REFRACTORS

Plus Points:

• Easy to use.
• Generally give high contrast and sharp images.
• Not usually prone to thermal currents from the observer's body heat.

Minus Points:

• Most expensive design, inch for inch of aperture.
• Can suffer from "chromatic aberration" (false colour), unless sophisticated (expensive!) glass is used.
• Difficult to mount in large apertures (over 5 inches), unless weight and mass is not a consideration.
• Can be unwieldy and heavy in larger apertures, unless of the apochromatic design. Large apochromats are extremely expensive, but are capable of exquisite imaging. If Achromatic, then required focal ratios can result in extremely long tubes, which are difficult to mount solidly. There are, however shorter focus achromats now being produced that have chromatic aberration sufficiently well subdued to please many observers, and at surprisingly reasonable prices.

REFLECTORS

Plus Points:

• Completely free from false-colour.
• Cheapest design inch-for-inch of all telescopes.
• When well made will deliver sharp and contrasty views.
• Easy to manipulate and rigid in the Dobsonian form.
• Portable up to 8 inches aperture in the Dobsonian.

Minus Points:

• Prone to thermal currents, not only in the tube, but from the observer's body. These can "muddle" the image temporarily.
• Reflectors need to cool down to ambient temperatures before they perform at their best. Moderate sized reflectors (no larger than 8 inches) can take up to a half hour to "cool down" when moving the telescope from a heated indoor environment out into the cold outdoors.
• To perform at their best, reflectors need to be collimated (the optical elements have to be accurately aligned with each other). A simple process, but can be intimidating for the terminally non-technical!
• A large reflector (anything larger than 10") can be a formidable beast to move around when equatorially mounted. Most people would agree that an equatorial 8" would be the limit for portability.

COMPOUND

Plus Points:

• Amazingly compact for their apertures. No other telescope design can pack so much into so small a space.
• The telescope of choice for photographers.
• Extremely well supported for accessories.
• Almost always motorised, and often available as go-to models.
• In the Maksutov design, images are generally very sharp and quite contrasty. In the Maksutov-Newtonian design, the imaging is superb, and rivals anything available (but cool-down is an issue...).

Minus Points:

• Portability generally comes with a price tag...
• Contrast can be an issue with the Schmidt-Cassegrain design. This is generally a result of the design itself, which uses a large secondary mirror to fold the light back down the tube. Large secondaries are contrast thieves, and this detriment is particularly noticeable when looking for fine details on the Planets.
• As these telescopes are sealed (air cannot easily enter) cool-down is an important factor. This cool-down factor is emphasised as the image is formed by folding the rays inside the tube - images are attacked by heat currents more than once before they reach the eyepiece.
• Collimation is essential if these telescopes are to perform at their best.
• Although the standard Schmidt Cass. can have its focal ratio reduced to a useful F6.3, most Cassegrains and Maksutovs have focal ratios of between F10 and F15. Long focus designs like these tend not to be able to produce the wide-field views so beloved by the owners of short focus refractors. Again, the Maksutov-Newtonian is exempt from this criticism, as it generally has a focal ratio of between F5.5 and F7.