Magnifications - Meade 8" LX100 Series Instruction Manual

-32-
Focusing the telescope from its nearest possible focus point (on an object about 25 tt. away with the g,
LX100, or about 50 ft.
with the 10'
LX100) to an object at infinity requires a
fairly large number of rotations
of
the focus knob. The focuser
is designed to provide an extremely
sensitive means
of bringing an
object
into precise, sharp
focus.
After
a
specific
object has been brought into focus, closer objects require turning
the
focus knob clockwise;
more distant objects require turning the focus knob counterclockwise.
It is
possible that you may notice
a
slight shifting
of
the image
as
you focus, particularly at high
powers.
This
image shift
is caused
by
very small lateral motions
of
the primary mirror
as it
moves toward
or
away from the
secondary
mirror during the focusing procedure.
3. Magnifications
The
magnification,
or power,
of the telescope depends
on two optional characteristics:
the focal length of
the
main telescope and the focal length
of the eyepiece used during
a
particular
observation.
For example,
the focal length
of the
8'
fl10 telescope
is fixed
at
2000mm;
the
focal
length
of
the
10" f/10 telescope is fixed
at 250Omm. To calculate the power
in use
with a particular eyepiece, divide the focal length
of the eyepiece
into the focal
length
of
the
main
telescope.
For
example,
using the SP 26mm eyepiece supplied with the
8"
f/10, the power is calculated as follows:
Power=
43F'
= 77x
zbmm
The type of eyepiece
(whether
"MA"
Modified Achromatic,
'OR'
Orthoscopic, 'SP" Super
Plossl, etc.)
has
no bearing
on magnifying power
but
does affect such optical characteristics as field
of view, flatness
of
field
and color correction.
The
maximum
practical
magnification is determined by
the nature of the object being
observed and, most
importantly,
by
the
prevailing atmospheric
conditions.
Under very steady atmospheric'seeing,'the 8'
LX100
maybeusedatpowersuptoabout500Xonastronomicalobjects,thel0'LXlOOuptoabout600X.
Generally,
however, lower powers
of
perhaps
250X to 350X
willbe
the maximum permissible,
consistentwith
high image
resolution. When
unsteady air conditions prevail
(as witnessed by
rapid
'twinkling'
of
the stars), extremely
high-power eyepieces result
in
"empty
magnification,'where
the object detail observed
is
actually diminished
by the excessive
power.
See page 33 for additional information on eyepieces.
When beginning obsevations on a
particular object, start with
a
low power eyepiece; get the
object
well-centered in the field of view
and
sharply focused. Then try the
next step up in magnification.
lf the
image starts to
become
fuzzy as you work
into higher magnifications,
then back down to a lower power
-
the
atmospheric steadiness
is not sufficient to
support
high powers at the time
you
are
observing.
Keep
in
mind that
a
bright, clearly resolved
but smaller image
willshow
far more
detail than
a
dimmer, poorly resolved
larger image.
Because
of certain characteristics
of
the human
eye
(in
particular, eye pupil diameter)
and because
of
optical
considerations inherent in the design of a
telescope, there exists minimum practical power levels
also.
Generally speaking, the lowest usable power is approximately 4X per
inch of telescope
aperture, or about
32X in the case
of the 8" telescope, about
40X
in
the case
of
the
1
0"
telescope.
During the daytime, when
human eye pupil diameter is reduced,
the minimum
practicalpower
with the
8'
LXlOO
is increased
to about
60X
and
to aboutT5X with the 10'LX100; powers lowerthan this levelshould be
avoided during daytime
observations. A reasonable
magnification range for daytime terrestrial observations
through the
8'
LX100
is from about 80X
to 190X; through the 10" LX100 from about
100X to 200X.
Accessories are available both to
increase and decrease the
operating
eyepiece power of the
telescope.
See
your
Meade dealer for information on accessories.