Friday, March 29, 2013

Say Hi to J065514.3+540858

For the last couple of years, I have been working on building a 5 meter radio telescope for educational purposes in my free time. Its primary purpose is to map neutral hydrogen distribution in the milky way. Hydrogen, the simplest atom, shines at the radio frequency of 1.42 Ghz (or 21 cm line), and we use multistage amplifiers to boost the very weak radio signal to something that can be processed by the electronics of a spectrometer.

Using INDI + KStars as the control platform, the user can command the telescope to slew and track objects. Slewing to an object is quite trivial. The telescope is equipped with absolute optical multiturn encoders that provide the positional feedback to the control system. If we know the home position of the telescope and the encoder to degrees ratio, it would be possible to command the dish to move to a new position and stop whenever the new desired angle is reached. Tracking, however, is another matter.

The telescope motion is in Alt-Az (Altitude-Azimuth) while objects rotate due to the motion of the earth from east to west at sidereal rate. Hence, both altitude and azimuth axis must move in a step-wise ladder direction to keep up with the sidereal motion. Due to limitation in the mechanical system of the dish, the minimum speed of both axis is still a lot faster than the sidereal rate, and therefore, tracking was developed to keep the object within the beamwidth of the radio dish, which is 3 degrees, at all times via a user configurable tracking threshold. Whenever the threshold is exceeded, the dish corrects itself using the minimum speed possible.

The following is a screenshot taken for the creatively named J065514.3+540858 radio source in the constellation of Lynx. It is a relatively bright radio source with integrated flux of 1371 mJy @ 1.42 Ghz.

In the next few days, the goal is to fine tune the pointing accuracy and to calibrate the overall system noise. Finally, I'd like to thanks the developers of Kst for making such a great program! I use Kst to stream real time measurements from the continuum spectrometer, and this saved a lot of development time and effort. Kudos to the Kst team!