An Integrated Program for An Integrated Program for GBT Focal Plane Array GBT Focal Plane Array

DevelopmentDevelopmentSteven WhiteSteven White

Based on discussions with:Based on discussions with: Rick Fisher, John Ford, Matt Morgan, Roger Rick Fisher, John Ford, Matt Morgan, Roger

Norrod, Kamaljeet Saini, Amy Shelton, Richard Norrod, Kamaljeet Saini, Amy Shelton, Richard Prestage, John Webber, and others Prestage, John Webber, and others

SummarySummary

A pathfinder focal plane array (FPA) frontend is the A pathfinder focal plane array (FPA) frontend is the necessary first step in an integrated development necessary first step in an integrated development program.program.

K-band is the appropriate frequency, for reasons K-band is the appropriate frequency, for reasons which will be outlined.which will be outlined.

We have the necessary resources and expertise ($, We have the necessary resources and expertise ($, more importantly appropriate staff effort) ready more importantly appropriate staff effort) ready and eager to go.and eager to go.

We need to start now to demonstrate results within We need to start now to demonstrate results within ~ 2 years (production use in ~3 years).~ 2 years (production use in ~3 years).

IntegratedIntegrated FPAFPA Development ProgramDevelopment Program The Science Case is the primary driver for frequency selection The Science Case is the primary driver for frequency selection

and detailedand detailed specificationsspecifications..

Development Program ComponentsDevelopment Program Components Frontend Frontend (EM Components, Cryogenics, HEMT Amp, Calibration, (EM Components, Cryogenics, HEMT Amp, Calibration,

Downconversion)Downconversion) Digitization and IF TransmissionDigitization and IF Transmission SpectrometerSpectrometer SoftwareSoftware

Scientific desires must be tempered by technical realities and Scientific desires must be tempered by technical realities and costs of each component .costs of each component .

True measure of a successful program for the GBT:True measure of a successful program for the GBT: Competitive instrumentation installed on short time scales and Competitive instrumentation installed on short time scales and

in production use by external observers.in production use by external observers.

Instrument Development RealitiesInstrument Development Realities GBT SpectrometerGBT Spectrometer

~5 year development time with an additional 5 year integration time.~5 year development time with an additional 5 year integration time. Finally debugged August 2007Finally debugged August 2007

Ka-band ReceiverKa-band Receiver Extensive refurbishments required for each of three successive observing Extensive refurbishments required for each of three successive observing

seasonsseasons

HARP/ACSIS FPA program on JCMTHARP/ACSIS FPA program on JCMT HARP – 16 pixel 345GHz frontendHARP – 16 pixel 345GHz frontend ACSIS – 16 x 2 x 1GHz spectrometer backendACSIS – 16 x 2 x 1GHz spectrometer backend Similar specifications and logistical challenges to our proposed systemSimilar specifications and logistical challenges to our proposed system ~10 year development time~10 year development time IF distribution became critical path itemIF distribution became critical path item

Commissioning FE/IF/BE simultaneously is enormously Commissioning FE/IF/BE simultaneously is enormously complicated!complicated!

SolutionSolution Select challenging but realistic component deliverables.Select challenging but realistic component deliverables. Develop on 1-2 year timescales.Develop on 1-2 year timescales. Integrate each new component into the production system to allow immediate, Integrate each new component into the production system to allow immediate,

productive astronomy.productive astronomy.

Development Path: 1Development Path: 1stst Step Step

Frequency SelectionFrequency Selection λλ < K-band: physically too large for Gregorian focus < K-band: physically too large for Gregorian focus

(needs beam-forming array at prime focus).(needs beam-forming array at prime focus).

λλ = Q, W-band: existing IF capacity inadequate; upgrade = Q, W-band: existing IF capacity inadequate; upgrade path unknownpath unknown..

λλ = Q, W-band: telescope performance still under = Q, W-band: telescope performance still under development (should be delivered on 2-3 year timescales, development (should be delivered on 2-3 year timescales, but not a given).but not a given).

λλ = W-band: significant detector and other R&D = W-band: significant detector and other R&D development required.development required.

Where is GBT Unique?Where is GBT Unique?

K-Band AdvantagesK-Band Advantages

A scientifically exciting but technically A scientifically exciting but technically realistic focal plane array.realistic focal plane array.

Can do production science with existing IF Can do production science with existing IF system and spectrometer.system and spectrometer.

Parallel and collaborative spectrometer Parallel and collaborative spectrometer developments possible but not critical.developments possible but not critical.

Realize cost savings and enhanced performance Realize cost savings and enhanced performance for future expansion of instrument.for future expansion of instrument.

K-Band AdvantagesK-Band Advantages Develop on 1-2 year timescalesDevelop on 1-2 year timescales

Integrate each new component into the production system Integrate each new component into the production system to allow immediate, productive astronomy.to allow immediate, productive astronomy.

Technical staff available for two years and ready to begin.Technical staff available for two years and ready to begin.

Frontend: minimal R&D effort required.Frontend: minimal R&D effort required.

Current capability to demonstrate a prototype within one Current capability to demonstrate a prototype within one year.year.

Concurrent software development with a usable instrument Concurrent software development with a usable instrument within 2 years.within 2 years.

K-band FPA Deliverables K-band FPA Deliverables

FrontendFrontend Cryogenic PackageCryogenic Package Modular Downconverter Modular Downconverter Modular Noise CalibrationModular Noise Calibration Mechanical PackagingMechanical Packaging

SoftwareSoftware Package for Engineering and M&CPackage for Engineering and M&C Package for data analysisPackage for data analysis

Future FPA Developments Future FPA Developments

Significantly enhanced spectrometer with this Significantly enhanced spectrometer with this new array and existing IF system.new array and existing IF system.

New digital I.F. distribution system.New digital I.F. distribution system.

Expanded focal plane array (perhaps at a Expanded focal plane array (perhaps at a different frequency)different frequency)

Phased, expandable software development to Phased, expandable software development to match hardware capabilities.match hardware capabilities.

SpectrometerSpectrometer

We have a new approach – CICADA programWe have a new approach – CICADA program

Configurable Instrument Collaboration for Agile Data AcquistionConfigurable Instrument Collaboration for Agile Data Acquistion Collaboration between GB, CDL, UC Berkeley CASPER group, Collaboration between GB, CDL, UC Berkeley CASPER group,

WVU, University of Cincinnati, othersWVU, University of Cincinnati, others Initial production instrument: pulsar backend (“Scott’s Dream Initial production instrument: pulsar backend (“Scott’s Dream

Machine")Machine") Design of a 2GHz bandwidth spectrometer already identified as a Design of a 2GHz bandwidth spectrometer already identified as a

deliverable for FY2008deliverable for FY2008 Construction could commence in FY2009 given resourcesConstruction could commence in FY2009 given resources Excellent candidate for external funding:Excellent candidate for external funding:

Correct technology area (innovative hardware / networking / Correct technology area (innovative hardware / networking / computing)computing)

University collaborations University collaborations

=> “prototype” backends are under development; new => “prototype” backends are under development; new Spectrometer development should start in Fy2009Spectrometer development should start in Fy2009

IF SystemIF System

Current analog IF system limits expansion.Current analog IF system limits expansion. Analog modulators costly with increased complexity due Analog modulators costly with increased complexity due

to stability requirements.to stability requirements. DownconversionDownconversion scheme limits bandwidthscheme limits bandwidth

Digitization at antenna with filtering/compression schemes Digitization at antenna with filtering/compression schemes probable solution for IF transmissionprobable solution for IF transmission..

Commerical technology advancements in digital transmission Commerical technology advancements in digital transmission reduces cost and increases performancereduces cost and increases performance..

Development in this area is extremely active, driven by SKA Development in this area is extremely active, driven by SKA and SKA-pathfinder initiativesand SKA-pathfinder initiatives

SoftwareSoftware

Similar systems do exist at other telescopes.Similar systems do exist at other telescopes.

Challenge in software is to transfer approach or actual Challenge in software is to transfer approach or actual implementation to your specific telescope.implementation to your specific telescope.

We will certainly leverage work done elsewhere, and We will certainly leverage work done elsewhere, and collaborate with other groups.collaborate with other groups.

Using existing GBT Spectrometer makes problem much Using existing GBT Spectrometer makes problem much more tractable (many GBT-specifics already solved).more tractable (many GBT-specifics already solved).

=> Software development is an integral part of => Software development is an integral part of FPA program addressed early in the project.FPA program addressed early in the project.

Development Path Development Path SummarySummary

Processing software:Processing software: issues well understood issues well understood

Backend:Backend: current system can accommodate a current system can accommodate a modest array (at K-band), issues and upgrade modest array (at K-band), issues and upgrade path well understood.path well understood.

I.F. system:I.F. system: current system can accommodate current system can accommodate modest array, upgrades premature at this timemodest array, upgrades premature at this time

Frontend:Frontend: Proposed K-band. Proposed K-band.

A pathfinder focal plane array (FPA) frontend is A pathfinder focal plane array (FPA) frontend is the necessary first step in an integrated the necessary first step in an integrated development program.development program.

K-band FPA SummaryK-band FPA Summary

K-band is the appropriate frequency.K-band is the appropriate frequency.

We have the necessary resources and We have the necessary resources and expertise ($, more importantly appropriate expertise ($, more importantly appropriate staff effort) ready and eager to go.staff effort) ready and eager to go.

We need to start now to demonstrate We need to start now to demonstrate results within ~ 2 years (production use in results within ~ 2 years (production use in ~3 years)~3 years)

Backup SlidesBackup Slides

Backend

Data Transmission System

Telescope Performance

Weather

Backend Strawman DesignBackend Strawman Design

Strawman SpecificationsStrawman Specifications 3 GHz analog bandwidth3 GHz analog bandwidth

6 GS/s6 GS/s 8 bit ADC, 6 bit ENOB8 bit ADC, 6 bit ENOB

183 KHz resolution@ 3 GHz bandwidth 183 KHz resolution@ 3 GHz bandwidth (16K channels)(16K channels)

80 millisecond minimum integration 80 millisecond minimum integration timetime

200 MB/s aggregate data rate to disk for 122 200 MB/s aggregate data rate to disk for 122 IF channelsIF channels

Backend Strawman DesignBackend Strawman Design

Technology ApproachTechnology Approach Off the shelfOff the shelf ADC chipsADC chips FPGA DSP hardwareFPGA DSP hardware Off the shelf Data Collection Computer Off the shelf Data Collection Computer

SystemsSystems Optional locationsOptional locations

Receiver roomReceiver room Equipment roomEquipment room

Backend Strawman DesignBackend Strawman Design

CostCost $20K/pixel sampler and DSP costs(2 $20K/pixel sampler and DSP costs(2

channels)channels) $2.44M for 61 pixels, scales linearly with # pixels$2.44M for 61 pixels, scales linearly with # pixels

$30K Data collection and storage computers$30K Data collection and storage computers $70K data transmission to disks (all DSP at $70K data transmission to disks (all DSP at

front end)front end) 6 FTE-years effort (3 people, 2 years)6 FTE-years effort (3 people, 2 years)

Total $3.3 million.Total $3.3 million.

Data Transmission SystemsData Transmission SystemsEVLA and ALMA Data Transmission EVLA and ALMA Data Transmission

Systems are not appropriateSystems are not appropriate GBT needs 4 bit samplersGBT needs 4 bit samplers

EVLA 3 bitsEVLA 3 bits ALMA 2 bitsALMA 2 bits

ALMA and EVLA multiplexing too ALMA and EVLA multiplexing too complex and expensive for GBTcomplex and expensive for GBT

=> => Advantages in allowing technology Advantages in allowing technology to develop.to develop.

Telescope PerformanceTelescope Performance

Azimuth Track Replacement Azimuth Track Replacement ProjectProject

Field work completed on Monday Field work completed on Monday 33rdrd September to specification, on September to specification, on schedule and within budgetschedule and within budget

Now in the process of developing Now in the process of developing new pointing model; initial new pointing model; initial results extremely promising.results extremely promising.

Very first observation after outage complete. Measured pointing offsets (3”, 1.5”) in (az,el).

Local tilt (pitch) of the antenna as measured by inclinometers mounted on the elevation axle. Both large and small scale track effects are significantly reduced.

14GHz half-power track14GHz half-power track

sec12 arc

)/'10,/'2(,

)5,1(),(

)58,290(),(

mmt

El

t

Az

ElAz

ElAz

Surface PerformanceSurface Performance OOF technique can easily OOF technique can easily

measure large-scale wavefront measure large-scale wavefront errors with accuracy ~ 100errors with accuracy ~ 100µmµm

Large scale gravitational errors Large scale gravitational errors corrected via OOF look-up tablecorrected via OOF look-up table

Benign night-time rms Benign night-time rms ~ 350~ 350µµmm

Efficiencies:Efficiencies:

43 GHz: 43 GHz: ηηSS = = 0.67 0.67 ηηA A = 0.47 = 0.47

90 GHz: 90 GHz: ηηSS = 0.2 = 0.2 ηηAA = 0.15 = 0.15

Now dominated by panel-panel Now dominated by panel-panel errors (night-time), thermal errors (night-time), thermal gradients (day-time)gradients (day-time)

Summary – Current Summary – Current PerformancePerformance

WeatherWeather

lowers the SNR lowers the SNR exp(- exp(-) / ) / TTsyssys

Atmospheric Absorption and

Emission(mediocre)(excellent)

Slide Courtesy Jim Condon

Observing Limits (accept/reject)Observing Limits (accept/reject)

Atmospheric Atmospheric efficiencyefficiency

Atmospheric stabilityAtmospheric stability

Absolute hour angleAbsolute hour angle

Zenith angle < 85 Zenith angle < 85 degdeg

Tracking flux errorTracking flux error < 10% < 10%

Atmospheric Atmospheric efficiencyefficiencySlide Courtesy Jim Condon

Stringency Stringency 1 / (fraction of time 1 / (fraction of time OK)OK)

Stringency Stringency measures the measures the difficulty of difficulty of scheduling an scheduling an observation on the observation on the GBT. It depends GBT. It depends strongly on the strongly on the observing observing frequency frequency and and also on the source also on the source elevation at elevation at transit. transit.

Slide Courtesy Jim Condon