Radio Astronomy - Science topic

K. M., I haven't read the Jansky paper, so can't address the question directly, but a related 1930 paper by Heinrich Barkhausen, "Whistling Tones from the Earth" may be of interest : https://ieeexplore.ieee.org/document/1670709 . Barkhausen's paper was about an investigation into a well-known but (at that time) unexplained phenomenon of "whistling tones" in radio background noise. Barkhausen's explanation was that lightning impulses excited standing EM waves around the Earth's surface. It's a long time since I read the paper but from memory, he first noticed these tones when tapping enemy field telephone communications during WW1. He noticed that they were stronger when there were thunderstorms nearby and made the connection between the two things. I would expect though that there are multiple interference effects from lightning.

I did confuse confuse bandwidth with delay a bit, but not in recent comments, I think, because I realised I was doing it.

Unless the entangled pairs are prepared on earth and then half are sent to Jupiter on the satellite, which would require coherence lifetimes of years (many orders of magnitude above present lifetimes), a signal has to be sent from Jupiter at lightspeed (for instance a photon entangled with the state on Jupiter) to convey the entanglement information between the two sites before the quantum measurement, so that the half of the entangled state can be set up on earth. This means that the actual data rate is still limited by the bandwidth of the lightspeed signals, even if data can be sent with no delay,

I guess if zero delay was possible, it would still be useful, for things like steering a remote vehicle, or conversation.

I expect that each entangled state will carry 1 bit, one Q-bit.

I am not sure instantaneous communication will ever happen. It may. I don't understand it enough to be sure it can't work. I hope it may be possible. It would also raise lots of interesting problems with relativity. I wouldn't invest my pension on it, but I might invest fun money.

Yes, definitely same antenna can used for receiving the purpose, as the antenna reciprocal device. Hence as per reciprocity theorem: The receive and transmit properties of an antenna are identical. Hence, antennas do not have distinct transmit and receive radiation patterns - if you know the radiation pattern in the transmit mode then you also know the pattern in the receive mode.

How are you relating size of the universe and it's age? Universe didn't expand with speed of light.

Cool project! Reproducing the first detection of the 21 cm line by Ewen and Purcell in 1951.

But why re-invent the wheel, in software terms? CASA (https://casa.nrao.edu/ ) is the "industry standard" software for radio astronomy data analysis. Works on Linux and Mac OSX, with an iPython interface. And it's free.

Your horn antenna data could be analysed in CASA in the same way as "single dish" data - the only real difference is that a horn has poorer angular resolution (larger beamwidth) than a parabolic dish of the same collecting aperture.

Thank you. For a non-scientific committee, how can I present this? What is the impact of having a VLBI station in a country? Per-sharp this could contribute to the transfer of skills to the local population? etc.

David,

If this is in response to our discovery of possinle anomalies in Paracelsus C, please send an email to skyking42@gmx.com

Fran Ridge

Try to find them in the NIST data base. e.g. 

Omar,

Yes.

But the problems of unpredictable latency mean that one would be unable to use an Internet-connected array to perform interferometry - so you won't get a more detailed picture.

At best one would simply 'stack' the images in a non-coherent fashion - akin to making a telescope with a higher sensitivity, but not a higher angular resolution. The picture will just be brighter - still not a bad idea.

I don't know of any, and according to Ellingsen et al. 2007 IAUS and Breen et al. 2010, MNRAS 401, 242, and the references therein, there are indications that OH masers likely occur after water masers.

Dear Mohamed

I am in a similar situation to you.

One of the most helpful review papers that i have seen about this topic is here:

It is short but covers a lot of distinctions between the different kinds of masers and their roles in star forming regions. It also contains references to more detailed analyses in the bibliography.

Kind regards

Ross

500 MHz is no longer very high.  I believe the Xilinx Virtex 7 or UltraScale will handle it.  Each chip has 2880 digital signal processing slices, and usually with signal processing you can expand the signal chain easily with as many chips as you need.  The clock rate of these 20nm devices should allow a multiply-accumulate operation to be done far faster than 500 MHz.  The older Virtex 6 would handle that rate.  My last project did Lidar real time coordinate conversion and re-gridding at 200 MHz in a Virtex 5 without taxing it at all.  I was able to use full floating point at that rate (see paper linked below).

Yacoub,

I'd say you are correct in thinking that "supernova 1054 may have had some inspiring role in the discovery of pulsars."

I was the second graduate student who worked under Antony Hewish from 1959-1962 on his project that started in 1953. For the first nine years of his project. Hewish studied the powerful radio source in the Crab Nebula as it was occulted each year by the extended solar corona. In 1962 another radio source, 3C123 was additionally studied as it was occulted. These observations allowed Hewish to confirm that the sun's atmosphere extended outward by more than 50 solar radii. See paper by Hewish and Wyndham.

As an outgrowth of this research, Hewish designed and built the Interplanetary Scintillation array which began operating in July,1967. This 4 acre radio telescope was designed to "measure the high-frequency fluctuations of radio sources, originally for monitoring interplanetary scintillation" (Wikipaedia) and to investigate the angular structure of compact radio sources. Jocelyn Bell, the fourth of Hewish's research students working for a PhD, was the one who noticed the signs on recording charts made by the first discovered pulsar, CP1919. This first-discovered pulsar has a right ascension of 19h 19m, whereas the Crab Nebula, which also includes a pulsar, has a right ascension of 5h 34m. So, the Crab Nebula was not the first discovered pulsar. The date for the discovery of CP1919 is given as November 28, 1967 and was announced in a paper by Hewish et al. in 1968.

Regarding the controversy over Jocelyn Bell being excluded from the 1974 Nobel Prize, there are at least two sides to this argument. A few years ago, I tried to present another side in the Wikipaedia pages for Jocelyn Bell and Antony Hewish, but my comments were immediately removed by another writer or the editor. Here are the comments I tried to add:

Fred Hoyle’s condemnation of the Nobel award to Hewish that omitted Bell must be balanced by the fact that Hoyle, a member of a different group, was not really a “fellow astronomer” of Hewish, but a bitter rival in his struggle to promote the Steady State Theory of the universe as opposed to the Big Bang Theory that was favored by Martin Ryle with whom Hewish worked. Hoyle lost that struggle. Moreover, the remarks by Fred Hoyle and Iosif Shklovsky, ignore the fact that Bell was the fourth graduate research student (this writer was the second) in a series of students who worked under Hewish who in 1953 started the line of research that led to the discovery of pulsars. Hewish designed the equipment and directed the research throughout the 14 years that preceded the discovery. Hewish had previously joined Ryle in 1945 in conducting radio astronomical research. The Nobel Prize was rightly awarded for Hewish’s consistent effort over many years.

Hewish’s caution in interpreting the initial pulses was more than justified by the previous history of man-made interference experienced by the radio astronomers in the Cambridge countryside. Further observations that the source of the pulses was sidereal in nature established that it was not man made, and enabled the next step in deciding whether the pulses were a sign of extraterrestrial, intelligent life, or were a natural phenomenon.

Bell Burnell was diligent in her research under Hewish’s direction. But this diligence is expected of a graduate student seeking a PhD. Her efforts must be seen in the context of Hewish’s sustained effort and innovative techniques over many years for studying the solar corona that resulted in his design of the Interplanetary Scintillation Array and the discovery of pulsars.

In the course of this survey, one of his graduate students, Jocelyn Bell, discovered the radio source which was ultimately recognized as the first pulsar. The paper announcing the discovery had five authors, Hewish's name being listed first, Bell's second. Hewish and Martin Ryle were awarded the Nobel Prize in Physics in 1974.

A previous editor of this entry has stated: “The Nobel award to Ryle and Hewish without the inclusion of Bell as a co-recipient was controversial, and was roundly condemned by Hewish's fellow astronomer Fred Hoyle. It is now universally recognized that Jocelyn Bell's supervisor and head of department "won the Nobel prize for Physics for a discovery which was essentially hers. Some people call it the No-Bell, Nobel prize because they feel so strongly that Jocelyn Bell Burnell should have shared in the award."[1]”

The statement in the previous paragraph is objectionable for several reasons: Fred Hoyle is not Hewish’s “fellow astronomer,” having no connection with the Cambridge radio astronomy group of which Hewish was a member. Hoyle was on the opposite side of a long-running and acrimonious debate that pitted his Steady State Theory against the Big Bang Theory of the universe favored by Ryle, Hewish and others, a debate that Hoyle lost.  There is no universal recognition that Hoyle’s statement is balanced or fair.

Jocelyn Bell was the fourth graduate student (this writer was the second) in a series of graduate students that Hewish took on to further his own project, started in 1953, a project that finally led to the discovery of pulsars. Hewish designed the equipment and supervised the research for each stage of the 14 year project that ultimately led to the discovery. It is not surprising that a new and more powerful radio telescope, the Interplanetary Scintillation Array (IPS Array), would lead to new discoveries.

Bell Burnell joined this ongoing project in 1965 and was fortunate enough to be the graduate student involved in Hewish’s project  when this discovery was made. Furthermore, Hewish began working in radio astronomical research with Martin Ryle just after World War 2 in 1945. Nobel prizes reflect (or should reflect) a substantial effort over a long period, not a chance discovery made on the shoulders and efforts of someone who directed the whole effort for many years. Bell Burnell herself said as much, in an after dinner speech in 1977: “I believe it would demean Nobel Prizes if they were awarded to research students, except in very exceptional cases, and I do not believe this is one of them.”  Those who claim otherwise are ignorant of how research of this nature is conducted.

are you talking about the integrated H I profile of a cluster of galaxies. This is possible for nearby clusters - what is your beam size. The Pegasus I cluster is full of H I rich spiral galaxies. But be warned, substructure in nearby clusters is high and very much complicates the interpretation of the dynamics.

It depends on how "raw" your raw data is. The number of triggers received in a given time period will be Poisson distributed. This is true for most kinds of cuts you have on your data; for example, an energy threshold. If you have downstream non-linear data processing or electronics effects, you can push this off Poisson, but it's pretty robust. Now, the measured energy is a different beast, which will be entirely detector dependent. If you have a measured energy dispersion function (ideally but rarely measured by shooting a known monoenergetic beam into the detector, and measuring the distribution of the reported energy) your on-orbit data will be the true energy spectrum convolved with your energy dispersion function.

See Gregory and Loredo, ApJ 398, 146, 1992, or Feigelson and Babu, "Promise of Bayesian Inference for Astrophysics" (a book of articles), or WF Tompkins, "Applications of LIkelihood Analysis in Gamma-Ray Astrophysics," a PhD thesis from Stanford -- pretty sure it's on arXiv.

You need a dewar. Please go to this link.

ND-3 has hold time of 24 hrs for LN2. Capacilty of LN2 is 0.5 liters. Due to evaporation during filling, I would estimate that you need at least 4 times of that amount, say 2 litres of LN2.

This is something I was confused about, but I think I have figured it out now. When you first read about polarization, you read about linear or circular polarization, which are two different ways of representing the same thing - specifically, two different bases for representing the same two-dimensional space of possible polarizations of light. Then you read on and suddenly there are four Stokes parameters for representing what seems at first to be the same thing. For that matter, in the original definition, it's not even possible for light to be unpolarized. The key difference is that between monochromatic light, which always sits somewhere specific in the two-dimensional space of polarizations, and light with a finite bandwidth, which is a mix of many monochromatic components, each of which may have a different polarization state. Stokes parameters summarize such a mix - they can specify the degree to which the mix is biased towards a particular polarization state, and they can also specify which state the mix is biased towards.

Radio telescopes also have a similar duality. The very frontend of the receiver measures (effectively) the electric field in two different directions as a function of time (assuming linear polarization in the receiver). This gives you tremendous detail on the incoming radio wave, and usually an unmanageable amount of data. What we normally want is the average power over some modest time interval and frequency range. If we're going to compute the average power, we're in a similar situation to the above - we're now describing a mix of different polarization states, and now it takes four Stokes parameters to describe the state of this mix. In practice, what you record is cross-correlations. Let's suppose your feed gives you two linear polarization states. Then if you wanted to measure the power in some linear or circular polarization. You would take an appropriate linear combination of your two linear feeds and then record the average power. But if you don't know which linear combination you're going to want, or if you're going to want to study all the linear combinations, you can actually do the averaging first: you record not just the average of feed one squared and feed two squared, you record the average of feed one times feed two and the average of feed one times the complex conjugate of feed two. With these four averages, you can reconstruct the average of any complex linear combination of the two feeds. This is also the information you need to recover the Stokes parameters.

In short, the raw voltages you measure are just two complex data streams, corresponding to two linear (or circular) polarization states. But if you want to average down to a reasonable number of frequency channels and time resolution, and you also want to keep a statistical description of the mix of polarization states coming in, you need to record more than just the average power in each channel: you also need to record the average power in two cross-multiplied channels. These four average powers contain enough information to compute the Stokes parameters for that frequency range and time span. The conversion process is a sort of four-dimensional rectangular-to-polar transformation (which you can look up).

Another good reason to use secondary flux calibrators is that sometimes there are not many primary flux calibrators on the sky for a particular observing time. This is often the case in sub-mm since sometimes planets are low or below the horizon. In these cases secondary calibrators (which, as Rainer said, have been calibrated against the primary calibrators) are the only chance to get flux-calibrated your science targets.