BLAST: The Balloon-Borne Large Aperture Submillimeter Telescope
Super BLAST-Pol Instrumentation
BLASTPol results indicate that examining molecular cloud magnetic fields is important to understand star formation. SuperBlastPol will build on the success of BLASTPol using many of the tested and proven systems and software designed for the previous flights. However, the many upgrades, including the optics, cryostat, and detectors will be redesigned to have 16 times the mapping speed and a much longer 28 day flight time. These new systems will allow Super BLASTPol to probe the inner workings of star formation with unprecedented resolution, sensitivity, and scope.
The Super BLAST-Pol gondola will borrow from the BLAST gondola that flew five times since between 2006 and 2013, however a number of systems will undergo significant redesigns to accommodate the primary and the receiver which have both increased in size significantly. The gondola comprises two major mechanical systems (see Figure 3). The first is an outer frame that remains that allows the telescope to rotate in azimuth with respect to the balloon. This is achieved via a high moment of inertia reaction wheel located below the receiver, and a pivot motor that couples the flight train to four aircraft cables located at the corners of the frame. The outer frame also houses the flight control electronics, solar array batteries and charge controllers that power the receiver, and supports the inner frame on large ladder-like supports. The inner frame is a stiff structure that supports the mirror assembly, cryostat/receiver, and readout electronics, and can be precisely pointed in elevation with respect to the outer frame by a high-torque geared elevation motor. Both the azimuth and elevation motors are dynamically controlled, using proportional-integral-derivative (PID) feedback loops, which account for changes in balance of the inner frame and other external forces, in order to stabilize the pointing.
In-flight pointing is determined to an accuracy of ~30" using a number of fine and coarse pointing sensors. These include fiber optic gyroscopes, optical star cameras, a differential GPS, an elevation encoder, inclinometers, a magnetometer and a Sun sensor. Post-flight pointing reconstruction uses only the gyroscopes and day-time star camera (http://arxiv.org/abs/astro-ph/0605039). Post flight absolute pointing accuracy for BLAST-Pol was found to be < 3".
A series of aluminized mylar baffles and sunshields on both the inner and outer frame protect the telescope from thermal changes during flight and shield the sensitive optics from direct sun. These baffles will be optimized to allow Super BLAST-Pol to point to within 40 degrees in azimuth of the sun, over a range in elevation from 20 to 55 degrees.
The primary mirror will be 2.5 meters in diameter in a Cassegrain configuration with a ~0.5 meter diameter secondary mirror with a focal point behind the primary mirror near the window of the cryostat. The primary and secondary mirrors are currently being developed through a partnership with Vanguard Space Technologies, under a NASA Small Business Innovation Research (SBIR) grant. The primary will be the largest mirror ever flown on a balloon, and the largest submillimeter carbon fiber mirror in operation. The telescope will have a resolution range of 22" to 42" with a ~22' diameter field of view. The cold reimaging optics are currently being redesigned to improve the polarization efficiency and compactness of the instrument.
Given the high sensitivity of the detector arrays it is essential to keep them at very low temperatures and shielded from as much stray light and thermal emission as possible. In order to achieve this we use a Helium cooled cryostat to keep the optics bench with the re-imaging optics near 4 K. We will also have a He3 refrigerator on the 4 K stage that will keep the detector arrays at temperatures below 300 mK.
Additional thermal isolation of the 4 K stage from the 300 K exterior is provided by two vapor cooled shields. These use heat exchangers to extract cooling power from the Helium vapor that is boiled off during the normal course of operations.
Our experiment's flight duration is limited by the quantity of Helium that it can hold so we will have a 250 liter Helium tank that will give us a hold time of about 28 days. This will give us time to observe a rich sample of targets across the southern sky.
Both BLAST and BLASTPol used arrays of neutron-transmutation-doped (NTD) germanium thermistors bonded to spiderweb absorbers. The spiderweb absorber converts the incoming light into a rise in temperature. The NTD thermistor, which is basically a very sensitive thermometer, changes resistance with the temperature increase.
These thermalizing detectors have been replaced by even more sensitive transition-edge sensors (TESs).
Thermalizing detectors have several disadvantages. First, the spiderweb absorbers are very fragile and are prone to breakage during installation and handling. They are also difficult and expensive to fabricate. Finally, each detector requires two wires to measure the resistance of either the TES or NTD 'thermometer'.
The next-generation SuperBLASTPol experiment will utilize arrays of microwave kinetic inductance detectors (MKIDs). MKIDs differ from TESs and NTDs as they are pair-breaking detectors. MKIDs work on the principle that photons absorbed in a superconductor breaks the cooper pairs of electrons and change its surface impedance in a process called the kinetic inductance effect. This change can be accurately measured by placing this superconducting inductor in a lithographed RLC resonator. A microwave probe signal is tuned to the resonant frequency of the resonator, and any photons which are absorbed in the inductor will affect the phase and amplitude of the probe signal. Since the quality factor Q of the resonators is high and transmission off resonance is nearly perfect, multiplexing can be accomplished by tuning each pixel to a different resonance frequency by either changing the size of the inductor or capacitor in the RLC resonant circuit. A comb of probe signals can be sent into the device, and room temperature electronics can recover the changes in amplitude and phase without significant cross talk.
Due to their inherent multiplexing scheme, an array of MKIDs can be read out with just a pair of coax cables. Another benefit of these types of detectors is that they are relatively simple to fabricate. While TES arrays require tens to hundreds of layers, MKID arrays require only one deposition layer of the superconducting film. A single lithography defines the resonators and microstrip line, and a deep reactive ion etch (DRIE) produces the backshort. This enables the cost per detector to be dramatically reduced to ~ $10 - $100; an order of magnitude reduction from TESs. This significant cost reduction will also aid in the realization of the Inflation Probe.
The next-generation Super BLAST experiment is further pushing the boundaries of MKIDs. By orthogonally crossing the absorptive inducting element of two MKIDs, polarization measurements can be made. Super BLAST will fly 2000 MKIDs, or 1000 dual-polarization sensitive pixels, in three bands: 250, 350, and 500 μm. BLAST will be able to read out ~800 MKIDs per set of readout electronics, one of the highest limits set to date. In addition, BLAST will be the first experiment to demonstrate MKIDs on a balloon payload, gaining valuable information towards applying these detectors on a Satellite experiment, such as the Einstein Inflation Probe.