Things to consider before purchasing an astronomical telescope.
1) Size of telescope based on level of the user:- Ordinarily we advice novice to opt for Binoculars with wide field of view but a binocular is incapable of showing someone the beauty of the deep sky. Moreover, binoculars cannot manage high magnifications as a telescope can. A binocular is normally with 7X-16X magnification but a telescope could easily work at high magnifications and resolve things at over 100X magnification. It is better to have a binocular along with a telescope.
a) Beginner (Almost no knowledge or little knowledge of sky and constellations):-
A refractor with 70mm (about 3 inches)-100mm (4 inches) lens or a reflector with 114mm
(4½ inches)-150mm (6 inches) mirror should be enough for a novice. These telescopes can easily show you the craters of the moon, all the planets and some satellites too. One can easily see Andromeda Galaxy with any of these telescopes. A 150mm(6 inches) reflectors can show almost all the Messier’s objects easily. If you have a high budget, you may consider purchasing a Catadioptric telescope too.
b) Advanced beginner (One has a good knowledge of the constellations, knows how to use skymaps very well, capable of handling any telescope with ease, can locate Messier’s objects like Galaxies and globular star clusters):- Any reflector 6 inches or above should be satisfactory for an advanced beginner. We recommend a minimum aperture of 8 inches reflector as it is capable of splitting the stars in a globular star cluster. For
c) Serious enthusiast (One who wants to discover new objects in the sky like comets and asteroids):- Some people go the extra mile to start looking for new objects on their own. They should look for at least 12 inches Reflector or a 14 inches Catadioptric telescope with a GOTO mount.
d) Observatory grade or research grade telescopes:- These are telescopes with 20 inches and above diameter mirrors. Since these telescopes are huge, complex and mostly immovable, they need a special building and trained user to operate. They also require very powerful computers to manage the data produced each night during observation
2) Make of telescopes:- One would always be confused about which telescope choose. Each make of telescope has its own share of pros and cons.
a) Refractor telescope:- Refractor are the most popular telescopes among the beginners, they hardly cost much and are good to observe the planets, moon and bright stars.
Refractors are what the average person instantly visualizes when he hears the word “telescope”: a long, thin tube where light passes in a straight line from the front objective lens directly to the eyepiece at the opposite end of the tube.
- Easy to use due to the simplicity of design
- Perfect for lunar, planetary, and binary star observing especially in larger apertures
- It can be used terrestrial viewing. Some of the refractors give erect image too.
- High contrast images as there is no secondary mirror.
- Good colour correction in achromatic designs. The apochromatic, fluorite, and ED (Extra low dispersion) designs are nearly flawless.
- Sealed optical tube reduces image degradation due to air currents.
- Objective lens is permanently fixed and aligned.
- More expensive per inch of aperture than Newtonians or catadioptrics
- Large aperture refractors are heavier, longer, and bulkier than Newtonians and catadioptrics of equivalent aperture.
- Their light gathering capacity is lesser than equivalent aperture Reflectors or Catadioptrics, thus, they are not very useful for imaging deep sky objects like faint galaxies.
- Focal ratios for traditional refractors are usually long (f/11 or slower) making photography of deep sky objects more difficult. Short focal-length (fast f/number) apochromats are now available that change this limitation.
- Some colour aberration in achromatic designs (doublet)
- It needs proper care if used in areas with high moisture like coastal areas. Improper care may lead to development of fungus within the objective lens and eye pieces.
b) Reflector Telescopes:- Value per square inch of the aperture is the best in reflectors. They are cheap and easy to maintain. Their light weight makes them portable and shorter tubes make them less cumbersome to carry. These makes telescopes are preferred by beginners as well as advanced beginners who want to see deep sky objects. Serious enthusiasts with low budgets opt for Reflectors.
A concave parabolic primary mirror at the back of the telescope collects and focuses incoming light onto a flat secondary (diagonal) mirror that in turn reflects the image out of an opening at the side of the front end of the main tube and into the eyepiece. For large f/ratio Newtonians, the primary mirror often has a spherical shape or a parabolic shape.
Advantages of Newtonian reflector optical design:
- Lowest cost per inch of aperture compared to refractors and catadioptrics.
- Very low optical aberrations
- Images are bright due to small f ratios like f/4-f/8. These are mostly suited for observation and photography of deep sky objects like nebulae, comets, faint galaxies, etc.
Disadvantages of Newtonian reflector optical design:
- Unfit for terrestrial applications
- There is light loss due to obstruction by secondary mirror and its supporting structure.
c) Catadioptrics:- The most reliable and maintainance free equipment. These telescopes have both advantages of reflector and refractor which practically makes them the best among all the makes. The usual catadioptrics are the Schmidt-Cassegrain Telescopes. There are other Optical tube Assembly (OPT) like Maksutov-Cassegrain, Dall-Kirkham, etc.
The Schmidt-Cassegrain telescope (SCT) uses a combination of mirrors and lenses (catadioptric optics) to fold the optics and form an image. The light enters through a thin aspheric Schmidt correcting lens, then strikes the spherical primary mirror and is reflected back up the tube and intercepted by a small convex spherical secondary mirror which reflects the light out an opening in the rear of the instrument where the image is formed at the eyepiece. These catadioptrics are the most popular type of instrument with the most modern design, marketed throughout the world in 3.5 in and larger apertures.
Advantages of the Schmist-Cassegrain optical design:
- Best all-around, all-purpose telescope design. Combines the optical advantages of both lenses and mirrors while canceling their disadvantages.
- Excellent optics with razor sharp images over a wide field
- Excellent for deep sky observing or astrophotography with fast films or CCDs
- Very good for lunar, planetary and binary star observing or photography
- Excellent for terrestrial viewing or photography
- Focal ratio generally around f/10, making it useful for all types of photography. Avoid faster f/ratio telescopes (they yield lower contrast and increase aberrations). For faster astrophotography, use a reducer/corrector lens.
- Closed tube design reduces image degrading air currents
- Most are extremely compact and portable
- Durable and virtually maintenance-free
- Large apertures at reasonable prices and less expensive than equivalent aperture refractors
- Most versatile type of telescope
- Superior near-focus capability compared to other types of telescope (approximately 20 ft or 6 m)
Disadvantages of the SCT optical design:
- More expensive than Newtonians of equal aperture
- Short-tube appearance is not what people expect a telescope to look like
- Slight light loss due to secondary mirror obstruction compared to refractors
Understand simply:- (Telescope preference is mentioned in decreasing order)BEST >BETTER > GOOD
- Terrestrial applications(Photography and observation):- Refractors> Catadioptrics > Reflectors
- Low Optical aberrations:- Reflectors> Catadioptrics > Refractors
- Highest useful magnification per square inch(High transmittance of light):- Refractors (above 90%)> Reflectors(75%-85%)> Catadioptrics(65%-75%).
- Weight (Heavy) Tube weight:- Reflectors(Light) > Catadioptrics(Heavy) > Refractors (Heaviest)
Certain fundamentals to know before buying a telescope
- Higher transmitting of light of light results in high resolution which is achieved by good optics. e.g:- an 8 inches telescope with cheap optics cannot match a 6 inches XLT coating Celestron mirror.
- Larger the aperture of the lens or mirror, more light is gathered, thus, improving resolution. An 8 inches mirror telescope can split the stars of a globular star cluster at high magnification which a mirror with smaller aperture like 5 inches might not be able to resolve.
- Usually heavy mounts are sturdy. A sturdy mount is absolutely necessary to get a clear image. Shaky mounts can ruin a great telescope with good optics.The mounts have been arranged according to their increasing order of stability. Alt-Azimuth mounts AZ1< AZ2< AZ3< Celestron Nexstar mounts < Celestron CPC mounts (Note:- Meade has similar mounts too with some different names.) Equatorial mounts CG1< CG2< CG3< CG4< CG5 < CGE< CGE Pro mounts. Dobsonian mounts are sturdy but are not very useful or balanced like Equatorial mounts.
- Good accessories are must for any telescope. Good eyepieces improve observation experience by multi-folds.