Nimisha G. Kantharia

Click here for a little on my academic background.

Publications

1. Click here for my publication list as listed by ADS.

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 15 can be found here .

Scheduling Software Using my experience in scheduling at GMRT, we have started a collaborative project to generate a scheduling software. Presently NCRA does not have a scheduling software and the GMRT time scheduling is manually implemented - this has been working well. However it is time consuming and I spend 2-4 weeks preparing the schedule for 4-5 months. It would help to delegate more of the work to the computer which this software when complete should be able do. It will not do away with human involvement totally but can come up with reasonable solutions. The software should also have other parts useful to the user, scheduler and GTAC. Update: A collaborative project with TRDDC, the research wing of TCS has been started in April 2006 for developing an optimisation algorithm. TRDDC has delivered a prototype software for testing in late 2006 which we have tested. Since I was on leave for a good part of last year the project got delayed on the NCRA side.

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.

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. We (NGK,RN,VKK), who comprise the group who worries about pointing are trying to work towards this. Click here for a technical note that I have prepared for this. Click here to see the activities of the pointing group.

Click here to see the webpage of the Observatory software group of which I am a member. We have been trying to organise the software. We are also trying to ensure easy accessibility to help documentation on various locally developed offline software.

Characterizing Polarization Isolation

Imaging with GMRT data

Some common data errors

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.