Telescope Reviews: How much magnification?

OK, here a rundown of useful power ranges. The power will be per inch of aperture (i.e. if you have a six inch telescope, 10x per inch of aperture would mean that you would be using 60x).

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 in moderate to large apertures 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 in larger telescopes 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.

Clear skies to you.

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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info