Doink posted:

How can I determine limiting magnification of my scope or any scope? I have
an 8" 1000fl f/4.9 reflector and a 4" 1000fl f/8.3 achro refractor.

Could I use a 3mm Radian Eyepiece to look at the moon? Better to use a 6mm
and 2X Barlow?

There is a very rough "rule of thumb" about maximum magnification: about 50x
to 60x per inch of aperture (although it is not a rigid rule by any means).
Thus an 8 inch might be useful up to 400x to 480x, and a 4 inch might have a
high power range of 200x to 240x. Again, these are very rough figures, and
your maxima may depend on not only seeing, but the quality of the instrument
and the specific item you are viewing. Here is some information about useful
power ranges.


USEFUL MAGNIFICATION RANGES FOR VISUAL OBSERVING
IN ASTRONOMICAL TELESCOPES

LOW POWER (3.6 to 9.9x per inch of aperture)(7mm to 2.6mm exit pupil):
Useful for finding objects and for observing ones of large angular size
like open clusters, large faint nebulae, or some larger galaxies. For
lunar work in modest apertures, it is generally somewhat on the low
side, but can show the crescent moon with background starfields well.
This is also the range where Nebula filters tend to perform the best.

MEDIUM POWER (10x to 18.9x per inch of aperture)(2.5mm to 1.3mm exit pupil):
Useful for observing somewhat smaller deep-sky objects such as galaxies, some
diffuse nebulae, smaller open clusters, and moderate to large planetary
nebulae. Also useful in apertures 6 inches and larger for getting at least
partial resolution on the brightest globular star clusters. Often used in
moderate to large apertures for detecting very small galaxies which may be
invisible at low powers and for revealing details in some galaxies like dark
lanes, mottling, and star-like nucleii. Very useful for wide area views of
the moon, or for showing the moon systems and some of the larger features of
the planets.

HIGH POWER (19x to 31.9x per inch of aperture)(1.3mm to 0.8mm exit pupil): A
very useful power range for observing fine planetary and lunar detail. This
is the range where the full theoretical resolving power of the telescope is
becoming visible. Also useful for getting better star resolution in tight
globular clusters or for viewing detail in the smaller planetary nebulae, as
well as resolving tight double stars. This power range is sometimes
compromised in apertures larger than 5 inches by seeing effects (ie:
disturbances in the Earth's atmosphere which can blur fine detail).

VERY HIGH POWER (32x to 46.9x per inch of aperture)(0.8mm to 0.5mm exit
pupil): Useful for study of certain specific planetary details, and resolving
double stars near or just above the resolution limit of the instrument. Also
useful for resolving the cores of some very tight globular clusters or for
detecting the finer detail and faint central stars in the smaller planetary
nebulae. Quite useful for telescope collimation tests or rough star-testing.
This power range is not as frequently usable with larger apertures due to
seeing disturbances. For planetary viewing, eye defects like motes and
floaters (along with the somewhat lower overall light level), begin to become
visible and slightly annoying in the upper half of this range.

EXTREME POWER (47x to 75x per inch)(0.5mm to 0.3mm exit pupil). Mainly
used for resolution of double stars at the resolution limit of the
instrument, or for detecting elongation of unresolved doubles. Powers up to
60x per inch are sometimes usable in rather small instruments for making
gross planetary detail easier for beginners to see (ie: Jupiter's main belts
or the Cassini Division in Saturn's rings). This power range is not often
used in apertures above 6 inches due to seeing limitations, and requires very
good optical quality in the instrument. Even when conditions are good, lunar
and planetary views using this power range can sometimes seem less pleasing
overall than at somewhat lower powers due to the lower light intensity and
increasing interference from eye defects like floaters. However, this range
can be somewhat useful for certain *specific* targets or details which
require extreme scale. Examples include (for large apertures) seeing Encke's
Division in Saturn's rings, the central star in M57, detail in some brighter
planetary nebulae, or for resolving a few small specific lunar details.
Powers from 75x to 90x per inch are occasionally used for very close double
star elongation, micrometer measurements, or for optical testing, but
otherwise, powers well beyond 75x per inch can often be nearly useless,
especially in inexpensive "department store" telescopes.

EMPTY MAGNIFICATION (100x per inch and above). Nearly useless powers,
mainly used as a marketing ploy by unscrupulous telescope retailers or
manufacturers to sell small over-powered telescopes to beginners.


--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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