For serious lunar, planetary, globular cluster, and binary star observing – as well as for surprisingly good views of the brighter Messier, NGC, and IC catalog objects – many amateur astronomers prefer the crisp, high-contrast, diffraction-free images of a good refractor.

Under average seeing conditions, a useful rule of thumb in astronomy is that a good 3″ to 4″ refractor will usually outperform an average 6″ to 8″ reflector or Schmidt-Cassegrain for seeing details on the Moon and planets, splitting binary stars, and resolving globular clusters. The situation becomes a little more complicated when comparing refractors to Maksutov-Cassegrains or Maksutov-Newtonians, but (with a few high-priced exceptions) a good refractor will usually equal or outperform a Mak-Cass or Mak-Newt of equal or slightly larger aperture

Why? Unlike reflectors and catadioptrics (Schmidt-Cassegrains, Maksutov-Cassegrains, etc.), refractors do not have a secondary mirror obstruction or multiple-reflection optical path to introduce light-scattering diffraction and internal reflections that brighten the sky background, reduce contrast, and smear images.

Refractors also have the highest light transmission – the percentage of the light gathered by the scope that actually reaches your eye. Refractors can transmit 90% or more of the light they collect, compared with the 77% to 80% transmission of reflectors and 64% to 75% of catadioptrics. (The reflector and catadioptric percentages only concern mirror reflectivity. They do not take into account the light blocked by a reflector or catadioptric’s diagonal or secondary mirror, which can reach a hefty 15% to 20% additional light loss in some scopes.)

Unlike reflectors and catadioptrics, which lose 1% to 1.5% of their reflectivity per mirror surface per year as their aluminum coatings gradually oxidize, the light transmission of a low maintenance refractor rarely deteriorates significantly with age. Century-old refractors are still used, and highly prized, by discerning amateurs, and the world’s largest refractor – the Yerkes Observatory’s massive 40″ – has been in constant professional use since 1897.

The result of a refractor’s lower diffraction and higher light transmission? Given favorable seeing conditions, a modestly-sized refractor can show you subtle lunar and planetary features with a wider and more easily observed contrast range, and with more sharply etched detail, than is possible with the light-scattering optics of many larger reflectors and catadioptrics. This is especially true on nights of less-than-perfect seeing, when the details visible in a larger scope are often blurred by turbulence in our atmosphere. A smaller refractor looks through less of our unstable atmosphere and its images are consequently less affected by this turbulence. A good 80mm refractor, for example, can reveal more lunar detail than you can sketch in a lifetime of observing.

Diffraction spikes on a reflector’s star images, caused by its diagonal mirror’s spider vanes, are absent in an unobstructed refractor. With no diffraction spikes to hide faint binary star components or smear globular clusters, refractors can resolve close-spaced stars more precisely than the typical reflector.

Since the Moon and planets are all brightly lit by the Sun, a large light-gathering capacity is not as important as high magnification within the solar system. The relatively small aperture of a refractor is therefore often an advantage for this kind of observing, as is the high magnification capability of its long focal length, as there is less glare from brightly lit planetary surfaces to wash out faint detail.

For purely visual lunar, planetary, binary and star cluster observing, an altazimuth refractor with slow motion controls may be perfectly adequate. If a family shares the telescope, however, an equatorial mount with a motor drive will keep objects centered in the field of view so all can share the same view. Close-up lunar and planetary photography requires such a mount and motor drive. Due to the limited light gathering of the smaller refractors, long exposure deep space nebula and galaxy photographs are rarely attempted with this type of telescope.

The drawbacks of a refractor? Except for very expensive apochromatic designs, all refractors suffer from chromatic aberration (or “spurious color”). This is an optical defect that produces a faint, and normally unobjectionable, pale violet halo around bright stars, the limb of the Moon, and the planets. Chromatic aberration becomes more visible as the aperture increases and the focal ratio decreases, although modern optical systems minimize the problem in two-element achromatic refractors – and virtually eliminate it in three to four lens apochromatic systems.

While they are light in weight and economical in smaller sizes, refractors become bulkier and considerably more expensive than reflectors or catadioptric scopes as apertures hit 4″ (102mm) and above. A premium 4″ apochromatic refractor typically costs and weighs four to eight times as much as a 4.5″ reflector or 3.5″ Maksutov-Cassegrain.

But these drawbacks aside, and if sheer light grasp is not essential – for hunting very faint galaxies, for example, where a larger reflector would have the light-gathering edge – the clarity, contrast, and sheer image quality of a good refractor is well worth your consideration.

REFRACTOR REPORT CARDS
(used in excellent seeing conditions and with no light pollution; adapted from Astronomy Magazine):

E = excellent; VG = very good; G = good; F = fair; P = poor.

Small aperture (2″ to 3″) “toy store/bargain” refractors:
Price range: $100-$200
Portability: E
Ease of setup: E
Ease of use: F
Performance on the Moon: F
Performance on comets: P
Performance on double stars: P
Performance on galaxies and nebulas: P
Performance on planets: P

Small aperture (3″ to 4″) achromatic refractors:
Price range: $200-$800
Portability: E
Ease of setup: G
Ease of use: G
Performance on the Moon: E
Performance on comets: F
Performance on double stars: VG
Performance on galaxies and nebulas: F
Performance on planets: VG

Medium aperture (4″ to 5″) apochromatic refractors:
Price range: $700-$10,000
Portability: VG
Ease of setup: E
Ease of use: VG
Performance on the Moon: E
Performance on comets: VG
Performance on double stars: VG
Performance on galaxies and nebulas: G
Performance on planets: VG

Large aperture (5″ to 8″) achromatic refractors:
Price range: $800-$3200
Portability: F to VG
Ease of setup: G+
Ease of use: VG
Performance on the Moon: E
Performance on comets: VG
Performance on double stars: E
Performance on galaxies and nebulas: G
Performance on planets: E

Large aperture (6″ to 8″) apochromatic refractors:
Price range: $5000-$27,000 and up
Portability: F
Ease of setup: F
Ease of use: VG
Performance on the Moon: E
Performance on comets: VG
Performance on double stars: E
Performance on galaxies and nebulas: G
Performance on planets: E

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