Overview of Radio Astronomy

射电天文学概况

沈志强

参考书目:

 

Interferometry and Synthesis in Radio Astronomy(Thompson, Moran & Swenson) 1986

 

VLBI Techniques and Applications (eds. Felli & Spencer) 1989

 

Very Long Baseline Interferometer (Takahashi, Kondo, Takahashi & Koyama) 2000

 

Synthesis Imaging in Radio Astronomy ASP Conf Ser vol 6 (eds. Perley, Schwab & Bridle)

 

VLBI and VLBA ASP Conf Ser vol 82 (eds. Zensus, Diamond & Napier) 1995

 

Synthesis Imaging in Radio Astronomy II ASP Conf Ser vol 180 (eds. Taylor, Carilli & Perley) 1999

 

Tools of Radio Astronomy(Rohlfs & Wilson) 3rd edition, 2000; Problems & Solutions

 

Radio Astronomy(Kraus) 2nd edition, 1986

 

Radio Telescope (Christiansen & Hogbom) 2nd edition, 1985

 

An Introduction to Radio Astronomy(Burke & Graham-Smith)

 

Galactic and Extragalactic Astronomy(Verschuur & Kellermann) 2nd edition, 1988

 

Radiative Processes in Astrophysics(Rybicki & Lightman)

 

The Fourier Transform and its Applications(Bracewell) 3rd edition, 2000

 

Data Reduction and Error Analysis for the Physical Science(Bevington & Robinson) 2nd edition, 1992

       1932年开始诞生了射电天文学,开始讲了一系列的射电天文学简史

 

针对射电天文学简史,可以做个思维导图

 

上海65m的八个波段

 

L波段20厘米/1.4GHz

 

S波段10厘米/2.3GHz

 

C波段6厘米/5GHz

 

X波段3厘米/8.4GHz

 

Ku波段2厘米/15GHz

 

K波段1厘米/22GHz

 

Ka波段9毫米/32GHz

 

Q波段7毫米/43GHz

 

 

望远镜  观天巨眼

 

天文望远镜主要是观测下面几个量:

•Angular distribution as a function of I(α,δ)

•Frequency distribution as a function of I(ν)

•Temporal characteristics as a function of I(t)

•Polarization characteristics: I, Q, U, V

 

射电望远镜天线系统的组成

clip_image002

最初的天线设计

clip_image004

反射面类型

clip_image006

 

VLBI

Very Long Baseline Interferometry

l  高分辨率的天文观测

l  高精度的大地测量和天体测量

l  高精度的航天器的导航

挺好的一个关于VLBI的示意图。

 

VLA very large array,甚大阵。

clip_image008

CVN

clip_image010

接收机等。

The DiFX Software Correlator at IRA and Italian VLBI

by John Morgan

Institute of Radio Astronomy, Bologna

IRA Coffee Talk

28 March 2008

相关器

       每个scan每个天线一个文件。

相关后得到UV数据(fits文件)

l  Time 时间

l  Baseline 基线

l  (IF)

l  (channel) 通道

l  (polarisation) 偏振极化

 

硬件处理机还是蛮通用的,比如:

天线

l  ALMAThe  Atacama Large Millimeter/submillimeter Array (ALMA), an international partnership of Europe, North America and East Asia in cooperation with the Republic of Chile, is the largest astronomical project in existence. ALMA will be a single telescope of revolutionary design, composed initially of 66 high precision antennas located on the Chajnantor plateau, 5000 meters altitude in northern Chile.

clip_image002

l  EVLAThe Expanded Very Large Array Project: A Radio Telescope to Resolve Cosmic Evolution

clip_image004

l  e-MERLINThe Multi-Element Radio Linked Interferometer Network (MERLIN) is an interferometer array of radio telescopes spread across England. The array is run from Jodrell Bank Observatory in Cheshire by the University of Manchester on behalf of STFC as a National Facility.

clip_image006

VLBI

l  Bonn

l  JIVE

n  The Joint Institute for VLBI in Europe was formally established in 1993 by the Consortium for VLBI in Europe. The Institute is located in Dwingeloo, the Netherlands, and hosted by ASTRON, the Netherlands Institute for Radio Astronomy.

clip_image008

l  VLBA

n  The Very Long Baseline Array (VLBA) is an interferometer consisting of 10 identical antennas on transcontinental baselines up to 8000 km (Mauna Kea, Hawaii to St. Croix, Virgin Islands). The VLBA is controlled remotely from the Science Operations Center in Socorro, New Mexico. Each VLBA station consists of a 25 m antenna and an adjacent control building. The received signals are amplified, digitized, and recorded on fast, high capacity recorders. The recorded data are sent from the individual VLBA stations to the correlator in Socorro.

clip_image010

 

软件相关处理正变得越来越流行。比如国际低频阵列LOFARlow- frequency acquisition and ranging),运行在IBMblue gene上。

也有通用的就是DiFX

l  在澳大利亚的斯温伯恩研发

l  是的任何的x86集群都可以作为相关器

l  许多的机构正在探索这个软件的潜在价值

n  Bologna 博洛尼亚

n  Cagliari 卡利亚里

n  Bonn 波恩

n  NRAO

软件相关处理的优势

l  在通道数目和积分时间上没有硬件限制

n  Spectral-line VLBI

n  Wide-field VLBI

l  相关的速度仅仅受限于计算机(在目前集群的发展下不用顾忌)

n  e-VLBI is possible if the correlator is real-time

l  DiFX也有pulsar binning 模式

l  不同的相关器可以使用不同的参数

n  Can correlate the calibrator first

n  Can correlate several times shifting the center of the field

软件相关的流程

       相关本身是比较简单的,就是需要输入的文件比较多。

对基于NRAO-DiFX版本的:

l  Collection of software tools

l  Designed to give identical output to the VLBA Correlator

l  However VLBA observations use different control files

clip_image012

 

使用python脚本来控制进程。也可以通过一个脚本来控制整个进程从宽频带数据到结果图像。

意大利的VLBI

SRT

l  Sardinia

l  Medicina

l  Noto

l  Matera

 

clip_image014

clip_image016

 

 

Phase-Switched Interferometer

Phase-Switched Interferometer

clip_image001Phase Switching is a system used to increase the discrimination and sensitivity of an interferometer; where an extra half-wave path difference is switched in, at well defined frequency, between the two interfering signal sources. In-phase signals then become out of phase and vice versa, so that the signal output becomes modulated by the switching frequency, and can be more easily filtered from the internally-generated noise. (The discrimination is highest for sources which are small compared with the interferometer-fringe spacing)

In the block diagram a phase switch is used to introduces either 180° or 360° (equivalent to 0° zero degrees) of phase shift into the right-hand transmission line, at a specific frequency easily demodulated by the receiver detector.

This switching signal generator is a square wave which changes periodically from one state to the other, many times per second, so that at one instant the first interference pattern is obtained, and at the next a second pattern. The same switching signal is applied to a phase-sensitive detector, which acts in synchronism with the phase switch and so subtracts the second pattern from the first. The resulting pattern is shown in the third part of Figure 1.; below, each maximum of the first pattern appears as a positive peak, but each maximum of the second pattern appears as a negative peak.

The instrument known as a phase-switching interferometerwas invented by Sir Martin Ryle in 1951, and is one of the major innovations in radio astronomy for which he was awarded the Nobel prize in 1974. This principle and later versions are now very widely used in radio astronomy.

You might ask, why go to so much trouble? Why not just use the simple adding interferometer and eliminate the phase switch and phase-sensitive detector? Although it is true the same information about a cosmic source is available in either type, the phase-switching instrument can be made much less sensitive to variations inherent in the receivers’ electronics. Also, the phase switch causes the cosmic signal to be modulated at the switching frequency before it is amplified in the receiver. The noise generated by the receiver has the same general character as the cosmic noise, and this switch modulation acts as an identification tag helping to distinguish it from the receiver noise.

Also, the phase-switching interferometer responds less to extended sources, such as the general background radiation of the Milky Way, which might otherwise obscure the fainter radiation from weaker and small-diameter sources. In summary phase switching interferometers or more current versions, the correlator interferometer, are vastly superior to a simple adding interferometer and now universally used in preference to it.

“Phase switching is a clever trick how to make a basically additive interferometer behave like a multiplicative one. A plain additive interferometer has some undesired traits, like it outputs the fringes on a big DC pedestal, and needs the channels to be reasonably amplitude balanced to give good results. The phase switching is a way to avoid these.”Marko Cebokli

Refer : http://www.hardhack.org.au/book/export/html/71

 

 

The early “adding” interferometers operated by adding two voltages, then detecting their sum using a square law detector with the result

clip_image002


This has the disadvantage that the autocorrelated signals are often orders of magnitude stronger than the cross-correlated signals. In the 1950’s, Ryle developed the technique of phase switching, in which the phase of one of the voltages is periodically flipped, so that the signal flips back and forth rapidly between clip_image003 and clip_image004. By taking the difference, the signal becomes clip_image005, which is the desired cross-correlation only.

Refer : http://scienceworld.wolfram.com/physics/PhaseSwitching.html

clip_image006

Information based on articles from Sky & Telescope by G.W. Swenson & W. Swenson, Jr.