radio telescope question

I was looking at this
http://www.atnf.csiro.au/news/press/.../LO9V1095p.jpg

and got to thinking, does the notion of f ratio make sense in radio
astronomy ? This looks to me like close to f1. Considering the analogies
to optical astronomy raises some questions. Is there an analog to an
eyepiece in radio astronomy, can radio astronomers select different
magnifications and FsOV ? How would this be done ? Would it be done at
the telescope, or in computer processing of the signal data later ? The
list goes on and on ! Does the notion of "seeing" apply ? Can computer
processing eliminate (or even mitigate)vibration in the "mount" ? At
these wavelengths, does it even matter ?

It just seemed to me like something that could be discussed here at least
as much as some of the other things I've seen.

Tom Rauschenbach wrote:

Comment inline


I was looking at this
http://www.atnf.csiro.au/news/press/.../LO9V1095p.jpg

and got to thinking, does the notion of f ratio make sense in radio
astronomy ? This looks to me like close to f1.

Fast F ratio dishes are used because it is easer to shield the beam from
outside interference if the feed or secondary does not see the ground.



Considering the analogies
to optical astronomy raises some questions. Is there an analog to an
eyepiece in radio astronomy, can radio astronomers select different
magnifications and FsOV ?

imaging single dish systems are usually in the sub mm wavelengths where
the have a small array of detectors instead of the usual single feed.
look up SCUBA, IIRC its in Hawaii at the JCMT

How would this be done ? Would it be done at
the telescope, or in computer processing of the signal data later ? The
list goes on and on

Otherwise interferometers are used to build up images with higher
resolution and in less time than a single dish,single feed system.

! Does the notion of "seeing" apply ?

Yes, it is due to water vapor in the radio and temperature variations in
the optical.
Can computer
processing eliminate (or even mitigate)vibration in the "mount" ? At
these wavelengths, does it even matter ?

Sort of, one can look at the mount error to see where the antenna was
pointing say during a wind gust.
It just seemed to me like something that could be discussed here at least
as much as some of the other things I've seen.

Not many amateurs have a large mm wave dish

d.

I was looking at this
http://www.atnf.csiro.au/news/press/.../LO9V1095p.jpg

Spectacular image of the Parkes single disk I think.

and got to thinking, does the notion of f ratio make sense in radio
astronomy ? This looks to me like close to f1. Considering the analogies
to optical astronomy raises some questions.

Yes. Engineering considerations make short f ratios preferable for
radio astronomy. Although a handful of single dishes with unusually
long prime focus have been built. Most systems use a Cassgrain
arrangement for longer focal lengths and at higher frequencies (with a
reflector just before the prime focus). This has an extra advantage in
that the receivers face the sky which is cold and dark whereas the
ground is warm and noisy. See for example

http://www.jb.man.ac.uk/booklet/Anatomy.html
http://www.atnf.csiro.au/education/i...ion/index.html

Is there an analog to an eyepiece in radio astronomy,

Not really - it is more like prime focus imaging. And reflective
surfaces are much easier to work with at RF.

can radio astronomers select different magnifications and FsOV ?

Only by changing frequency on a given telescope (and then the object
may look different!). Double the frequency and you halve the available
field of view. Or in aperture synthesis you can double the spacings
between dishes to obtain higher resolution. Choosing the right scope
and configuration for the target object can be critical. VLA can
operate in A,B,C or D array configurations with the scope dishes
practically overlapping in D array.

How would this be done ? Would it be done at
the telescope, or in computer processing of the signal data later ? The
list goes on and on ! Does the notion of "seeing" apply ?

Seeing applies in various forms. At low frequencies the ionosphere
plays a part and at higher frequencies water vapour and air pressure
temperature fluctuations. There are tricks to get good observables in
aperture synthesis once three or more scopes are being correlated (more
scopes is better). This has led in a few circumstances to major scopes
suffering damage when they stayed on target during storms for longer
than they really should have.

Can computer
processing eliminate (or even mitigate)vibration in the "mount" ? At
these wavelengths, does it even matter ?

Vibration in the mount isn't much of a worry. The angular resolution of
a single dish at radio frequencies is pathetic. Clever enginineering of
modern dishes means they distort under gravity but maintain a parabola
rather then having to be made completely rigid. Seeing can be largely
eliminated in aperture synthesis though by cunning post processing and
for aperture synthesis it is crucial to know exactly where the dishes
are positioned.

The nicest demo of selfcal where Rick Perley's observation of Cygnus A
first revealed the jet is no longer online (that I can see) the next
best illustration of the stages in data reduction for aperture
synthesis is at:

http://archive.ncsa.uiuc.edu/Cyberia...orm.html#steps

Regards,
Martin Brown

Originally Posted by Tom Rauschenbach

I was looking at this
http://www.atnf.csiro.au/news/press/.../LO9V1095p.jpg

and got to thinking, does the notion of f ratio make sense in radio
astronomy ? This looks to me like close to f1. Considering the analogies
to optical astronomy raises some questions. Is there an analog to an
eyepiece in radio astronomy, can radio astronomers select different
magnifications and FsOV ? How would this be done ? Would it be done at
the telescope, or in computer processing of the signal data later ? The
list goes on and on ! Does the notion of "seeing" apply ? Can computer
processing eliminate (or even mitigate)vibration in the "mount" ? At
these wavelengths, does it even matter ?

It just seemed to me like something that could be discussed here at least
as much as some of the other things I've seen.

The massive [and static] Arecibo Dish is spherical in shape which in turn produces spherical aberration at prime focus eg central part of the dish come to focus futher from the dish than the perimeter. This is resolved by the mosquito-like antenna probed into the beam that copes with difference radii of the dish along its length. The dish tracks on object by moving the antenna thus using a slightly different axis on the dish. This clearly can’t be done if the dish is a parabola which must physically track the object!

The Arecibo also has a Cassegrain and/or Gregorian secondary to correct aberrations etc much like an optical telescope.

Nytecam

"Tom Rauschenbach" wrote in message
news


I was looking at this
http://www.atnf.csiro.au/news/press/.../LO9V1095p.jpg

and got to thinking, does the notion of f ratio make sense in radio
astronomy ? This looks to me like close to f1. Considering the analogies
to optical astronomy raises some questions. Is there an analog to an
eyepiece in radio astronomy, can radio astronomers select different
magnifications and FsOV ? How would this be done ? Would it be done at
the telescope, or in computer processing of the signal data later ? The
list goes on and on ! Does the notion of "seeing" apply ? Can computer
processing eliminate (or even mitigate)vibration in the "mount" ? At
these wavelengths, does it even matter ?

It just seemed to me like something that could be discussed here at least
as much as some of the other things I've seen.

If you want to have a play with image synthesis take a look at
http://www.narrabri.atnf.csiro.au/ww...onomy/vri.html
It is a virtual radio interferometer and has some good information about
interferometry.

Terry B