The Instrument
Telescope
PAPPA has a 60
cm diameter off-axis primary mirror that focuses light
through an array of corrugated feed horns and onto the cryogenic focal plane.
These corrugated feeds have minimal cross-polar response and a demonstration
array of 1000 feeds at 1mm wavelength exceeded the criteria for PAPPA's design
criteria of a small cross polar response. To the left is a drawing
of the mirror and the telescope. Below is a photograph of the feed horn
array which was made at GSFC.
Planar Polarimeters
The new technology developed for this and later missions depends on rapid
modulation of the phase of the signals in the arms of the polarimeters. In
order to break down incident light into orthogonal components, switches
will periodically inject a phase delay in the incident light, which will
allow for measurement of linear polarization. Each polarimeter will have TES bolometers as
square-law detectors. Both the polarimeter and the bolometers will be
built on the same wafer and will operate on a 100mK cold focal plane using a
3-stage ADR developed at GSFC. Also, in order to avoid the
atmospheric lines in the typical spectrum of the sky, PAPPA will use
superconducting transmission line filters to define the 3 bands at 89, 212, and
300 GHz. A picture below shows the typical spectrum of the atmosphere at
35 kilometers, as well as the PAPPA bands, defined in color.
The
filter passbands are able to reduce the atmospheric emission contribution to
less than 10% of the CMB power in 89 and 212 GHz bands, and less than 35% in the
300 GHz band.
The Gondola
PAPPA will fly at an altitude of 35 kilometers using a shielded telescope on
a pointed platform. Below is a picture of the gondola design for the
telescope, which is based on the designs for two other previous missions, QMAP
(Miller et al. 2002) and BLAST (Devlin et al. 2004). The inside
consists of the mirror, baffle, and cryostat, and is supported and protected by
the external gondola. This frame is pointed in azimuth by a flywheel and
active pivot. The telescope will slew at 1 degree per second. While
it is not that important to know where the telescope is pointing during the
flight, the ability to accurately reconstruct that data after the flight is, and
the
telescope has several pointing sensors such as a star camera (which is
programmed to determine with high accuracy where the telescope is pointing),
several fast, low-drift gyroscopes, a quad-GPS system, tilt sensors, and a
magnetometer. The software on the telescope takes each sensor's data into account in a
hierarchical manner and can shift to the next available sensor's data if it
detects a problem.
