archive: Re: SETI Question...

Re: SETI Question...

Dave Barstow - SSL ( davedoes@magicnet.net )
Tue, 22 Sep 1998 10:08:08 -0400

Yuri Yuryev wrote:

> Hello SETIzens,
>
> I have a question in regard to pictorial representations of different parts of
> the sky in radio spectrum. And the question is:
> How is it done?
> I understand (on a basic level) how you can produce a picture in the optical
> spectrum... The OF waves enter trough a telescope into your eye (or camera,
> etc) and get projected onto an aretna or pixels, if you will, both of which
> have a two-dimensional area, out of which a picture can be produced.
> However, in radio astronomy you only get one signal, one wave, which enters an
> LNA! So how do you produce a two-dimensional picture out of one signal without
> "wiggling" your dish?
>
> Thank you in advance,
>
> Yuri

The "image" created by a radio telescope is sometimes refered to as an isophot
which is essentially the same as a topographic map with isogonic lines of equal
elevation. In the radio image the lines would represent points of equal signal
amplitude.

If we are running in drift-scan mode, letting the earth's rotation scan our beam
in right asention, we can record the signal strength as the beam crosses the area
of interest. If we change the declination of the telescope slightly each day we
will scan successive strips of sky.

Once we have enough strips to cover the area of interest we can arange them so
that the horizontal axis lines up in right asention and the vertical axis is
aligned in declination. If we now connect the areas of equal amplitude, and fill
the enclosed area with a color representing that amplitude, we will have a form of
"photo" of the area.

The same sort of "scan" can also be done by moving the antenna so as to scan
successive strips.

I hope this gives you an idea of how an image is created with a radio telescope.

Regards,

Dave Barstow