SOFIA: The Flying Telescope

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SOFIA: The Flying Telescope
                    Image Credit: NASA/Carla Thomas

 The first patent of the telescope was recorded in 1608. After that many improvements have been made in the telescopes. Today we have telescopes of all sizes. Telescopes about the size of a car or a bus are successfully launched in the outer orbit. Telescopes are also sent to distant planets like Mars, Jupiter, Saturn etc. The Voyager duo has even crossed the solar system. Large telescopes are also installed on the land, bigger than a building. Today, telescopes are successfully installed in aeroplanes. The flight would take off and will complete its observation while in the air and finally gets landed after completing the observation.

Stratospheric Observatory for Infrared Astronomy (SOFIA) is a joint mission by NASA and German Aerospace Center (DLR). SOFIA is basically a telescope of 2.5 m diameter installed in Boeing 747SP. It is not the first time when NASA installed a telescope inside an aeroplane. The first use of an aircraft-mounted telescope was in 1965 when Gerard Kuiper installed a telescope inside a Convair 990. Later in 1975, NASA developed Kuiper Airborne Observatory (KAO). The KAO mission finally ended in 1995. The average flight time of SOFIA is 10 hours. During that time it observes stars and their magnetic fields, planets, the galactic centre etc, all with its infrared instruments. 

The Boeing 747SP was modified such that its large door can be opened during flight for the telescope. The SP (Special Performance) version was developed by Boeing for very long-range flights, hence it was the best to fit for SOFIA. The weight of the aircraft reduced by removing some of the sections of the fuselage and by modifying other heavy components. So now the 747SP could fly even at a higher altitude. The telescope was designed for infrared spectroscopy at an altitude of about 12 km. The main reason for doing observation at such a high altitude is to get rid of all the components that block infrared rays. Water vapour plays a major role in blocking infrared rays. Also, SOFIA can travel to any place above the Earth, hence observations from both southern and northern hemispheres are made easy.

The aircraft was purchased by NASA in 1997. After some test flights, the work for modification was given to Raytheon Company. In 1998, Raytheon installed a 5.5 by 4.1 door in the aft left side of the fuselage for the telescope. The telescope is installed in a special pressurised section. The open fuselage was designed in such a way that it has almost no effect on the aircraft's aerodynamics. The centre of the aircraft has a mission control and operation section.

The telescope used in SOFIA was manufactured by Schott AG company. A special material called Zerodur, which has almost zero thermal expansion was used in building the telescope. The polishing of this 2.5 m diameter telescope was completed in 1999. In 2002 the remaining components of the telescope were assembled. In August 2004, the SOFIA finally conducted its first ground test by capturing the image of Polaris (a star located about 330 light-years away). SOFIA made its maiden flight on 18th December 2009, with its doors fully opened. However, in this 79-minute flight, the doors were opened for only 2 minutes. SOFIA's telescope was finally operated in an on-sky mission on 26th May 2010 when it captures the images of M82 (a galaxy located 12 million light-years away) and the heat escaping from Jupiter's clouds. SOFIA started working in routine with full capability in December 2010. By 2014, the aircraft had an average of 100 flights per year.  

 The mission took a very long time than expected to become fully operative. The project was proposed long ago, but due to a budget cut by NASA, the mission was delayed by 5 years. In 2001 the project was further delayed when United Airline due to bankruptcy, withdrew from the project. Also, the subcontractors responsible for the construction of the door took a longer time than initially expected. In 2006, the cost of the mission was increased by $145 million. This budget issue led NASA to suspend the mission for few months. 

More about the Telescope:

SOFIA uses a reflector telescope with a Cassegrain reflector design having a parabolic primary mirror and a hyperbolic secondary mirror. German Aerospace Center (DLR) has a 20% partnership in the project. Hence the construction and assembly of the telescope were given to the DLR. The telescope has 8 main components capable to perform infrared observations ranging from 1--655 micrometres. The main instruments are,

  1. FIFI-LS (Far Infrared Field Imaging Line Spectrometer)
  2. FORCAST (Faint Object InfraRed Camera for the SOFIA Telescope)
  3. FLITECAM (First Light Infrared Test Experiment Camera)
  4. FPI+ (Focal Plane Imager)
  5. EXES (Echelon-Cross-Echelle Spectrograph)
  6. GREAT (German Reciever for Astronomy at Terahertz frequencies)
  7. HAWC+ (High-resolution Airborne Wideband Camera + Polarimeter)
  8. HIPO (High-speed Imaging Photometer for Occultations
Here are a few more details about the instruments.

1. FIFI-LS: It is a spectrometer operating at a far-infrared wavelength. The instrument consists of two different spectrometers. One of them is a short wavelength spectrometer (blue channel) operating between 50 to 125 micrometres and another is a long-wavelength spectrometer (red channel) operating between 105 to 200 micrometres. FIFI-LS has a beam rotator that allows the telescope to rotate in three axes. 

2. FORCAST: It is a dual-channel mid-infrared camera operating between 5-40 micrometres. The FORCAST instrument is made up of two identically designed cryogenically cooled camera. Atmospheric blocking affects the sensitivity of these cameras. The FORCAST has the best-measured image quality between 7-11 micrometres. The image quality also reduces sometimes due to the vibration of the telescope.

3. FLITECAM: It is a cryogenically cooled infrared camera operating between 1-5.5 micrometres. FLITECAM was designed to co-mount on the HIPO. The camera is highly sensitive while operating above 4-micrometre wavelength. 

4. FPI+: The FPI is the standard tracking camera installed in the SOFIA. It is permanently installed on the telescope. The best use of the FPI system is to measure the exoplanet transit. The FPI system also enables SOFIA to operate even in the cloud-covered regions. The FPI+ operates between 360 to 1100 nanometers. The camera also has a daylight filter to easily observe the bright stars.

5. EXES: The EXES operates in 4.5-28 micrometres at all three resolutions (low, medium and high). The EXES has a three-layered radiation filter. It is also a cryogenic cooled instrument. 

6. GREAT: The GREAT is an instrument that provides high-resolution spectra in the 1.5 to5 THz range. In GREAT the image and signal bands both are equally sensitive to the incident radiations. 

7. HAWC+: THe HAWC is a high infrared multi-wavelength camera. The main work of this instrument is to generate a more filtered image of incoming radiations. 

8. HIPO: It is a special-purpose instrument designed to provide high-speed imaging photometry at two wavelengths simultaneously. The main purpose of this instrument is to measure stellar occultations. The instrument is also designed to work perfectly in cloud-covered regions. It is also used to measure the transit of exoplanets. 

If you like this article then go check our other articles too. You may find something amazing and unknown to you.  

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