A New Spectrometer for the Green Bank Telescope

Transcription

1 A New Spectrometer for the Green Bank Telescope Marty Bloss, Patrick Brandt, Hong Chen, Jayanth Chennamangalam, Jeff Cobb, Ramon Creager, Paul Demorest, Glenn Jones, Randy McCullough, Jason Ray, D. Anish Roshi, Simon Scott, Andrew Siemion, Mark Wagner, Galen Watts, Dan Werthimer, Mark Whitehead Atacama Large Millimeter/submillimeter Array Expanded Very Large Array Robert C. Byrd Green Bank Telescope Very Long Baseline Array

2 Outline Science Case Summary Spectrometer Requirements Architecture and Design Status Summary 2

3 Science Drivers Star Formation - order of magnitude increase in efficiency to improves measurements of molecular cores in star forming regions Pulsars in galactic center - pulsar scattering effects are mitigated at >10GHz, while hopefully at low enough frequencies to still be observable Are the Fundamental constants constant? -Highly accurate measures of some fundamental constants such as the electron/proton mass ratio, to determine if they ve been changing over time Comets as relics of Solar System formation Fast dump times and large FOV allow improved observations for a deeper understanding of solar evolution

4 Molecules and Comets Molecular emission maps of comet HaleBopp made in HCN (left) and HCO+ (right) with the 14 m telescope of the FCRAO, whose angular resolution is about the size of the cross in the right panel. Overlaid dots show the footprint of a 16-pixel GBT array in these lines. Figure courtesy A.J. Lovell. Eight new biologically significant molecules were discovered in the interstellar medium in a two-year period using the GBT. 4

5 Why a New Spectrometer? New Spectrometer Old GBT Spectrometer Polyphase Filter Bank for RFI rejection Autocorrelation Spectrometer 256 level samplers 3 level samplers 16 high-speed samplers 8 high-speed samplers 1350 MHz bandwidth/sampler 800 MHz bandwidth/sampler 800 MB/s max disk I/O rate < 25 MB/s max disk I/O rate New technology (FPGA based, Virtex-{5,6} Designed and built in early 1990 s 5

6 Specification -- Inputs Spectrometer IF Inputs 16 IF inputs Matched to the GBT Focal Plane Array pilot programs of 16 IFs per instrument Usable with the existing GBT IF system 150 to 1500 MHz input bandwidth Allows coverage of extragalactic line observations up to ~3000 km/s at 90 GHz 150 MHz is the low frequency cutoff in GBT IF system 1500 MHz is the Nyquist frequency for 3 GHz sampling rate Isolation GBT low frequency feeds have polarization isolation of more than 35 db, so we specify more than 35 db isolation, and stability of one hour to allow for calibration 6

7 Specification -- Inputs Dynamic Range Requirements Chosen to allow observations with RFI present in the band Integration time of 12 hrs assumed RFI must be ~10x less than RMS noise of the spectral observation in 12 hrs integration, implying: Spurious-Free Dynamic Range (SFDR) of 89 db Intermodulation product rejection (IMD) of ~ -89 dbfs Our available 3/5 GHz ADC modules provide only about 52 db SFDR and -52 dbfs IMD 89 db would require >14 bit ADCs 7

8 Specification -- Outputs Support for frequency-switched and position-switched observations, and pulsed cal signals. Up to 6 switching states will be available Controlled by the spectrometer or by an external switching master Minimum switching state width of 25 milliseconds Full-Stokes available if required for polarization studies Modest pulsar capabilities ~ 4096 channels, dumped at a maximum rate of 500 microseconds Full-Stokes Maximum disk output of 100 MB/sec per IF pair, full Stokes 8

9 Summary of Observing Modes 9

10 Specification Processing High-resolution modes OH Maser Observations Require about 100 km/s range, and about 0.01 km/s resolution 1 MHz bandwidth 32,768 channels Extragalactic H2O Maser Observations Require about 5000 km/s range and about 0.3 km/s resolution 370 MHz of bandwidth 16,384 channels 10

11 Specification Processing Wide-bandwidth modes Extragalactic spectral line observations of, e.g. 12CO Require about 3000 km/s range and about 5 km/s resolution 900 MHz bandwidth 1024 channels 11

12 Specification -- Processing Multiple Spectral Windows For many observations, several narrow lines are separated across the bandpass by large expanse of uninteresting or RFI infested spectrum. Recombination line transitions in the 342 MHz band Multiple NH3 transitions at ~22 GHz Requires ~8 sub-bands to cover these Bandwidth Recombination lines require about 500 km/s range and about 0.5 km/s Requires about 1 MHz and 2048 channels NH3 requires about 100 km/s range and 0.04 km/s resolution Requires about 14 MHz and 4096 channels 12

13 Design 3 GS/s max sampling and PFB/FFT calculations Heterogeneous Computing Approach Divide processing into front/back ends Use FPGAs to fully process bandwidths greater than 400 MHz Use FPGA front-ends to pre-process, split and packetize data, then GPUs to provide fine channelization on narrower chunks Software Design Adapting concepts and code from the Green Bank Ultimate Pulsar Processing Instrument (GUPPI) Adding support for the K-band FPA processing pipeline Integrating with the GBT Monitor and Control System for user ease. 13

14 Overall Block Diagram 14

15 Front-end Hardware 15

16 Computing Nodes 16

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19 HPC Data Flow: High BW Modes

20 HPC Data Flow: Low BW Mode

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24 Status and Summary Hardware construction (almost!) complete Shielded Rack 8 ROACH-2 boards 16 5 GS/s ADC boards 16 Analog Processors Networking and file servers Computing nodes under testing M&C Software integration complete M&C Manager and Coordinator ASTRID integration Config Tool support 24

25 Status and Summary, Continued Firmware under development Mode 1 in testing Mode 13 (8-window modes) in development High-resolution single window modes to come later All of the developments are available on github. 25

26 Thanks!