Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.atnf.csiro.au/whats_on/workshops/synthesis2001/material/alma.pdf
Äàòà èçìåíåíèÿ: Tue Jan 15 06:15:08 2002
Äàòà èíäåêñèðîâàíèÿ: Wed Dec 26 09:02:20 2007
Êîäèðîâêà:

Ïîèñêîâûå ñëîâà: m 63
ALMA
The Atacama Large Millimeter Arra y

ALMA site

5 km altitude at the foot of the high Andes
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ALMA telescopes (EU)

ALMA telescopes (US)

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ALMA specifications
· 64 antennas, at 5km height · 12m diameter, ±20 µm, 0.6" in 9m/s wind · arrays of 150m to 12km · 10 bands in 31-950 GHz + 183 GHz WVR. Initially:
· 86-119 GHz · 211-275 GHz · 275-370 GHz · 602-720 GHz

ALMA specifications
· all bands online · any 1 + 0.5 bands accessible · filled (150m) to ring (12km) and log-spiral or ring · data rate: 1M visibilities/s average
(= 6Mb/s average; 60Mb/s must be sustainable)

· 8 GHz , dual polarisation, 4096 channels/IF
(Note: var ying numbers mentioned throughout project) ·
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·all data archived (raw + images) · AC (Compact or Complementary)A
· 6-10 antennas of 6-8m dia meter in ring or hexagon for short spacings

600 M$US
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Sensitivity goals
GHz 35 90* 140 230* 345* 650
*

Other arrays
C o m p a ris o n w ith o th e r m m a r r a y s
A r ra y To t a l A re a (m 2 ) 290 Mo s a i c S pe e d (n D ) 61 SSB Ts y s @ 90 G Hz 1 50

S ( µJy ) 20 27 39 71 120 849 1260

S (mJy) @ 1km / s 5.1 4.4 5.1 7.2 10.0 51.0 66.0



a

F re q . ra n g e ( G Hz ) 7 0 -1 1 5 , 2 1 0 -2 7 0 8 6 -1 1 6 , 2 1 0 -2 7 0 8 5 -1 1 6 , 1 2 6 -1 5 2 , 2 1 3 -2 3 7 8 0 -1 1 5 , 2 1 0 -2 5 0 8 5 -1 1 0

Po l

BW ( G Hz )

Ma x . B s ln (k m ) 1 .5 0 .2 0 .4 0 .4

Li n e s e ns . (m J y ) 23

Co n t. s e ns . (m J y ) 1 .4

B IMA ( 1 0 *6 . 1 m ) O VRO ( 6 *1 0 . 4 m ) NMA ( 6 *1 0 m ) IR A M P d B ( 5 [ 6 ]*1 5 m ) A TC A ( 5 *2 2 m )

0 .7

1

0 .8

510

62

2 50

0 .7

1

1 .0

23

1 .3

470 880 [1 0 6 0 ] 1 9 00

60

4 00

0 .6 5

1

1 .0

43 8 .2 [6 .7 ] 7 .9

2 .4 0.63 [0 .5 ] 1 .0

7 5 [9 0 ] 1 10

1 50 2 50

0 .7 0 .4

1 2

0 .5 0 .2

0 .4 3 .0

850

At 50 deg elevatio n and b est 25 % weath er for <1 mm; best 75 % for >1 mm

S e n s itiv ity e s t im a t e s a re f o r 1 h r in te g ra tio n a t 9 0 G H z , a ll p o ls . c o m b in e d . L in e s e n s itiv it y is fo r a 1 0 k m /s c h a n n e l. A c tu a l s e n s itiv it y w ill d e p e n d o n a t m o s p h e ric p h a s e .

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ALMA map

ALMA local area

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ALMA topo

ALMA barometer

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ALMA humidity

Atmospheric transparency

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ALMA temperature

ALMA wind direction

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ALMA wind speed

ALMA bands

(court esy Wo lfgang Wi ld)

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Noise regimes
1000.

System noise source

SKA-l o Temp erature in K 100. SKA- mid

SKA- hi

ALMA

1. 0.01

10.

0.1

1.0

10.

100.

1000.

(court esy Wo lfgang Wi ld)

Frequ ency in GHz

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Receiver optics

ALMA schedule
Proposed schedule: · 2 prototype (US and EU) antennas in August 2002 · 6-9 months delayed · January 2002: Construction (Phase 2) start · delayed due to US president budget stop for new projects
· possible move of astronomy from NSF to NASA · future projects uncertain at the moment

· April 2003 decision on antennas (+ 0.5 year?) · 2006 interim operations · 2011 full science operations
(court esy Wo lfgang Wi ld)

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ALMA status
Decisions: · ESO (+E) December 2001 (when and if US decision) · USA (+CA) Hopefully next budget · JP Contract signed in April 2001 · First talks about work division for 3 partners held in Paris · For computing 3rd partner adds 12% to cost · US proposes to cut project by 20% · ESO wants no cuts, at most 10% - accepted by all parties · JP still hopes to add extra bands; next correlator · JP also talking ACA · Fight for money started (e.g. software/computing is 32M$) by partners
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Field-of-view
SKA 20 cm

15 Mpc at z = 2

SKA 6cm
HS T

ALMA

Primar y beam from 180" (30GHz) ­ 6" (900GHz)
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ALMA mosaicing
Many objects to be observed by ALMA, such as nearby galaxies and molecular clouds in our own galaxy, will be diffuse and much larger than ALMA's primary beam. Mosaicing will have to be done. However, mosaicing places stronger constraints on the antennas than single pointing interferometry. Wh y not build a 70~m single dish to observe these big sources? Mosaicing is faster than single dish observations, mainly because of the multiple synthesized beams which can be formed within each primary beam.

Mosaicing limits
Pointing: Because the emission spans beyond a single primary beam in mosaicing, small antenna pointing errors can have a large effect on the observed flux of a feature which lies near the half power point of the beam. Pointing accuracy of about 1/25th of the beamwidth will permit mosaics of about 1000:1 dynamic range (linear with ). Surface Accuracy: Surface errors will scatter radiation into the primary beam sidelobes, and unmodeled primary beam sidelobe structure will limit the quality of mosaic images. While surface accuracy of 1/16th of a wavelength only degrades the dish efficiency by a factor of 2 from Ruze losses, 1000:1 dynamic range mosaics will require surface accuracies of about 1/40th of a wavelength (quadratic with ).
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Mosaicing limits
Getting Very Short Spacings: The homogeneous array concept requires that the antennas be fairly close together (ie, 1.3 times the dish diameter for zenith observations) to be able to measure spatial frequencies in the range of the dish diameter. However, the antennas can actually smack into each other if the separation is less than about 1.5 dish diameters (depending upon the design). To improve the short spacing capabilities (i.e. the large scale structure) the ACA has been proposed with about 10 antennas of about 6m diameter.

Phase stability
Inhomogeneously distributed water vapour results in different electrical path lengths above the different antennas, or phase error. The phase errors scatter flux, limiting the dynamic range, and also cause decorrelation, which artificially decreases the source amplitude. The initial calibration is planned with a 183GHz spectral line WVR (cf the ATCA 22GHz WVR).

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Calibration possibilities
The complete ALMA array, with 64 telescopes has about 2000 baselines, many more than any other existing telescope. This enables the use of algorithms different from used in today's mm instruments. E.g.: ·use of redundant and quasi-redundant baselines ·use of parameterized models for the phase errors across telescope aperture ·use of pointing correction model parameters

ALMA

With NGST, ALMA and SKA in the second decade of this century the electro-magnetic spectrum from 1 m till 10m will be available to the next generation of astronomers with a resolution of about 0.02", and high sensitivities.

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