Small radio telescope for hydrogen 21cm emission line

by Kimmo
Tags: 21cm, emission, hydrogen, line, radio, telescope
Kimmo is offline
Mar6-13, 02:57 PM
P: 1

I am trying to figure out if there is call for a small, commercial, educational radio telescope to measure the 21cm emission line of hydrogen.

We have been developing an instrument with the following characteristics:

- The antenna would have a size of about 1m x 1m.
Because the beamwidth of such an antenna is relatively large, about 14 degrees, it is enough to have a 'manual stand' for the antenna.
Furthermore, it is not necessary to track the motion of the sky during observations; a typical integration of the HI line would take a couple of
The stand would have simple digital setting circles, similar to those used with Dobsonian telescopes. A simple computer program transforms the desired galactic coordinates to altitude and azimuth, and the user then manually points the antenna.

- A low-noise amplifier is attached to the antenna.

- The receiver itself transforms the original signal at around 1420 MHz to a lower frequency, say, between 0-3 MHz, which is then read by an analog-to-digital converter. The receiver is kept as simple as possible so that it can be used to demonstrate the basic components of a radio receiver (local oscillator, mixer, amplifier stages etc.)

- Analog-to-digital converter is used to digitize the signal. An ordinary computer is used to calculate the spectrum of the digitized signal
by using fast Fourier transform. The width of the spectrum could be about 3 MHz, so that it includes the hydrogen line plus some background
emission around the line. Software on a computer is used to integrate (sum up) the spectra so that good enough signal-to-noise ratio
is achieved.

- The spectra can be saved so that external programs can be used to analyze the spectra.

- The most interesting project that can be made with this telescope is probably the measurement of galactic rotation curve.
As an example of this project see

- In addition, one can use this instrument to measure radio emission from the Sun. With some luck, one can detect radio bursts from the Sun. Other sources than the Sun are difficult to observe with this small telescope.

- This instrument is intended to measure hydrogen emission mainly along the galactic plane where the emission is at strongest.

- This instrument could be used at schools, colleges and universities.

So, in principle the telescope would be similar to the SRT (Small Radio Telescope) that was developed at Haystack observatory
(see ), although smaller, lighter and without motorized tracking.
Consequently, the price would be much less than the price of about $6000 for the SRT (SRT is not produced anymore).

What is your opinion, do you think that teachers and students would find this instrument useful and interesting ?

Best regards, Kimmo Lehtinen
University of Helsinki, Finland
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