The mm-wavelength sky reveals the initial phase of structure formation, at all spatial scales, over the entire observable history of the Universe. Over the past 20 years, advances in mm-wavelength detectors and camera systems have allowed the field to take enormous strides forward – particularly in the study of the Cosmic Microwave Background – but limitations in mapping speeds, sensitivity and resolution have plagued studies of astrophysical phenomena. In fact, limitations due to inherent biases in the ground-based mm-wavelength surveys conducted over the last 2 decades continue to motivate the need for deeper and wider-area maps made with increased angular resolution. TolTEC is a new camera that will fill the focal plane of the 50m diameter Large Millimeter Telescope (LMT) and provide simultaneous, polarization-sensitive imaging at 2.0, 1.4, and 1.1mm wavelengths. The instrument, now under construction, is a cryogenically cooled receiver housing three separate kilo-pixel arrays of Kinetic Inductance Detectors (KIDs) that are coupled to the telescope through a series of silicon lenses and dichroic splitters. TolTEC will be installed and commissioned on the LMT in early 2019 where it will become both a facility instrument and also perform a series of 100 hour “Legacy Surveys” whose data will be publicly available. The initial four surveys in this series: the Clouds to Cores Legacy Survey, the Fields in Filaments Legacy Survey, the Ultra-Deep Legacy Survey and the Large Scale Structure Survey are currently being defined in public working groups of astronomers coordinated by TolTEC Science Team members. Data collection for these surveys will begin in late 2019 with data releases planned for late 2020 and 2021. Herein we describe the instrument concept, provide performance data for key subsystems, and provide an overview of the science, schedule and plans for the initial four Legacy Survey concepts.
We describe a custom time-to-digital converter (TDC) designed to time tag individual photons from multiple single photon detectors with high count rate, continuous data logging and low systematics. The instrument utilizes a taped-delay line approach on an FPGA chip which allows for sub-clock resolution of <100 ps. We implemented our TDC on a Re-configurable Open Architecture Computing Hardware Revision 2 (ROACH2) board which allows continuous data streaming and time tagging of up to 20 million events per second. The functioning prototype is currently set up to work with up to ten independent channels. We report on the laboratory characterization of the system, including RF pick up and mitigation as well as measurement of in-lab photon correlations from an incoherent light source (artificial star). Additional improvements to the TDC will also be discussed, such as improving the data transfer rate by a factor of 10 via an SDP+ Mezzanine card and PCIE 2SFP+ 10 Gb card, as well as scaling to 64 independent channels.
For many years, acoustic systems have been used as the primary method for underwater communication; however, the
data transfer rate of such systems is low because sound propagates slowly through water. A higher throughput can be
achieved using visible light to transmit data underwater. The first issue with this approach is that there is generally a large
loss of the light signal due to scattering and absorption in water, even though there is an optimal wavelength for
transmission in the blue or green wavelengths of the visible spectrum. The second issue is that a simple communication
system, consisting only of a highly directional source/transmitter and small optical detector/receiver, has a very narrow
field of view. The goal of this project is to improve an optical, underwater communication system by increasing the
effective field of view of the receiving optics.
To this end, we make two changes to the simple system: (1) An optical dome was added near the receiver. An array of
lenses is placed radially on the surface of the dome, reminiscent of the compound eye of an insect. The lenses make the
source and detector planes conjugate, and each lens adds a new region of the source plane to the instrument's total field of
view. (2) The receiver was expanded to include multiple photodiodes. With these two changes, the receiver has much more
tolerance to misalignments (in position and angle) of the transmitter.
Two versions of the optical dome (with 6" and 8" diameters) were designed using PTC’s Creo CAD software and modeled
using Synopsys' CODE V optical design software. A series of these transparent hemispherical domes, with both design
diameters, were manufactured using a 5-axis mill. The prototype was then retrofitted with lenses and compared with the
computer-generated model to demonstrate the effectiveness of this solution. This work shows that the dome design
improves the optical field of view of the underwater communication system considerably. Furthermore, with the
experimental test results, a geometric optimization model was derived providing insights to the design performance limits.