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I. Since the crew of astronomers in this Observational
Techniques course already has some experience
with "optical telescopes" and a working knowledge of optics
and diffraction theory, it's useful to apply that experience
to radio telescopes. Some comparing and contrasting is
worthwhile.

Consider the Australian astronomical icon, "The Parkes Dish":
When it was built, it had a 210 foot diameter; now it's
64 meters. The focal ratio f/D is 0.41.

The telescope is a favorite to Australian astronomers for
observing the 21cm spectral line of atomic hydrogen.

Some obvious questions to ask are:

What is the angular resolution of Parkes for 21cm line work?
(Most radio telescopes operate at "the diffraction limit.")
(How does this compare to typical resolution for ground based
optical telescopes?) Are there obvious ways to improve the
radio resolution? Do radio wavelengths have analogous
problems to optical "seeing"?

Suppose you're signed up to build a "radio camera" to put at the
prime focus at Parkes. Instead of a CCD with tiny pixels,
you'll need an array of radio detectors. How far apart should
they be spaced in the focal plane to sample the image in a
similar way to the CCD detector does?

How smooth does a mirror need to be?

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II. During the Observational Techniques meeting on Friday 27 April
we'll talk about sensitivity of radio telescopes, units for radio
intensities from celestial sources, and a little bit about radio
emission mechanisms. The following is a exercise to do after that
discussion.

The idea is to use the Parkes All-Sky Survey in the 21cm line to
estimate some parameters for your favourite galaxy. (Don't have a
favourite galaxy yet? well, now's the time to get one...)

If you don't have one yet, I'd suggest choosing
one of those gorgeous, photogenic spiral galaxies.
It is advantageous for it to be in the southern hemisphere,
for the obvious reason that Parkes could only survey the sky
south of +30 deg declination. (There are heaps of beautiful
galaxies south of +30)

Once you have a name, like NGC 4650A or M83 or some RA,Dec
coordinates for the galaxy, you can visit the HIPASS website to
get a hydrogen line spectrum for it:

http://www.atnf.csiro.au/research/multibeam/data_archive/data.html

Then, go to "HIPASS data"
Then you can either enter the name of the galaxy in "Object Name"
to obtain the celestial coordinates from the NED or LEDA data bases,
-OR- you can enter the coordinates directly in the RA, Dec blanks
on the webform.
Then "Plot"
Adjust velocity range on the horizontal axis, if desired.
There are download options so you print a copy of the
spectrum.

The kind of information that you can derive for your galaxy
includes:
0. Oops,... there's no signal!... why not?
0.a. For purposes of this
exercise, you'll have to choose another favourite galaxy
until you have one with some signal.
1. use the spectrum to estimate a distance to the galaxy
(what kinds of things can go wrong with this estimate?)
2. use the spectrum to estimate the rotation speed of the
galaxy. (What can go wrong if you only have the HI spectrum
to work with? If you have more info - like an optical image -
could you do better?) Can you get a measure of "dynamical mass"?
3. estimate the amount of hydrogen mass in the galaxy.
(what might have gone wrong with your estimate? ... lots of things
might...)
4. Suppose you'd like check whether the HIPASS survey is correct,
so you're going to head back to Parkes and reobserve.
How long must you observe the galaxy in order to "detect" it,
based on the signal strength that you saw in the HIPASS data?