15.3 Radio Practicals
211
using the cloud velocities formula as derived from the shift in their 21 cm line and
using a model of the rotation of the clouds around the galactic centre.
You should look at your planetarium package with the Milky Way turned on, to
determine what parts of the galaxy you will be able to see and where in the sky it is
in galactic coordinates. As the telescope effectively has only one pixel, you will be
scanning the Milky Way by taking a spectrum and then moving the telescope half
a beam width before taking the next spectrum. This will be needed for the whole
visible region, so it is best to plan how to do this in advance.
Aims
The principal aim of the practical is to give the user experience in using a radio
telescope and taking data. However, the observer will also be determining the rotation
curve of the Milky Way and mapping the HI distribution.
Observation
Once at the telescope, start it up and slew to a bright radio source, for example Cas
A or the Sun. Once there, take spectra with the radio telescope and confirm that you
are seeing a signal. You might have to adjust the gain or integration time to see detail
in the spectra.
Once you are happy with the operation of the telescope, slew to the first region
of the Milky Way. Once on target, take the spectra. Note the velocity position of
the peak in the curve and its intensity as well as its position in the sky in galactic
coordinates. If there is more than one peak, you might be seeing two or more clouds,
at differing distances, in which case note them all. Save the plot to a suitable medium.
Slew the telescope half a beam width to the next region of the Milky Way that
you are going to examine and take other spectra. Do this until you have covered at
least a 20
◦ section of the galaxy.
You might wish to take multiple spectra of each section if you have time, as this
will help identify your uncertainties.
At the end of the observation run do not forget to park and shut down the telescope.
Analysis and Write-Up
We now wish to map the HI in the galaxy and find the galaxy’s HI rotation curve. The
local standard of rest is the origin of our reference system, i.e., the current position
of the Earth compared to the galactic centre at any one time. However, the Earth is
moving in relation to the local standard of rest as it orbits the galactic centre. Hence,
any Doppler shifted signal we receive from an HI cloud contains this movement as
well as that of the cloud around the galactic centre. Figure 15.3 shows the relationship
between the galactic centre (GC), the radial velocity of the Sun relative to the local
211
using the cloud velocities formula as derived from the shift in their 21 cm line and
using a model of the rotation of the clouds around the galactic centre.
You should look at your planetarium package with the Milky Way turned on, to
determine what parts of the galaxy you will be able to see and where in the sky it is
in galactic coordinates. As the telescope effectively has only one pixel, you will be
scanning the Milky Way by taking a spectrum and then moving the telescope half
a beam width before taking the next spectrum. This will be needed for the whole
visible region, so it is best to plan how to do this in advance.
Aims
The principal aim of the practical is to give the user experience in using a radio
telescope and taking data. However, the observer will also be determining the rotation
curve of the Milky Way and mapping the HI distribution.
Observation
Once at the telescope, start it up and slew to a bright radio source, for example Cas
A or the Sun. Once there, take spectra with the radio telescope and confirm that you
are seeing a signal. You might have to adjust the gain or integration time to see detail
in the spectra.
Once you are happy with the operation of the telescope, slew to the first region
of the Milky Way. Once on target, take the spectra. Note the velocity position of
the peak in the curve and its intensity as well as its position in the sky in galactic
coordinates. If there is more than one peak, you might be seeing two or more clouds,
at differing distances, in which case note them all. Save the plot to a suitable medium.
Slew the telescope half a beam width to the next region of the Milky Way that
you are going to examine and take other spectra. Do this until you have covered at
least a 20
◦ section of the galaxy.
You might wish to take multiple spectra of each section if you have time, as this
will help identify your uncertainties.
At the end of the observation run do not forget to park and shut down the telescope.
Analysis and Write-Up
We now wish to map the HI in the galaxy and find the galaxy’s HI rotation curve. The
local standard of rest is the origin of our reference system, i.e., the current position
of the Earth compared to the galactic centre at any one time. However, the Earth is
moving in relation to the local standard of rest as it orbits the galactic centre. Hence,
any Doppler shifted signal we receive from an HI cloud contains this movement as
well as that of the cloud around the galactic centre. Figure 15.3 shows the relationship
between the galactic centre (GC), the radial velocity of the Sun relative to the local