- Cycle 8 NRAO ALMA Development Study Proposal – Regularized Maximum Likelihood Techniques for ALMA Spectral Line Imaging
- Ian Czekala, PSU
Regularized Maximum Likelihood (RML) imaging techniques have been demonstrated to achieve higher angular resolution with sub-mm observations of continuum sources while maintaining superior image fidelity, for example with the recent Event Horizon Telescope images of M87. There is great promise in applying RML imaging techniques to ALMA spectral line measurement sets, since the probabilistic framework can naturally treat diverse array configurations while regularizing away artefacts originating from incomplete u-v sampling. However, it remains to be seen whether prior formulations, like entropy and sparsity, that have worked well in other domains will be successful for the varied morphologies of spectral line emission. We propose to develop and implement RML algorithms for ALMA spectral line imaging, focusing on achieving high image fidelity when utilizing multi-configuration aggregate datasets. As the ALMA archive continues to mature, techniques that can accurately image large and diverse quantities of data will drive science forward in key areas that require sensitivity and angular resolution, such as the kinematic detection of planets in protoplanetary disks and astrochemical domains. We will use our GPU-accelerated open source code MPoL
to implement and rigorously test promising new prior choices like Gaussian process velocity-space regularization, and power spectrum regularization. Best-practices learned throughout this study will be disseminated to the community through publications in leading astronomical journals and open source software.
- Cycle 8 NRAO ALMA Development Study Proposal – ALMA Band 6v2 SIS Mixer-Preamp Development
This proposal will continue the development of the components for a future ALMA Band 6 receiver upgrade – referred to here as Band 6v2. The goals are: (i) to increase the IF bandwidth from the present 4 GHz per sideband per polarization to 12 GHz (4-16 GHz) or 16 GHz (4-20 GHz); (ii) to reduce and flatten the noise temperature across the full IF band; and (iii) to expand the usable RF band from the current 211-275 GHz to 211-280 GHz. While the current ALMA correlator and IF transmission system can only accommodate 4 GHz per sideband per polarization, the new receiver will take advantage of future upgrades enabling increased bandwidth.
A number of options are being explored, and a best design will eventually be chosen for the upgrade. The options include the use of SIS junctions with aluminum nitride tunnel barriers (Nb/Al-AlN/Nb) instead of the usual aluminum oxide barrier (Nb/Al-AlOx/Nb) to give increased RF bandwidth; balanced sideband-separating (2SB) mixers instead of the current single-ended 2SB mixers, to reduce LO sideband noise; the use of ferrite isolators or balanced IF amplifiers to flatten the receiver characteristics across the full IF band.
It had been planned to use the very-wide-band low-noise cryogenic IF amplifiers made by Low Noise Factory for the B6v2 receivers. However, significant gain variation observed in LNF amplifiers makes them unsuitable for use as IF amplifiers. LNF has been aware of this problem for some time but has so far been unable to find a remedy. Recently, the NRAO CDL has had success using transistors made by Diramics in experimental IF amplifiers for the current Band-6 SIS mixers. It is believed that these devices could be used to make IF amplifiers for 4-16 GHz and probably 4-20 GHz, and it may be possible to make balanced IF amplifiers using the same devices. Continuing this work on IF amplifiers is critical to the success of the B6v2 receiver upgrade, and a study proposal to fund that work is being submitted in parallel with this one.
Under a current ALMA study proposal an improved Band-6 LO source with low sideband noise is being developed. Success in that would remove the need for balanced SIS mixers, thereby simplifying the project significantly, and reducing the cost of producing the ~140 mixer-preamps.
Because the physical constraints of the ALMA cartridge restrict the configuration of the mixer-preamplifiers, magnets, and orthomode transducer (OMT), all of which must be located near the output of the feed horn but must not protrude into the optical path which traverses the 4-K section of the cartridge, it is important that these components be developed in close collaboration with the optics design team.
The essential work on the SIS mixers, the superconducting IF hybrids and components on silicon membranes will be done in close collaboration with the University of Virginia Microfabrication Laboratory (UVML) through separate funding.
- Cycle 8 NRAO ALMA Development Study Proposal – Beyond Black Hole Images: Extending New Imaging Techniques from EHT to ALMA
- Lynn Matthews, MIT
The scientific demands of high angular resolution, high-fidelity imaging at millimeter and
submillimeter wavelengths are a fundamental driver for future ALMA development. The current
ALMA development plan aims to achieve an additional factor of 2 to 3 improvement in angular
resolution, demanding either observing at high frequencies with the current longest baselines or
at intermediate frequencies with 2 to 3 times longer baselines. However, both options are
technically challenging for high fidelity imaging, due to larger calibration errors and/or much
less uniform uv-coverage on the longest baselines. A key technical frontier is therefore the
development of robust, high-fidelity imaging algorithms which can deal with larger calibration
uncertainties and effectively use the planned outrigger stations for enhancing the angular
resolution of ALMA images.
In the last decade, the technical challenges of imaging with the Event Horizon Telescope (EHT)
have accelerated the development of new imaging techniques, collectively known as regularized
maximum likelihood (RML) methods. RML methods have strong potential to overcome and
improve current and future challenges of ALMA imaging in three ways: (1) allowing highfidelity
reconstructions, even at modest super resolution 2-3 times finer than that of traditional
CLEAN; (2) the capability to reconstruct images directly from closure quantities, free from
antenna-based calibration errors; and (3) the ability to handle intrinsically multi-dimensional
emission, such as time-variable emission structures. The aforementioned advantages of RML
methods over CLEAN have now been demonstrated not only for very long baseline
interferometry, but also for single-band ALMA continuum observations. However, since the
current RML packages were designed for performing single-band continuum observations with
only a few antennas, the general application of RML methods to ALMA observations will
require further developments of both software packages and imaging algorithms.
This ALMA Study project aims to improve the effective spatial resolution and image fidelity of
the current ALMA array and its planned extended array, by extending of RML methods from the
EHT to ALMA. In the Study, we aim to develop the RML imaging algorithms to make them
more general, flexible, and powerful, including: (a) the extension of imageable dimensions,
enabling multi-band or multi-spectral channel imaging, and (b) developments of gridding-based
RML imaging techniques involving minor and major cycles to accelerate the algorithm.
Algorithms developed in the Study will be implemented in SMILI, one of the RML packages
which is publicly available, open-source and python-interfaced. We will also implement IO
functions for CASA Measurement Sets into SMILI, so that it can work as an external library of
CASA. We will provide documentation, including an online manual and tutorials with example
imaging scripts for ALMA, allowing the broader radio astronomy community to utilize RML
imaging techniques for various interferometric data sets.
- Cycle 8 NRAO ALMA Development Study Proposal – Extending IF Bandwidth of Band #6 SIS Mixer-Preamps to 12 GHz and 16 GHz with Optimal Noise Performance: An Experimental Demonstration
- Marian Pospieszalski, NRAO
This proposal has two main developmental goals:
1) Development of new designs of broadband IF amplifiers using commercially available devices from Diramics, Inc. that are integrable with the existing and future designs of Band# 6 SIS mixers. That should include 4-16 GHz and 4-20 GHz versions. The state-of-the-art cryogenic low noise performance of Diramic’s devices has already been demonstrated in existing 5-10 GHz IF amplifiers as well as in in 35-52 GHz Band #1 amplifiers. The noise performance of Diramics devices equals on the average the best ever demonstrated in any technology.
2) Demonstration of direct integration with optimal noise performance of these IF amplifiers with the existing Band #6 SIS mixers. This should allow for extending instantaneous bandwidth of an individual mixer to 24 GHz or 32 GHz per polarization. Extending the instantaneous IF bandwidth of the existing Band #6 receiver from 5-10 GHz to 4-12 GHz has already been experimentally demonstrated. Extending the highest frequency within the IF bandwidth always has to result in some penalty in IF bandwidth averaged noise temperature of an SIS receiver. An experimental comparison of Band #6 mixer optimally integrated with IF amplifiers having 8, 12 and 16 GHz instantaneous bandwidth should settle the question whether extending the IF bandwidth at the cost of receiver noise is scientifically justifiable. It therefore should help guide any future SIS mixer development for ALMA.