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How high is too high? Choose the right magnification for your planetary observing.

For planetary observers, selecting the optimal magnification requires striking a balance between the size and sharpness of the image while preserving sufficient image brightness.

A minimum magnification is required for the human eye to fully exploit the resolving power of any telescope. Most sources cite a rather modest value of 13x per inch of aperture that was derived from studies that used test charts of very high contrast--black markings on a white background. Of course, the contrast of most planetary markings is far more subdued. When low-contrast test charts that closely mimic planetary subjects are employed, this value increases by a factor of two or even three, corresponding to at least 26x to as much as 39X per inch of aperture. If we use the latter figure, to fully exploit the resolving power of a 4-inch telescope requires a magnification of 156x, while a 16-inch telescope requires no less than a whopping 624X.

Provided that a telescope has sufficient optical quality, the single most important factor in determining the clarity of the planetary images that it will deliver is the tranquility of the Earth's atmosphere. "The atmosphere," lamented the French astronomer Andre Couder, "is the worst part of the instrument." "Seeing" is caused by turbulent air cells at altitudes ranging from several hundred feet to ten miles that have different temperatures and hence different indices of refraction. At most observing sites these atmospheric cells usually range in size from four to eight inches in diameter, although research by atmospheric physicists has revealed that they can vary tremendously in size. Each cell acts as a lens, changing the focal position of the image by bending incoming rays of light differently.

If the aperture of a telescope is large enough to receive light that has passed through many air cells, a blurred, "washed-out," or "boiling" image will result. But when the aperture of a telescope is approximately the same diameter as the air cells, the image will be well defined, although sharpest focus may change as individual cells drift across the light path.

The larger the aperture of the telescope, the smaller the probability that the air mass over it will be optically homogeneous at any given moment. When it comes to resolving planetary details, telescopes of modest aperture (from 8 to 10 inches in diameter) are disproportionately efficient compared to large instruments, resolving to their theoretical limits on a far greater number of nights. In 1885, the American astronomer Asaph Hall remarked: "There is too much skepticism on the part of those who work with large instruments in regard to what can be seen with small ones."

The remarkably high efficiency of modest-aperture telescopes is accurately reflected in a five-decade-old formula devised by the German planetary observer Gunter Roth. According to Roth, the maximum practical magnification for planetary observing corresponds to 140 multiplied by the square root of the aperture in inches, as tabulated below.

The experience of Edward Emerson Barnard, the preeminent American planetary observer of the 19th century, lends credence to Roth's formula. According to Barnard, the best planetary images through the 36-inch Lick refractor were obtained with magnifications of 360X to 540X (10X to 15X per inch of aperture), with no improvement beyond 1000X (28X per inch of aperture) even on exceptionally steady nights.

Barnard's contemporary, the Greco-French astronomer Eugene Antoniadi, also preferred to use comparatively low magnifications with the 33-inch Meudon refractor at the Paris Observatory. His best views of Mars and Jupiter were obtained with magnifications of only 320X to 540X (10X to 16X per inch of aperture).

If large telescopes are so handicapped by atmospheric turbulence that they usually fail to resolve details beyond the capabilities of a 12- to 16-inch aperture, why do they often deliver more revealing views of the planets than smaller instruments? The answer lies in image brightness, which is essential for perceiving subtle differences in both tone and hue.

The various luminances (the apparent brightness per unit area) of the planets compared to the full Moon are shown in the table to the left.

With large apertures, the excessive surface brightness of the Moon, Venus, and Mars results in troublesome glare that must be reduced using neutral density or color filters. However, the apparent surface brightness of Jupiter is only one-third that of Mars, and the apparent surface brightness of Saturn is in turn only one-third that of Jupiter. The features of the gas giants consist of a palette of delicate pastel hues of rather modest contrast. Jupiter's belts and zones typically differ in apparent brightness by 10% to 20% percent, while the contrast of Saturn's belts and zones, muted by the presence of high-altitude atmospheric hazes, usually amount to only 5% to 15%. If the image is large but dim, it becomes difficult to perceive subtle differences of contrast and color.

The leading British Jupiter observer of the late 19th century, William Frederick Denning, was equipped with an excellent 12.6-inch Newtonian reflector. He found that magnifications of 205x and 225x gave superb definition but images that were too small. The best power for Jupiter (and planetary observing in general) proved to be 315x, while 404x and 450x offered no advantage except on the very best nights. Denning's results are representative of the general consensus that has emerged among experienced planetary observers.

The old Roman axiom De gustibus non es disputandum ("In matters of taste there can be no disputes") certainly holds true when it comes to magnification. Individual differences in visual acuity and even personality or temperament play a role. Some observers prefer to use comparatively high magnifications even under rather adverse seeing conditions, waiting patiently for those fleeting moments when the seeing momentarily improves.

The selenographer Johann Heinrich von Madler was an extreme example of the "wait it out" school. He routinely used 400x with his 4-inch Fraunhofer refractor when observing the Moon to produce (in collaboration with Wilhelm Beer) Mappa Selenographica and the first decent map of Mars. On the other end of the spectrum was the keen-eyed Philipp Fauth, one of the leading German lunar and planetary observers of the early 20th century. Fauth usually employed magnifications of only 25x per inch with his 6-, 7-, and 15-inch refractors, reserving the highest magnifications of only 38x per inch only for the finest nights. "Sharpness is more important than image scale," he wrote. Every observer has his or her own personal equation, and it may change over the years.

Thomas Dobbins began observing the planets when Lyndon Johnson was President of the United States.

Maximum Magnification

Aperture    Maximum useful    Power
(inches)    magnification    per inch

4                280X          70X
6                343X          57X
10               443X          44X
16               560X          35X
24               686X          29X
36               840X          23X


Planet       Luminances

Full Moon       1.0
Mercury         5.8
Venus           9.6
Mars            0.55
Jupiter         0.15
Saturn         0.045
Uranus         0.013
Neptune        0.005
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Author:Dobbins, Thomas A.
Date:Jan 1, 2016
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