Expert: Tom Whiting - 8/8/2006

Question
i have a 600mm refractor telascope. if i put a 1.25 focuser on it do i
need to tack the meatle plate out of the telascope?" "and

"what is the most power you can get out of a 10inch telascope?and is
(f/6.0) moore powerfull then a(f/4.0)  "  

Answer
Hi Jody,
Sorry, but I can't help you with your first question because
I know nothing about refractor telescopes....never owned
one.  I only know about Newtonian reflector (parabolic mirror) telescopes.

A 600 mm refractor (?)......a 24 inch diameter lens up front?
That's huge!....that's a permanent observatory telescope!

OR,  something's wrong here....perhaps you are confusing
focal length (F.L.) of your telescope, with lens diameter.  All telescopes are rated by mirror diameter,  or front lens diameter
(also called the objective) if it's a refractor, and not the focal length.  Focal length (FL) is a meaningless term, except for finding the power of your various eyepieces, because power is determined by the focal length of the scope divided by the
focal length of the eyepiece, as you probably know.
But, nevertheless, I can't help on that question because I
have no idea what "metal plate" you are talking about.
But I am very familar with the second part of your question.

Part 2 -- The maximum power of ANY telescope, reflector or refractor, (called the maximum effective magnification),  for a telescope that has excellent optical surfaces, say 1/10 wave or better, is 50 power per inch of aperture....so by simple arithmetic, 50 x 10  =  500 X (power), for your 10" telescope
example.  Above that value, you are getting what is called
"empty" magnification, because you are exceeding the optical
limits of your scope, and you are simply magnifying all the
inherent optical defects of the system.
(NOTE: the focal length does not enter into the maximum magnification equation...it's all based on the diameter of the objective).

Where the focal length comes in.....to achieve that maximum
power of 500 X,  the f6 gives a 10" scope F.L. of 1500 mm.
An f4  10 inch gives a scope F.L. of  1000 mm.
So to achieve the 500 power with the 1500 mm example,
you need to insert a 3 mm eyepiece  (1500 divided by 3), and
to achieve the 500 power with the 1000 mm example,
you need to insert a 2 mm eyepiece (1000 divided by 2).
It's that simple.....so No, an f6 is not "more powerful" than
an f4....it's just that you can achieve more magnification
per the same eyepiece, with the f6 compared to the f4.
But then with the higher powers, you lose 3 things...see the
last paragraph below......

Of course, you can rarely use the maximum effective
magnification....a sky condition called "seeing" or steadiness
of the atmosphere,  is one of the biggest determinants in
using high powers.  Another is the quality of the optics;
these figures I used are for 1/10 wave (equal or better than
2 millionths of an inch of the grind).  If you have poorer or
cheaper optics, like 1/4 wave (5 millionths of an inch)
then you have to drop the 50 power per inch of aperture
to maybe 20 or 30 power per inch of aperture.
Another is collimation of your optics (alignment of mirrors
and eyepieces have to be near-perfect for high powers, not
so much for low powers) but the cone of light has to precisely come down the tube exactly centered, for high power use.  Otherwise, you are getting an off-axis view which will smear a high power image, even under the BEST of all
conditions.

Another is temperature....scopes have to be at, or no more
than a few degrees above the ambient temperature and reach
equilibrium,  before you can achieve your highest powers.....
also remember to keep your body heat out of the optical plane.  
Also remember that as you go up in magnification, we lose
3 things (Sorry, but it's just the laws of Optics and physics) ...field of view decreases dramatically, you lose
light (image is dimmer),  and we lose a little resolution (fine details).  This is why most of us use our lowest, or next lowest eyepiece about 90% of the time, because we'd rather maximize the other 3 items, at the sacrifice of only one thing...magnification.

Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA

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