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ALMA Development-II
September 25th 2009, Charlottesville
• Crystal Brogan & Mostly Todd Hunter
Atacama Large Millimeter/submillimeter Array
Expanded Very Large Array
Robert C. Byrd Green Bank Telescope
Very Long Baseline Array
Increase the Data Rate
• ALMA – Wide bandwidth (8 GHz/pol), but baseline peak data rate (64 MB/s) utilizes
~6% of correlator capability (1 GB/s). Restricted by network hardware, ability to
process and archive:
• High frequency Spectral Line Surveys – requires wide bandwidth coupled with
high spectral resolution, at high frequencies integration times must also be short
• Interferometric On-the-Fly (OTF) maps - Requires fast mapping = short
integration times = high impact on data rate and compute power
• Transients – Requires high time resolution, some applications like the Sun also
require high spectral resolution
Examples:
• Transients - 16 ms dump time – good for transient science, but with current data
rate constraint must average to ~ 1 second or use a small number (~100) of
channels.
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Increase data rate -II
Infrared Dark Clouds:
A Galactic web of star formation
• Large area surveys can answer fundamental questions about the distribution of
gas in galaxy clusters, how gas cycles in and out of individual galaxies, how
molecular clouds form, and how stars form
3.6 mm
4.5 mm
8.0 mm
N2H+ at 93.1 GHz from
CARMA
0.5o
• ALMA – OTF map of 1 square degree in CO(3-2) in 50
hours would require ~2x the current maximum data rate
(64 MB/s). Raw data ~ 20 TB. Image would have 100 Mpixels
per channel!
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Increase data rate - III
• Spectral lines are critical tools for probing physical
conditions, understanding astrochemistry, and
discerning the chemical building blocks of life
– Requires: wide bandwidths plus high spectral and
angular resolution. Many diagnostic lines observed
simultaneously, removing calibration uncertainties and
increasing science throughput.
– Data rates of ~1 GB/s are required
– Assuming 50 antennas and 64 MB/s, one is restricted
to longish integration times from a fast-switching –
high frequency perspective
– Cannot utilize full 8 GHz of BW
Mode
BW per
sideband
Channels per
sideband
Velocity Res.
(km/s) 230 GHz
Min.
Integration**
7 (dual)
2 x 2 GHz
2 x 4096
0.64 2 GHz of bandwidth
5 sec
toward a massive star
12 (dual)
2 x 62.5 MHz
2 x 4096
0.02
13 (cross)
2 x 2 GHz
2 x 2048
1.28
5 sec
18 (cross)
2 x 62.5 MHz
2 x 2048
0.04
5 sec
70 (cross)
2 x 2 GHz
2 x 64
40.8
0.1 sec
forming region (Brogan et al. in prep.)
5 sec
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Upgrades to Increase Data Rate (beyond
archiving, data delivery, and processing)
• Data rate of 1 GB/s needed for increasing mapping speed, spectral line surveys,
and transient phenomena. Required facility upgrades include:
– 1 Gb/s connections upgraded to at least 10 Gb/s
– Upgrades to network equipment including Network Interface Cards
– Improvements to operational computers and software (quality assurance and real
time calibration)
– Data transmission speed to the local “spool” archive
– Control software for on-the-fly mapping
• Cost of development + upgrades on the order of $5M
• The cost of processing, archiving, shipping media etc would also increase significantly, but
has not yet been costed
For processing will need ~1000 CPUs instead of current ~100
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Increase Band 3 Sensitivity
Band 3 plays a very important role for both science and calibration of other
ALMA bands through phase transfer
When Band 3 was designed SIS mixers gave best performance then available at
this frequency – this is no longer true and substantial gains can be made by
switching to HEMT technology
• Change from SIS mixer to HEMT amplifier (note for higher frequencies SIS is
the only way to go)
• Decrease in system temperature is equivalent to 40% reduction in observing
time for a given sensitivity
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Improvements to IF system
• Some of the receivers exhibit a significant slope in output power across the IF
band, reducing sensitivity in some parts of the spectrum.
• This could be mitigated with the installation of bandpass equalizers (as in
the EVLA) with a 3-4% gain in sensitivity (i.e. worth 2 antennas)
• Install a second attenuator path in each of 4 basebands per antenna
• Because receiver output power can vary significantly between bands, band
switching will require resetting IF attenuators each time. Unfortunately, these do
not meet the original specification on settling time.
• Having a second path would eliminate the need for resetting and should
improve system stability.This may prove critical for accurate band-to-band phase
transfer, and thus may improve image fidelity for high angular resolution
observations at high frequencies
• Combined cost in few $M range
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SIS Technology Development
• In the past, two SIS foundries served the US radio astronomical community: JPL and UVa
– warning signs that JPL may not continue, and there are only a few in the whole
world
• We currently give modest support to UVa lab ($2M over five years), but that contract
ends in 2012
• Without reliable sources of SIS mixers NO FUTURE ALMA RECEIVER UPGRADES
WILL BE POSSIBLE
• Improvements in the SIS chips themselves could bring significant gains to performance,
also still room to improve SIS mixer design in the receiver labs (i.e. CDL):
– Even the best ALMA receivers are still about 4x the quantum limit
– For example, Band 6 performance at band edges is less than hoped for – cannot get
12CO(2-1) and 13CO(2-1) simultaneously. With better SIS mixers performance at
band edges could make this possible
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Sideband Separating Mixers for Band 9 & 10
• For current double sideband (DSB) design both sidebands come out of the
receiver mixed together and can only be separated easily with cross correlation
(other techniques exist but are tricky and unproven).This may significantly
affect ability to add single dish to interferometer for the high frequency bands in
the presence of bright line emission.
• System temperature is increased in DSB receivers because sky noise from both
sidebands add
• With upgrade:
– For a given sensitivity observing time is reduced by a factor of 2 due to
rejection of nearly half the atmospheric contribution to the system
temperature
– This upgrade improves line confusion problem by rejecting other sideband
by a factor of at least 10.
• Cost uncertain but on the order of $50-100k per antenna (better cost
estimate for Band 9 at least should be available soon)
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Don’t Forget Algorithm Development
• Algorithm development that is required to:
– Process data beyond baseline ALMA also need to be included (high data rates,
OTF mapping etc)
– Implement newly discovered techniques
Need to be part of development too!
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