Check out any online telescope store and the first thing you’ll notice is that there’s such a vast, bewildering choice that picking the right one may prove somewhat problematic. Purchasing a telescope is an investment and as such it requires proper research. This beginner guide will help you analyse all the vital aspects to consider when buying the first telescope. I hope it will help you make the right choice!
A personal choice
“What is the best telescope to buy?” I’ve read this question many times on astronomy or astrophotography forums and the answer to that is very simple. As astronomers like to say: “The best scope is the one you’ll actually use!”. While this may sound like an annoying answer at first, in reality it says it all. I’ve seen many people starting with a lot of enthusiasm only to end up locking their telescope in a basement to gather dust. So, before rushing into a store, it is good practice asking yourself these simple questions:
- What will I use my telescope for? Visual astronomy and astrophotography are two distinct hobbies, each with their own requirements. For instance, in visual astronomy the mount is not a crucial aspect, but the telescope’s aperture makes all the difference in the world. For astrophotography instead, a good, steady mount is essential and it’s where most of the money should be invested. Thus, before buying a telescope, make up your mind about your goal.
- What kind of astrophotography am I in? If you decide to take on the astrophotography journey, this is the next question you need to ask yourself. Planetary imaging and DSOs astrophotography have, again, different requirements: for planetary you would have to get the largest scope you can afford and a high frame rate camera whereas for deep sky astrophotography you would need a cooled camera and filters, and the best mount you can afford.
- Where will I use my telescope? Another important aspect to take in consideration is the weight of your gear. If you need to load your car and drive in the middle of nowhere with your telescope, you’ll find that moving a small scope is way easier and much more sustainable in the long run. If you’re lucky enough to have your own backyard or a terrace, perhaps you may opt for a larger telescope on a beast mount.
- What’s my budget? This is an obvious one but after you answered these first question you need to decide how much you want to invest before hitting the store. This can be an expensive hobby, and it’s very easy to get carried away so determining the budget in advance will help you get the best combination of mount, telescope and accessories, without breaking the bank!
- Where to buy a telescope? Today’s telescopes are a mix of fine, delicate optics and high-tech electronics so where you buy your telescope is as important as the telescope itself. Buying from a professional store will provide you with the best service and invaluable advice from experts in the field whereas getting a telescope off Amazon, for instance, may be cheaper but offers no service at all.
What you need to know
To choose the right telescope for your needs, it’s best to familiarise with some of the technical terms you will encounter and that may sound complicated at first; here are the most important ones.
Aperture. Often considered the most important characteristic of a telescope, this is the first notion to assimilate. A telescope’s aperture is the diameter of its main optics (lenses or mirrors) expressed in millimetres or, for larger apertures, in inches. The aperture determines the telescope’s light-gathering ability (how bright a target appears) and its resolving power (how sharp the target appears); in general terms, larger apertures show fainter objects and finer detail compared to smaller ones.
Take it easy though and try not to catch the “aperture fever”! The larger the aperture is, the bigger and heavier (and pricier) the telescope gets. Also, if you consider that the human eye has an average aperture of 7mm, even the smallest telescope represents a huge improvement. Apertures commonly recommended for beginner telescopes range anywhere from 60mm to 12” (300mm).
Focal Length. Another crucial feature in a telescope is the focal length, which measures the distance in millimetres from the main optic to the point where the image is formed (right into the eyepiece or camera). It’s the focal length that determines the scope’s field of view: shorter focal lengths will give a wider field of view (FoV) whereas longer focal lengths provide narrower FoVs.
As a rule of thumb, longer focal lengths are better suited for planetary imaging and astrophotography of small, faint objects such as galaxies and star clusters while short focal lengths are useful to image large targets, such as nebulas or large galaxies.
It is also important to remark that the focal length doesn’t necessarily equal the length of the telescope: certain types of telescopes, such as the Schmidt-Cassegrain (see below) use clever optical layouts to squeeze long focal lengths into small optical tubes.
Focal ratio. A key aspect for astrophotography, this is a quantitative measure of the optics’ “speed”. Written as “f/” followed by a number, the focal ratio is calculated by dividing the scope’s focal length by its aperture. For example, a scope with a focal length of 2000mm and an aperture of 200mm will have a focal ratio F/10 (2000/200=10).
Therefore, all other factors being equal, a scope with a lower focal ratio will produce brighter images than a scope with a larger one but will have a narrower field of view. In astrophotography this is quite important as it means that slow optics (> F/7) need longer exposures to collect the same amount of light as faster optics.
Magnification. This notion is quite straightforward and, as the name suggests, a telescope’s magnification is its ability to enlarge the object observed. This information is mainly useful for visual astronomy, as it helps decide the best eyepiece to use, and it’s obtained by dividing the telescope’s focal length by the eyepiece’s focal length. For example, a telescope with a focal length of 2000mm paired with an eyepiece of 8mm will deliver a magnification of 250x. (2000/8=250).
As a rule of thumb, a telescope’s maximum useful magnification is 50 times its aperture in inches or twice its aperture in millimetres,. Note that this is already an optimistic estimate, as external factors such as atmospheric conditions will likely reduce this number. When magnification is too high, the view will be too blurred to resolve any detail so stay away from telescopes that advertise unlikely magnifications such as 500x or more.
The different types of telescopes
Now that you have made up your mind on what your goal is, and you are familiar with the main aspects of a telescope, let’s see what our options are. Amateur telescopes can be divided into three main classes (refractors, reflectors, and catadioptrics), each with their own pros and cons.
Refractors. Ask any kid to draw a telescope and they will draw a refractor: a long tube with a lens at the front and the eyepiece (or camera) at the back. Refractors were the first type of optical telescope ever designed and their popularity has remained intact for centuries.
Refracting telescopes deliver the finest, sharpest images per aperture (a consequence of the fact that they use lenses rather than mirrors) and are best suited for wide field astrophotography. Generally featuring small apertures (up to 150mm) and short focal length, these telescopes can capture vast areas of the night sky to include large objects.
On the other hand, however, the typically short focal lengths means that refractors are not ideal to image galaxies, globular clusters, or planets. Photography of the Moon and the Sun is still possible, although with less details compared to larger-aperture telescopes.
Refractors usually represent a good choice for beginners as they require virtually no maintenance, and their wide field of view is somewhat more forgiving than larger telescopes when it comes to mounts’ tracking imperfections. Besides, these are the most compact, lightweight telescopes, which are perfect for astronomy on the go.
If this type of telescope suits your needs, make sure you opt for an apochromatic (APO) refractor. These are telescopes that use lenses made with extra-low dispersion (ED) glasses and other materials to reduce the false colour typical of achromatic refractors. Although more expensive than achromatic refractors, the price of APOs has decreased over the years becoming way more affordable.
On the downside, the cost of refractors with apertures over 115mm is prohibitive and their weight and balance quite unmanageable, especially for beginners. Also, these telescopes often require a field flattener to counteract the field curvature of the optical system and improve edge sharpness.
Reflectors. As astronomers usually say, “aperture is king”. This is to say that telescopes with larger apertures gather more light and thus show more details, making aperture one of the most regarded feature when buying a telescope. Well, reflector telescopes offer the larger aperture for the money!
Compared to refractors, in fact, reflectors (or Newtonians) don’t have a lens at the front but use a specially curved primary mirror at the bottom end of the telescope to gather light. From here, the light is reflected and pointed to a diagonal secondary mirror, which then directs it to the side of the tube, where they are met by an appropriately placed eyepiece or camera.
As mirrors are generally cheaper than lenses, this design allows for much larger apertures than are possible with refractors, at a fraction of the price. Reflectors, however, require continuous adjustments to ensure that the internal mirrors be perfectly aligned; this practice is called “collimation” and is often regarded as the main disadvantage of such optics.
At mid-size aperture, reflectors usually work best with focal ratios between f/4 and f/8, useful for wide field astrophotography. With large apertures, instead, reflectors are generally optimized for visual observations of faint, deep-sky objects such as nebulae and galaxies, but require much sturdier mounts for astrophotography.
Catadioptrics. The third class of telescopes was invented out of the desire to mix the best characteristics of refractors and reflectors: as a result, these telescopes, called catadioptrics, use a combination of lenses and mirrors and have very specific designs.
There are several types of Catadioptric telescopes but the most common among amateur astronomers are undoubtedly Schmidt-Cassegrain and Maksutov-Cassegrain. These telescopes have a lens at the front, called corrector plate, and a curved mirror at the back; similar to a refractor, the eyepiece is placed at the back.
The main characteristic of such telescopes, and the most evident, is their very short and compact design, which makes them more portable than reflectors or refractors when it comes to large apertures.
Catadioptric telescopes usually come with very long focal lengths and slow focal ratios (> F/10), thus working great on small targets such as planets or galaxies and star clusters. Besides, most models also allow for the addition of focal reducers to shorten the focal length turning the telescope in a sort of all rounder.
Catadioptric telescopes provide sharp images but have a few drawbacks. First, like Newtonians, they need to be always perfectly collimated to ensure that the lenses and mirrors be properly aligned. Also, due to their short, compact design, these telescopes are particularly subject to dew formation on the corrector plate, thus needing extension tubes and electric dew heaters, which add up to the cost.
Finally, catadioptric scopes take longer than other telescopes to cool down to the temperature of the night, which is vital for producing high-quality images. This means that these telescopes must be left outside for hours before you can actually start using them.
The different types of mounts
Once you know you have decided which telescope you want to get, it’s time to focus on the most critical aspect: the mount. If you are interested in DSOs astrophotography, here’s where you have to invest most of your money: any telescope, even the best one, is not worth much without a stable, precise mount.
Some telescopes, especially those for beginners, are sold with a mount and tripod as a package and could represent a quick, easy choice. This solution can be convenient for visual astronomy or for planetary astrophotography.
However, if your goal is DSOs astrophotography, I’d recommend to ditch this option and choose a proper equatorial mount straight away, which will save you quite some money as you won’t outgrow it too soon. As a saying goes: buy cheap, buy twice!
Quick note: in this guide, I will only refer to computerised mounts (“GoTo” mounts) rather than manual ones, it is the 21st century after all! These mounts have a built-it computer with a database of celestial objects and their coordinates, so after a first star alignment, the mount can slew to virtually any object in the sky and track them as they appear to “move”. There are two broad categories of mounts (with some variants), let’s explore them.
Alt-azimuth mount. As the name suggests, an “Alt-az” mount is a simple two-axis device that tracks objects in the sky by moving in two directions: up/down (altitude) and left/right parallel to the horizon (azimuth).
These mounts are a popular choice for beginners as they are incredibly easy to set-up: you simply have to level the tripod to the ground (most mounts come with a handy level bubble), input some data (day/time and GPS coordinates) and perform a simple star-alignment.
Some Alt-az mounts come with a built-in GPS system, so that the initial data input is no longer required. Besides, in most cases these mounts are relatively light and portable and are normally cheaper than other designs.
Apart from their standard version, Alt-az mounts come in several variants, the most famous being Dobsonian mounts. In this version, rather than on a tripod, the Alt-azimuth mount is fitted on a simple wooden platform placed directly on the ground, which rotates 360°. This type of mount is typically cheaper and is extremely convenient as it allows for very large Newtonians at an affordable price.
Alt-Az mounts work quite well for planetary imaging and visual astronomy but have some limitations when it comes to deep sky astrophotography. This is because they do not compensate for Earth rotation, limiting the exposure times to 20-30 seconds. At longer exposures, “field rotation” becomes visible and the stars appear elongated, forming what is referred to as “star trails”.
While DSOs astrophotography is still possible on an Alt-az mount (many astrophotos on this blog are done with an Alt-az), an equatorial mount will guarantee way better results.
Equatorial mount. As mentioned before, if your main goal is deep sky astrophotography then you definitely need to invest in a good, sturdy, and precise equatorial mount. Similar to an Alt-az, an EQ mount moves on two axes, one called right ascension (east/west) and the other called declination (north/south); the difference is that one of the axes, the right ascension, is aligned (parallel) to Earth’s rotational axis through a process called “Polar alignment”.
When polar aligned, an Eq mount will compensate for Earth’s rotation while tracking an object in the sky, effectively cancelling the field rotation effect, and thus allowing for long-exposure photography, which is essential to get deep sky objects’ faintest details.
One of the most important feature in Eq mounts is their maximum payload, which is the maximum weight they can support while tracking objects in the sky. As a rule of thumb, for astrophotography it is advised to use no more than 60-70% of the maximum payload to minimise tracking errors.
For example, if a mount has a maximum payload of 30 kg (66 lbs), it is recommended to load not more than 14 kg (31 lbs) worth of gear. This includes the telescope’s weight as well as all the other components such as the camera, the guiding system, and any other accessory.
For this reason, equatorial mounts tend to be larger and way heavier than Alt-az, which drastically reduces their portability, something to consider in case you have to bring your scope out at night. Alos, Eq mounts are usually more expensive than comparable Alt-az mount and require important investments.
Expectations vs. reality
If you decide to be a visual astronomer, there is one last thing to keep in mind. And this one is probably the most important: you need to lower your expectations!
Observing the Moon, the Sun (only with the appropriate solar filter!) or planets with a telescope is a breath-taking experience. Viewing Saturn’s rings or Jupiter and its four larger moons is something you will never forget!
However, when it comes to faint deep sky objects, you need to be aware that our eyes are not as sensitive to light as a camera is, and the views will NEVER match the images we can produce with long exposure photography.
Due to the long distance and the impact of light pollution, deep sky objects such as nebulas or galaxies will look like fuzzy, featureless blobs (pretty much as if somebody sneezed on the eyepiece), even with the most powerful telescopes. The human eye is just incapable of capturing faint details or colours.
That said, objects like double stars or star clusters will deliver enchanting views with the right telescope. Observing under dark skies, far from light pollution helps getting better views, and some filters can be used to increase contrast in objects like nebulas or galaxies.
So lower your expectations and remember that you are looking at objects that are thousands or even millions of light years away! Astronomy has a steep learning curve, it requires a lot of patience, but it’s one of the most rewarding hobbies.