The Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII): An Experiment for High Angular Resolution in the Far-Infrared

In developing the concept for BETTII, the underlying scientific goal was to be able to carry out FIR observations that sample the SED peaks of cool (∼50 K) material at low spectral resolution, but with sufficient angular resolution to differentiate emission from a group of closely-spaced sources that are unresolved by single mirror systems. This objective leads directly to the three top-level requirements on BETTII: it must operate at high altitude (120,000 feet), where high atmospheric transmission makes FIR observations possible; it must be a spatial interferometer to provide high spatial resolution; and it must be a double-Fourier instrument in order to obtain astrophysically interesting spectra from sources across a wide field of view.

These top level requirements, as well as the specific science cases that motivate BETTII, lead to several specific technical requirements on the experiment (Table 1). The majority of these requirements, such as instrument field-of-view, are self-explanatory, but two parameters in particular are worthy of some additional discussion. First, the required pointing stability is 1.5'', over timescales of 10 s. This stability constrains drift of the beam on the sky to under 10% of a detector pixel, on timescales longer than a single delay line scan (2.5 s). This prevents distortion of the fringe pattern due to changes in the relative pointing of the system; such distortion leads to a significant reduction in signal-to-noise ratio (S/N) and to errors in the reconstructed spectrum.

Second, high frequency pointing jitter must be tightly controlled. High frequency jitter in the mechanical system is manifested in the interferometric data directly as phase noise; unmitigated phase noise, in turn, effectively "washes out" the interferometric fringes, reducing the S/N of the fringes. The 0.1'' requirement ensures that jitter-induced instrumental visibility loss is < 5%.

The warm optical system (the system located outside of the instrument cryostat) is designed to collect two 50 cm diameter wavefront samples and to provide compressed 2.5 cm diameter, co-phased, commonly rotated beams to the central double Fourier beam-combining instrument. To achieve this purpose, the system includes the metering truss, the siderostats, the telescope assemblies, the k-mirror, and the warm delay line.

The metering truss (Fig. 2) is designed to serve as an optical bench. As such, it must be stiff. In addition, the truss must be well-behaved (i.e., it should not twist or warp) over large temperature changes; on the ground, prior to launch, temperatures can be as high as 35° C. At float, the temperature is expected to be ∼-40°C. In order to achieve the high altitudes desired for high atmospheric transmission, the truss must be lightweight. Finally, to meet requirements of NASA's Columbia Scientific Ballooning Facility, it must survive a 10-g vertical load.

Zoom In Zoom Out Reset image size

The primary material for the metering truss is carbon fiber; such structures have been shown to be strong, stiff, and light (Ogasaka et al. 2005). The structure itself has reflection symmetry for the two arms of the interferometer. This symmetry ensures that structural changes due to thermal effects are nearly identical for both arms of the interferometer. The individual members of this structure are 3-inch carbon fiber tubes, with a stainless-steel "nose-cone" attached in each end using two-part epoxy. Each tube was tested by placing ∼2000 pounds of stress along the length of the tube (twice the maximum load on a tube during a 10-g acceleration), to verify both the carbon fiber tube strength and the epoxy bond strength. Only a single nose-cone bond failed (out of a total of 250), giving a high level of confidence in the individual members of the metering truss. The metering structure consists of individual tubes of three different lengths, and the tubes are connected via stainless steel nodes. Only two different types of nodes are used within the structure. One advantage of the design is the low number of unique part designs; this simplifies replacement of damaged components, simplifies construction, and minimizes the numbers of spare components needed. Finite element analysis shows a first vibration mode for the structure at 24.8 Hz; this has been measured in the laboratory at 24.0 Hz. This mode is symmetric, consisting of vertical "flapping" of the two arms of the structure. The second mode is similar to the first, but antisymmetric, with one arm moving upward while the other moves down. This mode is predicted at 30.6 Hz, and measured in the laboratory at 28.0 Hz. Additional details on the metering truss, as well as both the modeled and measured performance, will be described in an upcoming paper.

One consideration in the design the optical truss is the time required for thermalization of the structure. While, for the first BETTII flight, scientific data acquisition will occur at night, the launch itself will occur during daylight hours. Illumination from the sun can result in thermal gradients within the structure, and hence deformation of the truss. However, this is largely mitigated through the low CTE of the carbon fiber structure; the structure should meet all requirements even during daytime operation. In addition, the truss is coated with a white epoxy paint, in order to minimize heating of the structure. The concern with thermal gradients is also relevant for the telescope structure; this concern drives the desire for BETTII to operate after sunset. The telescope optics and structure are all aluminum, and the 1/e thermalization time for these components is 6 minutes; 20 minutes after sunset, the telescope structure should be nearly isothermal.

The metering truss serves a single purpose: to act as a stable 8-meter long optical bench for the warm optics. The external optical design is shown in Figure 3. Light arriving at the interferometer from an astronomical source impinges upon two siderostats mounted at opposite ends of the metering truss; each is attached to a rotation stage which allows elevation pointing. Light reflects off the siderostats and enters the two nominally identical telescope assemblies, which compress the collimated 50 cm input beams by 20:1 down to 2.5 cm collimated output beams. Each of the telescope assemblies consists of four mirrors: a primary mirror, a flat mirror, a secondary, and a tertiary (Fig. 4). This four-mirror design meets the BETTII wavefront requirements (RMS errors of < 0.2 μm), and has reasonable alignment requirements (positions to within 75 μm, angles to within 7''), dominated by wavefront requirements in the short wavelength science channel. The structure of the telescope assembly consists of a mounting ring for the primary, a mounting ring for the flat mirror, separate 3-point mounts for the secondary and tertiary, and a connecting "trough". The rings isolate the mirrors, limiting optical distortion transmitted from the sag in the trough. The trough provides both high stiffness and high strength at low mass. The entire assembly, including both the structure and the individual mirrors, is made of aluminum, so that it undergoes homologous contraction over the large temperature change expected from launch-to-float (i.e., the optical prescription is conserved with changes in temperature).

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

On the ground, the two telescope assemblies can be aligned to within ∼10''; by adjusting flat mirrors in the warm delay line and the K-mirror assemblies, these beams can then be aligned to within ∼2''. On flight, in order to maintain high interferometric fringe visibility, the relative alignment needs to be maintained to within ∼0.5^'. This is achieved by using identical, symmetric mounting structures for both telescopes, and having them both mounted to the stable metering truss, which was designed for this purpose.

Following the telescope assemblies, the layouts of the two arms of the interferometer differ. In one arm of the interferometer, the collimated beam travels through a warm delay line (the WDL) consisting of four mirrors, all mounted at 45° angles relative to the incident beam (Fig. 5). Two of these mirrors are mounted on a linear travel stage, allowing changes to the optical path length but without any impact on the alignment of the beam. The warm delay line is used to compensate for variations in the optical path length difference between the two arms of the interferometer. We expect that pendulation will dominate phase variation for BETTII, although lower amplitude vibrations in the structure can also be compensated by the warm delay line, as described in section 2.4, below. Following the warm delay line, the beam continues to the cryostat.

Zoom In Zoom Out Reset image size

In the other arm of the interferometer, the collimated beam enters a K-mirror assembly (Fig. 6), the purpose of which is to rotate the image to match that of the arm with the warm delay line. The K-mirror consists of three mirrors, arranged such that the axis of the incoming and outgoing beams from the assembly are aligned. The assembly is mounted to a rotation stage, enabling rotation of the field-of-view by up to 360°. The K-mirror assembly is needed for BETTII because the differing orientations of the two siderostats produce opposing field-of-view rotations. Following the K-mirror, the beam continues to the cryostat.

Zoom In Zoom Out Reset image size

As shown in Figures 5 and 6, both the warm delay line and the K-mirror assembly include a fast tip-tilt stage. These tip/tilt stages respond to signals from the BETTII control system to align the pointing of the two arms of the interferometer, as described in section 2.4. To achieve the scientific goals of the BETTII experiment, all optical elements of the warm optical system must have RMS surface roughness smoother than 0.1 μm, with figure errors of less than 0.1 μm. The external optical system operates at ambient temperature (∼240 K at float altitudes of 120,000 feet). These "warm" optics limit the overall sensitivity of the instrument (see section 3).

The second major BETTII system is the instrument itself. The instrument is defined, in this context, as the cryostat and all components within it, including the cold optics, the cryogenic delay lines, cryogenic systems, and detectors, as well as external components needed for operation of these elements.

The BETTII cryostat was custom designed at NASA Goddard. This approach was driven by several requirements: long (24-hr) hold times were required for operation during a balloon flight; the cold (helium-cooled) volume must be sufficiently large to hold the science instrument, with adequate volume for a Helium-3 refrigerator (to cool the detector arrays to 300 mK); there must also be a cool (nitrogen-cooled) volume for the near-infrared channel. A cutaway view of the cryostat is shown in Figure 7. The cryostat has a 15-liter tank for liquid nitrogen and a 22-liter tank for liquid Helium. Manostats are used to maintain atmospheric pressure within them while the instrument is at float altitude, in order to reduce boil-off of the cryogens, increase cryogenic hold time, and to provide thermal stability. Inside the cryostat, there are two distinct instrument channels: a science (far-infrared) channel, and a tracking (near-infrared) channel.

Zoom In Zoom Out Reset image size

The cryostat is attached to the metering truss with an assembly constructed of titanium and aluminum. This structure maintains the position of the dewar window relative to the beam from the external optics to within 200 μm even through the large expected temperature change.

The beams from the external optics enter the cryostat through thin (15 μm) polypropylene windows. These windows provide good transmission at both near- and far-infrared wavelengths while also minimizing thermal emission. The windows will deform significantly with a 1 atmosphere pressure difference, but tests have shown that the material reliably holds to several atmospheres of pressure. The first cold optical system element is the near-infrared beamsplitter. The beamsplitter reflects light into the BETTII science channel, transmitting the NIR light into the tracking channel.

2.3.1. The Science Channel

A block diagram of the optics in the FIR channel is shown in Figure 8, and the corresponding physical layout is shown in Figure 9. The FIR beams travel through an optical relay, creating a cold pupil inside the cold volume for use in stray light control. A flat mirror then steers the beam into the cold delay line (described below). Following the cold delay line, another flat mirror steers the beams to the far-infrared beam combiner. This beam combiner, a hot pressed wire mesh beamsplitter, has close to 50/50 transmission/reflection across the entire science band, 30–90 μm, and will be fully characterized at Cardiff University prior to delivery. Both of the outputs from the beam combiner travel to spherical mirrors which serve as the camera optic for the short wavelength (30–55 μm) channel. The converging beam then impinges upon a FIR dichroic, which splits the FIR light into two bands. There are now four beams within the instrument, beams from both output ports of the beam combiner for each of the two wavelength channels. The short wavelength beams reflect off the dichroic to their detector arrays, while the long wavelength (55–90 μm) beams are transmitted by the dichroic to a final powered mirror, and then to the detector arrays. This final powered mirror is needed to adjust the overall focal length of the long-wavelength channel, and hence the plate scale on the detector arrays.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

One of the key components of the instrument is the cold delay line (CDL). The CDL is used to modulate the relative phase between the two arms of the interferometer (Fig. 10). This structural design is based upon Variable Polarization Modulators (VPMs: Chuss et al. 2006, 2012) built at Goddard. The majority of the CDL structure (Fig. 10) is made out of aluminum, with titanium flexures to maintain linearity of travel. At the center of the CDL is the aluminum mirror assembly. This comprises two rooftop mirror pairs built back-to-back. This provides optical delay of four times the physical stroke of the delay line. The CDL is driven by a voice coil and has a physical stroke length of 2.5 mm, providing 10 mm of optical path change. Two capacitive sensors are used to measure the position of the delay line to within a few tens of nanometers.

Zoom In Zoom Out Reset image size

In scientific operation, the CDL will conduct a single scan every 2.5 s, and successive scans will be co-added for a total observing time on target of 18 minutes. Observing with BETTII will be broken into 18-minute observing blocks; during the first flight, we plan to interleave observing blocks on scientific sources with observing blocks on external calibrators, to ensure that we are obtaining good quality fringes, and to understand and characterize the performance of the system and measure the instrumental visibility loss. Ultimately, however, the system should be sufficiently stable that observations of a single source can continue over multiple observation blocks.

The BETTII instrument is a multichannel bolometric camera. The FIR light efficiently couples to the detectors using resonant absorption. BETTII has four detector arrays, one for each of two bands on each of two output ports of the beam combiner. Each array is a 9 × 9 close-packed planar array of multiplexed superconducting transition edge sensor (TES) bolometers (Benford et al. 2008) incorporating the Backshort Under Grid (BUG) architecture (Allen et al. 2006). The bolometer arrays are designed as scaled versions of existing bolometer arrays that have been used for ground-based instruments (e.g., GISMO; Staguhn et al. 2006, 2014). By scaling predominantly only leg geometry for the same transition temperature, the BETTII arrays have increased saturation power and faster response times relative to GISMO, both needed by BETTII. As the sensitivity of BETTII is limited by the emission from the ambient temperature optics, the background loads are relatively high; the detectors must be capable of handling up to 200 pW of illuminating power. Fast response time allows for high speed detector readout, which is needed to sample interferometric fringes during the fast delay line scans described above. Table 2 lists all of the relevant parameters of these detectors.

The BETTII bolometers will be read out by an advanced linear SQUID multiplexer with heritage from prior instruments such as GISMO (Staguhn et al. 2008). A key advance is the use of high-order radiometric coils that reduce the sensitivity to environmental magnetic fields by orders of magnitude compared to early SQUID multiplexers. Representative noise is around , where ϕ₀ ≡ h/2e is the magnetic flux quantum, corresponding to . The 81 pixels in each of the four BETTII detector arrays are electrically addressed with 4 × 22 multiplexed readouts (Fig. 11). The additional seven channels provided by the readout electronics are used for two unilluminated pixels, one always-superconducting TES sensor that will monitor pickup in the detectors, and four 'dark SQUID' channels that monitor electronic pickup in each amplifier chain. Having these extra channels permits the removal of correlated noise sources, a technique which has been shown to reduce the low-frequency 1/f noise knee to below 0.1 Hz (de Korte et al. 2003).

Zoom In Zoom Out Reset image size

The individual column outputs are amplified by an array of SQUIDs in series that provide a gain of ≃200 to the signal. This approach has been used for decades (Welty & Martinis 1993), but the modern improvement of a low-power SQUID design and gradiometric input coils permits the amplifiers to be run in Earth's magnetic field. These SQUID series arrays are cooled to 4 K and operate in superconducting Nb shielding tubes to prevent modulation of the magnetic field as the payload rotates in the Earth frame.

The detectors and SQUID multiplexers operate at 300 mK. Cooling to such low temperatures is achieved by using a two-stage closed-cycle Helium-4/Helium-3 refrigerator. The refrigerator was manufactured by Chase Cryogenics, and is similar to refrigerators flown on previous balloon missions and used in GISMO (Staguhn et al. 2008).

The set of four TES and SQUID focal plane arrays is digitally controlled and read out by a single ambient-temperature Multi-Channel Electronics (MCE) unit developed by the University of British Columbia (Battistelli et al. 2008). An associated fundamental crystal clock operating at a rate of 50 MHz synchronizes the MCE data with that of other critical BETTII systems. We shift between rows after a preset number of clock cycles such that each of the 24 rows (22 addressed to the multiplexer plus two blank rows that read out only warm electronics noise) is biased on for .∼5.2 μs with a revisit rate of ∼8.013 kHz. The data stream is filtered with a four-pole digital Butterworth filter at ∼200 Hz and down-sampled to ∼400 Hz. This provides a reasonable tradeoff between fast switching (which risks pushing against switching transients) and slow (which would increase the amount of aliased noise). As it is, the total detector system noise from all sources is predicted to be . Of this noise, 8% comes from aliased high frequency noise, which could be decreased to 4% by doubling the switching rate or increased to 17% by halving it. The MCE is connected via an optical fiber to a computer that acquires the data and that can be used to dynamically reconfigure the readout system.

2.3.2. The Tracking Channel

In addition to the FIR science channel, the BETTII instrument includes a tracking channel that operates at near-infrared wavelengths. This channel has been referred to in previous papers as the fringe tracking channel (Rizzo et al. 2012), but since the publication of that paper, the tracking channel has been redesigned and the fringe tracker component has been eliminated, leaving only the angle tracker.

The fringe tracker was designed to measure the relative phase between the two arms of the interferometer. However, it was realized that the scientific success of BETTII could be achieved without the fringe tracker. In concept, the fringe tracker would allow for correction of residual phase errors which are uncorrected by the control system. The dominant source of error, however, is the slow pendulation of the gondola. The high bandwidth of the attitude control sensors on BETTII provides confidence in the ability to reliably estimate attitude motions and "freeze" the fringes within the control system, both within the timescale of each individual scan and on the timescale of minutes. This will allow for co-adding interferograms post-flight, when a complete pointing solution is derived from all the sensor data. However, it is critical to periodically readjust the phase to avoid long-term drifts that would be caused by thermal changes. Observing calibrators, where fringes are seen in a single scan, will allow for this adjustment, and will provide valuable information on phase drift over the course of the flight. Second, by using the FIR channel for this purpose, the requirements on the optical elements within the system are greatly relaxed. In order to ensure high-quality fringes in the NIR, the quality of individual mirrors and the complexity of the mechanical design (to maintain alignment of optics, ensuring good wavefronts) must be increased. This becomes the major cost driver for the experiment. The cryostat is designed to allow for the incorporation of a fringe tracker into follow-up BETTII flights, which will help set the stage for other potential balloon interferometry experiments.

The angle tracker is designed to monitor the positions of a guide star with high accuracy, allowing BETTII to correct for differences in pointing between the two arms of the interferometer. For each arm of the interferometer, an image of the tracking star is placed on a subarray in one quadrant of an HAWAII-1RG (written in full as HgCdTe Astronomy Wide Area Infrared Imager with a 1 K × 1 K resolution, Reference pixels and Guide mode, but further abbreviated to H1RG) detector array. By monitoring the centroid of these images, changes in the relative pointing of the two arms of the interferometer can be observed. This provides feedback for correcting both slow pointing drifts and for correcting fast (∼100 Hz) motions.

Optically, the design of the angle tracker is very simple (Fig. 12). The NIR beam is transmitted by the dichroic that reflects the FIR beam. The NIR light is then relayed by a flat mirror to a single off-axis parabola camera, which creates an image on the H1RG array for each arm of the interferometer. The full array is read out during acquisition mode, while in tracking mode only the subarrays within the two quadrants that contain the tracking star images are read out to greatly improve the speed of the tracking sensor. We use a simple centroiding algorithm to measure the position of the tracking star, and convert deviations into the error signal that drives the tip/tilt mirrors in both the warm delay line and the K-mirror assembly. By monitoring the two arms separately, the angle tracker ensures that both common and noncommon pointing errors in the two arms are measured and corrected at 100 Hz.

Zoom In Zoom Out Reset image size

The H1RG in the angle tracker is a 1024 × 1024 pixel infrared detector with complementary metal oxide semiconductor (CMOS) readout, sensitive to NIR photons from 0.6 to 5.0 μm. The H1RG multiplexer allows for the operation of the device either at low or high speeds using full frame or windowed readout. Full frame readouts are used to identify the location of the guide star images from both arms. Subsequently, two subarrays are read out in high-speed mode to track changes in the positions of the guide star images. The frame readout is about 10 ms at its highest speed (100 Hz control update rate). High-speed readout of multiple windows in an analogous configuration has been shown by Bezawada & Ives (2006), who were able to readout 32 20 × 20 square windows (ibid., Fig. 4) at a pixel rate of 100 kHz, in approximately 140 ms.

The readout is accomplished using a San Diego State University (SDSU) GENIII (Leach 1994, 1995) controller, which is ubiquitous across astronomical observatories and as such has excellent heritage and fidelity. The Leach controller is designed to be a universal CCD and CMOS readout system that requires only alteration of the Motorola digital signal processor (DSP) code, and the changing of a few connections, to operate various devices. The controller provides the power, clock level translations, video acquisition, gain adjustment, and FITS image generation. For our controller configuration, we have the standard 250 MHz timing board and IR clock driver board. However, our PC interface card is PCIe versus the standard PCI card, allowing a maximum throughput of 512 MB/s, which is required for high-speed data acquisition. Finally, we have an 8-channel IR video board for readout of the H1RG.

The final major BETTII subsystem is the exoskeleton and control system. The exoskeleton itself (shown in Fig. 1) consists of aluminum framing manufactured by 80/20 Inc., and serves as the structure for mounting flight electronics, computers, as well as the main azimuth actuator (a compensated control moment gyro, or CCMG). This structure must meet the same 10-g load requirement as the metering truss, but is not required to have the same level of stiffness. The load requirement has been verified by finite element analysis. The metering truss sits inside the exoskeleton, but the four load bearing points (at the bottom of the truss) are separated by passive vibration dampers, in order to prevent transmission of vibration from the exoskeleton to the optical system.

The three sensors critical to the control system are a star camera, three fiber optic gyroscopes (OptoLink SRS-2000), and the angle tracker. The actuators are the CCMG (azimuth), the rotation stages for the siderostats (elevation), and the tip/tilt mechanisms (fine pointing control). There is also a momentum dump mechanism.

The star camera was built in-house and is described in Benford et al. (2012). This provides 1'' RMS pointing knowledge in both R.A. and decl., and 40'' RMS knowledge of roll around the boresight. Individual frame exposures are 100 ms; astrometric computation dominates the pointing determination rate, with new pointing solutions determined every 1–2 s. However, individual frames will be acquired at the maximum readout rate, and the additional frames will be used on the ground for post-flight reconstruction of pointing. It should be noted that these results are based on ground-based observations in Greenbelt, MD, where the seeing conditions are generally unfavorable; we anticipate better results when the camera is operated in a more benign environment (such as the high altitude environment of a balloon flight). The star camera consists of a PCO.Edge camera, using a CMOS detector array, a commercial 300 mm camera lens with a mechanical focusing system, a baffle tube, and the star camera computer. The star camera is mounted on the metering truss, but the computer for reading out the camera and calculating pointing position is mounted on the exoskeleton. The star camera collects data on a 3.18 × 2.68° field-of-view, and uses algorithms developed at Columbia University for the EBEX experiment (Oxley et al. 2004), modified at Cardiff University, to achieve the desired pointing accuracy. The star camera will be used in the control system to correct for gyroscope biases and drifts. It also provides "lost-in-space" pointing determination capability.

BETTII uses three single-axis fiber optic gyroscopes mounted orthogonally. The gyroscopes provide fast readout (400 Hz, with bandwidth of ∼50 Hz) and specifications for drift and noise are 0.005 arcsecond/second and , respectively. The gyroscopes serve as the high-speed sensor for gondola motion. By combining these data with results from the star camera, it will be possible to control the gondola pointing to within a few arcseconds in order to acquire a guide star on the angle tracker. In the laboratory, this level of pointing stability has already been achieved. It should be noted, however, that pendulum motion representative of flight conditions is difficult to reproduce, and therefore these results are not necessarily representative of on-flight performance. A more detailed discussion of these results, and the overall function of the BETTII control system, is presented by Rizzo et al. (2014a).

When the guide star is in the angle sensor's field-of-view (within about 10" of field center), the angle tracker uses it to determine position via a simple centroiding algorithm. The deviation from the desired star location on the detector, corresponding to perfect optical alignment of both arms, is then fed into the tip/tilt mechanisms.

Azimuth control of the gondola is provided by the compensated control moment gyro (CCMG; Fig. 13). The two wheels in the CCMG spin at a constant rate of 3051.8 RPM (synchronized with the readout from the TES detector arrays), providing 10.4 Nms of angular momentum. These are mounted on a gimbal which changes the orientation of the wheels. In the null-torque position, the two wheels are oriented back-to-back. By changing the orientation of the wheels by equal and opposite amounts on the gimbal, they combine to provide a torque that rotates the gondola. The constant speed operation of the wheels provides two major advantages: it minimizes power usage, and it results in a time-constant vibration profile, allowing for effective mitigation. Control of the pointing elevation is provided by rotating the siderostats using rotation stages. Effectively, the pointing methodology for the interferometer is equivalent to an altitude/azimuth telescope. The CCMG has been fully tested under similar conditions as expected during flight (i.e., both cold and at low pressure), and meets all requirements for the BETTII control system.

Zoom In Zoom Out Reset image size

The pointing control system has four modes of operation, as detailed in Table 3. Two of these (tracking mode and slew mode) are diagrammed in Figure 14. Lost mode simply uses the star camera to determine gondola pointing. In slew mode, where no guide star appears on the angle tracker yet, we use the star camera and the gyroscopes to generate the attitude errors. This is a common pointing configuration, used on other balloons as well as on spacecraft, where the star camera information is used to estimate and correct for the gyroscope bias drift. Once the gondola is correctly pointed, the system switches to acquire mode, where the gondola pointing is locked through readouts from both the star camera and the gyros. While locked, the image of the guide star is located on the H1RG array, and subarray readout is configured for tracking mode. With the guide star acquired and locked by the angle sensor, the control signal changes. The deflection of both tip/tilt mirrors required to keep the guide star locked in both arms is now the best estimate of BETTII's attitude. Both the control signal and the physical motions of the tip/tilt mirrors, as measured with encoders on the tip/tilts, are recorded. Since this measurement is tied to the optical system, it does not drift over time. Combining the tip/tilt position, the gyros, and the star camera can be used in post-processing to build a robust estimator that can estimate the attitude to 0.1'' rms over a few minutes. This is investigated in detail by Rizzo et al. (2014a).

Zoom In Zoom Out Reset image size

BETTII also has an optical path difference control loop, with the purpose of freezing the fringes as the payload is pendulating. On short timescales, the phase noise is almost exclusively caused by attitude errors. Hence, measuring the attitude errors provides a good estimator of the phase errors. These phase errors are fed to a fast linear actuator, the "warm delay line", which compensates for varying external optical path length between the source and the siderostats. This has the effect of freezing the phase before light enters the cryogenic section of BETTII, where double-Fourier interferometry can happen. The warm delay line was discussed previously in this paper (see section 2.2). It is important to remember that, for BETTII, pointing knowledge is more important than the actual pointing of the gondola. Since pointing differences result in path differences between the two arms of the interferometer (1" of pointing change results in ∼40 μm of phase change), knowledge of small pointing changes is necessary to accurately remove these phase changes using the warm delay line.

Finally, BETTII includes a momentum dump mechanism which can rotate the flight train with respect to the gondola, effectively dumping momentum through the balloon ladder into the balloon. As the CCMG is used to slew the gondola, the momentum dump mechanism will be activated through a simple PID loop. This will prevent the possible build-up of angular momentum in the gondola. This dump mechanism is based upon designs which have flown previously (Fixsen et al. 1996).

Additional sensors attached to the exoskeleton will provide ancillary housekeeping data. These include a sun sensor, accelerometers, magnetometers, and inclinometers. In addition to the general housekeeping purposes, these sensors will be used for post-flight assessment of the payload's performance and for data quality determination (e.g., magnetometer data can be used to monitor magnetic field conditions which can impact detector readout). None of these sensors are integral to the control system or to the acquisition of science data. Other such housekeeping systems include thermometry (∼60 thermometers will be located at various positions on the gondola, with additional thermometers inside the cryostat) and laser/quad cell systems for measuring sag and vibration of the gondola.

The material presented in this paper is based upon work supported by NASA Science Mission Directorate through the ROSES/APRA program, with additional support provided by NASA's Goddard Space Flight Center. Work by T. Veach was supported by an appointment to the NASA Postdoctoral Program at GSFC, administered by the Oak Ridge Associated Universities under contract with NASA. Contributions to this project were also made by a large number of undergraduate students, including: S. Gomillion, J. Doiron, S. Gore, B. Hoffman, W. Tierney, R. Curley, T. Kale, T. Handleton, S. Shapoval, M. Canaparro, D. Andrade, J. Stokes, C. Gibbons, S. Weinreich, J. Alcorn, A. Rau, S. Padder, P. Nehme, L. Oliviera, H. Spooner, C. Wagner, A. Cotto, N. Mihalko, Y. Okafor, P. Taraschi, J. Gibson, and Y. Huertes-Morales. The BETTII program at Cardiff University and UCL is supported by an STFC PRD grant.