A Telescope Buyer's Guide - All About Astro.com

Sidebar: Basic Optical Tube Design

My fifth grade daughter had a science assignment recently that required her to know that light can do 3 different things when it encounters an object:  it can bounce (be reflected), bend (be transmitted), or be absorbed (causing heat).    A telescope is designed to utilize one or two of these dynamics to refocus light onto your eyeball.   Diagram of three main telescope designs. Courtesy of Andrew Johnston at http://www.eaas.co.uk/ This gives rise to THREE different scope designs (above):  one that uses mirrors (reflectors), one that uses lenses (refractors), and one that uses BOTH mirrors and lenses (catadioptrics or "cats").   All of these designs require some way of mounting it, which is the subject of the previous SIDEBAR.  ​

One thing in common, regardless of the design, is that light has some distance to travel through the scope once it enters the scope.  More precisely, from the moment it hits the primary element, whether a mirror (in reflectors and cats) or a lens (in refractors), the light begins to be focused toward its "focal point."   This path from first contact (not including the corrector plate in an SCT) to the focal point is known as the focal length of the telescope.  The focal ratio (or f-number) will be the overall focal length divided by the scope's aperture.  In essence, this is a measure of how "steep" the cone of light is...or at what rate the light finds its focal point.

With lenses, where light can only be bent, it must travel along the full length of the optical tube and out the other end.   Thus, refractors are typically longer than most scopes at comparative aperture sizes.   With concave mirrored primaries, both the reflector and cat scope (most notably SCTs), will fold the light back in the direction the light came to hit a secondary mirror placed in the center of the scope's aperture end (see the note above on "central obstruction").  In a typical newtonian reflector, that smaller secondary mirror is also concave, angled 45 degrees to bounce the light out the side, a short distance to the eyepiece.  Compared to the refractor, the cutting-off of the light to the side of the scope as well as the typically short focal length of the primary mirror - lenses have typically longer focal lengths - means that the overall OTA of a newtonian reflector will often be shorter. 

A catadioptric, like a Schmidt (SCT) or Maksukov (Mak) Cassegrain, has a convex secondary mirror, which bounces light straight back toward the primary again, but this time the light "cone" is small enough to go through a hole in the primary and through the back of the telescope.   The eyepiece awaits on the back side of the scope.   In effect, the light has been twice folded back upon itself, which lends most "cats" the distinction of having a "folded" design.  More importantly, it allows the tube to be greatly shortened, making the design far more compact.  Similarly, because the secondaary is convex, it pushes light even further out of the back of the scope, meaning that longer focal lengths can be created with even shorter tube designs.   The net result is an OTA that's about 1/5th shorter than the focal length of the instrument; hence, the compact design.

We should be cautious about reflectors, however, since non-newtonian types of reflectors might do something different.  For example, a classical cassegrain, Ritchey-Chretien (RC), and Dall-Kirkham (DK) telescope, while being twice folded like a Schmidt or Maksukov (Mak) Cassegrain, are actually reflectors since they lack a lens element or corrector plate.   Interestingly, telescope-maker,  Planewave, markets their "CDK" , or "corrected Dall-Kirkham."  This design includes a pair of optical elements (lenses) just before the focal plane, making it a catadioptric design. 

Even more confounding, most any of the reflectors when used in imaging, especially RCs, will have optional field-flatteners/correctors sold as accessories, user-installed just before the focal plane.  Similarly, TeleVue produces their Paracorr, which is a correcting "eyepiece" that goes ahead of the typical eyepieces to correct the "coma" aberrations natural to a newtonian design.   What this goes to show is that the lines between the types of designs become very blurry in practical usage, since there is often a benefit to utilizing both lenses and mirrors as add-ons to traditional designs. 

Therefore, if you are confused by everything, then join the club!   

But rest assured none of that really matters right now, especially for the beginner.  However, since you've undoubtedly seen all the lingo in your research of prospective scopes, this discussion gives some context about what it all means.  

Refractors come in both achromatic and apochromatic designs.  This is necessary because refractive elements (lens) bend light differently at different frequencies.  You can see this with a typical prism, where light is spread out into a rainbow across its different frequencies.  As such, a telescope of a single lens element would be unable to focus all wavelengths at the same spot (a longitudinal error), meaning that red light (long waves) would be out of focus compared to the blue light (short waves).   Similarly, there is a component of lateral error as well, meaning that a lens cannot necessarily assure that all of a specific wavelength is focused on the same focal plane, since any distortion or magnification variance within the lens is wavelength specific as well.  

This, of course, is a problem...one in which mirrored scopes do NOT have...a big advantage with reflectors. 
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The solution for this "chromaticism" within refractors is three-fold:  1.) use more elements, 2.) use elements of higher quality glass (extra-low dispersion or ED glass), and 3.) make the focal length long enough to increase the "zone of focus."  

An achromatic refractor, typically with two elements (made of low-cost crown and flint glasses), is designed to bring light to focus in two broad wavelengths, typically red and blue.  These "doublets" are easy to manufacture and are cost-effective, but they do not work to focus ALL the visible frequencies of light, most notably "violet."   As such, on bright objects like the moon and planets, purple fringing is typically seen on the edges.   This is called "spurious color" or "chromatic aberration."  This is even more obvious if they make the scope too short.  For this reason, many good achromatic doublets will be LONG, with f-ratios in excess of f/12 or f/15.   Longer scopes (high f-ratios) increase the critical zone of focus or depth of field (see image below), making it possible for all visible waves, albeit dispersed, to be contained within that focal zone.  Such scopes are good performers because they make best use of the design, working around its limitations.  

Problematic for the first time scope buyer are those doublets known as "rich-field" refractors, which are made at f-ratios of f/5 to f/8.  These, as you would expect, have abundant color fringing!   If used on dim targets like star clusters and Milky Way vistas (hence the term "rich-field"), then there isn't too much issue, but if you hope to use such a scope to get good views of planets and the moon, then you will likely be disappointed.  Likewise, if you bought the scope because of the "fast" f-ratios to do astrophotography, then you will be disappointed when most all the stars in the image, especially those at the edges of field, are a nightmarish purple mess.  

As mentioned, color performance can be improved in refractors by adding more elements of glass, which of course raises the material and design cost of the instrument.   This gives birth to the "triplet" refractor.  Such a design is typically "apochromatic," meaning that it will be able to bring three broad wavelengths of light to focus, typically red, green, and blue.  This should also bring "violet," and all other visible wavelengths, into focus as well.  Though it should NOT be reasoned that all triplets are inherently apochromatic.  Typically, true "apo" (or APO) performance requires the use of one or more of the elements being made of a special, extra-low dispension or ED glass.   FPL-53 is the typical "ED" glass found in most of these refractor types, so if you have seen it listed in the specifications for an instrument, then now you know what it does!  

In some cases, a doublet-design can also be apochromatic in performance.  Takahashi, in particular, once made a line of fine "fluorite" element doublet refractors (they still do in short production runs).  Fluorite, which is a remarkably low dispersion glass that is grown in a lab, is now in too short supply to make these refractors in abundance; however, such fluorite doublets are wonderful, high-contrast instrument without a hint of spurious color.   Takahashi also utilized fluorite in their triplets and quadruplets, which if you are lucky enough to have, are some of the best visual scopes on the planet.   To my knowledge, the small volume-maker, TEC, and Takahashi (in a couple of short-production run scopes) are the only companies that still makes APO refractors with fluorite elements.  

Be careful of doublet refractors being marketed as ED APO scopes.  A doublet with a single ED element of FPL-53 glass might not be truly apochromatic in the technical sense of the term, especially if the scope is on the "fast" side...let's say below f/7 or f/8.  These scopes, because of their performance value are very attractive to buyers since they advertise APO performance at a bargain price.  However, when compared side-by-side with a good APO triplet, it's easy to see that their color-fidelity is somewhat lacking.   Today, because of consumer blow-back by those who know better, many distributors of these mostly Chinese-made "value" scopes have backed off the "ED APO" branding, choosing to advertise these doublets as "just" ED scopes.   However, these scopes are a great middle ground, a good value option, particularly for those wanting a good photographic instrument and are willing to compromise just slightly on performance. 

To clarify another aspect of refractors, especially triplets, lens elements can be configured as either "air-spaced" or "oil-spaced."   In the air-spaced design, lenses are only separated by a thin amount of air between them.  This means light must pass through six surfaces before it exits the lens "cell."   Each time it passes through an optical surface, from the dense glass to low density air, there is the possibility of internal reflections that can create a slight ghosting effect as you get farther from the center optical axis.  This is fixed by using coatings on the surfaces of those elements, which most modern triplets do today.

But when the elements ARE spaced with oil in between them, then the oil fills the gaps, making it behave as if the light passes only through two surfaces, the front of the first and the rear of the last.  No need to use surface coatings, albeit it's much more difficult to assure both chromatic AND spherical aberrations are tamed.  

A good oil-spaced triplet is delight to use, high-contrast and very well corrected for aberrations, though it typically takes an advanced observer to appreciate it.  It also comes to thermal equilibrium very quickly!   An "oil-spaced" triplet is a rare breed today, since companies can get great performance in all aspects with the cheaper, air-spaced design. But companies like TEC still utilize the expensive "oil-spaced" design in their scopes.   Having used their TEC 140FL, TEC 180FL, and TEC 210FL models, which are also "fluorite" triplets, these are the very best views I've ever had through any telescope at their given apertures. 

Other oil-spaced triplets that I've used in the past include the William Optics FLT-110 f/6.5 and various Astro-Physics models.  Even today, for me, they represent some of the very BEST in optical triplet performance.   

Finally, another refractor design, also apochromatic, is the 4-element Petzval design.  It typically utilizes a color-correcting lens pair up front, with one-or-both elements of fluorite or FPL-53 glass, and an additional "field-flattening" pair of elements in the back of the scope.  Obviously expensive and wonderfully pristine in performance, it's typically the best refractor you can find if astroimaging is your desire.  The Takahashi FSQ-series of scopes utilizes this design, as does the TeleVue NP-series of APO Refractors.   Per inch of aperture, these scopes are the best optical instruments there is when you demand the best color-correction combined with the ultimate in field-flatness.   Both traits are highly desirable when imaging with today's larger sensors, so much so that most people purchase "field-flatteners" to be used with the already pristine doublet and triplet apos. 

Sidebar: Chinese Value Options

Americans often swell with pride when we see the "Made in the USA" label.  And maybe we should.  Companies like Celestron and Meade; boutique telescope-builders like Astro-Physics and Stellarvue and TEC and Planewave; and scope/eyepiece juggernaut, TeleVue, are all companies that originate right here in America.  

American pride must seem strange, or even narcissistic, to my non-American readers. They probably are better aware than we are that American companies outsource seemingly everything from overseas.   If it's not the whole product, then it's likely the assembly.  Apple iPhones are less "American" than Toyota pick-up trucks.        

It's those aspects of this hobby that lead to confusion...simply put, the majority of budget products (and even some pricey ones) that you are seeing in your online research from American companies are predominantly Chinese made.   

While there are indeed great astronomy items that are conceived, designed, and manufactured in the USA, those likely aren't the items that a newbie to the hobby is shopping for.   More importantly, for you the uninformed, you need to know how this hobby works; how it HAS worked for ages.   

Compare the following mounts from 5 major retailers... Each of these mounts is the EQ-3 type mount, produced in China by Taiwai-based Synta Corp.  All prices similarly in a base configuration, a touch of paint, labels, and perhaps the tripod are all that sets them apart.   Some companies might have found ways to add electronics to this mount, as Orion has in this picture, but the consumer has to know that they are essentially the same thing.  

Synta has produced these mount for ages (beginning in ), ranging from the EQ-1, which comes as the most basic of any company's cheapest telescope offerings, to the EQ-6, which is a hefty mount that can exceed the $ price tag in certain situations (it's a very configurable mount from an electronic aspect). 

Traditionally, the most popular has been the EQ-5 type of mount, that many people might know better as the Celestron AVX or Meade LX85.  Again depending on maker and the options if had, this is essentially the same mount...and almost all telescope retailers have a version priced somewhere in the $700 to $ range.  

In most cases, the origin of the mount can be identified within the model name for each company.  If you see CG-5, EQ-5, HEQ-5, or any mount with a 5, it's likely the same mount.  Same with alt-az mounts, AZ1, AZ2, AZ3, and AZ4...yep, all Synta. 

Of course, they make most of the OTAs as well.  If a company advertises a budget 70mm refractor, a 4.5" reflector (also known as the 114mm), all the way up in size...it's Synta.    Anything sold by Orion telescopes is Synta, and that includes the XT-series of Dobs and the 80ED and 100ED doublet ED/APO refractors.   Again, NOT everything "Synta" is bad.  Those refractors, which I criticized for the "ED/APO" label elsewhere in this Guide, I also praised for being a really good value performer.  And more than once in this Buyer's Guide I have recommended the Orion XT-8 dob as my "favorite recommendation" for a serious, beginning observer.   Oh, and the Skywatcher 8" Classic Dob is the same scope.  Compare below... Two of my favorite scopes to recommend to serious beginners are the Sky Watcher 80" Classic Dobsonian (on the left) and the Orion SkyQuest XT8 Classic Dob (on the right). Well, actually they are the same scope, with only a slight change to the rocker box. - Click to see bigger. Incidently, Synta isn't responsible for everything, nor is China.  Many American companies also source from Guan Sheng Optical (GSO) in Taiwai.   All those accessories like focusers, eyepieces, finderscopes, and adapters that come WITH all those telescope bundles?   That's GSO.    Oh, and how about all those Ritchey-Chretien (RC) tubes you see on Orion.  Heck, those are even labeled "GSO" in the model name. 

And any of the other items on those webpages you are surfing, like some of the 6" and 9" newts or the 93mm refractors...you have GSO to thank for that.    

Most people who've been in this hobby know this information.  Some are bothered by it; yet others like me celebrate it, because we know that such value options help grow the hobby that we love.   Now that YOU know it, use it....look for similarities in products, shop more specifically at an online dealer/distributor because you hear good things about the company or the "product line."  In essence, within the first $ in this hobby, much of what you see from a company is ALSO offered by somebody else...or many somebody's.   

There are USA-made scopes from Meade and Celestron, most notably the SCTs and Maks, the success on which those companies were built.  But to compete with retailers, Meade and Celestron are also forced to source many of the budget telescopes on their website; sales of which are responsible for the bulk of their income.

Don't be disappointed in this fact.  Instead, celebrate that open competition in these markets gives you many more choices at much lower prices.  While the choices are overwhelming sometimes, it's not a bad problem to have!

As promised in my recent video, “The $50 Gift That Made Me A Scientist”, I wanted to give you a few telescope options for budgets ranging from $50 to over $250.

You will get efficient and thoughtful service from EXTENTOOL.

While my first telescope, obtained in , cost about $50, inflation (and not the cosmological kind ????) has taken those costs to over $100 but at least with some of these instruments come with features scarcely imaginable 37 years ago, like ‘smart adapters’ and GPS receivers!

Here’s a quick list of options starting with the least expensive, but still fine for beginners.

$55: This is the absolute cheapest option I will recommend. It’s compact, but I can’t vouch for much more than that. It comes with a smart adapter and has a decent number of 4–5 star ratings on Amazon.

$65: This scope is the closest to the one I used as a 13 year old kid…no frills, but a lot of fun!

$78: Slightly bigger and better than the scope above is this one, from Walmart: Celestron AstroMaster LT 70AZ. This comes with a Bluetooth Remote!

$200: The Celestron — PowerSeeker 80EQ Telescope —is a Manual German Equatorial Telescope for Beginners with a 80mm Aperture. It is Compact and Portable — and comes with BONUS Astronomy Software Package

Want more information on Telescopic Tools? Feel free to contact us.

$270: This is an amazing instrument, but a lot more expensive than the previous options: Orion StarMax 90mm TableTop Maksutov-Cassegrain Telescope

$400: This instrument, the Celestron StarSense Explorer DX 130AZ Smartphone App-Enabled Newtonian Reflector Telescope is a serious 6" diameter refracting (using a mirror, instead of lenses). It has a ton of features not available on the budget scopes above…and the price reflects that!

Once you have your new telescope in your little one’s hands, check out this ‘beginners’ guide to using your telescope’ from my friends at Space.com. There’s even a helpful video you can watch while you wait for the sun to go down!

Lastly, for older kids, say 12 and up, make sure to get a simple “lab notebook” so your budding young astronomer can record his/her observations. Have them use good practices. A lab notebook is a complete record of procedures (the actions they take), the steps and tools used, the observations they make (these are the data), and the relevant thought processes that would enable another scientist to reproduce their observations. This generally includes an explanation of why the observations were done, including any necessary background and references.

It could even be closely scrutinized if it documents your kid’s claim to a new discovery. Long after your kids have moved on from the their first forays, their notebook will remain and may be referenced, eg by younger siblings! Others will be building on the research that you are doing now and it is imperative to treat what they do such that others can replicate what your kids have done. A proper notebook will allow those who come after you to do that. A poorly kept notebook will not. Ultimately, your lab notebook is how you will be remembered during this time in your career!

Send me a message — how did your kids (and you) react the first time you saw the moon or planets through your new telescope?

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