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The eSMA
The eSMA
"Submm Interferometry with the combined SMA-JCMT-CSO array"
Introduction
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The eSMA is a collaboration between the SMA, the JCMT, and the CSO to
join into a single sub-mm interferometer array with approximately
twice the collecting area of the SMA resulting in an increased
sensitivity. The eSMA will operate part-time and in the 345 GHz window
taking full advantage of Mauna Kea's excellent observing conditions
and prolonged periods of superb weather at that frequency.
The first initiatives for the eSMA started in 1996, well before the
completion of the SMA itself, and the special fibre-optics connections
were installed several years ago. Following the commissioning and the
start of operations of the SMA, the practical implementation of the
enhanced interferometer has now begun. In May of 2005 the JCMT took
delivery of the interface equipment, built by the Smithsonian
Astrophysical Observatory, consisting of a SMA-style antenna computer
and LO and IF signal processing units. These enable the JCMT receivers
to tune to the modulated LO signal originating from the SMA and to
ship the resulting IF signal back to the SMA correlator. Eventually a
dedicated interface will be installed between the SMA antenna computer
and the JCMT telescope operating system that will enable the SMA to
directly take control of the JCMT during eSMA operations. Similarly,
coaxial relays are being build that will allow the JCMT to switch
between single-dish operation and eSMA observing without having to
swap cables. The photo on the left shows the LO/IF interface equipment
during testing at the SAO in Boston.
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Submm Valley Mauna Kea
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Lab tests at the SAO
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The eSMA equipment was installed in a dedicated bay in the JCMT cabin
during June. Mark Bentum (ASTRON/WSRT, The Netherlands) visited the
JAC to take charge of this effort with financial backing from a NWO-M
grant awarded to Prof. Ewine van Dishoeck (Leiden) in support of the
development of the eSMA. In spite of having to sort out a number
communication and receiver issues, this work proceeded quickly and
first fringes between the SMA and JCMT were obtained at 267 GHz on
July 12, 2005 at 11:07HST, well ahead of the target-date in
mid-August. For these inital tests only a single LO was available at
the SMA forcing a choice of receivers with compatible LO
multiplication factors: i.e. the 345 GHZ receivers at the SMA and the
230 GHz receiver at the JCMT, which have an overlap in frequency
coverage from about 260-270 GHz. The photos below show the first
fringes with the JCMT configured as 'antenna 8' and the team
celebrating their success. Since the exact baseline with the SMA had
not been determined yet, the fringes on the JCMT baselines start
wrapping with time.
The eSMA interface equipment for the CSO was delivered in September
and first fringes with the whole eSMA were obtained on
Oct. 31. Illustrating the benefits of the large collecting area the
first baseline to show clear fringes was the JCMT-CSO one!
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First fringes!
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First fringes of the SMA-JCMT interferometer observed on July 12,
2005: phase as a function of time. The JCMT is configured as antenna 8
of the array. Due to an imprecise baseline solution at the time, the
fringes on the JCMT baselines start wrapping rapidly as time proceeds.
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Commissioning
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During the autumn of 2005 eSMA commissioning was carried out on
several evenings, first with the JCMT alone, later including the CSO.
Initial baseline tests proved to be discouraging, since a proper
solution seemed to remain elusive: 'decimeters' uncertainty had to be
reduced to 'millimeters' before an attempt could be made to further
refine the solution by observing astronomical calibrators. This is
less straightforward than it may appear: the baselines are between the
axes-intersections of the telescopes rather than the nominal survey
coordinates for the observatories or the antenna locations of the GPS
receivers. Taco (Ken Young) managed to do the impossible by finding
the solution in spite of what later turned out to have been a mix-up
of axes in the transformation of 'Hawaiian survey' to astronomical
coordinates.
Taco also undertook another major development: a complete revision of
the SMA tuning software in order to be able to exactly line up the sky
frequencies of receivers driven by two different LOs. During the early
autumn the SMA implemented their second LO system primarily intended
to support dual frequency observations e.g. 230 GHz and 690 GHz. The
same system could in principle be used to obtain interference between
the 230 GHz receivers at the SMA and and JCMT which use different
multiplication factors and thus require a different LO frequency to
tune to the same sky frequency. However, for successful interferometry
of the IF coming from the respective receiver the two LO frequencies
had to be fine-tuned and the sky frequencies aligned using secondary
oscillators in the system. This required a major update of SMA's
tuning software, but first spectra at 230 GHz were successfully
observed in early October.
A number of basic operations of the eSMA have been checked out
successfully and the baselines were refined to mm accuracy. After the
JCMT shutdown commissioning we plan to continue with phase-stability
and imaging tests and the switch-over to 345 GHz. This will be
followed by a 1-year pilot programme. A workshop on the pilot
programme is being planned for the autumn of this year (details to
follow).
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The first spectra with the eSMA of the CO(2-1) line in CRL2688. The
figure shows the JCMT baselines (configured as antenna 8), which to a
varying degree resolve the source. The total integration time was
about 10 minutes.
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RxW conversion
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Late this year and early next year the SMA will add 400 GHz receivers
to the frequency line-up. These receivers overlap with their 300 GHz
receivers over most of the 345 GHz window, but observe the orthogonal
polarization. For the eSMA to take advantage of this
dual-polarization capability, a state-of-the-art dual-polarization
receiver is needed at the JCMT with an IF frequency of 5 GHz. HARP by
contrast is a single-polarization receiver. RxB3's mixers are no
longer state-of-the-art and its mechanics would require a significant
overhaul and after a careful analysis it was decided that a better
option would be to convert one of the frequency channels of RxW to 345
GHz. In March the JCMT Board gave the go-ahead to convert RxW's 460
GHz channel.
At the time of writing, development of new 345 GHz mixers and a
re-design of the optics has started at MRAO. The new mixers are based
on the HARP mixers and should provide the JCMT with a superb
dual-polarization single-pixel 345 GHz receiver. We are also
investigating the possibility of upgrading the 690 GHz mixers with
ALMA new junctions from SRON (Groningen, The Netherlands), although
this is a more speculative endeavour. We hope to be able to reinstall
a converted and upgraded RxW in the autumn of this year, and, once
in place, it will become the primary receiver for observing with the
eSMA interferometer.
Remo Tilanus,
eSMA Project Manager JCMT
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RxW optics inside the cryostat in front of the focusing lens and
460 GHz mixers (top); proposed redesign of the optics to fold
the beam into the 345 GHz HARP-style mixer block using a curved mirror
(bottom).
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The SMA
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