To take advantage of new opportunities for long-duration circumpolar ballooning (LDB), we have developed a novel instrument concept: TopHat. Designed to provide reliable, quantitative measurements of the CMBR anisotropy, TopHat is optimized to reject both systematic and foreground spurious signals. We achieve this by placing the telescope on top of the balloon, creating an observing environment unequaled in any sub-orbital CMBR experiment performed to date. By mounting the telescope on top of the balloon, the entire sky above the instrument will be free from supporting structures which could scatter radiation into the sidelobes of the optics---a critical source of systematic uncertainty for anisotropy measurements at the 10^-6 level of sensitivity. Two versions of TopHat are being constructed: the Pointer and the Spinner.

The TopHat Pointer is similar to MSAM both in the Cassegrain style of the telescope and its ability to perform pointed integrations. The beam size makes this instrument sensitive to angular scales as fine as 0.33 degrees. The TopHat Pointer will be able to observe the same region of sky as MSAM2 with similar sensitivity except at shorter wavelengths. This will provide unprecedented spectral coverage of a significant portion of the sky. Not only does this yield an excellent check of systematic errors which can plague any experiment of this sensitivity, it also will enable us to understand the nature of the far-infrared foreground spectrum in high galactic latitudes as never before. We originally planned to fly this experiment from mid-latitudes in 1998. Following the recommendations of an independent scientific review, we have deferred this flight in order to advance the schedule for the TopHat Spinner.

This illustration shows the main components of the TopHat Pointer. All the components in this picture are on top of the balloon.

This picture shows the engineering test package (ETP) for the TopHat Pointer (right) and the hardware for the actual scientific flight payload (left). The ETP is a fully functional unit with mockup optical components, and was intended to be flown in a test flight. We have now deferred development of the TopHat Pointer in favor of the TopHat Spinner.

The TopHat Spinner will map a 48 degree diameter disk of sky above the Southern Polar Cap at 20 arcminute resolution. To accomplish this, the telescope, tipped 12 degrees off the zenith, simply spins at a constant rate about the vertical axis. Each rotation sweeps out a circle of 24 degrees. As the Earth rotates, the entire polar cap is observed each day. The TopHat Spinner will be launched from Antarctica as a Long Duration Balloon (LDB) flight and will have approximately 2 weeks of observation time. This will permit a substantial fraction of the flight to be dedicated to studying and characterizing systematics in-flight while still maintaining high sensitivity to CMBR anisotropy. Observing in five spectral bands between 5 and 21 icm (150 and 630 GHz, 0.5 to 2.0 mm) with anisotropy sensitivity on 0.33 to 48 degree angular scales, a single TopHat Spinner flight is expected to measure over 1800 square degrees of sky and about 20000 independent points.

This integrated program reflects a central lesson underscored by the COBE/DMR experience---that extremely careful understanding of systematic errors and foreground sources, and not simply raw detector sensitivity, is the key to any unequivocal measurement of the CMBR anisotropy.

The measurement of CMBR anisotropy is a very active field, with many experiments, current and planned, that are providing clues to solve this important puzzle. For more information on these experiments, please (view this excellent summary http://www.sns.ias.edu).