ALMA Project Book, Chapter 2.2

ALMA ENGINEERING SPECIFICATIONS

Darrel Emerson & Jaap Baars
Last revised 2000-Dec-12

Revision History
2000-12-01: First near-complete version

Introduction

In this chapter we give a summary of the engineering specifications of all aspects of the ALMA project. The material is drawn from other chapters of this Project Book, but is collected here for convenience. This chapter is intended to be completely consistent with all other chapters of the book. If discrepancies are found, please notify the editors DTE or JWMB as soon as possible.

Tables of specifications

Table 2.1 ALMA Science Flowdown to Technical Specifications

Science Requirement and Examples

Technical Requirements Needed to Achieve

1. High Fidelity Imaging

  • Imaging spatial structure within galactic disks;

    • • Imaging chemical structure within molecular clouds;

    • Imaging protostars in star formation regions
  • Reconfigurable Array
  • Robust Instantaneous uv-coverage, Nant > 60
  • Precision Pointing, 6% of the HPBW
  • Antenna Surface Accuracy RMS = 20 microns
  • Primary Beam Deviations < 7%
  • Total Power and Interferometric Capability
  • Precise (1%) Amplitude Calibration
  • Precise Instrumental Phase Calibration (<10 degrees rms)
  • Precise atmospheric phase calibration (<15 degrees rms) with compensation using both fast switching and water vapor radiometry
    • 2. Precise Imaging at 0.1" Resolution
    • Ability to discriminate galaxies in deep images;
    • Imaging protoplanets orbiting protostars;
    • Imaging nuclear kinematics
  • Interferometric baselines longer than 3 km
  • Precise Instrumental Phase Calibration (<10 degrees rms)
  • Precise atmospheric phase calibration (<15 degrees rms) with compensation using both fast switching and water vapor radiometry
    • 3. Routine Sub-milliJansky Continuum Sensitivity
    • To enable imaging of the dust continuum emission from cosmologically-distant galaxies
    • To enable imaging of protostars and protoplanets throughout the Milky Way
    • To enable astrometric observations of solar system minor planets and Kuiper-belt objects
  • Array site with median atmospheric transparency < 0.05 at 225 GHz
  • Quantum-limited SIS receivers
  • Antennas with warm spillover <5K, and aperture blockage <3%
  • Antennas of aperture efficiency > 75%
  • Wide correlated IF bandwidth, 16 GHz
  • Dual polarization receivers
  • Array collecting area, ND2 > 7000 m2
    • 4. Routine Milli-Kelvin Spectral Sensitivity
    • Spectroscopic probes of protostellar kinematics
    • Spectroscopic chemical analysis of protostars, protoplanetary systems and galactic nuclei
    • Spectroscopic studies of galactic disks and spiral structure kinematics
  • Array site with median atmospheric transparency < 0.05 at 225 GHz
  • Quantum-limited SIS receivers
  • Antennas with warm spillover < 5 K, aperture blockage <3%
  • Antennas with aperture efficiency > 0.75
  • Wide correlated IF bandwidth, 16 GHz
  • Dual polarization receivers
  • Array collecting area, ND2 > 7000 m2
  • Array collecting length, ND > 700 m
    • 5. Wideband Frequency Coverage
    • Spectroscopic imaging of redshifted lines from cosmologically-distant galaxies
    • To enable comparative astrochemical studies of protostars, protoplanets and molecular clouds
    • To enable quantitative astrophysics of gas temperature, density and excitation
  • Receiver bandwidths matched to the width of the atmospheric windows
  • Tunable local oscillator matched to the bandwidth of the receivers
  • Cryogenic capacity > 1 W at 4 K
    • 6. Wide Field Imaging, Mosaicking
    • Imaging galactic disks
    • Imaging the astrophysical context of star formation regions
    • Imaging surveys of large angular regions
    • Searches for dusty and luminous protogalaxies
    • Searches for minor planets in the solar system
    • Solar astrophysics
  • Compact array configuration, filling factor > 0.5
  • Instantaneous uv-coverage that fills more than half the uv-cells, Nant > 60
  • Precision pointing, 6% of HPBW
  • Antenna surface accuracy 20 microns
  • Total power and interferometric capability
  • Precise amplitude calibration, 1%
  • Precise Instrumental Phase Calibration (<10 degrees rms)
  • Correlator dump time 10 msec
  • Capability to handle data rates > 100 Mbyte/sec
    • 7. Submillimeter Receiving System
    • Measurement of the spectral energy distribution of high redshift galaxies
    • Chemical spectroscopy using CI and atomic hydrides
    • Determination of the CII and NII abundance in galaxies as a function of cosmological epoch
  • Array site with median atmospheric transparency < 0.05 at 225 GHz
  • Quantum-limited SIS receivers
  • Antennas with warm spillover < 5 K, aperture blockage <3%
  • Antennas with aperture efficiency > 0.75
  • Precise Instrumental Phase Calibration (<10 degrees rms)
  • Precise atmospheric phase calibration (<15 degrees rms) with compensation using both fast switching and water vapor radiometry
    • 8. Full Polarization Capability
    • Measurement of the magnetic field direction from polarized emission of dust
    • Measurement of the magnetic field strength from molecular Zeeman-effect observations
    • Measurement of the magnetic field structure in solar active regions
  • Measure all Stokes parameters simultaneously
  • Cross correlate to determine Stokes V
  • Calibration of linear gains to <1%
    • 9. System Flexibility
    • To enable VLBI observations
    • To enable pulsar observations
    • For differential astrometry
    • For solar astronomy
  • Ability to phase the array for VLBI
  • Sum port on the correlator for external processing
  • Sub-arraying, 4 subarrays simultaneously
  • Optics designed for solar observations

    • Calibration Requirements

    The precision imaging to be attained by the ALMA will be achieved through accurate calibration. The types of calibration are summarized in Table 3.1.
     Table 3.1 ALMA Calibration Requirements.
    Pointing 0.6" absolute
    Primary Beam 2-3%
    Baseline Determination  0.1 mm
    Flux Calibration 1% absolute flux accuracy goal
    Phase Calibration 0.15 radian at 230 GHz
    Bandpass Calibration 10000:1 to 100000:1
    Polarization Calibration  10000:1
    Single Antenna Calibration Employed

    Antenna Specifications

    The ALMA radiotelescope is currently planned to consist of a goal of 64 antennas, each of 12 m diameter.  In this chapter we outline the general requirements for the antennas and the detailed specifications can be in the contract for the prototype antenna  ( NRAO, 2000 ) and in the Interface Control Documents (ICD, 2000) which are part of the contract. The principal requirements for the antennas are shown in Table 4.1.
     
    Table 4.1 ALMA antenna principal performance requirements.
    Configuration Elevation-over-azimuth mount, Cassegrain focus
    Frequency range 30 GHz to 950 GHz
    Precision performance conditions Nightime: wind 9 m/s    Daytime: wind 6 m/s and sun from any angle
    Reflector surface accuracy  20 microns rms, goal; 25 microns rms, spec
    Pointing accuracy, rms 0.6 arcsec (offset, 2 deg in position and 15 min time), 2.0 arcsec (absolute)
    Fast switching (settle to 3 arcsec pointing) Move 1.5 deg in position in 1.5 seconds
    Phase stability 15 microns rms
    Close packing  1.25 dish diameters (15.0 m) between azimuth axis
    Solar observing Allowed
    Transportability Transportable on a rubber-tired vehicle

    The antennas will be designed and built by one or more commercial companies. Prototype antennas are being built for the US and European ALMA partners by Vertex Antenna Systems LLC (Santa Clara, CA) and European Industrial Engineering (EIE) (Mestre, Italy) respectively.

    Receiver Specifications

    The document Specifications for the ALMA Front End Assembly (latest version) contains the detailed specifications. Portions of this have been approved by the AEC. The main specifications are:

    For details, see the full Specifications for the ALMA Front End Assembly (latest version).

     

    Table .1 – Frequency bands for ALMA

    Band

    from (GHz)

    to (GHz)

    1

    31.3

    45

    2

    67

    90

    3

    89*

    116

    4

    125

    163

    5

    163

    211

    6

    211

    275

    7

    275

    370

    8

    385

    500

    9

    602

    720

    10

    787

    950

    * change to 84 GHz has been proposed

    LO specifications

    The LO subsystem also forms part of the array master clock, in cooperation with a computer of the monitor-control subsystem. It does this by providing an interface to an external time scale (currently GPS) and by measuring the difference between external time and array time. Measures of time larger than 48 msec are obtained in the MC system by integration. Further details are given in the LO chapter.

    Table 1: Specification Summary
    Item Specification Goal (if different)
    Frequency Range, 1st LO 1st LO: 27.3 to 938 GHz (see Table 2)

    2nd LO: 8-10 and 12-14 GHz
    Output Power 1st LO: band dependent (see Table 3)

    2nd LO: +10dBm ea. to 2 converters.

    		 100 µW
    Sideband Noise, 1st LO 10 K/µW 3 K/µW
    Amplitude Stability, 1st LO .03% <1s; 3% between adjustments .01%; 1%
    Phase Noise (>1 Hz) 63 fsec (18.9 µm) 31.4 fsec (9.4 µm)
    Phase Drift (<1 Hz) 29.2 fsec (8.8 µm) 6.9 fsec (2.1 µm)
    Tuning step size, maximum On the sky: 250 MHz

    SIS mixer 1st LO: 500 MHz
    Subarrays with independent tunability TBD (3 or more) 5
    Simultaneous different sky frequencies 1 per subarray
    Time for frequency change,

    maximum

    		 Within .03% (freq switching): 10 msec

    Otherwise: 1.5 sec

    		 1 msec

    1.0 sec

    Repeatability 1. Phase-unambiguous synthesis

    2. Stability specs apply across frequency changes.

    LO Output Power

    The local oscillator must provide adequate mixer drive power for both HFET and SIS based receivers. A conventional balanced mixer used in a millimeter-wave HFET front-end requires approximately 5 mW of LO power. However, 20 mW may be required if a sideband-separating mixer follows the low noise HFET amplifier.



    Chapter 7, Table 3: First LO Power Requirements
    ALMA

    Receiver

    Band

    		 LO Tuning

    Range

    [GHz]

    		 Type of

    Receiver

    Front-End

    		 Number of SIS

    Junctions

    		 Minimum

    Required Mixer

    Power

    		 Required

    Power at

    Input of

    -20 dB

    Coupler of SIS Mixer

    		 Required

    Power at

    LO port of an

    Image -Reject & Balanced

    Mixer

    		 LO Power

    Specification

    of

    50% Over

    Worst-Case

    1

    		 27-33 HFET --- 5 mW --- 10 mW 15 mW
    2 71-94 HFET --- 5 mW --- 10 mW 15 mW
    3a

    3b

    		 101-104

    101-104

    		 HFET

    SIS

    		 --

    4

    		 5 mW

    0.10 µW

    		 ---

    10 µW

    		 10 mW

    0.40 µW

    		 15 mW

    15 µW

    4 137-151 SIS 4 0.15 µW 15 µW 0.60 µW 23 µW
    5 175-199 SIS 4 0.26 µW 26 µW 1.06 µW 39 µW
    6 223-263 SIS 4 0.46 µW 46 µW 1.84 µW 69 µW
    7 287-358 SIS 2 0.21 µW 21 µW 0.84 µW 32 µW
    8 397-488 SIS 2 0.40 µW 40 µW --- 60 µW
    9 614-708 SIS 2 0.42 µW 42 µW --- 63 µW
    10 799-938 SIS 1 0.37µW 36 µW 0.73 µW 54 µW



    In the worst-case scenario where only single-ended, two-port SIS mixers are used, a waveguide or quasi-optical LO coupler, having a coupling factor of -20 dB, will be required to combine the LO and RF signals appropriately. The LO power required at the input of the coupler is also given in Table 3. However, if a balanced mixer can be utilized, the LO power is supplied via a separate LO port on the mixer thus rendering the coupler unnecessary. Column #7 in Table 3 lists the power requirements for a balanced mixer configuration that is both image separating and balanced. The last column is a suggested specification per RF band based upon a 50 percent overhead for the worst-case conditions. The LO power goal will be 100 µW per band to ensure adequate power to overcome losses within the mixer block.


    The Downconverter

    In each ALMA antenna there will be two Downconverter modules, one for each polarization, and the two inputs to each module will carry upper and lower side-band signals. A block diagram of the Downconverter is shown in Figure 9.1.1 and the specifications are in Table 9.1.1 The input and output noise power spectral power distribution will be nominally flat over the passband as given in the specifications. The Downconverter will take the wideband 4 - 12 GHz input signals received from the front end subsystem and produce four output signal channels each with a passband of 2 - 4 GHz suitable for bandpass sampling at by the digitizers, which are clocked at 4 GS/s

     

    Table 9.1.1 Specifications for Downconverter

    DOWNCONVERTER MODULE

    SPECIFICATIONS for ALMA CONSTRUCTION

    Reference: Block Diagram, Document # ALMA06002KX0002

    * indicates interfaces

     

    Number of modules

    142 (2 x 64 antennas plus 14 spares)
       

    *Inputs from front end

    		  

    Number of inputs per module

    Two: USB, LSB (upper and lower sidebands)

    Frequency range, nominal

    4-12 GHz or 4-8 GHz

    Power level within any 2 GHz

    bandwidth when the antenna

    temperature is 290K

    -40 +/-3 dBm, less loss of coax and connectors between front end outputs and module inputs

    (3m of phase stabilized Andrew FSJ1P-50A ¼ inch diameter, attenuation = 2.4 dB @ 12GHz)

    Variation of power spectral density vs.

    frequency (flatness)

    <+/-1.5 dB across the nominal frequency range

    Headroom when the antenna

    temperature is 290K (see definition)

    >20 dB
       

    *Inputs from Second LO (LO2)

    		  

    Number of inputs per module

    Four (A, B, C, D), independently tunable

    Frequency range

    8.0-14.0 GHz nominal;

    frequency LO2 > frequency input

    Power level

    +13 +/-1 dBm

    Power level of spurious frequencies

    <-70 dBc, except <-40 dBc for 2nd harmonic
       

    *Outputs to digitizers

    		  

    Number of outputs per module

    Four (A, B, C, D)

    Frequency range

    2 - 4 GHz nominal

    Power level

    -TBD +/-TBD dBm plus loss of coax and connectors between output and input to digitizer module

    Headroom when the antenna

    temperature is 290K

    >20 dB

    LO2 spurious and leakage at

    outputs

    <(power level -40 dB) for all combinations of frequencies of LO2-A, -B, -C, -D

       

     

     

    Throughput from front end inputs to outputs to digitizers

    		  

    Input S11 reflection magnitude 4 – 12

    GHz

    <-20 dB (VSWR < 1.22) to minimize spectral ripples

    Input noise figure 4 – 12 GHz

    < 10 dB (2 610K); SPDC < -164 dBm/Hz

    << SPFE = -133 dBm/Hz

    Image rejection

    >20 dB

    Filter, 4-12 GHz nominal

    bandpass for total power

    detection

    passband <4.0 GHz and >12.0 GHz at -1 dB,

    max ripple +/-0.5 dB;

    stopband 3.5 GHz and 12.5 GHz at < -20 dB,

    0-3.0 GHz and 13.0-18 GHz at < -40 dB

    Filter, bandpass image reject

    (may be revised after re-analysis of spurious mixer responses)

    4-8 GHz nominal

    passband <4.1 GHz and >8.4 GHz at -1 dB,

    max ripple +/-0.5 dB;

    stopband 4.0 GHz and 8.6 GHz at < -10dB,

    0-3.0 GHz and 10-18 GHz at < -40 dB

    8-12 GHz nominal

    passband <7.6 GHz and >12.0 GHz at -1 dB,

    max ripple +/-0.5 dB;

    stopband 7.4 GHz and 12.4 GHz at < -10 dB,

    0-6.0 GHz and 14-18 GHz at < -40 dB

    Filter, outputs A, B, C, D

    (may be revised after re-analysis of

    mixer and digitizer spurious responses)

    passband <2.1 GHz and >3.9 GHz at -1 dB,

    max ripple +/- 0.5 dB;

    stopband 0-2.0 GHz and 4.0-12 GHz at < -20 dB

    Passband amplitude ripple

    <1.0 dB peak-peak

    Passband deviation from linear phase

    <40 degree peak-peak

    Gain stability

    <0.1 dB peak-peak over 1 minute,

    <0.5 dB peak-peak over 60 minutes

    Phase/delay stability

    <10 degree peak-peak over 1 minute,

    <40 degree peak-peak over 60 minutes

    Headroom1 when the antenna

    temperature is 290K

    >20 dB

    Crosstalk (inverse of isolation) among

    any input and any unconnected output

    >40 dB rejection

    Attenuators in input path 4-12 GHz

    		  

    Steps

    1 +/-0.3 dB

    Range

    >30 dB

    Phase variation vs. attenuation

    <20 degree peak-peak over attenuation range

    0-20 dB

    Deviation from linear phase vs.

    frequency 4-12GHz

    <20 degree peak-peak over attenuation range

    0-20 dB

     

     

    Attenuators in output path 2 - 4 GHz

    		 

    Steps, nominal

    0.25 +/-0.15 dB over attenuation range 0-20 dB

    Range, nominal

    >30 dB

    Phase variation vs. attenuation

    <10 degree peak-peak over attenuation range

    0-20 dB

    Deviation from linear phase vs. input

    frequency 4-12 GHz

    <10 degree peak-peak over attenuation range

    0-20 dB

    Matching among all downconverters

    		  

    Amplitude vs. frequency

    TBD

    Phase vs. frequency

    TBD
       

    Total power detectors (TPD)

    		  

    Input path 4-12 GHz

    		  

    Number

    two, one for each input channel

    Response vs. input frequency at any

    LO2 frequency

    < 2 dB peak-peak.

    Output path 2 - 4 GHz

    		 

    Number

    four, one for each output channel A, B, C, D

    Response vs. input frequency at any

    LO2 frequency

    < 1.5 dB peak-peak.

    Linearity

    <1 % deviation from square law over range -6 dB to +13 dB relative to antenna temperature = 290 K

    Monotonic resolution of digitizer,

    minimum

    16 bits for 13 dB headroom above antenna temperature = 290 K

    *Readout

    2 millisec integrations and dumps to MC-AMBTP card via serial or parallel interface

    *Offset calibration

    MC sets the input power to zero by either setting the preceding attenuator to >(20 dB + headroom) or by removing bias to the preceding amplifier

    Stability of output relative to inputs

    from front end

    <50 ppm in 1 second, <500 ppm in 60 seconds

    *Interface to MC-AMBTP

    dedicated total power data link to antenna bus master (ABM)
       

    *MC control functions

    		  

    Set levels of input total power detectors

    1 byte for each of two attenuators

    Set levels of output of each total power

    detector and input level of each

    output digitizer

    1 byte for each of four attenuators

    Set to zero all inputs to total power

    detectors

    1 byte to remove bias to six amplifiers; or set all attenuators to maximum

    Set all 3 matrix switches (select image

    filters for each output)

    1 byte

     

     

    *MC monitor functions

    		  

    Total power detectors

    3 bytes every 2 milliseconds for each of 6 detectors

    Temperatures

    2 bytes every 10 seconds for each of 8 locations

    Supply voltages derived within module

    2 bytes every 10 seconds for each of 8 voltages
       

    *External power supply inputs

    +18 +/-0.5VDC @ <2.2A,

    -18 +/-0.5VDC @ <0.7A,

    +8 +/-0.3VDC @ <0.6A,

    +5 +/-0.1VDC @ <0.6A
       

    Internal voltage regulators

    		  

    Output voltages @ amperes

    +15 @ 2.2 (total of >1 regulator),

    -15 @ 0.6, +5 @ 0.6, -5 @ 0.1

    Output regulation plus noise

    0.01% peak-peak over time interval > 60 seconds
       

    Timing generator

    		  

    *Inputs from Reference Receiver

    25 MHz sine wave at 0 dBm;

    20.833 Hz positive edge, 5V differential

    Output for timing total power

    integration

    500 Hz TTL pulses of >1 usec width synchronized to 20.833 Hz timing reference

    Output for digitizer clock

    TBD MHz to match digitizer; synchronized to 20.833 Hz timing reference
       

    *Operational environment

    		  

    Altitude

    5000 meter (16,000feet)

    Shock

    Negligible

    Vibration

    TBD

    Temperature of air flow past sides of

    module

    Plenum temperature set 16 – 22 Celsius, variation < 2 C peak-peak

    Air mass flow rate past sides of module

    TBD

    Specific heat of air flow

    TBD
       

    Packaging

    		  

    Module

    3 to 6 width x 5U high x TBD depth standard module (ATNF) with extruded vertical heat fins on one side or both sides

    *Multi-pin connector (power,

    MC-AMB, MC-TP)

    One double density 100 pin D type [male]

    *Coaxial connectors

    12 OSP (M/A-COM) blind mating [male]

     

    1. Define headroom as the dB ratio of available power at 1% gain compression {P(-1%)} to the total system noise power {P}. Typically, P(-1%) is 16 dB less than the available power at –1 dB gain compression and 26 dB less than the available power at third order intercept. -end-

    The Digitizers

    9.2.1 Introduction, Top Level Specifications

    The analog-to-digital converters, or digitizers, installed in the antennas provide the flexibility required for the fiber optic transmission of the IF. Signal digital conversion is of course indispensable to the correlator in order to derive the correlation function as a function of digital lags for spectroscopy. The digitizers are thus crucial and single-point-failure elements in the system. The ALMA system incorporates 3-bit digitizers thus improving the overall sensitivity compared to the classical 2-bit case.

    The goal specifications are given in Table 9.2.1

    Input BW 2-4 GHz

    Sample clock 4 GHz (250 ps)

    Bit resolution 3 bits

    Quantization levels 8

    Aperture time ~ 50 ps

    Jitter a few ps

    Threshold indecision region a few mV

    Output demultiplexing factor 1/32 (125 MHz system clock)

    PLL Clock distribution 4 GHz, 125 MHz (system)

    Fine delay command

    Low power consumption


    ALMA Correlator

    This section describes the ALMA correlator. The design described here is for a lag correlator with a system clock rate of 125 MHz. The goals of Phase 1 are to produce paper designs and some simulations of all major correlator elements, including the correlator chip, and to fabricate and test prototype hardware.  The goals of Phase 2 are to produce a prototype minimally populated correlator, deliver such a prototype for use in the test interferometer, and deliver the complete correlator to the ALMA site.

    Table 10.1 ALMA Correlator Specifications


    Item
    Specification
    Number of antennas 64
    Number of baseband inputs per antenna 8
    Maximum sampling rate per baseband input 4 GHz
    Digitizing format 3 bit, 8 level or
    4 bit, 16 level
    Correlation format 2 bit, 4 level
    Maximum baseline delay range 30 km
    Hardware cross-correlators per baseline 1024 lags + 1024 leads
    Autocorrelators per antenna 1024
    Product pairs possible for polarization HH, VV, HV, VH (for orthogonal H and V)
    Table 10.3 Selected correlator modes


    # of Digitizers
    Bandwidth/ 

    Digitizer
    Cross-pol 

    Products?
    Channels/ 

    Product
    At 230 GHz, in velocity space: 

    Range         Resolution 
                     km/s
    8
    2 GHz
    Yes
    64
    9391
    40.8
    8
    2 GHz
    No
    128
    18783
    20.4
    8
    1 GHz
    No
    256
    9391
    5.1
    8
    500 MHz
    Yes
    256
    2348
    2.5
    8
    250 MHz
    No
    1024
    2348
    0.32
    4
    2 GHz
    Yes
    128
    4696
    20.4
    4
    1 GHz
    No
    512
    4696
    2.5
    4
    500 MHz
    Yes
    512
    1174
    1.3
    4
    250 MHz
    No
    2048
    1174
    0.16
    2
    2 GHz
    Yes
    256
    2348
    10.2
    2
    1 GHz
    No
    1024
    2348
    1.3
    2
    500 MHz
    Yes
    1024
    587
    0.64
    2
    250 MHz
    No
    4096
    587
    0.08

    ALMA Computing, principal requirements


    Sustained data rate, science data

    6 MB/s (Average)
    60 MB/s (Peak sustained)

    Image pipeline

    First-look images produced automatically for standard observing.

    Dynamic scheduling

    Nearly automatic scheduling of the array, accounting for current weather and other conditions, to optimize the scientific throughput of the array.

    Archiving

    Networked archive of all ALMA raw science data and associated calibration data and derived data products.

    Antenna configuration on the Chajnantor site

    Table 15.1 Guidelines for Configuration Design

    Main D&D Task

    Design a set of configurations which allow for a range of angular resolution and sensitivity

    Flexible design philosophy

    Configurations must allow for graceful expansion through possible collaboration

    Costing

    Optimize for shared stations to minimize cost

    Site placement

    Choose specific locations for antenna placement on Chajnantor site

    The table below outlines different designs up to 3 km maximum baseline. A larger, 12 km baseline will now be included as well; the beam size at 345 GHz will be approximately 0.013 arc seconds. The choice between a donut or a spiral configuration is being discussed.

    Table 15.3 Specifications for the ALMA strawperson configurations.

    Array

    Minimum

    Maximum

    Array

    Time for

    Natural

     

    Baseline

    Baseline

    Style

    FOC = 0.5

    Beam at 345 GHz

     

    [m]

    [m]

     

    [hours]

    [arcs]

    A

    30

    3000

    donut

    10

    0.050

    B

    24

    1430

    donut

    2

    0.101

    C

    18

    680

    donut

    0.1

    0.22

    D

    16

    325

    donut

    0.1

    0.47

    E

    16

    150

    filled

    0

    0.97

    Table 15.4 Specifications for the compact configuration N-S elongations.

    Array

    Min. N-S

    Elev. of first

    Min. observing

    Max. observing

    N-S

     

    Distance

    Shadowing

    Elevation

    Elevation

    Elongation

    E1

    1.3 D

    50 deg

    40-45

    90

    1.2

    E2

    1.9 D

    31 deg

    30

    50+

    1.6

    E3

    3.0 D

    19 deg

    14

    33+

    2.9