MNRF Upgrade: New ATCA stations and the North Spur

Project Personnel

Progress

Science Introduction

Detailed account of the project

Related Information

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This project provides additional stations (antenna placement points) for the Australia Telescope Compact Array. The extra stations comprise four built on the existing east-west rail track, and five on a new, short, north spur track which has now been constructed.

Project Personnel


Project Leader

  • Dr Bruce Thomas

Project Scientist

  • Dr James Caswell

Management and Advisers

  • Drs Robert Sault and Lister Staveley-Smith provided valuable input to the configuration studies, and John Brooks is responsible for the financial and day-to-day management of the project.


Progress

The new station placements were optimised to produce a versatile and extendable design. The north spur is placed near the centre of the 3 km east west track, and has 5 spur stations, with the most northerly station 214m north of the east west track.

The novel method of transferring the antennas to the spur was pioneered several decades ago in the Parkes 18m antenna. The 4 transporter bogies on each antenna are rotated individually, requiring only 4 small turntables, rather than one enormous (high cost) turntable. Final tender documents were complete by the end of 1997 and a bid for civil works was accepted 1998 March. The project involves major industrial and ATNF participation.

Construction began mid-1998, with substantial completion by 1998 Nov 26 (this marked the official opening of the North spur track, and celebrated the original Telescope's 10th anniversary). Much of the construction was carried out under very difficult weather conditions, with frequent delays due to an exceptional flood-prone winter. The construction will soon be complemented by the final electrical work, including the laying of new cables in trenches.

The Finished Product

Digital imaging (thanks to Stewart Duff) provides us with a picture of a configuration using the finished north spur.


Science Introduction

The main scientific drivers for the new stations are:
  • 1. The need to provide additional VERY compact array configurations. These are especially important for mm-wave observations, and will also enhance operations at the cm-wavelengths currently used.
  • 2. The need to utilise several short observing periods of only a few hours each, without impairing the uv-coverage. This is especially important at mm-waves where good weather requirements are more stringent and will often limit observations to less than the 12 hours that are currently preferred.
  • 3. The need to obtain good uv-coverage despite restriction to high antenna elevations (so as to minimise phase distortion effects and attenuation by atmospheric water vapour, which are acute at mm-wavelengths).
The new east-west stations address the first point, especially for objects in the far south, such as the Magellanic Clouds and the southern Milky Way. The north-south track will address the second and third points, and will achieve not only better observing efficiency, but also the ability to make good images at more northerly declinations such as the Galactic Centre, and even further north.


Detailed Account of the Project


Contents of this section:



1.Introduction

(Return to Contents: Detailed Account of the Project)

Four new stations are being constructed on the existing EW track and five stations on a new, short, NS track. Provision of four new EW stations (costing $0.5M) is mostly funded by the MNRF proposal. The NS track and its five stations is a CSIRO-funded capital works project, costing about $1.3M. The benefit will be much better configurations for array sizes smaller than 400m, and these will be especially useful at mm wavelengths. Engineering details are described in Section 2 .
Visualisation of the new capabilities of the ATCA is available on the ATNF website using the VRI tool as described in Section 3 below. The astronomical capabilities and the ways in which the new development will be used are described more fully in Section 4, together with the expected benefits. Earlier scientific considerations were given in a previous document (Caswell & Staveley-Smith AT31.06.7/012), and a brief description of the upgrade was given in the ATNF News of 1997 April (number 52, p11). Section 5 describes the rationale and constraints that were taken into account in the final design. These include consideration of the existing instrument, the desired improvements, some engineering constraints, the budgetary constraints, and the desire for flexibility with respect to further development.
Finally, an appendix gives a detailed summary of the project, and some additional background.

2. Engineering details of the layout and construction.

(Return to Contents: Detailed Account of the Project)

a) The EW track, existing stations, and new stations.

The track length is 3000m, and five antennas can be driven along the track so that they can be precisely located at any of 35 stations for astronomical observations. The station locations are at multiples of a basic increment of 15.306m (more precisely, 6000/392). Each station is referred to by its distance (west) from station 0 (at the east end of the track), in units of 15.306m.

In the vicinity of the new construction, original stations are at: W64(#12) W84(#13) W98(#14) W100(#15) W102(#16) W109(#17) W110(#18) W111(#19) W112(#20) W113(#21) W128(#22) W129(#23) and W140(#24). (For convenience, the original numbering of the stations is added in parentheses. W98=#14 is the middle of the track.) We will construct new stations at W104, W106, W124, and W125.

b) The North spur.

We have built a new short (230m) track extending north from the EW track at EW position W106 (see Section 4). Five stations are being built on the NS track, at positions north of the EW track by 2, 5, 7, 11, and 14 units of the standard 15.306m increment. Several methods of transferring antennas from the EW track to the NS track were considered. The scheme that was ultimately chosen involves a junction where each of the four bogies can be individually rotated through 90 to be repositioned onto the (intersecting) NS track. Modifications to each antenna are chiefly confined to the cabling to each bogie and are quite modest in cost.

3. Visualisation of the array capabilities using the VRI tool.

(Return to Contents: Detailed Account of the Project)

The array capabilities can be explored graphically using VRI, the 'Virtual Radio Interferometer' .Although full details of the new astronomical capabilities are given in Section 4, it is useful to see some examples first. The following examples with VRI are suggested as an introduction both to VRI and the capabilities of the upgraded ATCA.

Demonstration:

Select 'New ATCA' as the observatory. Restrict to 5 antennas (the 6km antenna will not then be considered; it is unlikely to be included in arrays for mm wave observations). Restrict to elevation limit 30degrees rather than 12 so as to limit observations to high elevations suitable for mm observations.
Example 1: Add arrays of EWvb367 for HA +/- 4.7h; EWva352 for HA +/-2h; NSub214 for HA +/- 4h. And view the uv coverage for a source at dec -30degrees (e.g. the Galactic centre).

Example 2: Add arrays H75, H168 and H214, all with HA coverage +/- 2h. And view uv coverage for a source at declination +22degrees. We will indeed be able to make observations at these northerly declinations with our new compact configurations!

4. New astronomical capabilities resulting from the new stations.

(Return to Contents: Detailed Account of the Project)

Operation at mm wavelengths will dramatically alter the capabilities of the ATCA, creating both new opportunities and new challenges. For example the field of view will be very small, with primary beamwidth to half power near 90 GHz of only 35 arcsec (the precise value will depend on the aperture illumination, and earlier documents cite larger values corresponding to illumination of only 15m diameter rather than 22m). Since many objects will be larger than this, we anticipate extensive use of a sophisticated mosaicing technique which has already been developed and used for lower frequencies (Sault Staveley-Smith & Brouw 1996 A&AS 120 375-384).

The synthesised beam at 90 GHz will be approximately 4 arcsec to half power when the maximum baseline is about 200m and thus high resolution will be achieved at quite short baselines. The original station layout did not provide good array configurations at such short baselines. For example, the 375m array yields only even multiples of the 15.306m increments, which leads to prominent 'grating lobe' artifacts. Such limitations are overcome with the addition of the new stations. We now describe some of the configurations that will become achievable.

Using just the EW array, it will now be possible to configure the five antennas as an ultracompact array EWua214 using stations W98 W102 W104 W109 and W112 to give the spacing set 4 2 5 3, and yielding baselines: 2,3,4,5,6,7,8,10,11,14 (see AT31.06.7/012). This configuration will have excellent brightness sensitivity and be very efficient at mosaicing large areas of sky. No satisfactory configuration of this type has previously been possible.

Two complementary very compact arrays extending nearly 375m, referred to as EWva352 and EWvb367 will also be possible. Each will be superior to the currently used 375m array; and together, the two new arrays give uv-coverage that has negligible gaps and very low sidelobes. In detail, the stations used, spacing set, and baselines are: EWva352 W102 W104 W109 W112 W125; spacing set 2 5 3 13. Baselines 2 3 5 7 8 10 13 16 21 23. EWvb367 W104 W110 W113 W124 W128; spacing set 6 3 11 4. Baselines 3 4 6 9 11 14 15 18 20 24.

At southerly declinations where there is no concern about shadowing, one would use EWva352, as a first choice, and complement with EWvb367 if needed. At declinations north of about -30degrees where shadowing is a problem, one could use EWvb367 as a first choice, and complement with a shorter run on EWva352. In general, at low frequencies (below 25 GHz) the best uv-coverage is obtained most efficiently by 12h runs on these EW configurations.

The North spur stations and hybrid arrays. At very high frequencies, approaching 100GHz, a major problem is that poor weather affects these frequencies much more than at lower frequencies. Consequently it is preferable not to observe at low elevations (typically, one would prefer to observe only at elevations greater than 30 degrees). Good uv-coverage can then be obtained either by adding together individual shorter observations of both east-west and north-south configurations, or making the observations entirely in hybrid arrays. The station positions have been chosen to provide both options, and the preferred choice will be made in the light of experience when mm-wave operations have begun.

Hybrid arrays (configurations with some antennas on the EW track and others on the NS track) can yield quite good uv-coverage for an extremely compact 75m array in as little as 4 hours observation. Such a compact array of course yields quite low angular resolution. If higher resolution corresponding to baselines of 168m is desired, a hybrid array of this size can be used to complement the 75m hybrid array. Highest resolution will be achieved with the 214m hybrid array; most often it will be used to complement the 75 and 168m arrays, and thus for observations at this resolution we will require a 4 hour observation at each of the 3 configurations. In detail, the H75 array uses stations W104 W106 W109 N2 and N5 to give baselines of approximately 2 3 and 5 units. The H168 array uses stations W100 W104 W111 N7 and N11 to give baselines of approximately 4 7 and 11 units. The H214 array uses stations W98 W104 W113 N5 and N14 to give baselines of approximately 6 9 and 15 units.

The precise uv-coverage of hybrid arrays is not easily envisaged since the coverage depends on both the declination to be observed and the HA range deemed to be adequate. As described in the section 3, the VRI tool is especially useful for exploring the results of combining the H75, H168 and H214 hybrid arrays.

Hybrid arrays are slightly less efficient than EW arrays in some respects. However they do provide other benefits, and allow useful observations at declinations more northerly than is practical with the EW array. Furthermore, the 75m hybrid array is also a powerful configuration for high sensitivity low resolution studies, offering important new capabilities even at low frequencies.

NS arrays. An alternative way of obtaining good uv-coverage while restricting observations to elevations greater than 30 degrees is to complement EW configurations, of typically 8h (symmetrical about transit), with similar length observations from a purely NS array. This is a scheme pioneered with the Fleurs Synthesis Telescope. Configuration NSub214 will use stations W106 with N2 N7 N11 and N14 (spacing set 2 5 4 3) to give a good NS ultracompact array, with baselines 2 3 4 5 7 7 9 11 12 14.

NSub214 will be a useful complement to EWua214. In some cases it will be an excellent complement to EWva352 and EWvb367 if fields are observed towards northerly declinations (e.g. north of declination -30). Indeed, the set of these three configurations may be the preferred choice for studying the Galactic Centre in greatest detail.

A smaller NS ultracompact array, NSua168, can also use four NS stations and the EW station at the intersection. The spacing set and baselines are: 2 3 2 4 and 2 2 3 4 5 5 6 7 9 11. It is generally less useful than the longer 214m array and was first devised when it appeared that funds might limit the north spur length to 168m. It may have some value in test observations since it provides duplicate spacings of 2 units and 5 units which will provide a good assessment of phase instabilities.

5. Rationale for the details of the design.


(Return to Contents: Detailed Account of the Project)

The general location of the new EW stations. Good arrays for the very compact and ultra compact configurations require a total of about 10 stations within a 450m region. We choose a region of the EW track where most of the stations already exist so as to limit the number of necessary new stations to three or four. As discussed in AT31.06.7/012, the region selected is one of the few that satisfy this criterion. Since we also desire that it be close to the control room rather than one extremity of the track, and preferably quite near the centre of the track to allow for possible future developments, the chosen region uniquely satisfies these requirements. Detailed location of the new stations is discussed in AT31.06.7/012.

The length of the north spur. The track should extend as far from the EW track as possible and is limited to about 200m by budgetary limitations. (These constraints together dictate that it should be essentially perpendicular to the EW track.) An additional 150m with an extra 3 stations is estimated to cost a further $1M. This is not possible at present, but the current scheme can readily be extended if funds become available.

The location of the NS junction will be at W106. It was chosen so as to be at a position where the density of EW stations is quite high, in order that satisfactory hybrid arrays should be possible. It needs to be fairly near the centre of the rail track in case there were a future possibilty of extending to a length as great as 1500m. It should also be near the control building, which minimises the cable length for LO cabling, and facilitates maintaining phase stability. The general region chosen is the only one satisfying these criteria.

The final choice of the precise position was dominated by the need for good hybrid arrays. It was not initially clear whether there could be a station precisely at the junction, and an offset by half a station to 106.5W was originally chosen (any small offset would have only small consequences for the hybrid arrays). The choice was finally determined from engineering considerations. These depended on the scheme adopted for transferring antennas from EW to NS track, and the NS junction was finally made coincident with new station W106.

Antenna locations on the north spur. Firstly, note that shadowing of the NS antennas does not even begin to occur (at the smallest 30.6m spacing) unless pointing to a source south of declination -75 (in which case an EW array alone is more effective). The stations have been chosen to give a stand-alone configuration which will complement EW arrays when hour angle coverage is less than 12 hours. (Limiting the HA coverage to less than 12 h, in both EW and NS configurations, is necessary if observations at low elevations are to be avoided.)

The antenna locations will be at the four NS stations N2, N5, N7, N11, and at W106 (the Nzero position at the junction). For a NS stand-alone configuration of 168m, there is some flexibility in the optimum station positions. The choice was then partially dictated by the need to allow future extension to a 214m length with one extra station (as actually built, despite initial uncertainties as to whether funding would permit it) and to 368m length with no more than four extra stations. The requirement that these same stations be a set able to provide good hybrid configurations (with three antennas on the EW track and two on the NS track) was also met after slight adjustments.

The proposed ultra compact NS configuration NSub214 will use stations W106 with N2 N7 N11 and N14 (spacing set 2 5 4 3) to give a good NS ultracompact array, with baselines 2 3 4 5 7 7 9 11 12 14. A shorter ultra compact NS array NSua168 uses stations W106, N2, N5, N7 and N11 to give spacing set 2 3 2 4 and thus baselines: 2 2 3 4 5 5 6 7 9 11. It has a repeat of baselines 2 and 5. These may prove advantageous in the early work at mm wavelengths since redundancy could be useful if phase stability is poor. It was originally devised as a compromise option if budget constraints had limited the NS track to 168m length.

This section has shown some of the flexibility available from the planned extension. Such flexibility is an essential requirement since some experience with mm-wave observing conditions at Narrabri will be needed before selecting the most profitable and widely requested configurations.

6. Appendix.


(Return to Contents: Detailed Account of the Project)

Summary of details and additional background.
A1.

Summary of new station locations and list of new array configurations, giving short name, stations, spacing set, and baselines.

New EW stations at W104(#38), W106(#39, at NS junction), W124(#40) and W125(#41) New NS stations, all offset North from W106 to N2(#42), N5(#43), N7(#44), N11(#45), N14(#46), N19(#47), N23(#48), N24(#49). (The last 3 will not be built in the first phase.)
Array configuration nomenclature. Each currently used configuration is designated by its maximum length (approximately, in metres if less than 1000, or km otherwise) followed by a letter to distinguish different members of a set. All of them are EW, and thus a distinguishing prefix to denote this was not formerly needed. However, the new arrays can be EW, NS, or Hybrid, and are thus most easily distinguished by prefixing EW, NS or H to their names. Previous documents have referred to ultracompact (uc) configurations (roughly 200m), and very compact (vc) configurations (roughly 400m). The recommended names, as used in the present text, are summarised here (followed in parentheses by the names used in preliminary documents):
EWua214 = (EWuc) W98 W102 W104 W109 W112; spacing set 4 2 5 3;
baselines 2 3 4 5 6 7 8 10 11 14

EWva352 = (EWvc1) W102 W104 W109 W112 W125; spacing set 2 5 3 13;
baselines 2 3 5 7 8 10 13 16 21 23

EWvb367 = (EWvc2) W104 W110 W113 W124 W128; spacing set 6 3 11 4;
baselines 3 4 6 9 11 14 15 18 20 24

NSua168 = (NSuc1) W106 N2 N5 N7 N11; spacing set 2 3 2 4;
baselines 2 2 3 4 5 5 6 7 9 11

NSub214 = (NSuc2) N2 N7 N11 N14; spacing set 2 5 4 3;
baselines 2 3 4 5 7 7 9 11 12 14

H75 W104 W106 W109 N2 N5; baselines at zenith:
EW 2 3 5; NS 2 3 5; plus four others.

H168 W100 W104 W111 N7 N11; baselines at zenith:
EW 4 7 11; NS 4 ~7 ~11; plus four others.

H214 W98 W104 W113 N5 N14; baselines at zenith:
EW 6 9 15; NS ~5 9 ~14; plus four others.

Possible only if there were a future NS extension to about 380m:

NSva352 = (NSvc1) W106 N5 N7 N19 N23; spacing set 5 2 12 4;
baselines 2 4 5 7 12 14 16 18 19 23

NSvb337 = (NSvc2) N2 N5 N11 N19 N24; spacing set 3 6 8 5;
baselines 3 5 6 8 9 13 14 17 19 22

H368 W100 W109 W124 N14 N23; baselines at zenith:
EW 9 15 24; NS 9 ~14 ~23; plus four others

A2.

The present proposal requires 4 new EW stations. The original MNRF proposal was for 3.5 new stations, which was to allow a small variation in the shortest available spacing to cope with possible shadowing problems (or to allow even shorter baselines since it was not clear at that time whether the surface upgrade of the antennas would proceed). The addition of a North spur removes the shadowing problem. However the need for good hybrid arrays requires that there be 4 new EW stations. Notionally, the MNRF budget provides the cost of the 3 stations, together with the extra cost of adding a station at the junction of the North spur with the EW track. The junction itself is part of the North spur costing.

A3.


Other existing and planned arrays show a wide variety of layouts. The different solutions adopted or explored for the arrays such as BIMA, Nobeyama, IRAM, MMA and SMA were all considered when selecting a solution appropriate to the ATCA. Some of the important differences are the number of antennas, the antenna size (large antennas require fewer stations but are costly to move), and, in the case of the ATCA, making best use of the existing layout, where many stations are already present along an EW track.

A4.


We clearly do not expect to regularly schedule all the new configurations, but it will require some experience to know which will be most valuable. There is some debate as to how much we will use the sum of individual NS and EW arrays and whether to prefer hybrids. Both options are well catered for by the choice of new station positions, and so the decisions can be made in the light of practical experience. It will be useful for potential users to see the range of options and explore them in VRI since it is so helpful for envisaging the effects of NS arrays at different declinations, adding them to EW arrays, and comparing with instantaneous hybrid arrays.

The following is a likely scenario for using the new arrays:

The current temporary 210 configuration will be replaced by EWua214. The existing 375 configuration will be replaced by EWva352 and EWvb367. The existing 122 configuration may well be replaced by H75 for most purposes and by EWua214 for others.

H168 and H214 will often be needed to complement H75 and provide higher resolution.

NSub214 and NSua168 will not both be used. NSub214 is preferred, and the latter will probably only be used to provide phase stability assessment.

So, in the first stage there will probably be 7 new configurations at most, but 3 of the existing ones would be dropped. Only 4 configurations will use the NS stations and will probably be unique to the 'mm-observing season'.

NSva352, NSvb337, and H368 will not be possible unless/until another $1M is available (perhaps 7 or 8 years time?) but it is important to show that an orderly expansion in a second stage would be fully integrated with the first stage plan.

A5.


There are some arguments in favour of using the EW stations as soon as possible, and if this needs temporary cable links, it is important to specify which stations will be required simultaneously. The relevant EW arrays are the ultracompact one EWua214, and the two very compact ones, EWva352 and EWvb367.
As a reminder:
EWua214 uses W98 W102 W104 W109 W112 ie includes new station W104 only and does not use nearby existing station W100.

EWva352 uses W102 W104 W109 W112 W125 ie uses new stations W104 and W125 and does not use nearby existing stations W100 W128 W129 or W124new.

EWvb367 uses W104 W110 W113 W124 W128 ie uses new stations W104 and W124 and does not use nearby existing stations W100 W102 W129 or W125new.

A6.


There is a possibility that an economical means will be devised to allow the sixth antenna of the ATCA, which is currently confined to the 6km site, to join the other 5 on the 3km track for part of each year. Configurations of 3km and smaller would benefit by the simultaneous measurement of 15 baselines rather than 10. The choice of stations now available has been assessed to ensure that they would allow efficient usage of the extra antenna, and reap significant benefits in observing speed and efficiency.




Related Information

Other MNRF Projects

Background to the MNRF Project

MNRF Management Structure


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Last update by Michelle Storey. 18/3/99

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