1. An Integrated Program for GBT Focal Plane Array Development Steven White Based on discussions with: Rick Fisher, John Ford, Matt Morgan, Roger Norrod, Kamaljeet Saini, Amy Shelton, Richard Prestage, John Webber, and others Rick Fisher, John Ford, Matt Morgan, Roger Norrod, Kamaljeet Saini, Amy Shelton, Richard Prestage, John Webber, and others

2. Summary A pathfinder focal plane array (FPA) frontend is the necessary first step in an integrated development program. A pathfinder focal plane array (FPA) frontend is the necessary first step in an integrated development program. K-band is the appropriate frequency, for reasons which will be outlined. K-band is the appropriate frequency, for reasons which will be outlined. We have the necessary resources and expertise ($, more importantly appropriate staff effort) ready and eager to go. We have the necessary resources and expertise ($, more importantly appropriate staff effort) ready and eager to go. We need to start now to demonstrate results within ~ 2 years (production use in ~3 years). We need to start now to demonstrate results within ~ 2 years (production use in ~3 years).

3. Integrated FPA Development Program The Science Case is the primary driver for frequency selection and detailed specifications. The Science Case is the primary driver for frequency selection and detailed specifications. Development Program Components Development Program Components Frontend (EM Components, Cryogenics, HEMT Amp, Calibration, Downconversion) Frontend (EM Components, Cryogenics, HEMT Amp, Calibration, Downconversion) Digitization and IF Transmission Digitization and IF Transmission Spectrometer Spectrometer Software Software Scientific desires must be tempered by technical realities and costs of each component. Scientific desires must be tempered by technical realities and 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 in production use by external observers. Competitive instrumentation installed on short time scales and in production use by external observers.

4. Instrument Development Realities GBT Spectrometer GBT 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 2007 Finally debugged August 2007 Ka-band Receiver Ka-band Receiver Extensive refurbishments required for each of three successive observing seasons Extensive refurbishments required for each of three successive observing seasons HARP/ACSIS FPA program on JCMT HARP/ACSIS FPA program on JCMT HARP – 16 pixel 345GHz frontend HARP – 16 pixel 345GHz frontend ACSIS – 16 x 2 x 1GHz spectrometer backend ACSIS – 16 x 2 x 1GHz spectrometer backend Similar specifications and logistical challenges to our proposed system Similar specifications and logistical challenges to our proposed system ~10 year development time ~10 year development time IF distribution became critical path item IF distribution became critical path item Commissioning FE/IF/BE simultaneously is enormously complicated! Commissioning FE/IF/BE simultaneously is enormously complicated! Solution Solution 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, productive astronomy. Integrate each new component into the production system to allow immediate, productive astronomy.

5. Development Path: 1 st Step Frequency Selection Frequency Selection λ < K-band: physically too large for Gregorian focus (needs beam-forming array at prime focus). λ < K-band: physically too large for Gregorian focus (needs beam-forming array at prime focus). λ = Q, W-band: existing IF capacity inadequate; upgrade path unknown. λ = Q, W-band: existing IF capacity inadequate; upgrade path unknown. λ = Q, W-band: telescope performance still under development (should be delivered on 2-3 year timescales, but not a given). λ = Q, W-band: telescope performance still under development (should be delivered on 2-3 year timescales, but not a given). λ = W-band: significant detector and other R&D development required. λ = W-band: significant detector and other R&D development required. Where is GBT Unique? Where is GBT Unique?

6. K-Band Advantages A scientifically exciting but technically realistic focal plane array. A scientifically exciting but technically realistic focal plane array. Can do production science with existing IF system and spectrometer. Can do production science with existing IF system and spectrometer. Parallel and collaborative spectrometer developments possible but not critical. Parallel and collaborative spectrometer developments possible but not critical. Realize cost savings and enhanced performance for future expansion of instrument. Realize cost savings and enhanced performance for future expansion of instrument.

7. K-Band Advantages Develop on 1-2 year timescales Develop on 1-2 year timescales Integrate each new component into the production system to allow immediate, productive astronomy. Integrate each new component into the production system 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 year. Current capability to demonstrate a prototype within one year. Concurrent software development with a usable instrument within 2 years. Concurrent software development with a usable instrument within 2 years.

8. K-band FPA Deliverables Frontend Frontend Cryogenic Package Cryogenic Package Modular Downconverter Modular Downconverter Modular Noise Calibration Modular Noise Calibration Mechanical Packaging Mechanical Packaging Software Software Package for Engineering and M&C Package for Engineering and M&C Package for data analysis Package for data analysis

9. Future FPA Developments Significantly enhanced spectrometer with this new array and existing IF system. Significantly enhanced spectrometer with this 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 different frequency) Expanded focal plane array (perhaps at a different frequency) Phased, expandable software development to match hardware capabilities. Phased, expandable software development to match hardware capabilities.

10. Spectrometer We have a new approach – CICADA program We have a new approach – CICADA program Configurable Instrument Collaboration for Agile Data Acquistion Configurable Instrument Collaboration for Agile Data Acquistion Collaboration between GB, CDL, UC Berkeley CASPER group, WVU, University of Cincinnati, others Collaboration between GB, CDL, UC Berkeley CASPER group, WVU, University of Cincinnati, others Initial production instrument: pulsar backend (“Scott’s Dream Machine") Initial production instrument: pulsar backend (“Scott’s Dream Machine") Design of a 2GHz bandwidth spectrometer already identified as a deliverable for FY2008 Design of a 2GHz bandwidth spectrometer already identified as a deliverable for FY2008 Construction could commence in FY2009 given resources Construction could commence in FY2009 given resources Excellent candidate for external funding: Excellent candidate for external funding: Correct technology area (innovative hardware / networking / computing) Correct technology area (innovative hardware / networking / computing) University collaborations University collaborations => “prototype” backends are under development; new Spectrometer development should start in Fy2009

11. IF System Current analog IF system limits expansion. Current analog IF system limits expansion. Analog modulators costly with increased complexity due to stability requirements. Analog modulators costly with increased complexity due to stability requirements. Downconversion scheme limits bandwidth Downconversion scheme limits bandwidth Digitization at antenna with filtering/compression schemes probable solution for IF transmission. Digitization at antenna with filtering/compression schemes probable solution for IF transmission. Commerical technology advancements in digital transmission reduces cost and increases performance. Commerical technology advancements in digital transmission reduces cost and increases performance. Development in this area is extremely active, driven by SKA and SKA-pathfinder initiatives Development in this area is extremely active, driven by SKA and SKA-pathfinder initiatives

12. Software Similar systems do exist at other telescopes. Similar systems do exist at other telescopes. Challenge in software is to transfer approach or actual implementation to your specific telescope. Challenge in software is to transfer approach or actual implementation to your specific telescope. We will certainly leverage work done elsewhere, and collaborate with other groups. We will certainly leverage work done elsewhere, and collaborate with other groups. Using existing GBT Spectrometer makes problem much more tractable (many GBT-specifics already solved). Using existing GBT Spectrometer makes problem much more tractable (many GBT-specifics already solved). => Software development is an integral part of FPA program addressed early in the project.

13. Development Path Summary Development Path Summary Processing software: issues well understood Processing software: issues well understood Backend: current system can accommodate a modest array (at K-band), issues and upgrade path well understood. Backend: current system can accommodate a modest array (at K-band), issues and upgrade path well understood. I.F. system: current system can accommodate modest array, upgrades premature at this time I.F. system: current system can accommodate modest array, upgrades premature at this time Frontend: Proposed K-band. Frontend: Proposed K-band. A pathfinder focal plane array (FPA) frontend is the necessary first step in an integrated development program. A pathfinder focal plane array (FPA) frontend is the necessary first step in an integrated development program.

14. K-band FPA Summary K-band is the appropriate frequency. K-band is the appropriate frequency. We have the necessary resources and expertise ($, more importantly appropriate staff effort) ready and eager to go. We have the necessary resources and expertise ($, more importantly appropriate staff effort) ready and eager to go. We need to start now to demonstrate results within ~ 2 years (production use in ~3 years) We need to start now to demonstrate results within ~ 2 years (production use in ~3 years)

15. Backup Slides Backend Data Transmission System Telescope Performance Weather

16. Backend Strawman Design Strawman Specifications Strawman Specifications 3 GHz analog bandwidth 3 GHz analog bandwidth 6 GS/s 6 GS/s 8 bit ADC, 6 bit ENOB 8 bit ADC, 6 bit ENOB 183 KHz resolution@ 3 GHz bandwidth (16K channels) 183 KHz resolution@ 3 GHz bandwidth (16K channels) 80 millisecond minimum integration time 80 millisecond minimum integration time 200 MB/s aggregate data rate to disk for 122 IF channels 200 MB/s aggregate data rate to disk for 122 IF channels

17. Backend Strawman Design Technology Approach Technology Approach Off the shelf ADC chips Off the shelf ADC chips FPGA DSP hardware FPGA DSP hardware Off the shelf Data Collection Computer Systems Off the shelf Data Collection Computer Systems Optional locations Optional locations Receiver room Receiver room Equipment room Equipment room

18. Backend Strawman Design Cost Cost $20K/pixel sampler and DSP costs(2 channels) $20K/pixel sampler and DSP costs(2 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 front end) $70K data transmission to disks (all DSP at 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.

19. Data Transmission Systems EVLA and ALMA Data Transmission Systems are not appropriate GBT needs 4 bit samplers GBT needs 4 bit samplers EVLA 3 bits EVLA 3 bits ALMA 2 bits ALMA 2 bits ALMA and EVLA multiplexing too complex and expensive for GBT ALMA and EVLA multiplexing too complex and expensive for GBT => Advantages in allowing technology to develop.

20. Telescope Performance

21. Azimuth Track Replacement Project Field work completed on Monday 3 rd September to specification, on schedule and within budget Field work completed on Monday 3 rd September to specification, on schedule and within budget Now in the process of developing new pointing model; initial results extremely promising. Now in the process of developing new pointing model; initial 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.

22. 14GHz half-power track

23. Surface Performance OOF technique can easily measure large-scale wavefront errors with accuracy ~ 100µm OOF technique can easily measure large-scale wavefront errors with accuracy ~ 100µm Large scale gravitational errors corrected via OOF look-up table Large scale gravitational errors corrected via OOF look-up table Benign night-time rms Benign night-time rms ~ 350µm Efficiencies: Efficiencies: 43 GHz: η S = 0.67 η A = 0.47 90 GHz: η S = 0.2 η A = 0.15 Now dominated by panel-panel errors (night-time), thermal gradients (day-time) Now dominated by panel-panel errors (night-time), thermal gradients (day-time)

24. Summary – Current Performance

25. Weather

26. lowers the SNR  exp(-  ) / T sys Atmospheric Absorption and Emission (mediocre) (excellent) Slide Courtesy Jim Condon

27. Observing Limits (accept/reject) Atmospheric efficiency Atmospheric efficiency Atmospheric stability Atmospheric stability Absolute hour angle Absolute hour angle Zenith angle < 85 deg Zenith angle < 85 deg Tracking flux error < 10% Tracking flux error < 10% Atmospheric efficiency Slide Courtesy Jim Condon

28. Stringency  1 / (fraction of time OK) Stringency measures the difficulty of scheduling an observation on the GBT. It depends strongly on the observing frequency and also on the source elevation at transit. Stringency measures the difficulty of scheduling an observation on the GBT. It depends strongly on the observing frequency and also on the source elevation at transit. Slide Courtesy Jim Condon