SAFIRE: Instrument Design
1. Instrument Overview
The SAFIRE instrument is designed to be a highly modular system as shown
below. This permits the assembly and testing of each component separately,
which is necessary for programmatic reasons. SAFIRE is being developed
slowly, with components being assembled serially by a limited workforce.
As these items are completed, they are held until needed for later
integration and testing.
SAFIREÕs detectors are the most technologically advanced component, and
have received much of the attention to date because of their relatively
high development needs. A prototype instrument for the demonstration of
these detectors is described elsewhere. The detectors are cooled by an
adiabatic demagnetization refrigerator built for SAFIRE, and similar to
that used in the HAWC instrument on SOFIA. In order to modularize these two
components for testing, an insertable cryostat has been fabricated at the
Goddard Space Flight Center. This cryostat can be installed in the SAFIRE
cryostat, which is a custom-built LN2/LHe dewar from Precision
Cryogenics. An optics plate can be mounted in this cryostat on the 4.2K
surface, as described above. The two Fabry-Perot mechanisms and the filter
wheel, together with other optical components and baffling, are mounted on
this optics plate. A light-tight shield surrounds the optics plate. Almost
all the analog portions of the detector electronics are mounted on the
insertable cryostat, in order to keep them within a single
electrically-shielded environment; digital electronics are mounted in a
chassis on the side of the cryostat. Finally, software for the commanding
of the instrument, data acquisition and display, and housekeeping
information and control is being developed. The software, called Instrument
Remote Control (IRC), is designed to be a
platform-independent, extensible, modular environment usable for many SOFIA
and related instruments.
The major portions of the SAFIRE instrument; not shown explicitly are
optical mechanisms, electronics, and software.
2. Optical Design
SAFIRE achieves its high spectral resolution using a double Fabry-Perot
design, shown below. One of the design features is that the use of lenses
and flat mirrors reduces the aberrations induced by large field angles in
off-axis systems. Stray light is reduced by means of a set of stops at the
entrance to the 4.2K-cooled region, which is a light-tight enclosure. All
the optical elements are mounted on a removable plate, to allow convenient
room-temperature assembly and alignment. The only exceptions to this are a
speeding lens mounted at 77K and the detector array, mounted on a 1.3K
stage. The detectors are superconducting transition edge sensor bolometers
operated at ~200mK, which are described elsewhere.
3. Superconducting Bolometer Detector Array
The SAFIRE detector array uses superconducting transition edge sensor (TES)
bolometers. The thermal/electrical block diagram and a representative
current-voltage characteristic (IV curve) are shown below. These detectors
feature high sensitivity, high efficiency, and high speed. Additionally,
SQUID multiplexers exist that enable larger format arrays to be constructed
and read out. A schematic of the detector and its readout is shown further
below.
Our group continues to advance superconducting bolometer array technology,
and so this web page is never truly current. The most recent advance is in
the production of close-packed monolithic planar arrays. Shown below is a
recent 8x16 array with 2mm pixels, which can ultimately be scaled to a
16x32 array of 1mm pixels for SAFIRE.
A recent 8x16 superconducting bolometer array fabricated at NASA/GSFC.
Table of Contents:
|
