ALMA Holography CDR, report.


(Revision iv, 2000-10-19)

NRAO Tucson, Tuesday October 10 2000

Panel Report from the Critical Design Review


The formal review panel for this CDR consisted of:

There were many other participants, from within and outside of the ALMA project from NRAO, and also representatives from our future Japanese collaborators. For part of the meeting, there was a teleconference connection to Socorro, and to Europe.

Written material was prepared in advance of the meeting and made available on the WWW; this material can be found at:

The agenda for the meeting is given below.


NRAO Tucson, Tuesday October 10 2000


08:30 Coffee etc.

  1. 09:00 Welcome
    Darrel Emerson
  2. 09:15 General Overview
    Larry D'Addario
  3. 10:00 Transmitter Design
    Bill Shillue
10:30 Coffee Break

  1. 11:00 Receiver Design
    Antonio Perfetto, Larry D'Addario
11:45 Break for Lunch

  1. 13:00 Infrastructure (tower, cabling)
    Peter Napier
  2. 13:30 Real-time Software Design
    Mick Brooks
  3. 14:15 Data Analysis Design
    Robert Lucas

15:00 Coffee

  1. 15:30 Schedule
    Jeff Mangum
  2. 15:45 Options and extensions
  3. 16:15 Panel meets

17:00 Conclusion of Meeting

Since the material presented is covered in detail in the written material (see the above link), no attempt is made to summarize the presentations. However, comments from the panel and others present are included below.


Darrel Emerson reminded the group of the requirements and purpose of a CDR. Quoting from the ALMA guidelines (see

Critical Design Review (CDR)

To be held before expenditure of significant funds on the construction of field equipment which will be incorporated into the test interferometer.

The purpose of the CDR of an ALMA (MMA) Subsystem is principally to review 4 questions:

  1. Are the detailed requirements for the subsystem complete and adequate?
  2. Will the design selected for implementation on the test array meet the requirements?
  3. Are interfaces to other subsystems defined adequately and completely?
  4. Has adequate attention been given to the produceability and maintainability of the subsystem?

    General Overview

    Peter Napier pointed out that the spatial resolution requirement may be driven more by the need to allow for the high phase gradient left from the near field correction, rather than just the ability to resolve individual antenna panels.

    Robert Lucas stated that, if the antenna is not refocussed to allow for the near-field siting of the transmitter, then the needed scan range for holographic maps is the sum of the scanning distance that would be calculated for a transmitter at infinity, and the near-field spreading of the field response from the antenna. The latter approximates a 12-meter diameter tube. At the distance of our near-field transmitter, and if 0.1m resolution across the surface is required, the necessary map scanning distance is approximately doubled. However, this is less of an issue in practice since the intention is to put the holography receiver main dish feed to the near field focus position for the distance to the transmitter tower.
    This point was covered in more detail during the Data Analysis presentation later in the meeting.

    Bill Peters pointed out that it may not be correct to consider effects due to multi-path propagation just a sum of random, uncorrelated vectors. In practice the reflections may not be random, but rather may produce (for example) rapid fringes across the surface.

    Ukita (Nobeyama, Japan) thought that the level of cross-talk between signal and reference channels may be more important than we had considered; this had been found to be the case at Nobeyama with holography on the 45m telescope. He had specified 80 dB as the required isolation, rather than the 50-60 dB we contemplated. More investigation of the effects of cross-talk and the precision of antenna measurement is required.

    Transmitter Design

    Peter Napier noted that the current planning (i.e. the use of one of the Evaluation Receiver photonic phase reference assemblies as the photonic transmitter) did not permit interferometric holography using the terrestrial transmitter. However, Bill Shillue and Larry D'Addario responded that an easy workaround for this was possible - for example by splitting the fiber photonic reference to use one photonic synthesizer as a reference simultaneously for both antennas. Although not included in the design presented today, this would be a simple and cheap modification. There are other options as well.

    Peter Napier said that it may be useful to rotate the transmitter box about the axis of radiation, so that the signal polarization of transmitter can be varied.

    Dick Sramek asked about how the transmitter antenna would be rotated to illuminate different ALMA antennas. Bill Shillue responded is that the current design requires this adjustment to be made manually, at the top of the tower.

    Richard Simon asked if truly simultaneous single dish holography using both antennas would be possible. Given that only one holography receiver is being built, this is not possible, but nor is it a requirement. The decision was made some time ago to limit the holography implementation to a single receiver.

    Receiver Design (front end)

    Peter Napier asked if cross-polarization holographic measurements might be required: for example, to investigate the effects of the grooving of the panels for solar scattering in the Vertex antenna. The response is that the holography receiver is NOT designed to have different polarizations; the concensus at the meeting was that this is acceptable.

    Larry D'Addario remarked that it might be possible to use the holography transmitter, with little or no modification, to produce sufficient power at higher frequencies - say in the 211-275 GHz band - for the astronomical receivers to measure in interferometric mode. The astronomical receivers are much more sensitive, and the total collecting area is much greater than for the holography system with its small reference horn.

    Ukita suggested that all exposed surfaces at the receiver should be covered with absorber to cut down on reflections. We should be sure that this is permitted by our receiver design.

    Receiver (digital section)

    Brian Glendenning asked why FPGA had not been chosen for the real time digital processing, rather than a DSP chip. Larry D'Addario responded that the FPGA might be harder to implement, and the DSP approach had the advantage of being easy to modify and reprogram. (The interesting circumstance of Larry apparently defending a software approach, with Brian suggesting a seemingly more hardware oriented approach, was noted.)

    Infrastructure (tower, cabling etc.)

    The question was discussed of whether a 50m or 60m tower is required. No-one had any compelling arguments in favor of a 60m tower, so the decision is to keep to the plan of a 50m power.

    Real Time Software

    Ukita asked how well encoder reading is synchronized to data. The response is that both the encoder readout and the data integration periods are latched to 48 ms timing pulses, so that encoder readings and data are in synchronism to within 10 microseconds or better.

    Analysis Software

    Peter Napier suggested that we need a full-scale simulation of the near-field focus correction and its effects on phase error. Rather than performing a fast Fourier Transform and then applying corrections, might we be better off with a direct Fresnel transform? The power of modern PCs might make this feasible today.

    Test Schedule This needs to be integrated in with the schedule plan for the so-called zero-baseline interferometer electronics test (the Laboratory Interferometer or LI).

    The LO system installation milestone needs to be added to the TI plan.

    Panel Discussion

    A number of relative minor issues were discussed, and a list of action items was generated. In no particular order:

    1. Dish feed illumination taper. NRAO had planned on about -3 dB taper at the edge of the dish. Ukita pointed out that this gave higher susceptibility to reflections from behind the dish, in particular from the antenna mount. Ukita suggests that -8 dB is the best compromise edge taper to use.

    2. More investigation is needed of the cross-talk requirement between the receiver channels. This will be undertaken.

    3. The limitations and capabilities of using the photonic holography transmitter with astronomical receivers and in interferometric mode using both antennas need to be clarified. A minor change in the design might allow considerable more flexibility. This will be investigated.

    4. The danger of RFI being generated by all parts of the holography electronics, including front end, back end and computers, needs to be taken seriously. The required limits must be specified.

    5. The weight of the holography receiver: according to the antenna specifications for weight at the apex of the antennas, dummy mass may be required in the holography receiver to bring its total weight up to 110 kg. (Cheng is performing some calculations to see if the exact location of the centre of gravity of the receiver system is critical.)

    6. Fresnel calculations need to be done; this is in the plan of Lucas. A full simulation using Fresnel calculations should be done.

    7. Allowance for absorber around the receiver flange area must be made. Some thought should be given to problems of reflections around the transmitter area, and also from the ALMA antenna mount.

    8. The OTF observing mode should ideally support the relevant antenna coordinate system, rather than just Azimuth-Elevation. This is certainly not essential, but if the implementation is fairly easy then it should be provided.

    9. There is a desire to have the tower and transmitter installed well in advance of the delivery of the antennas, to allow plenty of time for checking out. The available budget may be a constraint. Peter Napier will check on deliver and installation timescales.

    10. The Laboratory Interferometer (LI) schedule, and integration with holography requirements, needs to be carefully defined.

    11. We should retain the potential for going to higher frequencies with a photonics transmitter, for possible future ALMA applications. This is not part of the holography system or requirements, but it may be easy to use the holography transmitter for this. Such a system would not need to be phase locked.

    12. Some more calculations or simulations are required to investigate further the effects of multi-path propagation.

    13. Close coordination with IRAM is required so that adequate time is allowed for the measurements on the holography feeds. This is not expected to be a problem, but needs to be watched.

    14. The panel layout and exact placement of the adjustors need to be documented for both the Vertex and EIE antennas in an ICD. This information is required to transfer the holography measurement result to the necessary panel adjustments.

    It was considered that none of the above action items were "show stoppers," and all could easily be resolved in a timely manner.

    The panel considered each question to be reviewed in an ALMA CDR (see the list at the head of this document.) It was felt that all questions had been answered satisfactorily.

    Additional points from correspondence outside the CDR meeting.