just waiting to be made.

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How much computer processing is done on the aircraft, and how much is done on the ground?

The SOFIA aircraft has computers on board that will record the incoming data, and will also record the necessary information about the status of the telescope, the instrument, and the observing conditions. The computers on board the aircraft can also do a small amount of analysis, so the scientists can get instant feedback to see if the observations are being made correctly.

However, most of the analysis of the data is done on the ground, back at the scientist's own university or institution. It often takes many months before the scientist can analyze the results from an observation and publish them in a scientific journal.

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