test scribefire 3.1

1) text no problem
2) for image(s): choose wordpress, include the image(s)

3) choose back blogspot
4) post

test (ScribeFire 2.3.2)

text with an image

UV Sampling for GMRT-like Distribution

I have attached images for the 31 antenna SG configuration and 10 hour synthesis. They are for a rotation of 20degrees and for a source at 19, 30, -30 & -45 deg declination respectively. The wavelength is 1m and the coordinates are also with respect to GMRT.

Notice the dense coverage close to zero baseline...

Final Images 1

Above images are for the 31 antenna SG configuration of a 10 hour synthesis.
They are for a rotation of 20degrees and for a source at 19, 30, -30 & -45 degrees declination respectively. The wavelength is 1m and the geographical coordinates are also with respect to GMRT.

Currently, I am working on the code to get the FFT Images.

test final : scribefire 2.3.1, Forefox 3, Linux

ScribeFire all works well for Wordpress.




For blogspot, do the following with Scribefire


1) type text in the text window

2) to upload an image, choose the wordpress blog in the right-side panel of ScribeFire.

3) once the image is uploaded, it will show up along with the text.

4) now, change the blog from Wordpress to Blogger (click in the right-side panel)

5) click "Publish to.."


this will upload the blog offline to blogger page (bitstelescope.blogger.com).

test2: Qumana 1.0.2 (Mac OS X)

text and image 2

Quamana is also available from Windows XP. so you can upload blogs offline :-)

5 Identical SG Configuration

The above images are for a 31 antenna SG configuration with a rotation of 5, 20, 55, 85 & 110 degrees respectively for a 10hour synthesis for a source at 30 degrees declination.

This configuration consists of 5 SG configurations with maximum baselines of 300m, 3000m, 9000m, 21000m &30000m having 6 similar antenna location in each of them. The uv coverage is uniform and well distributed and umax and vmax is around 7000.

Now, my next job would be to take FFT of these images and then compare them.

Identical - 3 SG Configuration

Above images are for a 31antenna configuration for a rotation of 5, 55, 80 degrees respectively for a 10hour synthesis.

The antenna configuration is made of 3 identical SG configuration with 10antennas in each. The only difference is that the maximum baseline in each is 300m, 3000m & 30000m. One antenna is placed at the reference point (0.0, 0.0), hence, a total of 31 antennas are present in the configuration.

The results are way better than the previous simulations but not comparable to the GMRT coverage. I believe that I have found a solution to this problem too and I shall upload the new images if they are good.

Increased Baselines

Above image is for a 42antenna SG configuration which is rotated by 55degrees for a 10h synthesis.

To compare our results with the GMRT coverage, we extended the maximum baseline to around 4000m from the existing 15m. The results obtained were very surprising as heavy redundancy was observed which can be seen in above image.

We have a new plan to resolve this issue. I shall put up the results soon !

Sierpinski Gasket

Above is an image of Level 5 Sierpinski Gasket. The more number of triangles you go on drawing inside each other, the number of Level goes on increasing.

Now for my configuration,
the primary configuration is a Level 3 SG with the orientation as seen above. North points to top, East to the right and so on. When I say "rotation of 'x' degrees, I mean that the above orientation (big triangle) is rotated by 'x' degrees anticlockwise.

SG - UV Maps II

The above images for a 42antenna configuration of Sierpinski Gasket.They are for a 10h synthesis of rotation of 35degree from original and for declination of 15, 19, 30, -30 and -45 degree declination respectively. I have chosen 35degree rotation as that has the maximum uniform coverage. The observatory location is assumed to be BITS, Pilani - Goa Campus.

Sierpinski Gasket UV-Maps

I have attached images of a 10hr. synthesis for a Sierpisnki Gasket (Level-3, 42 antennas) configuration. The original config. is such that the altitude is along the East-West direction. The four images are for a source at 30deg. declination and the observatory location is BITS, Pilani - Goa Campus. The four images correspond to the config. when the original config. is rotated by 10, 50, 90, 120 degrees respectively.

UV Simulator

The above images are for a 10hour synthesis of all 30antennas of the GMRT with +19, +30, -30 & -45 degrees declination respectively. One may refer to this GMRT Page for comparison.

The programme made to generate the uv patterns is 100% complete. I simulated the GMRT patterns and they have come as they have displayed on their website. I will now simulate the patterns for a Sierpinski Gasket configuration.

The above image shows uv-coverage for a configuration where the antennas are placed at the nodes of the Sierpinski Gasket (Fractal). I strongly suggest that we use this configuration for our proposed radio telescope in the institute.

Consider any point on the ground as reference and then the coordinates of the antennas are the following:

X (E)	Y (N)
4.500000	23.382675
18.000000	0.000000
-9.000000	0.000000
0.000000	15.588450

All distances are in meters. The X-axis is along the East direction and the Y-axis along the North direction.

It takes 4 to...

Simulations to obtain best u-v coverage possible with 4 antennae

Onkar has some preliminary results from his u-v plane coverage simulations. Here are mouth-watering diagrams first.

Onkar generates the positions of antennas using some logic. This is independent of other details. His top image (u-v coverage of 6 antennas) was made using a logarithmic spiral distribution of 4 antennae. The second image was created when 4 antennae were used in a Y-shaped array, like the VLA.

His (possibly) best result is kept for the last, when is that, Onkar?

Trying to be modular

After a long hiatus, got FFTW and PGPLOT working in harmony (at last!). Also, I'm starting to see the importance of modularity, and trying to document my code properly.

As of now, I have two cast-iron functions
i) One which takes 512 samples from a file on the hard drive, and returns their autocorrelation as an array of 1023 elements. (Note: The autocorrelation is just for convenience, the same procedure can be used for cross-correlation)

ii) One which takes the array returned by i), transforms it with FFTW, and gives magnitude and phase plots from the FFT.

I'm attaching some screenshots from the results obtained. The phase plots probably require some critical examination..

What to do next:-

Put i) and ii) in a loop on index M, where M is the number of frames to be superimposed.
Also, add some phase to the phase plots to correct for phase difference due to the baseline.

Software Correlator-III

In case of two-antenna interference, the cosine of cosine function for the fringe pattern was generated due to the geometric path delay between the two antennas. If we compensate for the phase difference electronically, or in software as we will, the fringes will disappear. This is called 'fringe stopping'.

Let us imagine, that for one source in the sky, fringes removed by the electronic circuits. Now, if there is another fainter point source anywhere in the sky, fringes due that source will be left behind. One can, thus, reason, that the observed fringes are due to superposition of fringes created by multiple point sources.

In synthesis imaging, we measure the cross-correlation coefficient of voltages V1V2* (note the complex conjugation) at two antennas. According to van-Cittert Zernicke theorem, the complex fringe visibility (cross-correlation coefficient) is the Fourier Transform of the normalised sky brightness.

For details of equations, and the transformation to l,m and u,v co-ordinates, see the paper by Thompson and Clark (Synthesis Imaging school at NRAO).


The diagram for synthesis imaging is as below.




we can express the complex cross-correlation coefficient between antennas as below.

Software Correlator - II

The angular resolution of radio telescopes can be improved by using the principles of interferometry. Interferometry depends upon detecting the combination of voltage signals assumed to be originating from the same source. In a simple two-element radio interferometer, the signals from a point source alternately arrive in phase and out of phase as the Earth rotates and causes a change in the difference in path from the radio source to the two elements of the interferometer. This produces interference fringes in a manner similar to that in Young's double-slit interference experiment, shown below.

The figure below shows how two waves from the same point source reach the two antennas on ground with a certain geometric path difference. If we multiple the two voltages generated at the antennas due to the waves reaching their respective locations, an interference pattern is observed (see third figure further down). Note how the appearance is exactly identical to the interference pattern generated by a two-slit interferometer using two slits as point sources, shown above.

The 'geometric' path difference due to spatial orientation of the source is similar to the path difference in 2-slit interference experiment. We probe phenomenon of 'temporal coherence' in 2-slit Young's experiment, that is we study the stability of phase difference between two sources (slits) over time. If the phase difference betwee two slits varied randomly, fringes will not have any absolute dark fringe ('zero' of intensity).

On the other hand, in 2-antenna interference, the phase difference arises due to different locations of the 2 antennas as seen by the source. If the source was not a point source on the sky, fringes will not be perfect (no absolute zero of intensity). Thus, we study 'spatial coherence' of waves, that is if our source has a finite size.

Software Correlator - I

(Hopefully, the first of many entries on this subject)

Mandar is working to create a program which will replace a bulk of old, creaking machines at GMRT and other similar synthesis-imaging instruments, which produce a stream of cross-correlations between different antennas that are separated from each other by a huge distance.

The first (baby) steps are to create a program to compute cross-correlations from a stream of numbers. Then, we develop the same algorithm for an input of quantized numbers. After correcting for source delays (fringe-stopping and fractional delay correction), we compute an FFT of the cross-correlation series, to obtain the power spectrum of correlations. Finally, we tie this bit with astronomical source co-ordinates.

Data Acquisition System

We would like to make radio maps using Earth-rotation aperture synthesis . This technique requires us to record raw voltages at two antennas and compute complex visibility (cross-correlation) for different frequency channels.

The block diagram of the receiver and acquisition is shown in the figure on the left.

Each of 4 antennas will have a heterodyne receiver system which will translate a signal at the radio frequency (RF) of 73.9-MHz to an intermediate frequency (IF) of 1.9 MHz.

Four such signals reach the data acquisition system marked in the diagram. Each signal will be converted to digital using a 12-bit ADC, and then sampled at the rate of 0.5 MHz using only the most significant 2-bits.

Aniket carried out feasibility studies and design of the DAS to acquire Nyquist-sampled voltages from a bandwidth-limited signal (0.25 MHz). These quantized voltages are stored on a computer disk. There is a provision of acquiring from 4 antennas, so total data rates are 0.5*4 = 2 M-samples/s. Each sample consists of 2 bits each. Hence it is 8 M-bits/s = 1 MB/s storage rate on a PC.