So, recently I was shopping for telescopes and I decided on the following telescope:


I had exactly $180 to spend, and that telescope fit the bill. However, I also noticed that it had a variety of options in sizes, mainly the following:

  • 60MM EQ Refractor
  • 70MM EQ Refractor
  • 80MM EQ Refractor
  • 114MM EQ Reflector
  • 127EQ Newtonian Reflector

What is the difference between all these sizes/types, how does a "Newtonian Reflector" act differently than a "regular" (for lack of a better term) reflector and what are the benefits/drawbacks?

Bonus Points: Did I choose the "highest-quality" option for the price?

Side-by-side comparison of all sizes:

Side-by-side comparison of all sizes


4 Answers 4


First off, I would not recommend any of these telescopes. Of the ones you listed, the 114EQ is the least bad choice. But, due to the poor quality of its mount, it's the better of the bunch.

Ok, before I get into the nuts and bolts, let me first offer a couple pieces of advice:

1.) Join an astronomy club or society. In the US, membership usually runs $50 or less per year. Most clubs offer guidance and advice to new members. They also often conduct frequent star parties. These are events where those who have them bring out telescopes and everyone spends the evening observing together. At such events, you can see a lot of different types of telescopes of all different designs, prices, manufacturers, etc... and talk to owners about them. Clubs also have other programs that can help those new to the hobby learn and better enjoy their observing time.

2.) The best telescopes I can recommend in the under $200 price range (which is where these seem to be) are the AWB OneSky 130 (also known as the SkyWatcher Heritage 130P outside of the US) or the Meade Lightbridge Mini 130. Both are 130 mm Newtonian telescopes with fairly good optics for their price range and stable mounts. Meade makes a 114 mm version, and so does Orion. I would not recommend any telescopes smaller than that, as there's little they can show you that a set of binoculars in the price range wouldn't do just as well or better.

Now, your initial question was about the differences between them. Please bear with me, I tend to be verbose and try to be thorough (in other words, grab a snack and settle in for the read).

First of all, let me clear up a common misconception many people who are new to amateur astronomy tend to have. The primary purpose of an astronomical telescope is not to magnify, but to gather light. The problem with astronomical objects is less one of apparent size (this is a secondary concern, and not unimportant, but not as critical) and more one of brightness. For example, the Andromeda Galaxy, also known as object 31 in the Messier catalog (M31), has an an angular size of about 3° by 1.5°. The full moon is about 1/2° wide - so M31 is a lot bigger in apparent size. But it's very faint in comparison. If you know where to look and have fairly dark skies, you can actually see it with your naked eye at times, but not very clearly.

All the light you see comes into your eye through your pupil. The pupil of an average adult eye, when fully dark adapted, has a diameter of about 6 or so mm. That's about 28 or so square mm of light gathering area. A modest 80mm telescope (80mm being the aperture) has an area of over 5,000 square mm - over 150 times as much light gathering area. All that light is then funneled down and concentrated on your eye. This makes dim objects much brighter. Magnification helps, but it doesn't matter how big an object appears if you can't see it at all.

The light gathering ability of the telescope is dictated by its aperture - the size of the main optical element collecting light (either an objective lens or primary mirror). As such, it's not really adjustable (well, you can adjust it downward by blocking the aperture, but you can't increase it). Magnification, however, IS adjustable. The magnification power of a telescope is calculated by dividing the focal length of the telescope by that of the eyepiece. For example, if you have a telescope with a focal length of 1,000 mm and an eyepiece of 25 mm, you get 40x magnification. If you swap out the eyepiece for a 10 mm eyepiece, you get 100x. Theoretically, you can magnify as much as you like, though you can only magnify so much before the image is a blurry mess (this is also a matter of aperture - larger apertures allow for higher magnification before blurriness sets in).

Thus, the most important consideration when choosing an astronomical telescope is usually the aperture of the telescope.

When most people who are inexperienced with telescopes hear the word telescope, they think of a refractor. A refractor, or refracting telescope, uses a lens to collect light. This lens is known as the objective lens (though sometimes referred to, erroneously, as the primary lens). When light is passed through a substance, such as glass, it is slowed slightly by the substance. An objective lens is shaped with a convex (outward) curvature. When light passes through the thicker, center part of the lens it is slowed more than that in the thinner, outer portion of the lens, This causes the light to refract, or bend. The symmetry of the lens causes the light to all bend toward a point along an axis through the center of the lens. The point where the light rays meet is known as the focal point. All the light it collects is concentrated here. The eyepiece sits here to gather the light and reconstruct the image for viewing.

Refractors were the first telescopes made, invented in the early 1600's in the Netherlands. People often mistakenly consider Galileo the inventor of the telescope. In fact, the man most frequently credited with the invention is Hans Lippershey (though two other men sometimes are given credit). Galileo was only the first to use it to examine the night sky.

Even in his day, however, there was a problem with the design. When you refract light, you cause the colors to separate. This is exactly how a prism works. In fact, in a real sense, a lens IS a prism. This is because different wavelengths of light, what we percieve as colors, are slowed at different rates. Shorter wavelengths, which are bluer light, are slowed more than longer, redder, wavelengths. What this means in a telescope is that the actual focal points of different colors of light are separated with bluer light coming to a focus a bit closer in than redder light.

We call this phenomenon chromatic aberration, or CA, and it can be seen in telescopes as ablueish blur or fringe around objects you're viewing. It degrades the quality of the image.

There are two main methods for reducing its effects. The first is to increase the focal length of the telescope. A lens with a longer focal length has less pronounced curve: the difference between the thinnest and thickest parts of the lens are less dramatic than in a lens with a shorter focal length. This means there's less dramatic bending, which leads to the focal points of the colors being closer together. If you look at early telescopes, like those used by Galileo, they tend to be very long indeed with high focal ratios. The focal ratio is the ratio of the focal length to the aperture. My 8" (203 mm) Schmidt-Cassegrain Telescope (SCT) has a focal length of 2,030 mm, making its focal ratio f/10. My 8" imaging Newtonian has a focal length of only 800 mm, making its focal ratio f/3.9. I believe some of Galileo's early scopes had ratios like f/20 - the tube was 20 times longer than it was wide. Mostly this was to reduce chromatic aberration (though it was also to increase magnification)

The other method of reducing CA is by using multiple lenses with different shapes and made of different glass materials. The most common option is to use two complementary lenses that work together to shift the focal points of bluer and redder light to bring them closer together. This is called a doublet or achromat. Using three lenses makes a telescope a triplet and and is often called an apochromatic refractor. Sometimes even more lenses are used. But this comes at a price. Each lens modifies the light, and while correcting, can also introduce distortion. It also requires 2 surfaces (front and back) per lens to be precisely configured with a specific curvature. This is costly and time consuming. Lastly, lenses are heavy, and with a refractor, they are mounted at the front, which puts the weight at the front, and makes larger and longer such scopes much more difficult to properly maneuver. The largest refractor ever made was only about 49 inches, and the largest used for research was only 40 inches.

Galileo and his contemporaries understood the problem and that it could be solved by using a mirror instead of a lens. Unfortunately, in those days, mirrors were much more difficult to make. They were made from solid metal, which is much more difficult to precisely form and polish. Most often,they were made of a metal known as speculum metal, which tarnishes very easy. It wasn't until Sir Isaac Newton came along about 60 years later that a reflecting telescope was first created.

Newton used a mirror with a parabolic curvature to gather light. It sat at the end of an open-ended tube. Light gathered by this mirror was bent and directed toward a secondary mirror, which, mounted at a 45 degree angle, would send the light out the side of the tube to the waiting eyepiece.

Because a mirror of this kind is all metal, there's no glass to pass through, so the glass does not separate the colors of light. This eliminated chromatic aberration (well, there was still some in the eyepiece, but not much). But, again, mirror technology was a problem. Still, a few people adopted the design, and even built very large reflecting telescopes. William Herschel's telescope was about 48 inches in diameter. It was much easier to maneuver such a telescope, as the weight was all at the rear of the instrument (not to say it was easy, but putting a 4 foot glass lens at the end of the 40 foot tube would have been exceedingly difficult to work with - very, very top-heavy). Herschel's mirrors, however, tarnished easily. He actually had multiple mirrors, so one was being polished while another was in use, that he could just swap out as needed.

In the mid 19th century, the process of depositing a thin layer of metal, originally silver, but now usually aluminum, onto glass was developed. Glass, while heavy, is much easier to configure into a precise shape. Once done, the reflective coating could be deposited onto the glass mirror blank. Because the coating is on the front-side of the mirror, light doesn't pass through the glass, just bounces off the coating. This made Newtonian reflecting telescopes much easier to produce. Only the one surface, the primary mirror, has to be precisely configured, and this made them more cost-effective. And the weight and balance issues made making larger and larger telescopes easier.

Newton's design wasn't the only. Laurent Cassegrain and James Gregory both designed, but probably never constructed, reflecting telescopes relying on mirrors with a hole in the center. The light is collected by the primary, sent to the secondary, and sent back toward the primary to pass through the hole. In Cassegrain's design, the secondary is flat. In Gregory's, it was curved. Both allowed for a shortening of the overall physical length of the instrument.

In more recent times, the use of mirrors and lenses together lead to the development of the Catadioptric telescope. The most common such designs are the Schmidt-Cassegrain Telescope (SCT) and Maksutov-Cassegrain Telescope (Mak).

Just as refractors suffer from Chromatic Aberration, Newtonian telescopes suffer from an aberration called Coma. Coma appears as distortions of the stars around the edge of the field of view. They appear to lengthen and have comet-like tails (hense the name, coma). As with Refractors, the effect is reduced by increasing focal length. In modern telescopes, corrective lenses are often used.

Catadioptric telescopes resolve this partly by using a different kind of mirror shape, a spherical mirror. The curvature of the parabolic mirror used in a Newtonian design varies along its profile. The curvature of a spherical mirror is a constant curve. This makes them easier to manufacture and reduces or eliminates coma, but it introduces another problem: spherical aberration. Catadioptrics overcome this by using the combination of lenses and mirrors to correct the distortions. However, the numerous surfaces then require precise configuring, which increases the cost to manufacture.

Each of these designs has its pros and cons. There isn't one "all-purpose" option that's best for everything.

As far as amateur telescopes go, however, the Newtonian design tends to be the most popular. This is largely due to the average cost per inch of aperture. For example, an 8" Newtonian can be found in the neighborhood fo $300 USD for the telescope tube alone. An 8" SCT tube alone is likely to cost about three or four times that. Very few 8" refractors are available, and those that are would likely cost several thousand dollars.

Now, as to the telescope you chose. The Celestron PowerSeeker 127EQ is actually not a true Newtonian. It is what we call a Bird-Jones (or Jones-Bird, depending on who you talk to) Newtonian. A true Newtonian has a parabolic primary mirror and a flat secondary mirror. The only other optical surfaces are in any eyepieces used. The Bird-Jones Newtonian uses a spherical primary mirror. To counter the spherical aberration inherent in the design, a small correcting lens, which is not much more than a Barlow lens, is included in the optical path (usually mounted inside the focuser mechanism). This allows for a more compact design (the PowerSeeker 127 is only 508 mm (20 inches) long, but has a focal length of 1,000 mm (almost 40 inches). But it also decreases the quality of the view. The optics on this telescope are pretty poor. Worse still, it is extremely difficult to collimate.

All telescopes require the optical elements to be properly aligned to produce the best image. This is collmation. In the case of refractors, it rarely needs adjustment. Most SCT's require it only occasionally, and Maksutovs rarely. But Newtonians require it frequently. Newtonians will have a set of adjustment screws or knobs on the back, behind the primary mirror, and on the front as part of the support that holds the secondary mirror (usually referred to as the spider). Most telescopes use a 3-screw system for adjusting the tilt of the mirrors. It can be a chore at first, but with a little practice, it becomes fast and easy to do. It also requires a tool to insert into the focuser. Traditionally, this is a collimation cap or Cheshire, which work like an eyepiece, but do not have lenses. They help you visualize the alignment of the optics so that you can adjust it properly. You can also get a laser collimator which uses a low-power laser to guide you. But none of these work with a Bird-Jones scope. The correcting lens makes them very, very difficult to collimate.

Furthermore, the mount on the PowerSeeker line, particularly the 127EQ, is not very stable. It wobbles and shakes and makes for difficult observing.

Sadly, the reviews on sites like Amazon make it out to be a great instrument. But these reviews themselves are flawed. Amazon's servers use review rankings to determine the display order of search results. If an item has higher ratings in its reviews, it appears toward the top of the list of results when you search. Most buyers don't look past the first page, and rarely much further than 2 pages. So if a seller wants to sell his or her product, it needs to be on the first page, preferably right at the top. This leads a lot of companies and individual sellers to push customers to add reviews. Some companies also falsify reviews (though it would be difficult to prove it) in order to game the system. As such, the reviews on Amazon (and many other sites) are nearly useless to the consumer. And the PowerSeeker telescopes are a prime example.

On multiple other sites I spend time on, this telescope is outright hated. It is sold by Celestron (a chinese-owned company) to unsuspecting buyers. In the long run, I think it causes more harm to amateur astronomy than good - because users often end up discoraged by its poor peformance and cheap construction.

If you've already purchased it, I'd strongly recommend returning it and getting something different (e.g. the two scopes I listed at the beginning). If you haven't yet, then please do not. The two telescopes I listed are significantly better in nearly every way for a similar price. Even the slightly smaller 114 mm versions are better. But the PowerSeeker line is nearly entirely composed of low-quality equipment that is not worth purchasing.

But even before purchasing, joining a club is the best thing you can do. Some clubs even have loaner telescopes that members can borrow to learn before they buy.

I wish you luck with whatever you end up purchasing. Clear skies!

  • 1
    $\begingroup$ How... how do you have 91 reputation when you type answers as beautiful as this? Youve given me about 20 new terms to research-- I suppose Ill be returning it then, I didnt realize it was a chinese company... Ive been absolutely SCREWED by China VIA Amazon over 20 times so... thanks for the warning, the history lesson and the astronomy lesson. (And, technically, economics lesson) $\endgroup$ Commented Aug 16, 2018 at 23:36
  • 2
    $\begingroup$ I only have a 91 reputation because I really haven't spent much time here yet. I spend a lot of time on reddit.com/r/telescopes and quora.com (on Quora I'm regarded as a "Top Writer" and according to their old stats, was in the top 3 most read writers for telescopes and astrophotography. I just like to help people and share what I've learned along the way. $\endgroup$ Commented Aug 17, 2018 at 4:47
  • $\begingroup$ Cassegrain secondary is a convex hyperboloid. Gregorian secondary is a concave ellipsoid. $\endgroup$
    – Mike G
    Commented Aug 17, 2018 at 13:54
  • $\begingroup$ My mistake. I thought Laurent Cassegrain's original design was a flat mirror versus Gregory's convex curvature. I stand corrected. $\endgroup$ Commented Aug 17, 2018 at 14:39
  • $\begingroup$ Great writeup but I wanted to point out a clarification. @J.M.Haynes mentions the AWB OneSky130 as an alternative to the Celestron PowerSeeker 127EQ. One of his concerns is that Celestron is a Chinese-owned company. Turns out that in 2018 the OneSky is manufactured by Celestron for AWB. Not sure if that was the case in 2016 when this answer was written. $\endgroup$
    – Carlos N
    Commented Nov 24, 2018 at 2:00

A "Newtonian" reflector has a main mirror which is curved and focuses the light back up the tube. It then has a secondary flat mirror at an angle which reflects the image sideways. A Newtonian reflector will have an eyepiece at the top, near the opening, and at 90 degrees to the tube. Newtonian reflectors are one of the simplest designs of reflector.

The other major design of reflecting telescope is the Cassegrain, which has a main mirror with a hole in it. It reflects light up the tube to a secondary which reflects the light back down the tube to an eyepiece at the bottom.

From the image posted I'd guess both the reflectors are Newtonian. I don't know why only one is marketed as a Newtonian reflector. The main difference, apart from the larger main mirror of the 127, seems to be the focal length. A shorter tube allows for a wider field of view, but optical defects tend to be more significant. A longer tube is more tolerant of such defects. Short tube telescopes are good for looking at larger and diffuse objects like galaxies. Long tube is better for bright but small or detailed objects like the planets or the moon.

The other smaller telescopes seem to be refractor telescopes with a lens, not a mirror. "Newtonian" applies only to a design of reflector telescope.

But since "Aperture Rules", normally one wants the biggest possible mirror.

  • $\begingroup$ So, I was correct in thinking "the larger the mirror the better" technically? Also-- the 60, 70 and 80 have the eyepiece at the end of the telescope. Only the 114 and the 127 seem to have the side-sight as you mentioned. Would that mean the 60, 70 and 80 aren't Newtonian or are you saying that 90 degree bend at the end also counts as a Newtonian reflector? Thanks-- by the way, for taking the time to answer. $\endgroup$ Commented Aug 15, 2018 at 15:39
  • $\begingroup$ That's right. As I said "Newtonian" is the basic design of reflector telescope. Refactor telescopes can't be "Newtonian". $\endgroup$
    – James K
    Commented Aug 15, 2018 at 15:51
  • $\begingroup$ Ahhh, so it boils down to refractor versus reflector-- that's mildly confusing! Didn't notice they were even different words the first read. I'll read the wikipedia entries for more information, this is fascinating, honestly. Thank you! en.wikipedia.org/wiki/Chromatic_aberration was also a cool resource. $\endgroup$ Commented Aug 15, 2018 at 15:54

Some telescopes are refractors (convex lens at upper end), and some are reflectors (concave mirror at lower end). This Sky & Telescope article discusses the pros and cons of these and other optical designs. Briefly, refractors are low-maintenance but require a multi-element lens to minimize chromatic aberration; reflectors can provide larger aperture at lower cost but may need a little work to maintain the alignment of the mirrors.

The essential feature of a Newtonian reflector is a flat diagonal secondary mirror near the upper end. It folds the light path at a right angle, relocating the prime focus from the upper end of the main tube to a smaller tube on the side. Besides the classical Newtonian design using a parabolic primary mirror, there are Maksutov-Newtonian and Schmidt-Newtonian designs using a spherical primary mirror and adding a refractive element at the upper end to correct spherical aberration.

Cassegrain reflectors instead use a convex secondary mirror at the upper end to put the focus behind a hole in the primary mirror. As with the Newtonian family, there are Schmidt-Cassegrain, Maksutov-Cassegrain, and classical Cassegrain designs. Other reflector designs, e.g. Gregorian, exist but are less common.

There is an important difference between the two Newtonian reflectors in question. The 114mm, whose focal length is the same as its tube length, is of the classical design Newton originated in 1668. The 127mm, with a tube only half as long as its focal length, is probably a Jones-Bird variant with a spherical primary mirror and a small corrective lens near the secondary mirror. Aperture wins in theory, but in practice some users of the latter type have been unhappy with it.

In this lineup, the 80mm refractor and the 114mm reflector may be better choices. In addition to the 20mm and 4mm eyepieces that come with them, you might want an 8-10mm eyepiece for medium magnification.

  • $\begingroup$ Thanks for the clarification between Jones-Bird Newtonian and classical Newtonian. Also-- you said "in another answer" but didn't link the answer, do you have the answer? I didn't find it on a preliminary search of "Telescope Variants", "Newtonian Telescope" and a few other queries. $\endgroup$ Commented Aug 15, 2018 at 18:43
  • $\begingroup$ I've expanded this answer. $\endgroup$
    – Mike G
    Commented Aug 17, 2018 at 13:50

I'm interpreting this question two ways which are (1) what is the difference between telescope types and (2) reading "between the lines" is the question of "How do I get better views and/or the best that I can from the funds available?"

What are the different types of telescopes?

There are three main categories, but each major category divides into sub-categories.

Briefly, they are:

  • Refractors: Telescopes which rely only on lenses to focus light.
  • Reflectors: Telescopes which rely only on mirrors to focus light.
  • Catadioptric: Telescopes that use a combination of lenses and mirrors.


While these use only lenses, there are two sub-categories.

  • Achromatic refractors use only two lenses arranged as an achromatic doublet. This means the first lens is a typical convex lens ... with a convex curve on both front and back. But the edges of this lens work a bit like a prism ... splitting the wavelengths of light into a rainbow (an effect called dispersion). Stars in the center of the field of view look normal ... but off-axis stars (near the edges of the field) would be strongly effected with severe color fringing. So a second lens is added which has a concave front side and a flat back side. The second lens is used in an effort to reverse the effects of dispersion. This reduces but does not eliminate the dispersion effects.

  • Apochromatic refractors (often abbreviated as APO) are named because they remove (or at least attempt to remove) all the effects of dispersion. They often use variants of glass that are known to have low-dispersion properties ... such as fluorite crystal. They may also add additional corrective elements. Or they may do both. But low-dispersion glass alternatives usually aren't cheap. Fluorite, for example, is a naturally occurring crystal ... but isn't found in large enough or pure enough amounts to use as a lens. This means the crystal needs to be synthetically grown in a kiln. When growing crystal in a kiln, the faster you attempt to grow the crystal, the lower the optical quality (e.g. optical inclusions occur). This means that growing crystals large enough and pure enough that they can be ground into lenses requires growing them in a kiln for ... possibly a few months to produce just one batch. This really drives up the price of the optics. For this reason, apochromatic refractors tend to be expensive ... sometimes extremely expensive.


As mentioned previously, these telescopes focus light via mirrors and do not contain lenses (Other than the eyepiece that might be used when doing visual observing. Those don't count and technically you could use a camera instead of an eyepiece ... and have no lenses anywhere.)

These also have numerous sub-categories.

  • Newtonian reflectors are the simplest and most common version (also the first). Light is focused using just a single parabolic mirror at the back of the telescope. There is a second mirror ... but it doesn't focus light. It's just mounted at a 45° angle to bounce light into the focuser and ultimately into the eyepiece or camera. Since these telescopes have just one focusing surface, they tend to be the least expensive design. These telescopes do suffer from off-axis coma (stars in the center of the field appear normal. Stars near the edge of the field develop a "tail"). For visual observing, this usually isn't bothersome. For imaging purposes, a "coma corrector" lens is typically added. These telescopes are well-suited for visual observing of deep-sky objects (galaxies, nebulae, etc.) and tend to have lower focal ratios (e.g. f/4 to perhaps f/6 or f/7).

  • Ritchey-Chrétien reflectors (often abbreviated 'RC') use two focusing mirrors. Both the primary mirror and secondary mirrors are hyperbolic mirrors and this design is used to eliminate off-axis coma. Famous RC telescopes include the Hubble Space Telescope and the Keck Telescopes. ESO's Very Large Telescope is also an RC. They are more often used for astro-imaging work rather than for visual observing.

There are a few less-common variants

  • Gregorian reflectors are similar to Newtonian reflectors but do not use a parabolic primary mirror (it's a simpler concave primary.
  • Cassegrain reflectors (not to be confused with Schmidt-Cassegrain Telescopes which are in the catadioptric family) use a parabolic primary mirror and a hyperbolic secondary mirror.
  • Dall-Kirkham reflectors use a concave elliptical primary mirror and a convex spherical secondary mirror. They tend to have flatter fields (focus is even across the field) but do have off-axis coma. (PlaneWave Telescopes use a variant of the design called a Corrected Dall-Kirkham ... the corrector eliminates coma and astigmatism so that the instrument projects an image which is flat with respect to focus, astigmatism, and coma to a fairly large diameter. They are primarily designed with astro-imaging applications in mind and build telescopes with apertures up to 1 meter (popular as research-grade telescopes used by universities).


As previously mentioned, these telescopes use a combination of lenses and mirrors. There are numerous sub-categories but the two most popular are Schmidt-Cassegrain Telescopoes and Maksutov-Cassegrain telescopes.

  • Schmidt-Cassegrain Telescopes (often abbreviated as SCT but also often referred to by the nickname "Cats") use a spherical primary. The spherical primary results in spherical aberration so the front of the telescope has a Schmidt corrector plate which is designed to eliminate spherical aberration. It also uses a convex secondary mirror. These telescopes tend to have long focal lengths relative to their physical size. Usually they have an 10:1 focal ratio (aka f/10) but there are variants that have other focal ratios. Celestron on and Meade are possibly the two most common manufacturers of SCT's for the amateur astronomy market.

  • Maksutov-Cassegrain Telescope's (often referred to by the nickname "Maks"). These telescopes use the Cassegrain design with a spherical primary mirror at the back, but the "corrector plate" at the entrance is a weakly negative meniscus shaped lens. The center of the lens is silvered to act as a convex secondary mirror. While the telescopes are relatively compact in size, they have rather long focal lengths and high focal ratios (typical 15:1 or f/15 ... or close such as f/14). They are well suited to planetary and lunar observing.

Most Common Entry-Level Telescopes

Of these designs, the most common and popular entry-level telescopes (telescopes on a budget) are the achromatic refractor and the Newtonian reflector.

Desirable Qualities and Features

For visual observing, telescopes focus light to an eyepiece. The telescope has a focal length ... but the eyepiece also has a focal length. The magnification factor is found by dividing the focal length of the telescope by the focal length of the eyepiece. E.g. if a telescope were to have a 1000mm focal length and you were to insert a 20mm eyepiece, then 1000 ÷ 20 = 50. This would provide 50x magnification.

This means the shorter the focal length of the eyepiece, the higher the total magnification. E.g. a 1000mm telescope with a 5mm eyepiece would result in 200x magnification.

Eyepieces have two standard barrel diameters. Smaller/shorter eyepieces tend to use 1.25" barrels. Longer focal length eyepieces and eyepieces with wider apparent field-of-view (AFOV) tend to use 2" barrel diameters.

Most telescopes have 1.25" or 2" focuser to receive these eyepieces and you can insert any eyepiece into the focuser as long as they use the same size. (There are adapters to allow 1.25" eyepieces to be inserted into 2" focuser). Brand doesn't matter (e.g. you can buy a telescope from one brand and get eyepieces made by other brands and that's fine... they work.)

But there are some practical limits. The maximum useful magnification has a simplistic guideline (possibly overly-simplistic) ... where the maximum useful magnification is 2x the telescope's aperture when measured in millimeters or 50x the telescope's aperture when measured in inches. E.g. a 4" aperture telescope (roughly rounded to 100mm) would have a maximum useful magnification of 200x. The caveat is this presumes flawless optics and perfect "seeing" conditions ("seeing" refers to the stability of the atmosphere). In other words, you have to have highly ideal (and rare) conditions to use such an aggressive amount of magnification. If you over-magnify the object will not appear to be well-resolved (it will appear as if the object is not in focus ... even though it is focused as well as possible).

A more practical limit is to expect the maximum enjoyable magnification to be roughly 1x the telescope aperture when measured in millimeters or 25x the aperture when measured in inches.

Detail Resolving

The conclusion you can draw from the previous section is... the larger the aperture of the instrument (physical aperture size) the more detail it can resolve.

My first instrument was a 3.5" telescope. It allowed me to view the "rings" on Saturn and the "bands" on Jupiter. My next instrument was a 5" telescope. That's when I noticed that if "seeing" conditions were good, I could sometimes see the Cassini Division (black band or gap) in Saturn's rings. Also I could detect a hint of swirling detail in Jupiters cloud bands. And as I would eventually have access to larger and larger aperture telescopes, the amount of detail increased accordingly.

Larger aperture general results in greater detail. I eventually owned a 14" telescope. The advantage is much better ability to resolve detail ... but the disadvantage was weight. The weight shouldn't be disregarded as trivial... this telescope weighed so much that it required two people in reasonably fit physical shape to setup and take down (this is getting into a size range where the telescopes are usually permanently mounted on an observatory pier). The take-away here is that telescopes heavy telescopes that require two people to set up ... may not get used as often (some astronomers will also buy a smaller "grab and go" telescope that they can use with no fuss.)

Stability (Mount)

The other downside to my first telescope was the mount. It had an aluminum tripod. It looked reasonably solid (looks can be deceiving). It turned out that the mount was not particularly solid. When I would touch the focusing knob on the telescope, the telescope would shake (vibrate) ... and this resulted in the object I was viewing (e.g. a planet such as Saturn) to shake violently in the field of view. It would shake so much ... that I was unable to tell if my focus adjustment was improving focus. This was frustrating. Having learned from this experience, future telescope purchases involved me being much fussier about mount stability.

"Dobsonian" reflectors: I put "Dobsonian" in quotes because that name actually refers to the mount ... not the telescope. The telescope is actually a Newtonian reflector ... on a Dobsonian mount. But the design is so popular that they are often called Dobsonian telescopes or Dobsonian reflectors or just the nickname "Dob". The mount is a cradle that rests on a turntable ... directly on the ground. It is commonly made out of wood (or even particle board) and does not have legs. But the design is extremely stable and yet... the construction cost is cheap. This is an extremely stable yet very low cost design. And since the telescope is a Newtonian reflector (one of the least expensive telescope designs), the result is that you can get a very stable, and yet fairly large aperture telescope ... for not a lot of money.

An 8" aperture Dobsonian ... is often somewhere in the price range of about $400 USD. That's not a lot of money for such a large aperture instrument.

Your local library might have a telescope lending program. If you join an astronomy club, the club might have a telescope lending program (my astronomy club does ... and I know of a few other clubs that do as well). Members are able to borrow a telescope from the equipment pool at no cost ... and use it for a few weeks. Given that membership in an astronomy club is usually pretty cheap ... it's a low-cost way to get access to reasonable decent telescopes (far less expensive than buying a telescope) and comes with the added bonus that club members will often help you learn to use the equipment and help you learn to find the most interesting objects in the night sky.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .