Transcript
ALMA – the Atacama Millimetre/ Sub-millimetre Array Richard Hills
Contents • Outline of Current Status • Hardware features relevant to solar observing: • • • •
• • • •
Antenna surfaces Solar filters Receivers Detectors
Single-dish observations Aperture synthesis observations A little on software Plans for solar science?
Front Ends
Reference
•
Correlator
To do millimetre aperture synthesis, the items you need are: Infrastructure (power, roads, buildings, pads…) Antennas, Receivers, a System to transport the signals, a Correlator and lots of Software.
The Scientists Level Block Diagram
Oct 2012 42 antennas
Only occupying a tiny fraction of the site
ALMA Current Status • 53 “array elements” have completed Assembly, Integration and Verification and have been handed over to the Commissioning team. 4 currently down for maintenance. • 25 Vertex antennas (12-metre) delivered to ALMA • 16 Melco antennas (12 7-metre and 4 12-metre) • 17 AEM antennas (12-metre) • ~65 Front-Ends delivered to Chile, all with bands 3,6,7 & 9 ~9 with bands 4 and 8, ~4 with band 5 and ~3 with band 10 • All of the Back-End complete and handed over including “antenna articles”, local oscillator, data transmission system and two complete correlators. • All water vapour radiometers, amplitude calibration devices and solar filters delivered to Chile, most installed.
Antennae Galaxies HST and ALMA
Ring of Dust around Fomalhaut – Band 7 (350 GHz) continuum
IRAS 16293 at 220 GHz
Pre-biotic Molecules: Jorgensen et al. 2012
IRAS 16293 at 695GHz
Note beam
C+ Emission Line in BR 1202 z = 4.69 (<1hr)
M100 – 47 point mosaic
47 pt mosaic This source was observed in Band 3 (115 GHz) for 6 hours with 13 antennae in September 2011. The image on the left shows the CO(1-0) integrated intensity, and the image on the right shows the velocity field.
13
Compact Array
Atacama Compact Array – fills in the short spacings. 11 (out of 12) 7m dishes in place. Plus 3 (out of 4) of the 12m dishes for total power.
The ACA is very Compact
Combining 12m and ACA data
Velocity field is much more complete
Special Considerations for Solar Observing: 1) Don’t Melt the Subreflector! • When pointing at the Sun each dish collects well over 100kW. Can’t reflect this onto subreflector. • Absorbing it would overheat panels – must scatter it. • So we have to make surface “rough” for optical and IR wavelengths but smooth and highly conducting for mm/submm wavelengths. • Initial studies of grooves, etc. Final solutions: • Chemical etching of aluminium, Vertex & Melco. • AEM: sand-blasting the mould on which surfaces are replicated by electroforming (Rhodium on Nickel).
Optical Reflectivity Results • Machined Aluminium Panel
Machined grooves • Need to be < λsubmm/2 spacing and smallish slopes:
Subreflector >
<
Chemical Etch – relies on the fine grain structure of the Aluminium • Initial trial – still too narrow
Chemical Etch • Final Version
Prototype Antennas pointing near Sun
Under-exposed shot gives better indication
Results • Rough primary works well – temperature rise of subreflector is modest: of order 50C. No danger but some effect on phase and focus. • Heat reaching receivers when pointing at Sun is negligible. The subreflectors are also roughened – not really needed. • Main problem is when pointing between about 5 and 15 degrees away from the Sun, so it shines past the subreflector. Might make some problems for observing Mercury, etc.
Membrane
Widget Space
Front End
Special Considerations for Solar Observing: 2) Don’t Completely Saturate the Receivers • On quiet sun the RF signal at 100GHz will be at least TA = 3000K. (Less at higher frequencies because of atmospheric absorption, etc.) On a bright flare they will be much higher: 10 SFU (105 Jy) will give about 30,000K, compared to Tsys ~ 60k on blank sky. • SIS mixers typically saturate as something like: Sout = Gzero / (1+ Tsys/Tsat) Sin where Tsat is ~5000K to 10,000K and Tsys includes the signal from the source. • So we need a solar attenuator (not really a filter) in front of the cold mixer. We must not overdo it because of the need to calibrate on quasars, etc.
Solar “Filter” – see Pavel Yagobov’s talk • Mounted on Amplitude Calibration Device (ACD) so it can be moved into the beam in front of whichever receiver is being used. Takes a few seconds. • Can not have filter in place at the same time as the hot or ambient loads. Complicates calibration somewhat – we can’t measure Tsys directly. • Water Vapour Radiometer beam will often be blocked by ACD. But WVR’s cannot be used when observing the Sun anyway they will be saturated (?) • Attenuation is mostly by reflection onto ambient absorber, so Tsys on cold sky is ~300K / Transmission.
Amplitude Calibration Device
View of “Widget Space”
Receivers – Bands and Atmosphere • Eventually almost all “windows” will be covered • For now 3, 6, 7 & 9 plus some 4 & 8 and a few 5 & 10
Receivers (2) • All bands are dual polarization and have upper and lower sidebands with (nominally) 4 GHz instantaneous band-width in each. • Exact RF frequency is not usually important for solar observations so one would choose good places in “windows”. • Designed for sensitivity (not relevant here!) but gain stability is good – parts in 103 so main limitations will usually be from atmosphere, plus perhaps some drifts from thermal effects.
Total Power Detectors – for Single Dish • These are in the IF processor. 6 per polarization • These can be sampled at 2kHz and have ~15 bits.
• We would use those after the second downconversion, i.e. four 2GHz-wide bands per pol. • Note the attenuators used to set the levels.
Digitizers – for Synthesis Observations • The signal voltages are also digitized at the antenna.
Only 3 bits so limited dynamic range, but in practice OK (?)
Single-Dish Observing • Only one beam and one frequency per telescope. • Can either stare – gives very high time resolution – or scan – modest mapping capability. • Presently only “raster” scans at a few seconds per row, hope to have more general forms, e.g. Lissajous, with frequencies up to about 1Hz in the coming months. • System limitations on “sub-arrays” will make it difficult to have more than about 4 frequencies or pointings at the same time. Multiple pointings is “just software”. • Some possibility of switching through several (three, possibly four) frequencies in a few seconds.
Raster Scan performance at 3 arc min / sec
Double-Circle and Lissajous scanning
• Scanning works – needs some software work to integrate it into the system.
Aperture Synthesis • Uses (digital) correlator. Always has some spectral resolution. Usually 120 channels per baseband (60 if full-Stokes is used). Can add these together after corrections for delay errors, etc., have been applied. (For some purposes averaging raw visibilities may be OK and this is available.) • At present we can sample at ~0.1 seconds but faster should be available in future (nominally 0.016 sec). • Instrument is not “phased-up” ab-initio. If a very bright compact object is present in beam we can self-calibrate. Otherwise we must observe a bright quasar. Barely possible with filters in. Discuss Tues.
Aperture Synthesis (2) • Critical considerations are: – Angular resolution required: sets maximum baseline. – Field of view: determines if a mosaic is needed: for 12m dishes field ~ 20 arcsec * λ (mm). – Complexity of image: determines how many antennas are needed. Recall that for most solar work we will be using “snapshots”. Earth Rotation improves images greatly but only if the source doesn’t change over timescale of hours.
• Mosaic – At present done point by point. Overhead is ~5 seconds per point. Should come down to ~2 seconds eventually. – On-the-fly mosaicing (using raster pattern or possibly Lissajou, etc.) is possible in principle.
Snapshot Coverage Configuration and UV Coverage as of Sep ‘12 for 32 antennas
Software • Obviously everything is under control of software. • “Manual” input (CCL). In practice use Python scripts. These can be used to make complete observations and in fact this is how all Solar work is done so far. • Automatic mode is based on Scheduling Blocks. This are files (actually xml) of parameters which are then converted into commands by a set of scripts (SOS). • The Scheduling Blocks are created by the Observing Tool. This is Phase 2 – the generation of the proposal is Phase 1. No Solar concepts in OT yet (?) • No real-time image. Data is stored in ALMA Science Data Model (ASDM) in Archive. Process it with CASA.
The way is open to start Solar Science with ALMA, but we must get organized. Lots of details remain to be sorted out.
The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA is funded in Europe by the European Organization for Astronomical Research in the Southern Hemisphere (ESO), in North America by the U.S. National Science Foundation (NSF) in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and in East Asia by the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Academia Sinica (AS) in Taiwan. ALMA construction and operations are led on behalf of Europe by ESO, on behalf of North America by the National Radio Astronomy Observatory (NRAO), which is managed by Associated Universities, Inc. (AUI) and on behalf of East Asia by the National Astronomical Observatory of Japan (NAOJ). The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.
