2024-09-11

South Plains Astronomy Club

Observing Under The Dark West Texas Skies

SCT fork mount

SCT fork mount

So You Want To Buy A Telescope

Admin 2021-03-16 14 min read

Picking a telescope is much like picking a vehicle.  There are many options available and it can be a bit confusing to narrow the field at first. Your ideal scope probably won’t match mine, but will be based on many factors, including:

  • Your budget
  • Your commitment level, willingness to learn
  • Your knowledge and skill level
  • The objects you want to observe
  • Where you’ll observe and if you’ll travel
  • Your storage space for the scope and gear
  • How much weight you want to carry/lifting
  • Design limitations of the telescopes
  • Design of the mount
  • Astrophotography

I wouldn’t recommend a super car when you live on a ranch and I wouldn’t want to recommend a large scope if you don’t have space to store it. Your best bet is to join our public star parties and ask our members about our telescopes. We’ll be glad to answer your questions, show our telescopes, and guide you in your choices.

I’ll give a quick summary of scope types, but please note, this is far from comprehensive. I’ll list additional resources at the end.

Please note that I’ve provided links to vendors, but this is only to provide examples of scope types. We do not have a relationship with any of the vendors or manufacturers. Ask questions, do your research, and make informed purchase decisions.

Telescope Types – Reflector, Refractor, & Compound

From Sky & Telescope Magazine

How do we classify amateur telescopes?

  • Basic optical design
    • Reflector – A concave mirror at the back of the scope focusing light, often to a secondary mirror that reflects light to the eyepiece.
    • Refractor – Two or more lenses focusing light straight through the tube to an eyepiece, often with a secondary mirror that makes the viewing more comfortable.
    • Compound – A combination of front lens and primary mirror, often with a pair of secondary mirrors. A Schmidt-Cassegrain Telescope (SCT) uses a spherical primary mirror, a convex secondary mirror, and a corrector plate that corrects spherical aberration. A Maksutov-Cassegrain telescope (Mak-Cass) uses a thick and heavy corrector plate with a convex secondary with a spherical primary mirror. Both SCT and Mak-Cass have the eyepiece exiting the back of the telescope. There are other designs that place the eyepiece to the side or a camera in front of the telescope, but these are less common.

Telescope Mounts

A telescope mount does a few things:

  • Stabilizes the telescope
  • Allows movement of the scope across the sky, but also freezes in place for viewing
  • Places the eyepiece at a reasonable (not always comfortable) height for use.
  • If motorized, tracks the sky to allow longer viewing of an object.
  • If computerized, finds objects in the sky and tracks the sky to allow longer viewing of an object.
From Great Galactic Space
  • Telescopes mounts –
    • Altitude/Azimuth (Alt/Az) –
      • Simple Alt/Az – The most basic Alt/Az mounts have a pivot for up/down and left/right like a camera tripod. These simple mounts on inexpensive tend to be very wobbly, but more expensive versions can be very stable.
      • Dobsonian mount (Dob) – Created by the sidewalk astronomer, John Dobson, this mount is now very common, especially in the largest amateur reflector scopes. A Dob is an Alt/Az telescope without a tripod. The Azimuth bearing is on base plate. Dobs come in two flavors:
        • Tubes – Round metal/fiberglass tubes that hold the mirrors and eyepiece holder, with the altitude bearing on the sides of the tube.
        • Trusses – A full tube in a larger scope would be difficult to handle, so Dobs larger than 15″ often replace the tube with two mirror boxes connected with lightweight pipes that would take down to a very compact unit for transportation. It takes longer to set up, but solves many problems.
      • Fork Mount – Often found on computerized compound scopes and some refractors, a fork mount is a very stable and compact Alt/Az design:
        SCT fork mount
    • Tabletop mount – Once only for toys, you can find a few telescopes meant for use on a table, including manual models like the Celestron FirstScope and computerized models like the Meade ETX series. These telescopes require a solid table with a flat surface. The small manual scopes are often great for kids.
    • Equatorial (EQ) Mount – One axis aligns with the north or south pole. Instead of up/down & left/right, you’re moving north/south and east/west. You will need to balance the telescope and eyepiece weight with a weight on the other side of the axis pointed at the pole. Equatorial mounts are a little more complicated, but follow the stars, making observing easier, especially with a tracking motor.
From Telescopes Plus
  • Movement
    • Manual – You do all the work finding objects and centering them in the eyepiece.
    • Motorized – The basic motorized mount is an EQ mount with a single motor that tracks the sky. You will need to align this scope with the pole and release the clutch to move to another object. If you have two motors, then usually it will both track the sky and move the scope to various objects.
    • Computerized – Manual or motorized scopes can be computerized. Computers help you find objects in the sky, though it really helps if you understand what’s up tonight, what your target looks like, and if your scope can realistically gather the object.

Factors To Consider

  • Objective size – The name of the game in telescopes is the size of the telescope’s lens or mirror. The larger the objective, the more photons and detail you gather, but that means a larger, heavier, and more expensive scope. Larger scopes tend to be higher magnification and narrower views while smaller scopes give wider views.
  • Focal Ratio – Focal ratio is usually expressed as f/# and it determines both the field of view (FOV) and the natural magnification of the scope. The FOV and natural magnification are set for a telescope so eyepieces will work within a range depending on the focal length (eyepiece to objective) and objective size. A 20mm eyepiece will give very different views in an 8″ f/10 SCT and a 4″ f/4 refractor.
    Here are some general rules:
    • f/4-5 is a wide-angle scope. “Short Tube” refractors are fantastic for wide views of the Milky Way. Binoculars are often a bit wider at f/2 to f/4. Large reflectors are often in the f/4.5 range to reduce the size of the ladder and widen the view. For example, a 30″ Dob can easily require a 10-12′ ladder to look at the zenith!
    • f/6 is an all-around performer. An 8″ f/6 Dob is a very flexible telescope that can view wide objects like open clusters, medium views of objects like open clusters, and with a Barlow lens, narrow views of the planets.
    • f/8-f/10 gives a high magnification view. An f/10 refractor or compound scope will give high magnification views of objects like planets, galaxies, and nebulae. I would also recommend a tracking or computerized mount so you can have longer to view before needing to move the scope.
  • Objects –
    • The objects you want to view will dictate the basic telescope specifications (focal length and objective size) for the best view of these objects.
    • Note the size and brightness of the types you like.
      • Human 20/20 visual acuity = 1′ or 0.000278°
      • The dimmer and smaller objects will require larger telescopes with longer focal ratio.
      • The brighter and wider objects will be best in small scopes or binoculars with wide views.
    • Here is a sampling of some of the with a wide range of sizes and brightnesses:
      • The Moon
        • Magnitude of -12.7 (very bright when full) and an angular diameter of 1/2°.
        • The moon is a familiar object, an easy target, and a good reference point for this discussion.
        • It’s bright enough that an ND filter makes viewing more comfortable.
        • You can use Barlow lenses to increase magnification to see fine details on the Moon.
        • Wide field, low magnification telescopes and binoculars give stunning views of the Moon.
      • M45, the Pleiades Cluster (very bright open cluster)
        • Magnitude 1.6 (bright) and an angular diameter of 1.5° (wide)
        • Visible to the naked eye and some see the miniature dipper shape of the main stars.
        • It is best suited for binoculars and small, wide telescopes.
        • Larger telescopes tend to lack the field of view to encompass the whole cluster.
      • M26 cluster (dim and small open cluster)
        • Magnitude 8 and an angular diameter of 0.0039°
        • Not visible to the naked eye.
        • Hard to find with small scopes and binoculars.
        • Larger scopes will see the about 25 stars against a background of dimmer stars.
      • M31, Andromeda Galaxy (very bright and very large galaxy)
        • Magnitude 3.44 and an angular diameter of 3° x 1°. (wide)
        • Visible to the naked eye as a small, fuzzy blob in dark skies.
        • Small, wide scopes and binoculars can see the central bulge and some of the disk
        • Larger scopes will have trouble fitting the whole 3° in the view, but will pick up more details of the disk, spiral arms, and even some of the objects in the arms.
      • M102 Spindle Galaxy (dim and tiny galaxy)
        • Magnitude 10.7 and an angular diameter of 0.083° x 0.083°.
        • Small scopes can see the galaxy is there, but it will lack detail.
        • Larger scopes will gather more detail and you’re likely to see why it’s called Spindle.
      • M42, Orion Nebula (very bright and large nebula)
        • Magnitude 4 and an angular diameter of 1° (wide)
        • Visible to the naked eye as a fuzzy blob in the middle of Orion’s sword.
        • Small, wide scopes will see the full extent of the nebula, but will lack details.
        • Larger scopes will see details in the gas cloud and the Trapezium.
      • M76 Little Dumbbell Nebula (dim and small nebula)
        • Magnitude 10 and an angular diameter of 0.00075°
        • Small scopes will struggle with this tiny, dim nebula.
        • Larger scopes will reveal the dumbbell or apple core shape.
      • M22, globular cluster (bright globular cluster)
        • Magnitude 5 and an angular diameter of 1/2°
        • Barely visible to the naked eye in dark skies (dim fuzzy most will miss)
        • Small scopes and binoculars can easily find the object, but may not resolve stars.
        • Large scopes will increase detail and show texture and stars.
      • M68 Cluster (small, dim globular cluster)
        • Magnitude 9.8 and an angular diameter of 0.003°
        • Small scopes can find the cluster will will see little detail.
        • Larger scopes should resolve the outer stars and see some texture in the center.
      • Jupiter
        • Magnitude 1.66 to -2.94 and an angular diameter of 0.008° to 0.014°
        • Jupiter appears as a brilliant yellow/brown to the naked eye
        • Small scopes and large binoculars will see bands of color on the planet and the 4 Galilean moons.
        • Larger scopes will see more bands, the Great Red Spot, and smaller storm details in the cloud.
      • Saturn
        • Magnitude 5.8 and an angular diameter of 0.33°.
        • To the naked eye, Saturn is a brilliant cream colored star.
        • Small Scopes will pick up the odd shape of the rings, but may only see the
        • Large scopes will show more divisions in the rings and clearly show some of Saturn’s moons.
      • Neptune (dim planet)
        • Magnitude 5.8 and an angular diameter of 0.33°.
        • Small scopes and binos will see a dim blue dot against background stars.
        • Large scopes will see a small disk of bright blue against the background stars.
      • Comets
        • Magnitude and angular diameter vary greatly.
        • Comets that make the news will be bright, naked eye objects.
        • Small scopes will see pleasing amount of detail in the well-advertised comets.
        • Large scopes can dive in to see additional details within the coma and tail.
      • Asteroids
        • Magnitude and angular diameter vary greatly, but all are small and dim
        • Invisible to the naked eye.
        • Small scopes may see a dim dot moving slowly against the background stars.
        • Large scopes will not see any further detail.
  • Travel – The number of objects you can view in the city are limited by the lights reflecting up in the sky, so unless you’re lucky enough to live out in the country away from city lights, you’ll need to travel to darker sites to view dim objects.
    • The cargo capacity of your vehicle will limit the size of the telescope you can haul. You won’t get a 30″ Dob into a 4-door coupe.
    • Remember that the scopes will often need cases, boxes, or carriers to protect the equipment.
  • Storage – As with travel, your storage space (house, garage, storage unit) limits the size of the telescope. While telescopes can withstand temperature extremes, some materials last longer if you keep the scopes inside the house.
    • House – If you have a closet in the house that doesn’t see heavy traffic, you have an ideal spot to protect the scope. Temperature control will help preserve the wood and less traffic keeps the dust down.
    • Garage – The garage is secure, but dustier and with temperature extremes. If you use the garage, you’ll need to do some additional maintenance and cleaning on your gear or keep most items in sealed containers.
    • Storage unit – Like the garage, a storage unit is probably dustier and with temperature extremes.
  • Astrophotography – Astrophotography can be complex, expensive, and frustrating, but the results can be very rewarding.
    • You can start with your DSLR (using adapters), but your telescope should have a 2″ eyepiece holder, at least a tracking EQ mount (computerized is better), a battery power pack, and remote software to allow your laptop to control your DSLR.
    • Planetary Astrophotography – This branch of astrophotography requires less equipment, but more processing. Amateurs have done citizen science and spotted dust storms on Mars and changes in the storms on Jupiter before scientists saw it from large observatories.
  • Citizen Science – Use your telescope to collect data for scientists.

Recommendations

After reading all this, I’m sure you’re wondering what I’ll recommend. I really recommend that you come out to our star parties and see the scopes in action before you plunk down your hard-earned dollars. However, I’ll go out on a limb and talk about a few that I think might work for beginners, along with some of the pros and cons. Remember, this isn’t comprehensive and is the author’s opinion only:

  1. 10x50mm Binoculars. This is the cheapest item on the list and ideal if you want to learn the sky. I’ve spent endless hours under the starry sky scanning the sky with my trusty 10x50mm binos. I love the wide view and intimacy of binos. If you have kids, buy a pair of inexpensive 10×50’s and let them spend the evening on a blanket under dark skies exploring the sky. Open clusters look like diamonds on velvet when observed from a dark site.
    1. PROS: inexpensive, very small, easy to use, fun, good for kids
    2. CONS: You’ll need to learn how to find objects in the sky, low magnification, you can’t keep the binos on an object to share with someone else.
  2. 25x100mm binos on a tripod stand or parallelogram mount. There are a number of 100mm binos on the market with a wide range of prices, so you’ll need to look at reviews and prices. These binos bridge the gap between binoculars and telescopes. The magnification is good for most of the Messier objects and still offer wide views of open clusters. These binos are too heavy to hold by hand and will require a mount, either a tripod alt/az or a parallelogram, so you can share the view with others, if you want. They provide wonderful bright views of wider objects.
    1. PROS: Easy to setup and use, some are reasonably priced, few accessories needed, great views of medium and wide objects
    2. CONS: No computer, you’ll need to learn how to find objects in the sky, lacks magnification for smaller objects.
  3. 6″ or 8″ f/6 Dobsonian. This is a good all-around scope that is easy to use and able to view both open star clusters and with a high magnification eyepiece, look at the planets. It’s pretty intuitive to use and quick to set up. Kids often like driving once they learn the sky. Large enough to see most of the Messier list from a dark site.
    1. PROS: Rugged, easy to use, reasonable price, few accessories needed.
    2. CONS: No computer, you’ll need to learn how to find objects in the sky, might be too big for your closet.
  4. 80-100mm APO Refractor (f/4-f/7) on an equatorial mount. This is an expensive option, but one that will give plenty of pleasure in a relatively small package. APO means apochromatic, a type of refractor that has excellent color correction, which is a problem in inexpensive refractors. The mount can be manual or computerized, but either way, you’ll need to learn how to align the mount with Polaris, the north star. This scope will tend to provide fantastic views of larger objects, but an f/7 is usable on the planets as long as you don’t mind a small image scale.
    1. PROS: Less space in the closet than the Dob, pleasing image (higher contrast), very low maintenance
    2. CONS: More expensive than the other types, small aperture gathers less light than other types, requires alignment with the north star (easy once you learn)
  5. 4″ to 10″ f/10 computerized SCT. This is a wide range of sizes and prices, but the SCT or Mak-Cass will provide wonderful high magnification views of planets, nebula, and other small objects. You’ll need to pan around wider objects, but there are many more small objects to explore. You will need to learn how to align the telescope (some GPS units align themselves like magic!) and still need to know something about the sky to know which objects are good targets for the scope. SCTs are often found on alt/az fork mounts, but you can also get it on equatorial mounts.
    1. PROS: low maintenance, compact size for the aperture, great planetary and small object views,
    2. CONS: higher price than Dobsonians, you need to learn the computer and alignment,

For more information about telescopes, I’d recommend the following sites:

For more information on observing:

For more information on what’s up in the sky tonight:

Star Charts

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