Description of the payload
The BLAST instrument is composed by a telescope with a primary mirror near two meters in diameter and a 50 cm diameter correcting secondary mirror, mounted in a rigid support structure for the secondary. This secondary has been designed to give diffraction limited performance over a 14' x 7' field of view at the Cassegrain focus.
The image detected by the telescope or better the radiation detected by the telescope pass through a vacuum window made of polypropylene near the Cassegrain focus and enters in the cryostat.
The cryostat makes possible the radiation detected to be re-imaged onto the three detector arrays (bolometers) using a pair of mirrors arranged in a "Gaussian beam telescope" configuration, which corrects for aberrations in the main telescope, and provides a flat focal plane. The longer expected duration of flight demands that the cryostat operate for at least 14 days with the detectors at very low temperature so it requires 37 liters of liquid helium for cooling purposes, along with a separate miniature pumped Helium pot, and a Helium recycling refrigerator.
The BLAST focal plane (detectors) consists of three bolometer arrays with 149, 88 and 43 detectors each at 250, 350, and 500 µm respectively. They are read out with an differential circuit and data is collected using a high-speed, flexible, 18-bit data acquisition system.
All the instrument's parts are mounted in a structure formed by a precision-pointed inner frame (containing the primary mirror, secondary mirror, near-field baffle, and cryostat) supported by an external gondola. The outer frame will be pointed in azimuth by a flywheel and an active pivot. The inner frame has an elevation mount and is driven by motors relative to the outer frame. Balance of the inner frame will be achieved using actively positioned weights for differential buoyancy and cryogen blow-off.
The cable-suspended gondola design is lightweight, with a high tensile strength and a high resonant frequency.
By providing the first sensitive large-area submillimeter surveys at these wavelengths, BLAST looks to address some of the most important cosmological and Galactic questions regarding the formation and evolution of stars, galaxies and clusters.
The main science goals include to measure photometric redshifts, rest-frame FIR luminosities and star formation rates of high-redshift starburst galaxies, thereby constraining the evolutionary history of those galaxies that produce the FIR/submillimeter background, also to measure cold pre-stellar sources associated with the earliest stages of star and planet formation and to make high-resolution maps of diffuse galactic emission over a wide range of galactic latitudes. A final goal is to observe solar system objects including planets, large asteroids, and trans-Neptunian objects.
Details of the balloon flight and scientific outcome
Launch site: Scientific Flight Balloon Facility, New Mexico, US
Balloon launched by: National Scientific Balloon Facility (NSBF)
Balloon manufacturer/size/composition: Zero Pressure Balloon Raven W 39.570.000 cuft - (0.8 mil) - SF3-29.47-.8/.8/.8-NHR
Balloon serial number: W29.47-2X-46
Flight identification number: 522N
The balloon was launched by dynamic method assisted by launch vehicle at 15:15 utc on September 28th after a lot of delays and last minute aborts.
After a nominal ascent phase the balloon reached float altitude of 122.700 ft at 17:37 utc and started a drift due the southwest.
After near 27 hours of flight was sent the cutdown command and the balloon was terminated at 18:24 utc on September 29th when flight over the SW corner of Arizona state. The landing was very good because BLAST managed to land squarely on its crushpads, and the only real damage from landing was one bent leg strut. Another minor damage was at chute shock when the pivot fell into the sun shields and sheared a cable or two and the inner frame got jossled for not being locked.
This was an engineering flight devoted to made a qualification of the system performance in preparation for two long duration trip's first in the Arctic and then in Antarctica.
The instrument performed fairly well but several troubles arose during flight. Once at float the gondola was commanded to perform the pre-schedule file routine which involved balancing the inner frame, unlocking it, and autofocusing the star camera. When the team unlocked the inner frame, thay discovered that the limit switches had failed. However, a backup manual override for the limit switches was activated unlocking the gondola's motor.
The star camera had problems too, crashing and rebooting during the first part of the flight, however, it would stay up long enough to get a few pointing succesfully done. Then, as the flight avanced the star camera focus frozen out of focus, which meant that making maps from the data obtained would be a more difficult task.
As the flight ended and while preparing the gondola for separation from the balloon, the team discovered that the inner frame lock didn't work, most likely due to it over extending itself when we unlocked it, due to the limit switches failing. After vainly sending the lock command several times, wisely was decided to pump all the fluid into the front tank of the balance system, making the inner frame front heavy and ensuring it would stay at it's lowest elevation thus avoiding ato hit the ground during landing.
The landing was quiete good: BLAST managed to land squarely on its crushpads, and the only real damage suffered was one bent leg strut.
External references and bibliographical sources