QUESTION:
What's the magnifying power of the HST?

ANSWER from Bill Gutsch on April 8, 1996:
Telescopes gather light by means of their primary and secondary mirrors.
By inserting different eyepieces into a telescope, different powers can
be achieved.  The formula for the power given by any particular
eyepiece is simply:

Power = The focal length of the main mirror / the focal length of the eyepiece

Thus, any telescope can yield any power given a short enough focal length
eyepiece.  But, every telescope is limited.  Typically, the highest
useful power you can get with a telescope is about 40 x the diameter of
the main mirror in inches.  You can get higher powers but what you see
is pretty fuzzy and dark and so you wind up seeing less rather than more.

So what focal legth eyepieces does the HST have?  Actually, it doesn't
have any eyepieces.  Instead, it just uses its mirrors to focus light
on to CCDs which are part of cameras or spectrographs.  If you have a
telescope at your school, you can see it form an image without an
eyepiece.  Just aim the scope at the moon and tape a piece of tracing
paper or wax paper over the end of the tube where you would normally
put the eyepiece and you'll see an image of the moon on the paper.

While amateur astronomer usually talk about power because they use
their scopes to look at stuff through them,  professional rarely refer
to power because they rarely use eyepieces in their work.  Telescopes
are used by professionals just to focus light.  But, using our little
formula above, if you had the HST in your backyard, on a real good night,
when the atmosphere was very steady I guess you could get up to about
40 x 94 inches = 3760 power.