With the huge variety of telescopes on the market, the new astronomer faces a daunting task to decide which one to buy. This guide will help you cut through the hype. Here’s what you’ll learn:
There are three important specifications to consider when choosing a telescope. You’ll see these numbers in advertisements and dealers will use them. So, it’s important to understand them.
Aperture is the diameter of the front opening (objective) of the telescope. It is the probably the most important specification because it determines how much light enters. If a telescope can gather a lot of light, then it can create bright, high resolution views. Think of it as a limiting factor. If a telescope is advertised as having huge magnification power, but there's not enough light available, then you're just going to be magnifying a blurry blob.
Focal length refers to the distance between the objective opening of the telescope and the point where the light comes to a focus where the eyepiece is. For some telescopes, the focal length is approximately the distance from one end of the telescope to the other. But for other telescopes, the light travels back and forth inside the tube so the focal length is actually longer than the telescope. We'll explore that later in the Optical Design section.
This number (sometimes referred to as ‘power’) indicates how many times larger the telescope can make an object appear. For example, a telescope with a magnification of 250X can potentially magnify an object to 250 times the size that it appears with the naked eye.
Determining the magnification
Telescopes with longer focal lengths have more magnification power. In fact, you can easily calculate the magnification power of a telescope by dividing its focal length by the focal length of the eyepiece that you are using. For example, if your telescope has a focal length of 1000mm and you use a 25mm eyepiece, the magnification is 40x.
A note of caution about magnification claims!
Most experts caution consumers about the extraordinary claims that manufacturers make about the magnification power of their telescopes, warning that they are trying to mislead or even scam buyers. I agree and disagree.
It's true that if you are hoping to view faint, deep-space objects, and you buy a telescope with a small aperture and a high advertised magnification power, then you will be disappointed with the fuzzy image you get. These faint objects need a wide aperture to gather enough light to be able to magnify them clearly.
But you might be more interested in viewing brighter objects like the moon and some of the planets. These objects are so bright that they can be viewed with fairly small apertures and still be magnified quite effectively. In this case, the bold magnification claims don't disappoint.
Here’s a rule of thumb. To estimate the ‘maximum useful magnification’ for a telescope, multiply it’s aperture (in inches) by 50. For instance, for an 80mm (3 inch) telescope, 3 x 50 = 150. So, on a very clear night, the upper limit of magnification where you could still see objects clearly would be 150x. If the package screams ‘400 times magnification!!’, then perhaps you should move on.
There are three categories of telescopes, and knowing their benefits and drawbacks is key to a good decision.
This was the first telescope design and the one that Galileo used. The light enters the front opening (objective) of the telescope and immediately passes through a convex objective lens which refracts (bends) the light so that it converges at a focal point at the back of the telescope. Here the eyepiece is positioned for viewing the image.
- These can be rugged and low maintenance, because the lenses rarely, if ever, need adjustment and dust cannot enter the closed system
- It produces an erect image, which makes terrestrial viewing and locating night sky objects easier.
- Images are a little bit clearer and higher contrast compared to other designs at the same aperture. For this reason, higher end refractors make excellent astrophotography telescopes.
- Air cannot travel inside the system, so temperatures are stable, making the images steadier
- Refractors cost more per inch of aperture because lenses are more expensive to produce than mirrors.
- Chromatic aberration is an issue because different kinds of light refract differently through the lens. Manufacturers compensate for this by using more complex mirror designs. Most medium-priced refractors are achromatic, which means much of the discoloration is corrected (fine for visual observation). Premium refractors are apochromatic, indicating that the problem is virtually non-existent.
- The closed system can take some time to adjust to the ambient air temperature.
Who should buy a refractor telescope? Smaller refractors make a good gift for a child or new astronomer because they are rugged and require no maintenance. Also, if you want to view terrestrial objects, choose this design, since its the only one that keeps the image upright. High quality achromatic and apochromatic telescopes are the best choice for many astrophotographers.
A reflector telescope, on the other hand, has no objective lens, so the light passes directly to the back of the telescope, where it meets a concave ‘primary mirror’. This mirror reflects the light back up the telescope tube toward a smaller, ‘secondary mirror’.
In Newtonian reflectors, the light is then reflected sideways towards an eyepiece placed on the outside of the tube. One of the most popular types of telescope amongst amateurs is the Dobsonian. This is a large Newtonian telescope mounted on a swiveling base for easy maneuvering.
- Reflectors cost less than refractors because mirrors are more cost-effective than lenses. This means for the same price you can buy a reflector with a larger aperture.
- No chromatic aberration because the light is reflecting rather than refracting.
- The profile of a reflector is relatively stable and manageable. Since the light is traveling back and forth inside the tube, the telescope can be half the length of a reflector and still have nearly the same focal length. That means you can have a very powerful telescope that sits easily in the back of your car and rests gracefully on your mount.
- The eyepiece position on the side of the tube makes for a comfortable viewing position.
- Periodic maintenance may be necessary, as the mirrors need adjustment (this is called collimation). Also, dust can enter the system.
- Newtonians suffer from a problem called coma distortion, where stars at the edge of the field of view have a tail effect, similar to comets. While this isn’t a concern for most visual astronomers, a device called a coma corrector can fix the issue.
- Reflectors usually are not suitable for serious astrophotography without modification. Recently, however, a few imaging Newtonians designed specifically for astrophotography have entered the market.
Who should buy a reflector telescope? Newtonian reflector telescopes are loved by the visual astronomer who wants as much light gathering power as their budget will allow. Dobsonian telescopes are affectionately called ‘light buckets’ because their apertures are so large. These are a great choice, not for young children, but certainly for enthusiastic teenagers and adults who want to see deep space delights that may be fainter.
Catadioptrics telescopes (compound telescopes)
We have seen that refractors use lenses, while reflectors use mirrors. Well, catadioptric telescopes use a combination of both lenses and mirrors. This allows manufacturers to adjust for some of the minor image aberrations common to either refractors or reflectors. This also allows them to ‘fold’ the light path, which means more magnification compared to other telescopes of similar size.
The most common types of catadioptric telescopes are called Schmidt-Cassegrains and Maksutov-Cassegrains.
- Because the light path is folded, catadioptric telescopes are much more compact than refractors or Newtonians.
- For the same reason, they often have very long focal lengths (coupled with relatively large apertures), making them powerful telescopes.
- Very little maintenance is required compared to Newtonians, as the mirrors and lenses rarely need adjustment.
- They make wonderful astrophotography telescopes.
- The longer focal length means a narrow field of view, which isn’t suitable for viewing some of the large deep space objects - great for planetary viewing though.
- These can be relatively expensive.
- The closed system can take some time to adjust to ambient air temperature.
Who should buy a compound telescope? These telescopes do a lot of things very well. If you want a low-maintenance, very portable telescope for both visual and photographic use, and you don’t mind paying a bit more, these telescopes are a great pick. Also, if you want a planet viewing specialist, this is the one.
The type of mount you have is just as important as the optical design of your telescope. This is not an exaggeration, so read on.
This is the most common type of mount for beginners. Its most similar to a camera mount in that it adjusts the orientation of the telescope in two directions, straight up and down (altitude) and left to right horizontally (azimuth). This makes it easy to move the view of the telescope around the sky as you desire.
Since stars don't move straight up and down or left to right, the equatorial mount was invented to follow their actual paths. This kind of mount is for more advanced or ambitious users since it must first be aligned with the North Celestial Pole and it adjusts along two different axes, namely the Declination and Right Ascension axes. This movement puts the telescope in awkward positions, so a counterweight keeps the center of balance over the mount. Many equatorial mounts are driven by a motor along the Right Ascension axis, which keeps the image in view for better photography and visual observation.
This is actually a type of altazimuth mount because it also moves horizontally and vertically. However, it looks very different and is used only with Newtonian telescopes. Instead of using a tripod, it uses a swiveling base, much like a Lazy Susan. This makes easy work of maneuvering larger telescopes.
GoTo Mounts are completely computerized mounts that can find objects in the sky and track them automatically. They exist in all the above forms: Altazimuth, Equatorial and Dobsonian mounts. They are very popular for astrophotographers who require long-exposure images, but they also make visual observation easier and even more social. Imagine inviting friends over for a star party and having your mount take you all on a guided tour of the sky tailored to the specific date and time.
Congratulations! Now you can identify the different types of telescopes and their mounts. You understand the specifications. And, you can make an informed decision about which one to purchase. If you have any questions, feel free to contact us. We’re happy to help.
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I bought this scope for an industrial client whose needs are industrial, not astronomical, and this Explore Scientific ED127 is intended to be a beam expander in various Interferometric testing setups. For this, optical quality must be high and the carbon fiber tube is a plus for thermal stability. My purchase of the Explore Scientific ED127 F7.5 APO Triplet through Telescope Space was straightforward and pleasant. Ryan made sure everything was smooth.
Upon receipt I tested the scope by looking at a power pole 700 feet away, one that I have used for many such tests. The star test from the insulator glints was good – the focus snap was crisp, the in-focus diffraction blur was round with a faint first ring. I did not look at the through focus balance with a narrow passband filter as it was obvious there was no SA3. Color correction is very good as you would expect with the glasses used. There is no color in focus, and only a hint of subtle color fringing in the just out of focus edges of details. Extended details on the pole were rendered as well as, or better than, with any similar scope I have used on this target – this includes TMB, Astrophysics, and Takahashi scopes.
The build quality of the carbon parts is gorgeous. The metal parts are good quality anodized machined. I would prefer machined tube rings as the cast ones supplied are a little chintzy compared to the rest of the scope. The rack and pinion focuser works well, but the feel is not as good as some stock friction focusers I’ve used. Considering the extra high weight capacity of the focuser the feel may be the price paid. Overall I think the mechanical appearance and function is in line with the price.
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As a higher-level amateur, I was looking for a decent scope & mount that I can modify for my needs. The 130mm Newtonian scope fits my needs quite well. It comes with a 25mm eye piece--which is great. I would consider purchasing other eye pieces at the same time to give you a variety of magnification options. I'd recommend something like a 12mm and 8, or 6mm eye pieces to complement the 25mm. Getting a 2X Barlow will double your eye piece viewing options. The higher number will give you a wider view, smaller number a tighter, more magnified view. In order to just resolve the bands & zones of Jupiter, I need to use an 8mm eyepiece. For exploring Lunar geography, or seeing the Galilean Moons, the 25mm is sufficient. Saturn is just discernable, until you switch-out the eye piece for a 12mm, or smaller. The heavy & stable tripod mount is easy to set up, and the large rotation nobs allow you to smoothly track your subject as the Earth rotates. The Red Dot spotting site is dead-on accurate (once you dial it in). If you want a good, intermediate telescope: one that's neither too cheap, nor one that's too expensive, the 130mm Newtonian should fit your needs quite well. (Image of the Moon has been rotated and adjusted with photo software).
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For an amateur astronomer, this scope fits quite nicely. Everything arrived about 2 days after ordering. Assembly took less than an hour, and that was with a cat continuously attacking plastic wrapped items (cat not included). After assembly, I spent about 30 minutes fund-raising the control placements, mount angles, etc.... It comes with a 25mm eyepiece, which is a decent, general purpose unit. Perfect for observing the geography (selenography?) of the moon. Was also able to observe Jupiter and 4 of its moons. Couldn't resolve the Jovian bands with the 25mm eyepiece, so will purchase other eyepieces for viewing the gas giants. The red dot spotter scope was easy to dial-in for accurate spotting. It came with a battery, so all good. I would recommend this scope for serious amateurs, consider buying other eye pieces at the same time for various magnifications that you might need.
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