If you’re choosing an astronomical telescope as a beginner astronomer, you may find yourself somewhat overwhelmed. Telescopes are advertised in various different ways with different numbers and properties. Let’s take a look at what the specifications of a telescope are and what they mean.
A Common Shorthand
A common shorthand for telescope specifications is “Aperture/Focal-Length” in millimeters. So a 6” Dobsonian would be a 150/1200, and a ShortTube-80 would be an 80/400.
Another common shorthand, borrowed from binoculars, is “Magnification x Aperture.” As we’ll see, advertising magnification is a red flag, since telescopes are meant to use multiple different magnifications. But for example, you might see a 450×70, meaning it’s a 70mm aperture with accessories that can be used up to 450x magnification. (which would be blurry and dim and unusable)
Some sellers, usually on Amazon, mix up these two systems, and you might see, for example, a 70/450 where the 450 refers to magnification, not focal length. So you’ll have to check to be sure.
This is the easiest to understand–it’s just “how much bigger is the image.” And that’s exactly why cheap toy telescopes prey on beginners by including misleading statements about magnification, claiming their telescope can do 200x, or 450x, or, heavens, 750x magnification!!!
Beware of any telescope being advertised on the basis of a high magnification
Any telescope can theoretically use any magnification, because the magnification actually has to do with a combination of telescope and eyepiece.
Magnification = Focal Length of Telescope / Focal Length of Eyepiece
So, cheap telescopes will include low-quality short-focal-length eyepieces and plastic barlow magnifiers to provide extremely high magnifications. But these high magnifications are going to look terrible because there is a maximum usable magnification given by the aperture of a telescope.
Maximum Useful Magnification = 2x per millimeter of aperture = 50x per inch of aperture
So the cheap 40mm or 60mm refractors advertising 400x magnification are blowing up dim, blurry images to much higher than 2x per mm. It’s like zooming in on an image on a computer until you see the pixels instead of the detail.
Some of the telescopes being advertised this way can be properly used at lower magnifications, but most of the telescopes advertised with magnification-first are essentially scams.
This is the single most important aspect of a telescope: How large is the front opening (or more specifically, the lens or primary mirror diameter, or even more precisely) of the telescope? The aperture determines the brightness of the image; the number of objects you can see and how easily you can find them; the limiting resolution (and therefore maximum usable magnification) of the telescope; and the overall size of the instrument. A larger aperture is better in virtually all respects.
This is more important than magnification but less important than aperture. Focal length is the distance from the objective of the telescope to the focal point, and it is often equivalent to roughly the physical length of the telescope.
Because magnification depends upon the focal length of the telescope and the focal length of the eyepiece, the focal length is often thought of as a proxy for magnification. A longer focal length telescope will tend to be used at higher magnifications, even when the aperture is the same, because most telescopes come with similar eyepieces, usually something around 20–25mm and something around 10mm. Just keep in mind that even short-focal-length telescopes can be used at high power! They just require somewhat more expensive short-focus eyepieces to compensate.
When evaluating a telescope, I find Focal Ratio, a specification which takes aperture into account as well, is more useful. For example, two telescopes with a 1200mm focal length could behave very differently if one is an 80mm refractor and the other is a 10” dobsonian. Both would show a 48x magnification with a 25mm eyepiece, but for the 80mm that’s high-to-medium power, and for the 10”, that’s low power. A 6” Dobsonian and a 10” Dobsonian would both show an image at 300x, but the 6 incher would be maxed out and the 10 incher is in its sweet spot.
The ratio between the focal length and the aperture of a telescope, usually expressed in the form “f/5” or “f/8”
Focal Ratio = Focal Length of Telescope / Aperture
The Focal Ratio is also called the “Speed,” and we can refer to fast and slow telescopes. Fast telescopes are those with a shorter focal length or a larger aperture, and Slow telescopes are those with a longer focal length or a smaller aperture. This comes from camera lens terminology–slow lenses have to use longer exposure times for the same image brightness.
Generally, optical aberrations, when present, are worse in Fast telescopes than in Slow ones. Slow telescopes have more gently curved optical surfaces, so it’s easier to make them precisely the correct shape.
Spherical aberration, miscollimation aberration, chromatic aberration in refractors, and astigmatism all make the image softer and blurrier, and they are usually worse in fast telescopes. They won’t be severe on a high-quality fast telescope. The trade-off is that fast telescopes generally provide a wider range of magnifications since they allow relatively low magnifications with wide fields of view to be used.
f/3-f/5 is usually considered “fast,” and f/8-f/15 is considered “slow.”
Fast telescopes are usually intended for wide fields of view and deep-sky-object observing, whereas slow telescopes are usually intended more for planetary and lunar viewing. However, each can usually do both as long as the optics are good.
Focal ratio determines the useful range of eyepieces. Slower scopes can use long-focus eyepieces at lower powers, and faster scopes can use shorter-focus eyepieces at higher powers. For more on this, check out our guide to the Best Eyepieces.
Telescopes come in three main types:
- Refractors, which use a large lens at the front of the telescope to focus light towards the back of the telescope, with an eyepiece made of smaller lenses at the back.
- Achromatic Refractors use a doublet or two-element lens, which can correct for chromatic aberration (false color fringing) that occurs in lenses by ensuring two colors of light come to the same focus. They don’t correct for it perfectly, but longer thinner achromats have less false color than shorter fatter ones. All refractors for beginners are Achromats.
- ED or Semi-Apo Refractors are doublets made of exotic glass that can greatly reduce the amount of chromatic aberration compared to a traditional achromat.
- Apochromatic Refractors use a triplet or three-element lens made of exotic glass, which can virtually eliminate all false color fringing from the image by making three colors of light come to the same focus.
- Newtonian Reflectors, which use a large concave mirror in the back of the telescope to reflect and focus light towards the front, with a flat diagonal mirror to fold the light path out the side and into the eyepiece, made of small lenses. Most Dobsonians are Newtonians. The exact figure or shape of the primary mirror matters a lot:
- Spherical mirrors are figured very cheaply. Fast spherical-mirror newtonians are virtually useless except for low magnifications, but slow spherical mirrors, around f/8 or so in the 3”-6” range, can actually show nearly perfect images, since the shallower curve of a slow spherical mirror nearly exactly matches that of a parabolic mirror.
- Parabolic mirrors are the “correct” figure for a Newtonian reflector. They are more expensive to make, especially to make good, and especially in fast telescopes. They show sharp, crisp, detailed images at high power.
- Cassegrain Reflectors, which use a concave primary mirror in the back, and a convex folding/magnifying mirror in the front, which then folds the light path back to an eyepiece behind the primary mirror. Almost all Cassegrains you’ll find will be Catadioptrics, either Maks or SCTs, but a few “True Cassegrains” and other variants can be found. Because of the curved secondary mirror, Cassegrains have much longer focal lengths than their physical length. Cassegrains tend to be very compact but also ‘zoomed in.’
- Catadioptrics. Catoptrics means refractors and Dioptrics means reflectors. A Catadioptric is usually a Cassegrain reflector telescope with a large corrector lens on the front, but the Mak and Schmidt plates can be applied to Newtonians too.
- Maksutov-Cassegrain (Mak): A relatively affordable “Cat” with all-spherical surfaces, including a thick corrector lens on the front. The secondary mirror is usually a silvered spot on the corrector lens. Optical quality on the cheaper versions ranges from so-so to very sharp, but it is extremely sharp on the more expensive models.
- Schmidt-Cassegrain (SCT): A Cassegrain where the primary mirror is a cheaply produced spherical curve, but it is corrected with a very thin corrector plate. Most of these are very good optically.
- Jones-Bird or “Catadioptric Newtonian”: It would be difficult to avoid these, and yet you must. These are some of the bottom of the barrel, cheapest, lowest quality telescopes you can get. Essentially, the gimmick is they put a barlow magnifier lens inside of the telescope’s focusing mechanism to make the focal length longer. The problem is that all telescopes sold with this scheme have cheap, fuzzy optics, so high power views will disappoint, but they also can’t ‘zoom out’ to low power, wide-field-of-view views. Beware of any so-called Newtonian Reflector which has a focal length which is significantly longer than its actual physical length.
- Mak-Newts and Schmidt-Newts: It is possible to correct a Newtonian with the same type of corrector lens used on Maks and SCTs,
Each of these telescopes have different advantages and disadvantages. Refractors can have very contrasty images, as long as the false color fringing is kept under control and the optical quality is good (rare in cheap refractors). Newtonians are the cheapest type of telescope to make, so you can get high-quality sharp optics in large apertures without breaking the bank, but some observers get annoyed by the struts which hold up the secondary mirror, as they cast diffraction spikes on all the stars. (Others love the spikes). Cassegrains and Cats can be very compact telescopes, but they have such large secondary mirror obstructions that they have slightly reduced contrast and resolution compared to the best long-focus Newtonians and refractors of the same size.
The truth is you’re unlikely to recognize the difference in optics in your beginner telescope. The important thing is that whichever optical configuration you end up with, you get one with a high quality set of optics.
Some telescopes are advertised with statements such as “1/10th wave optics” or “1/4th wave.” The way each manufacturer measures and formats these claims varies, but the gist of this is that they are referring to the surface accuracy of the optics. Telescope mirrors have to be incredibly precise, so precise that they must be within 1/1000th the width of saran wrap, microscopically perfect.
Light has a wavelength, depending upon its energy level. The wavelength determines what color we see, so blue light is a shorter wavelength and a higher energy than red light. When a telescope manufacturer claims the optic is 1/4th wave, their claim is something like “the mirror deviates from the mathematically perfect curve by 1/4th of a wavelength of light,” usually green light.
Because there’s multiple ways of measuring wave error and telling them apart tends to be confusing, most telescope sellers merely manufacture their telescopes to 1/4th wave or better and then guarantee that they are “diffraction limited.”
Ultimately a diffraction limited telescope is one that will show sharp images right up to the 50x per inch or 2x per mm maximum useful magnification. There are minor improvements to contrast and sharpness to be found in a telescope with a smaller wave error than 1/4th (i.e., 1/8th or better), but these are unlikely to be noticed by a beginner, and are usually only noticed by advanced astronomers when conducting sensitive optical tests and when the air is perfectly still and the stars don’t twinkle.
It is a very good thing for a telescope to be diffraction limited, so this property will almost always be advertised in some form or another–so if it isn’t advertised, that’s a red flag. Same with whether a Newtonian reflector has a parabolic mirror. That’s a selling point, and if it’s not advertised, it’s a red flag.
Eyepieces come in three sizes: 0.965”, 1.25”, and 2”. (there’s also 3 inchers but those are niche and rare). The differences between them are expanded upon in our Eyepieces Guide.
A telescope’s focuser size determines what eyepieces can be used in it.
A telescope with a 0.965” focuser is probably a cheap toy (or at least, very old). Most of those eyepieces are cheap and distorted, and it would be hard to find high-quality upgrades. Avoid telescopes with 0.965” focusers.
Telescopes may have 2” or 1.25” focusers. 2” focusers can take both 2” and 1.25” eyepieces, though an adapter (which is almost always included) will be necessary for the latter.
Because the 2” eyepiece format provides wider true fields of view, a telescope with a 2” focuser is designed to provide wider fields of view than a telescope with a 1.25” focuser.
Sometimes the focuser is a 1.25” because there’s mechanical or optical limitations which prevent 2” eyepieces from working. For example, Newtonians with small secondary mirrors (which produce a smaller obstruction and so a clearer image) can’t illuminate the full field of view of a 2” eyepiece. Small Cassegrains may not have a hole in the primary mirror large enough to illuminate the full field of view of a 2” eyepiece either.
Other times the focuser is a 1.25” purely because it’s cheaper that way. This is the case with many refractors (which could be modified to support a 2” focuser), and some Newtonians. Cassegrains often come with 1.25” visual backs, but they can be replaced with a 2” visual back.
In a few cases a 2” focuser is incorrectly provided in a telescope that can’t make use of it. For example the SkyWatcher Classic 150P uses a 2” focuser, but its secondary mirror is too small to use 2” eyepieces.
Some refractors and cassegrains come with a 2” focuser or visual back, but use a 1.25” adapter and a 1.25” diagonal. In order to make full use of the 2” focuser, you would also have to buy a 2” diagonal.