Nimisha G. Kantharia

Click here for a little on my academic background.

Publications

1. Click here for my publication list as listed by ADS. This gives a flavour of my research interests.

GMRT time scheduling: I have been scheduling GMRT observations since Cycle 6 once GTAC has approved and alloted time. This is an unforgiving job since it involves considering various constraints - both user and astronomical and coming up with an optimum schedule which makes use of most of the available telescope time in a cycle which runs for five months. Moreover there is always one upset investigator :-). The observing schedule for the ongoing Cycle 18 can be found here . Any user who has seen the Cycle 15 schedule will notice a big difference between that and the current Cycle 18. This is because several changes have indeed happened :). Firstly we collaborated with TRDDC (the research wing of TCS) and they developed a constraints optimisation algorithm (click here for the ADS abstract of a document on it) which we now use to generate the first cut of the observing schedule. This schedule, generated in ascii format, is then worked upon using algorithms developed by our telescope operator, Santaji and the final produce is the jazzy look which was implemented from Cycle 17 schedule. Additionally the same algorithms do several things. The schedule generated by the optimizer is not final since it needs to be 'repaired' mainly to account for fragmentation of long observing runs. Santaji's programme helps with this process. Although the change does not reduce the time I put into the process, the upside is that, in principle, the scheduling process can become person-independent.

Determining the GMRT primary beam Any image which 'synthesizes' the antenna primary beam needs to be corrected for the gain variation of the primary beam. Simply said, a 1 Jy source sitting at the peak of the primary beam will be seen as a 1 Jy source whereas at the half power point it will be seen as a 0.5 Jy source. Since this effect is due to the primary beam gain, the final image has to be divided by the primary beam gain to get back the actual flux density of the source near the outer parts of the beam. I have been using data from one-dimensional cuts across the source to obtain these coefficients which can be used in AIPS. Click here for the coefficients of an eighth order polynomial fit to the antenna primary beam which can be directly plugged into PBCOR in AIPS. The more recent addition in here is that alongwith Dave Green, we've been trying to see if we can obtain polarisation beam characteristics.

Pointing Model for GMRT Antennas At the low radio frequencies that GMRT observes, sub arcsec pointing accuracies are not required. At the GMRT observing band of 20cm, the rms pointing accuracy should be better than 0.5' to avoid pointing offsets limiting the dynamic range. If the pointing offsets are greater than this, then deconvolution errors due to point sources located on the outer parts of the beam increases the image plane rms errors and leads to reduced dynamic range.
At GMRT, most antennas show a systematic variation in elevation pointing offsets of 3'-4' (more recent results by Dave Green and Tim Garn show it to be larger and even our recent results point to variations of 4'-6' in many antennas) from rise to transit to set which can be modelled to give a pointing model for each of the 30 GMRT antennas. The pointing group has changed as two of the senior members who were part of it (RN and VKK) have now retired. Subhashis Roy and I comprise the group now. Subhashis has found and implemented the antenna pointing model at GMRT and has a report which can be accessed through the Library page. The other activities that we try ( we need more people so that we can do :> ) to do are
(1) the GMRT primary beam which I have outlined in the previous topic.
(2) update the pointing model.
(3) check for problem antennas wrt encoders, axis misalignment, sudden changes in offsets and transit problems.
(4) polarisation beam shape.
(5) 2-D primary beam and sidelobe levels.

Characterizing Polarization Isolation - this was something I did a while ago. The programmes were developed and the tests are now done by our telescope operators at the Observatory. Anyway, here is the link to the old old writeup.

Imaging with GMRT data

1. Cas A at 240 MHz from GMRT . The dynamic range of this image is about 70. This is a single channel (64 kHz bandwidth) image with data integrated over about 4 hours. The angular resolution is about 18". The data was taken on 19 May 2005. (NGK,NU,SS,AAK,AR,AAD - the Indo -Ukraine project on low frequency radio recombination lines)
2. Cygnus A at 240 MHz from GMRT . The dynamic range of this image is about 125. This is a single channel (64 kHz bandwidth) image with data integrated over about 1.5 hours. The angular resolution is about 18". The data was taken on 19 May 2005. (NGK,NU,SS,AAK,AR,AAD -the Indo-Ukraine Project on low frequnecy radio recombination lines)
   The main points relevant to the procedure used to obtain the above images are given here
3. Holmberg 124 in 21 cm HI . The column density map is shown here with an angular resolution is about 15". The lowest column density is 4.4 X 10^18 /cm^2. (NGK,SAK,RN,AH)
4. Holmberg 124 at 330 MHz . The lowest contour is 3 mJy/beam for a beam of about 15". (NGK,SAK,RN,AH)
5. The nova remnant GK Per at 330 MHz . The lowest contour is 2.1 mJy/beam for a beam of about 12". The cross marks the position of the nova. (NGK,GCA)

1. Click here to see the effect of bad baselines and radio frequency interference on a 610 MHz image.
2. Click here to get some hints for analysing GMRT data in NRAO AIPS.
3. Click here for a little on natural versus uniform weighting for GMRT.
4. Click here to get some hints on doing Tsys calibration for GMRT antennas.
5. Click here to get the expected correlation counts (Ta/Tsys * 1000) of the primary VLA calibrators with GMRT.
6. Click here to get a quick look at a few calibrators taken from the VLA calibrator list which have flux densities greater than 15 Jy at 330 MHz.
7. Click here for some interesting info.

Click here to go to the TGSS webpage.