Types of Telescopes : Pros and Cons

All Refractors

Pros:

• Collimation is not needed
• Thermally acclimate quickly
• Highest light transmission per inch of aperture (assuming fully multicoated lenses)
• No central obstruction, so highest possible contrast (assuming chromatic aberration is dealt with)
• Work well with binoviewers and cameras since they often are able to reach focus easily without the need to use a barlow for these devices.
• Optics never degrade, and are easy to clean.
• Typically can use binoviewers natively without the need for a barlow, but not always.

Cons:

• Limited aperture (tube gets too long, or gets too expensive)
• Moderately to extremely high cost per inch of aperture
• Equatorially mounted refractors can put the eyepiece and finder scope in awkard positions
• Dew / condensation is a problem. In humid climates, a dew heater is crucial.

Achromatic Refractors

Pros:

• Aside from the pros listed above, the only pro for an achromat is a cheaper cost per inch of aperture compared with an apochromat.

Cons:

• Often suffer from chromatic aberration or very, very long tubes to mitigate chromatic aberration.
• More expensive per inch of aperture than Newtonian reflectors.
• Long focal ratios are needed to make up for chromatic aberration, which reduces maximum true field of view and maximum practical view brightness.
• Long tubes often require equatorial mounts, and the long moment arm created by the long tube means vibrations are more pronounced in long tube achromats. This can make focusing a challenge.

Apochromatic Refractors

Pros:

• Pure, crystal clear, ultra high contrast views.
• The special glass used in apochromats allows for shorter tubes and focal ratios than achromats (though they are still comparatively long next to other optical designs)
  

Cons:

• Very, very expensive. The most expensive type of telescope per inch of aperture.

Newtonian Reflectors

Pros:

• Cheapest type of telescope per inch of aperture.
• Virtually no practical limit to their size. Apertures as large as 70″ have been constructed by amateurs. 8″ to 12″ apertures are common and affordable.
• Very short focal ratios are possible, resulting in wider true fields of view (relative to their aperture), and short photographic exposure times if a dedicated astrograph is used.
• The larger apertures permit higher magnifications, brighter views at a given magnification, and more visible detail for planetary and lunar viewing.
• Dew / condensation is rarely a problem.

Cons:

• Take longer to thermally acclimate than refractors
• The central obstruction and spider vane can cause diffraction effects that reduce contrast and rob the view of its purity. It also very slightly diminishes light gathering power.
• Most reflectors lack the focus range to natively focus binoviewers and cameras (though a dedicated astrograph can focus without needing a barlow)
• The mirror coatings don’t reflect as much light as a refractor’s lenses will transmit (typical reflectance is around 92% per mirror, so 92% of 92% is a net reflectance of 86%, vs 99% from a refractor of the same aperture.
• The mirror coatings can degrade over time due to oxidation, requiring them to be re-aluminized.
• Cleaning a reflector’s mirrors has to be done very, very carefully to avoid scratching it.
• Reflectors with short focal ratios exhibit an aberration known as coma. Coma increased by the cube as the focal ratio decreases. Coma makes stars near the edges of the field of view look like small comets instead of points of light. This can be corrected with the use of a coma corrector.
• Reflectors need periodic collimation to perform at their best. The shorter the focal ratio, the more important collimation accuracy is.

Dobsonian mounted Newtonian Reflectors

Pros:

• Cheapest telescope design of all
• Very easy and intuitive to use
• Permits large apertures
• Virtually free of vibration due to the low center of mass and comparatively stable mount
• Eyepiece always remains at a consistent orientation (though its height above the ground varies based on the angle of the telescope)
• Easy to set up

Cons:

• Hard to motorize, meaning most the sky must be manually tracked. Motorized dobsonians become heavier and harder to set up, and much more expensive.

Equatorially mounted Newtonian Reflectors

Pros:

• Can be aligned such that tracking the sky manually is as easy as turning one knob
• Are usually easy to attach a motor drive to, making it possible to provide basic tracking without much trouble.

Cons:

• Equatorial mounts will put the eyepiece and finder scope in awkward orientations, making it necessary to frequently rotate the tube to be able to look through the finder or eyepiece.
• The higher center of mass means vibrations are a lot more problematic for equatorial mounts. The long moment arms of a long tube Newtonian also means more torque is applied to the axes of the mount, and make vibrations worse.
• To get the most benefit out of an EQ mount, you must take the time to level and polar align the mount. It’s not as easy to set up as a dobsonian mount is.
• Because an EQ mount operates in Right Ascension and Declination, aiming a telescope on an EQ mount is not as intuitive or straight-forward as with a Dobsonian or other alt-az mount.
• Long tube Newtonians will often hit the tripod legs as you aim them near the Zenith. EQ mounted refractors have a similar problem.
• Equatorial mounts are often heavier and bulkier than the rockerbox of a dobsonian telescope. They are harder to transport, and require time to break them down and set them up.
• Equatorial mounts require you to fiddle with brakes/clamps/clutches in order to aim them. A dobsonian mount just uses simple friction and balance to keep the scope aimed at where it should be.
• Depending on the orientation and length of the tube, the slow motion controls may be out of reach.
  

All Schmidt Cassegrains

Pros:

• Short, compact tube makes them work well on a variety of mounts (fork mounts, equatorial mounts etc)
• Long focal ratio means they produce nice clean views even when cheaper eyepieces are used. However, this has its own drawbacks (see below).
• Collimation is required, but only the secondary mirror needs to be collimated, making collimation fairly simple and straight-forward.
• Practical design for planetary and lunar astrophotography.
• Secondary mirror is suspended by the corrector plate, so no diffraction spikes are present like they are with a Newtonian reflector.
• Can use binoviewers natively without the need for a barlow.

Cons:

• The long focal ratio and long focal length makes them challenging to use for deep sky astrophotography. The long focal ratio means much longer exposure times are needed, and the long focal length means a very sturdy mount is needed.
• Closed tube means the scope can take a long time to thermally acclimate
• Dew is frequently a problem, and requires a dew heater to correct it
• Image shift when using the focusing mechanism is a common problem in most SCTs. The image appears to shift side-to-side as you turn the focuser knob in different directions.
• The long focal ratio often means it’s hard to get a bright, wide field of view with most eyepieces. SCTs are decidedly not “rich field” telescopes.
• SCTs have large central obstructions which reduce contrast.
• Fairly heavy and expensive per inch of aperture (not as expensive as a refractor though!)

Fork mounted Schmidt Cassegrains

Pros:

• Usually all fork-mounted SCTs are computerized and will track the sky. This does require setup and alignment though
• Typically the cheapest and the most portable solution for an SCT
• Capable of doing planetary and lunar astrophotography

Cons:

• The alt-az arrangement of a fork mount makes a fork-mounted SCT incapable of imaging deep sky objects. Exotic solutions like field de-rotators are necessary.
• Cannot always easily add an external focuser to solve for image shift problems due to the limited clearance when aiming vertically
• Requires a balance kit if using heavy equipment on either end, else the motors may strain too hard and not track as accurately.

Equatorially mounted Schmidt Cassegrains

• Assuming an adequate equatorial mount, can be used for both planetary/lunar and deep sky astrophotography.
• The short tube of an SCT, as well as the location of the eyepiece at the back, makes SCTs much easier to use on EQ mounts than refractors, and especially newtonians.
• Because there are no clearance problems, an external focuser is easy to add if needed.
• Easier to balance than a fork-mounted SCT

Cons:

• Heavier and more expensive than a fork-mounted SCT

All Maksutov Cassegrains

Pros:

• Needs no collimation
• Often slightly smaller central obstruction than an SCT, meaning slightly better contrast than an SCT (though not by much).
• Like an SCT, the secondary mirror is part of the main corrector plate, so no spider vanes and diffraction spikes are present.
• Corrector plate is typically thicker than an SCT, which means it stays warmer longer and fights dew longer. The downside is it means it takes it longer to thermally acclimate. Active dew fighting system is still recommended.
• Compact design works well on equatorial mounts or alt-az mounts.
• Like SCTs, image shift can be a problem due to the internal focuser.
• Can use binoviewers natively without the need for a barlow.
  

Cons:

• The closed tube and thicker glass means Maks often take the longest to thermally acclimate of any optical design
• More expensive per inch of aperture than an SCT, and much more than a Newtonian reflector, but not as much as a refractor.
• Very long focal ratios (F/12 or higher) means it’s harder to get maximum brightness from the telescope compared to other designs with shorter focal ratios. True field of view will also be narrower due to inherently higher magnification from the longer focal length.
• Completely unsuitable for deep sky astrophotography, but great for planetary and lunar astrophotography.

There are many other kinds of telescopes, but the ones listed above are the most common.