archive: Re: SETI A hair-brained idea

Re: SETI A hair-brained idea

David Woolley ( )
Sat, 19 Sep 1998 12:33:54 +0100 (BST)

Dr Shuch wrote:
> At 07:47 PM 9/17/98 +0100, Colin Crook wrote:
> >Ok first,light is a form of radiation.So are radio waves.Now if a telescope
> >can magnafify light 400x,couldn't it also do the same with radio waves?You

Magnification in a telescope is only indirectly related to the increase
in light gathering power. Magnification allows one to have a larger
objective lens without the beam at the eyepiece, resulting from a
point-like source, exceeding the width of the dark adapted pupil of
a human eye. The size of this exit beam is called the exit pupil.
In fact, magnification is impossible to define if the detector is a CCD
array or piece of film, because you would be comparing an angle with
a length.

> >could focus your telescope on a star,put your reciver at the eye piece,and
> >get a signal 400x more powerful!If this would work,it would cost less than

[ Reasons why it wouldn't work explained in Dr Shuch's reply. ]

If the objective is 400x the diameter of the dark adapted pupil of the eye
the power gain for a human observer will be 160,000 not 400, for reasons
that Dr Shuch explains below.

Ideally binoculars or a telescope, intended for night time use, should
have an objective size which is the diameter of the dark adapted pupil
times their magnification. The objective size and magnification are
the two numbers used to characterise binoculars, and the ratio of the
first to the second is larger in binoculars designed for night time use
and smaller in cheap ones.

The increase in sensitivity is proportionnal to the relative area of the
eye's pupil and the objective lens, provided the magnification is at least
enough that the exit pupil is not larger than the pupil of the eye. Excess
magnification will not incease sensitivity (and may decrease it as the
image blurs through diffraction effects (Airey disks)).

> >a dish,and take up less space.So,would it work?
> Colin,
> You have just described exactly how both (Newtonian) optical telescopes
> and (parabolic dish) radio telescopes work. The reason the optical
> telescope mangifies light hundreds or thousands of times is that its mirror
^^^^^^^^^ You are confusing magnification with light gathering
power as well.