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May 2013 |
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This article attempts to explain why and how a radio telescope uses interferometry to achieve high sensitivity of performance in revealing the sky. SKA Telescope is such a radio interferometry telescope.
Why- The Britanica Encyclopedia defines Radio Interferometer as an apparatus consisting of two or more separate antennas that receive radio waves from the same astronomical object and are joined to the same receiver. The antennas may be placed close together or thousands of kilometers apart.
o The principle of a radio interferometer’s operation is the same as for an optical interferometer, but, because radio waves are much longer than light waves, the scale of the instrument is generally correspondingly greater. Parts of a radio wave reach the spaced antennas at different times. This time difference is compensated for by a variable-delay mechanism and the waves can be made to interfere.
o The simplest radio interferometer consists of two aerials connected together via lengths of coaxial cable to a multiplier and integrator. The voltages A and B from the aerials (proportional to the electric field at each aerial) are multiplied giving AB* at the output. The AB* term when averaged over several radio frequency cycles in the integrator is proportional to the cross - correlation R12 of the electric field received by the two aerials. The multiplier and integrator (and other electronics) are collectively called the correlator for this reason.
For the next step of information on the same subject matter, read this: 2013-06 Radio Interferometry Cross Correlation .
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May 2013 |
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This article was based on the SKA1 Survey Telescope Central Signal Processor design feasibility white paper dated 2013-0503 and was intended to indicate the magnitude of data rates involved. Figures shown below are for SKA1 Survey only. They may not be accurate and are for illustration only.
o The SKA1 Survey dishes will be equipped with Phased Array Feeds (PAF) and will cover the continuous frequency range from 650MHz to 1670MHz. Signals from the PAF will be beam-formed at the antenna to produce 36 dual polarisation beams in a single 500MHz wide instantaneous band. This beam data is transported to the Central Signal Processor (CSP) facility where it is further channelized to 2kHz channels and cross-correlations of all pairs of antennas are performed.
o The main inputs to CSP are the data for 36 beams from the 96 survey antennas and the main output are the visibilities generated by the correlator which are sent to the Science Data Processing located in Perth.
o The input into the SKA1-Survey CSP per beam per antenna is:
36 × 1GHz x (8 + 8 bits) x 8/7 oversampling = 76.6GB/s
o If 40Gb/s links are used to transport this data where each link achieves a throughput of 96% of capacity then 20 links are sufficient to transport the data from each of the 96 antennas
o Assuming an average for each baseline and polarization pair of 0.7927 dumps per second, and 32 + 32 bit summations results in:
36 beams x 4560 baselines x 4 polarization pairs x 0.7927 dumps/s x (32 + 32 bits) = 3.97MB/s dumped to the SDP per channel. For a 500MHz signal with 2kHz frequency resolution requires 250000 frequency channels, so using 218 frequency channels gives a total integrator output of 1.017TB/s.
Please read this page for an idea of finding a technology to handle this level of data rates.
END
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May 2013 |
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This article was based on the SKA1 Survey Telescope Central Signal Processor design feasibility white paper dated 2013-0503 and was intended to indicate the magnitude of data rates involved. Figures shown below are for SKA1 Survey only. They may not be accurate and are for illustration only.
o The SKA1 Survey dishes will be equipped with Phased Array Feeds (PAF) and will cover the continuous frequency range from 650MHz to 1670MHz. Signals from the PAF will be beam-formed at the antenna to produce 36 dual polarisation beams in a single 500MHz wide instantaneous band. This beam data is transported to the Central Signal Processor (CSP) facility where it is further channelized to 2kHz channels and cross-correlations of all pairs of antennas are performed.
o The main inputs to CSP are the data for 36 beams from the 96 survey antennas and the main output are the visibilities generated by the correlator which are sent to the Science Data Processing located in Perth.
o The input into the SKA1-Survey CSP per beam per antenna is:
36 × 1GHz x (8 + 8 bits) x 8/7 oversampling = 76.6GB/s
o If 40Gb/s links are used to transport this data where each link achieves a throughput of 96% of capacity then 20 links are sufficient to transport the data from each of the 96 antennas
o Assuming an average for each baseline and polarization pair of 0.7927 dumps per second, and 32 + 32 bit summations results in:
36 beams x 4560 baselines x 4 polarization pairs x 0.7927 dumps/s x (32 + 32 bits) = 3.97MB/s dumped to the SDP per channel. For a 500MHz signal with 2kHz frequency resolution requires 250000 frequency channels, so using 218 frequency channels gives a total integrator output of 1.017TB/s.
Please read this page for an idea of finding a technology to handle this level of data rates.
END
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May 2013 |
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This article is extracted from the official SKA baseline design document SKA-TEL-SKO-DD-001 revision 1 dated 2013-03-12. The purpose here was to outline how powerful the SKA telescopes are without involving deep technical terms. For full, accurate and updated information, please visit www.skatelescope.org. The driving concepts for the SKA have been to develop high sensitivity as well as high “survey-speed” telescopes.
o The SKA Organisation has selected 2 sites for the SKA- one in Western Australia and one in Southern Africa. The telescope facilities for SKA1 have been defined as SKA1-low (a low-frequency aperture array); SKA1-survey (a mid-frequency array of dishes equipped with Phased-Array Feeds), both arrays to be built in Australia; and SKA1-mid (a mid-frequency array of parabolic dishes) to be built in South Africa. SKA1 has been scheduled for operation in 2019 and the 2nd phase called SKA2 has been scheduled for operation in 2024.
o SKA1-low
The receptors will consist of an array of ~250,000 log-periodic dual-polarised antenna elements. The antenna array will operate from 50 MHz to ~350 MHz (covering a similar frequency to LOFAR telescope and MWA telescope). Its sensitivity will be ~1000 m2 / K at frequencies above 110 MHz at the zenith- an increase of more than an order of magnitude albeit being optimised for brightness temperature sensitivity. The elements will be grouped into 866 35m diameter stations and beam-formed to expose a field-of-view of ~20 deg2 in a single smooth beam.
o SKA1-mid
The telescope receptors will consist of 64 13.5m diameter dishes from the MeerKAT array and 190 15m SKA1 dishes. Three spiral arms will extend to a radius of ~100 km from the centre. SKA1-mid will cover the continuous frequency range from 350 MHz to at least 3050 MHz in three receiver bands. At the frequency of 1.4 GHz, SKA1-mid provides an order of magnitude better than existing instruments in terms of resolution, sensitivity and survey speed.
o SKA1-survey
The telescope receptors will consist of 36 12m diameter dishes from the ASKAP array and 60 15m SKA1 dishes. Three spiral arms will extend to a radius of ~25 km from the centre, although space is available for ~50-km arms, if needed. SKA1-survey will cover the continuous frequency range from 650 MHz to 1670 MHz in a single dual-polarised PAF in a 500 MHz wide instantaneous bandwidth. The PAF provide a constant Field-of-View of approximately 18 deg2 in 36 beams at the highest frequency. It is comparable with the JVLA telescope in SEFD (System Equivalence Flux Density), but the SKA1-survey PAF-enabled field-of-view, about 100 times larger, provides a huge increase in survey speed.
o Note: LOGAR, MWA, JVLA are operating telescopes as in May 2013 and are quoted here for reference.
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May 2013 |
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This article is extracted from the official SKA baseline design document SKA-TEL-SKO-DD-001 revision 1 dated 2013-03-12. The purpose here was to outline how powerful the SKA telescopes are without involving deep technical terms. For full, accurate and updated information, please visit www.skatelescope.org. The driving concepts for the SKA have been to develop high sensitivity as well as high “survey-speed” telescopes.
o The SKA Organisation has selected 2 sites for the SKA- one in Western Australia and one in Southern Africa. The telescope facilities for SKA1 have been defined as SKA1-low (a low-frequency aperture array); SKA1-survey (a mid-frequency array of dishes equipped with Phased-Array Feeds), both arrays to be built in Australia; and SKA1-mid (a mid-frequency array of parabolic dishes) to be built in South Africa. SKA1 has been scheduled for operation in 2019 and the 2nd phase called SKA2 has been scheduled for operation in 2024.
o SKA1-low
The receptors will consist of an array of ~250,000 log-periodic dual-polarised antenna elements. The antenna array will operate from 50 MHz to ~350 MHz (covering a similar frequency to LOFAR telescope and MWA telescope). Its sensitivity will be ~1000 m2 / K at frequencies above 110 MHz at the zenith- an increase of more than an order of magnitude albeit being optimised for brightness temperature sensitivity. The elements will be grouped into 866 35m diameter stations and beam-formed to expose a field-of-view of ~20 deg2 in a single smooth beam.
o SKA1-mid
The telescope receptors will consist of 64 13.5m diameter dishes from the MeerKAT array and 190 15m SKA1 dishes. Three spiral arms will extend to a radius of ~100 km from the centre. SKA1-mid will cover the continuous frequency range from 350 MHz to at least 3050 MHz in three receiver bands. At the frequency of 1.4 GHz, SKA1-mid provides an order of magnitude better than existing instruments in terms of resolution, sensitivity and survey speed.
o SKA1-survey
The telescope receptors will consist of 36 12m diameter dishes from the ASKAP array and 60 15m SKA1 dishes. Three spiral arms will extend to a radius of ~25 km from the centre, although space is available for ~50-km arms, if needed. SKA1-survey will cover the continuous frequency range from 650 MHz to 1670 MHz in a single dual-polarised PAF in a 500 MHz wide instantaneous bandwidth. The PAF provide a constant Field-of-View of approximately 18 deg2 in 36 beams at the highest frequency. It is comparable with the JVLA telescope in SEFD (System Equivalence Flux Density), but the SKA1-survey PAF-enabled field-of-view, about 100 times larger, provides a huge increase in survey speed.
o Note: LOGAR, MWA, JVLA are operating telescopes as in May 2013 and are quoted here for reference.
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