Description of the payload
Is an instrument created to detect and map emission from the Intergalactic Medium (IGM) at redshifts between 0.3 and 0.9. As IGM emission is faint and extended, and often much fainter than the foreground diffuse emission it is only detected by an instrument optimized for low surface brightness, widefield emission-line surveys in the presence of time and spatially variable diffuse foregrounds.
The FIREBALL experiment is comprised of three major subcomponents: the telescope, the spectrograph, and the gondola.
The telescope is a 1 meter diameter f/20 Classical Cassegrain with image quality requirement modest, in the order of 3-4 arcseconds. The secondary mirror mount provides active focus control, fine guiding, and periodic 1-10 arcminute chopping for on/off-source/background measurements by mean of three precision vacuum grade actuators. The incoming light passes through the telescope and first strikes a dichroic beam splitter which sends light of the primary wavelength to the high resolution spectrograph. The balance of the light (visible) goes to a guider system while a calibration lamp injects light into the system via the dichroic backside, with line spectra and flat field illumination using a diffuser.
The spectrograph is a fiber fed design, using approximately 400 fibers, each with a 100micron core. The fibers are optimized for UV transmission, ideal for the wavelength range (190nm-210nm) of the experiment. The sky coverage is approximately 2.6 arcmin on a side. This allows for 400 individual spectra to be taken of each point on the sky and then for reconstruction of the 3d structure over X, Y, and redshift. The spectrograph design is based on a holographic reflection grating and a compact single mirror system. A micro-channel plate NUV (similar to that used in the GALEX satellite experiment) is used as detector.
The FIREBALL gondola design is based off the French FOCA experiment -also devoted to UV research- which delivered an inflight pointing jitter of 2 arcseconds rms. The design uses a two-stage tracking concept. The housing of the gondola is passively and actively damped and is stabilized azimuthally to provide a reference platform for the second stage tracking.
In the second stage, the gimbaled telescope actuates with two degrees of freedom, elevation and cross-elevation. The gondola is azimuthally stabilized with a motorized suspension bearing. Gyros on the gondola provide velocity reference and a three-axis magnetometer provides the azimuth angle. The motorized bearing is torque limited and provides a viscous friction that damps torsional oscillations. Pendulum oscillations of the gondola platform are damped by a smaller 30 pound pendulum immersed in oil and mounted onto the gondola. The pendulations are also actively damped with angular momemtum coupled to the gondola. With these active and passive controls, the gondola achieves 2-4 arcminutes stability. The second stage controls the telescope and instrument with elevation and cross-elevation motors. The motor drive signal takes input from two sources.
Gyro sensors on the gondola feed forward the rate of large velocity perturbations at the gondola. Final fine corrections to the motor output come from a quadrant star sensor.
Details of the balloon flight and scientific outcome
Launch site: Scientific Flight Balloon Facility, Fort Sumner, (NM), US
Balloon launched by: Columbia Scientific Balloon Facility (CSBF)
Balloon manufacturer/size/composition: Zero Pressure Balloon
Flight identification number: 597N
The balloon was successfully launched by dynamic method using launch vehicle on June 8 at 17:46 UTC. During the nominal ascent phase the balloon followed a eastward path, bout once it reached float altitude of near 125.000 ft. it acquired a northwestward course that lasted during the entire flight as can be seen in the map at right (click to enlarge).
Separation of the payload ocurred at 16:00 UTC on June 9th while the balloon was flying west of Cedar City in Utah, after a total flight time of near 22 hours.
This was the second flight of the FIREBALL instrument.
Althought a small accident that ocurred at launch when the launch vehicle hit the gondola totally broke the front of the door that covers the instrument at launch and landing, nothing serious occured with the instrument itself.
The scientific team reported that they obtained all the data they had hoped for and are now in the process of reducing it in a process that will probably take a few more months.
As FIREBALL had a very smooth landing the scientists are considering to make a new flight in the upcoming Spring 2010 Australia campaign.
External references and bibliographical sources