3) Cassegrain (“SCT”) telescopes: these telescopes can be thought of as a “hybrid” of both reflecting and refracting telescopes because they often combine both mirrors and lenses to reflect and refract light at different points along its path from main opening to your eye. There are different designs that fall under this broad umbrella, such as Schmidt-Cassegrain; Maksutov-Cassegrain; and others, but they follow the same general idea as one another.

With these types of telescope delineated, which one is “best?” That is difficult to answer, because they each have their advantages and drawbacks resulting from how they focus light and how they are constructed, as well as a wide variety of specifications for individual models from different manufacturers. This table provides a general overview of the advantages and drawbacks of each.

No optical system is perfect; each one has benefits and drawbacks. For example, lenses and mirrors are curved by design in order to focus light into an image. What shape the lenses and mirrors take can lead to more or less apparent distortions: on the inexpensive side of manufacturing are spherical lenses and mirrors, which have the most-apparent distortions. These are typically found in “toy” telescopes and optical equipment that is maybe useful to show off the Moon and maybe a planet, but little else for anyone who wants to “really look” at objects in the sky. Parabolic mirrors and lenses are a step up in price to manufacture, but have a corresponding step up in the quality of the images they produce. Both spherical and parabolic mirrors and lenses can have their distortions reduced with additional corrective lenses placed somewhere in the light path of the telescope, but this adds additional manufacturing and design cost, which translates to additional price paid for the equipment.

Mounts

Perhaps even more important than your telescope itself is the thing that the telescope sits on, the mount. Depending on options and features, some mounts can cost as much as, if not more, than the telescopes they carry! Broadly-speaking, mounts can be categorized under “alt-az” or “equatorial” types:

Alt-Az: “What your head does”

Alt-az is short for “altitude/azimuth,” which refers to angles of measurement. “Altitude” refers to “how high above the horizon” an object can appear, while “azimuth” refers to an angle taken from zero degrees (North), ticking upward to 360 degrees as you rotate eastward until you reach North again. This kind of mount is fairly-intuitive, especially for beginners, as it mimics what our head does: it tilts up and down, and swivels horizontally.

One particular kind of alt-az mount that is very popular with beginners is the Dobsonian, which has been mentioned in other parts of this primer. The Dobsonian is typically a combination of a decent-sized reflecting telescope (beginning at around 6”) and sits on a base that can pivot up and down and swivel left and right. The design was invented by John Dobson in the 1960s, and made astronomy much more accessible to amateurs because of the relative ease of use: one can literally just grab the tube and swivel around manually to aim at and observe objects!

Equatorial: “Track the Stars”

The other kind of mount type is the “equatorial” mount, also referred to as the “German equatorial mount” (GEM). This kind of mount will also tilt and swivel, but is meant to be aligned in a particular way. With alt-az mounts, the altitude angle ranges from 0 degrees to 90 degrees, with zero being your local horizon and 90 being directly overhead. In the equatorial system, however, 0 degrees is meant to match Earth’s equator and 90 degrees is supposed to point directly at the North celestial pole (where Earth’s North geographic pole points, near the North Star, Polaris). This means that using an equatorial mount will require knowledge of where on Earth you are observing from—particularly, your latitude—as you will need to set the equatorial mount to correspond to your latitude, and make further small adjustments to align the mount itself with the true North pole. Each equatorial mount may be slightly different from one another, so the steps to make this alignment will vary, but the general concepts of the process are the same. Once properly-aligned, the equatorial system uses two angles like the alt-az system, but in equatorial coordinates, the “alt-az” is replaced with “Dec/RA”—that is, “declination” and “right ascension.”

When aligned, all the main objects that you might like to observe outside of the Solar System (double stars; star clusters; nebulae and galaxies) have relatively-fixed coordinates in the sky relative to one another, so finding an object—or at least getting very close—is as simple as turning the Dec/RA setting dials to the desired positions. What is more, as the night progresses and the Earth rotates, for a well-aligned equatorial mount, one can just slowly turn a knob on the mount to counteract the rotation and keep an object in view.

Which is “Better?”

As with many things in astronomy, which one is “better” depends. Both will require a bit of learning and skill development to be able to find objects in the sky. Alt-az mounts are generally more intuitive, especially for beginners, and so are usually recommended for those starting out in order to avoid layering on extra steps like getting proper alignment with an equatorial mount. In both cases, having “slow-motion adjustment” knobs is a nice-to-have feature that can help reduce frustration, as you can (usually) slowly turn the gears with your fingers to adjust your view at a much more desirable rate than ham-fisted motions. Such smaller, slower adjustments let you glance between star maps and your live eyepiece view to see if you are scanning to find the object you are looking for through star-hopping. Equatorial mounts are more likely to come with such slow-motion knobs.

Another potential consideration is the weight capacity of the mounts. Typically, alt-az mounts do not require additional weights, but equatorial mounts necessarily require at least one counterweight to balance the telescope to align effectively and not place unnecessary strain on the gears of the mount. These weights do not add to the capabilities of the scope, but will limit the size of telescope and other equipment that can be placed on the mount.

What About “Go-To?”

Many telescope mounts are now sold with what are called “go-to” capabilities, which are computerized systems that can allow the telescope to—as the name suggests—“go and point to” a desired object. Such systems require initial calibration, which includes knowing the date and time and some precision around your GPS coordinates (latitude and longitude). Both alt-az and equatorial mount types can come with these computerized systems. Alignment steps will vary between different systems and manufacturers, but typically, the steps will require first manually aiming at two or three identifiable stars, so will require at least a little knowledge on your part to align effectively. If well-aligned, motors inside of the Go-To mount can also automatically turn the telescope to counter Earth’s rotation, keeping objects in view for minutes at a time. Additional equipment can be purchased to keep objects not only in the view, but centered and static in the view, which would be essential for pursuits like astrophotography (beyond the scope of this primer).

Whether or not to use these systems is, as with almost anything in this hobby, a set of trade-offs. Buying a telescope with this capability will mean that, for a fixed budget, a compromise must be found between the size of telescope; quality of mount; and whether or not to purchase a mount equipped with the Go-To features. Additionally, those who wish to grow their skill in manually aiming at the sky may find the Go-To features hamper this skill development because a machine is doing the aiming, instead of you, the astronomer. Others who wish to simply view many different objects quickly and efficiently without much concern for finding the objects themselves may like the capability of the Go-To systems, but may also find frustration if they do not achieve good alignment and cannot recognize the field of view around their desired target, and so cannot so easily make the manual adjustments that could be needed to find the object(s) for which they were looking.

As said already: trade-offs abound in a hobby like astronomy. Only you can decide what budget you would like to set, what type(s) of objects you would like to see, and what you want to learn from engaging in this interest!

Accessories: Finderscopes, Eyepieces, Filters & More

Once you have the main body of your telescope selected, next come the accessories used either to help aim the telescope or the actual bits through which you look at images of the universe. In most cases, when buying a telescope, your purchase will come with at least a couple of these accessories.

1) Finderscope: this device sits on top of the main body of the telescope and is used, as the name suggests, to find objects in the sky. It generally has a low, fixed magnification on purpose: you should start with low magnification when finding objects, and adjust your view to a higher magnification if desired. When you first set up your telescope, especially after purchase, you will likely need to align your finderscope with the view in your main telescope; reach out to us for help, if you have questions! There are also alternative devices called “telrads,” which have no magnification at all, but sit in the finderscope spot and have little circles that can be used to “starhop” from one object to the next in even increments across the sky; such a device can be useful for those who really want to learn the sky visually and learn how to aim a telescope manually.

2) Eyepieces: this is the piece through which you look, regardless of telescope type. For respectable telescopes, the common barrel sizes are 1.25” and 2” in diameter, and any such eyepiece should fit into any telescope that accepts those widths. Most telescopes sold come with two eyepieces each with their own focal lengths; common focal lengths included in such bundles are 25 mm and 10 mm. The combination of focal lengths of the telescope and eyepiece determine magnification by their ratio: a 1,000 mm focal length telescope with a 25 mm focal length eyepiece will have a magnification of 40x. When considering making additional purchases of eyepieces, it is useful to consider having a spread of magnifications. You do not need to purchase every possible focal length of eyepiece! Eyepieces also have their own set of specification vocabulary to be aware of, with a couple of terms that are especially useful to know:

a) (apparent) field of view (aFOV): this is the width of the area that you think your eye can see while looking through the eyepiece. Your natural field of view with just your eyes is about 200^o horizontally (left peripheral to right peripheral) and about 135^o vertically (top of vision to bottom). Typical eyepiece aFOV is about 58^o; the bigger the number, the wider your view, and likely the more expensive the eyepiece.

b) Eye relief: this refers to the distance between the eyepiece and where the image it makes forms, which translates to how far away from the eyepiece you need to place your eye in order to see the image. The larger this number, the more “comfortable” the viewing experience will be, because you will not need to cram your eyeball up close against the equipment to see what you want to see.

There are more specifications that may be listed with eyepieces, such as “exit pupil,” but these two are likely the most important for beginning astronomers. When purchasing additional eyepieces, there is a wide range of designs (and names), which come in a wide range of quality, which come with a wide range of prices; some individual eyepieces may even cost more than some low-budget telescopes! It is very easy to get lost in the weeds of all the terminology on these. Seek advice if you want to compare different eyepieces with one another.

3) Filters: depending on type and use case, these may sit over the main aperture of the telescope or may be threaded into the bottom of the eyepiece. Wherever they are in the light-gathering process, they are meant to limit the light coming through the telescope for some reason or other. Some, like a Moon or polarizing filter, are meant to block a significant portion of light to lower the intensity while observing such bright objects (viewing a full Moon through a telescope can be surprisingly bright!). Others, like light pollution filters, try to block specific wavelengths of light associated with light from cities and streets (but also will block any natural light of those wavelengths, as well). Still others may block or allow only specific bands of colors, like red, green, blue, or even some infrared, which are useful for photographers to learn about. There are other specialty filters, like solar filters, that cover the whole aperture of the telescope to enable direct observation of the Sun. Exercise due caution and safety precautions when considering and using such devices; talk with some of our members or consult reputable suppliers and sources when considering doing this kind of astronomy!

4) Barlow lenses: these get a special mention because they are not quite an eyepiece and not quite a filter: they are made of a barrel that holds an eyepiece and contain a lens that sits between the main body of the telescope and the eyepiece along the path light takes to your eye. They have a static magnification factor, typically 2x, 2.5x, or 3x. This can make, say, a 10 mm eyepiece “behave like” a 5 mm eyepiece (in the case of a 2x Barlow). Some astronomers like Barlows because it reduces the number of physical eyepieces needed to have a range of magnifications, if planned strategically; others do not like them because, with the lens being another piece of glass through which light is passing, reduces the amount of light actually reaching your eye. Only you can know whether such tradeoffs are worth it, for your experience and budget.

5) Dew heater: as the air cools, it is possible for the air temperature to reach the dew point and water vapor will begin to condense into liquid water over everything, including your telescope and equipment. This can lead to distortions on your mirrors or lenses and affect what you see in your eyepiece, not to mention potential damage if not properly-cared for. A dew heater does what it suggests: heats your telescope (especially the hollow tube of a Newtonian reflector) to prevent dew from condensing on your equipment. This is not entirely essential equipment for most introductory users, but would become more important for applications like astrophotography.

6) Power supply: depending on whether your telescope and mount are computerized in some way, or if you get a dew heater, or other electronic accessories, you may need a power supply to ensure a stable source of electricity to run the motors in the gear at expected rates, especially over an entire night time of observation. Considerations for these are beyond the scope of this page, but it is just worth mentioning as a last afterthought.

7) Red lights: while working in the dark, your eyes will naturally open and adjust to allow more light to pass through. When you go to change something about your equipment (say, swapping an eyepiece), though, you will naturally want to be able to illuminate your equipment so you can see what you are handling. The problem that will arise is that if you use a standard flashlight to help you, the white light will cause your pupils to shrink, and you will lose your dark adjustment. However, if you use a red flashlight (a light designed to emit only red light), our eyes do not respond as strongly to red light as opposed to white light (which is composed of all colors), and so unless the red light is very intense, we can illuminate our workspace without affecting our dark adjustment too much. Red lights are sold individually, but in a pinch, a white flashlight covered with red cellophane can substitute fairly well.

8) Observing chair: one last thing to consider is a place from which to comfortably observe! Whether you opt for a chair that can recline (partially or fully—the latter of which is useful for meteor showers), a chair with a small tray or cupholder attachment, or even adjustable height is for you to decide, but definitely an easily-overlooked piece of equipment that can help extend observing sessions.

For the most part, all of these devices are interchangeable and able to be upgraded after your first purchase. Even the focuser—the thing that you adjust to actually achieve focus—can be swapped out and modified on some models of telescope, if so desired. It is not that uncommon for amateur astronomers to swap out parts of this or that telescope as if they were working on a hot rod in their garage. Some even go so far as to try to build their own telescope according to specifications that they want, instead of mass-produced manufactured equipment. As you become more familiar with what is out there in the field of astronomy, you, too, can decide for yourself what is worth your dollars to buy, upgrade, trade, sell, or build on your own.

Final Thoughts

We can end this primer where we began: “What telescope should you get?” The answer was, and still is, “the telescope you’re most likely to actually use.” But if you have read and understood most of what is on this page, hopefully you have some kind of mental image of what such a telescope may actually look like. You may know what kinds of objects you may want to look at, or if you want a telescope that is versatile enough to flip between observing different kinds of objects. You may even have ideas of what kinds of accessories may be included, or what you may need to look into purchasing to help you go look. We hope that at the end of this reading that the reasoning for our initial recommendations make some sense, and why we cautioned that those recommendations can only be general. After all, we are not sponsored by any manufacturers, and while we may each have our individual brand loyalties, it is difficult to say—for once and for all time—“buy this particular model of telescope” when a company may go bankrupt, or manufacturers may change quality with time, or even better individual products may come out in the future. Still, now you should at least have some tools and vocabulary that will empower you to make a more informed decision for yourself.

Once you have, please join us out on the field, and look up with us!