SWIPE is the Short
Wavelength Instrument for the Polarization Explorer
It is a Stokes Polarimeter,
based on a
cold (4K) HWP polarization modulator as the first
optical element, a simple 50 cm aperture refractive
telescope, a beamsplitting polarizer, and two large
focal planes, hosting 330 multi-moded bolometers at
140, 220, 240 GHz.
Everything is cooled by a
large L4He cryostat and a 3He refrigerator, for
operation of the bolometers at 0.3K
In the figure on the right
we sketch: the outer shell of the cryostat (blue),
the 170K shield (red), the 40K shield (orange), the
4He tank (yellow), the polarization modulator
(purple), the large polarizer beamsplitter and the
two multimoded horns arrays (green). The outer
diameter of the system is 140 cm. In the lower part
of the same figure we sketch the position of the 330
detectors, evenly divided in the the 3 frequency
bands, as projected in the sky.
The telescope ray tracing is
reported below.
Detector
system
The detectors of SWIPE are
multimode spiderweb TES bolometers (see table on the
right) and use multimode feedhorns to collect
radiation from the focal plane (see pictures of a
7-pixels prototype on the right).
Collecting many modes of the
incoming radiation on the same absorber we improve
significantly the sensitivity of the detectors (at
the expense of angular resolution, which is not a
driver for this measurement).
Compared to other
experiments, LSPE has less detectors for the same
sensitivity, resulting in simpler readout
electronics and instrument management.
Polarization
Modulator
• Is a cold (2K), large (50 cm useful dia.), wide band,
metamaterials HWP, placed immediately behind the window
and thermal filters stack (see picture on the right).
• HWP characteristics for the ordinary and extraordinary
rays are well matched:
(To-Te)/To < 0.001, Xpol<0.01, over the 100-300
GHz band.
• Its orientation is stepped by 11.25° or 22.5°
every few scans by means of a cryogenic rotator similar
to the one in the picture on the right.
Instrument
Performance Forecast
The presence of the HWP
modulator allows to fully exploit the sensitivity of
LSPE-SWIPE.
Realistic simulations are
used to assess systematic effects (mainly beam
asymmetries), which become irrelevant if the HWP is
used.
In red in the plot on the
left the angular power spectrum obtained in absence
of the HWP. In green the same obtained using the
HWP. The input power spectrum is in black. For both
E-modes and B-modes (top and bottom plots) is
evident that beam effects not cured by the HWP would
produce systematic effects much larger than the
noise of the instrument.
Assuming
instrument performance limited by the radiative
background on the detectors, we obtain the sensitivity
forecast in the table on the right.
SWIPE – LSPE - photon
noise limited performance
Band center
(GHz)
140
220
240
combined
Bandwidth
(%)
33
5
5
Detectors
#
110
110
110
l2 modes #
10
21
23
Total
background (pW)
9
4
13
Optical
NEP (aW/Hz1/2)
180
220
420
Optical
NET(mKCMB/Hz1/2)
50
55
115
Final
polarization survey sensitivity (mKCMB
arcmin)
(14 days, 20% of the sky)
