Rich's Telescope Primer Part 1
Sep 21, 2000 (Updated Mar 25, 2007)
Popular Products in TelescopesThe Bottom Line Telescopes have strengths and weaknesses depending on their design. No one type excels at every purpose, but good examples of the main types are immediately available.
I am writing this as a sort of sequel to my article titled Picking a Telescope. This article assumes the reader has read that article. This document is designed to give an overview of the basic types of telescopes currently offered and their inherent strengths and weaknesses. This article also includes some information on types of mounts and how they work. The idea is for the reader to have a fair grasp of the state of the art by the time they complete Part 2. Since the telescope industry has become increasingly dynamic over time, I revisit this article and update it from time to time to keep it current. The contents are:
Types of telescopes:
- Achromatic Refractors
- Long Focal Ratio
- Short Focal Ratio (Short Tubes)
- Apochromatic Refractors (APO)
- Newtonian Reflectors
- Long Focal Ratio
- Short Focal Ratio (Short Tubes)
- Schmidt Cassagrain Reflectors (SCT)
- Class Performance
- Maksutov Cassagrain Reflectors (MAK)
- Class Performance
- Maksutov Newtonian Reflectors (MAK-NEWT)
- Class Performance
- Unobstructed Reflectors
- Unonbstructed Newtonians
- Other variants
* Equatorial Mounts
- Traditional Versions
- Computer Controlled
* Fork Mounts
- Traditional Versions
- Computer Controlled (GOTO)
* Dobsonian Mounts
- Traditional Versions
- Computer Controlled (PUSH-TO)
The function of a telescope is to serve as a passive light amplifier. To perform this function, the telescope concentrates light rays from the viewing object to a point. From there, the rays diverge again to where they are brought into focus by either an eyepice or a photographic device. The two methods of concentrating light this way are refraction, where a lens bends the light, and reflection, where a mirror is used to focus the incoming light. Below is an overview of the different devices used for doing this function.
The refracting telescope is the oldest type of telescope. It operates by using a lens assembly to bring light rays to a common focus. The lens is, in effect, a varied prism with a stronger angle as you get closer to the edge, which bends the light as it enters and leaves the lens. Like a prism, this has the effect of spreading the light into its colors because refraction is dependent on the wavelength of the light being bent. This quickly causes colored fringes around objects seen in a simple telescope because the lens bends blue light to a very different focus from green, yellow, and red light. So, if you focus in green, where your eye is most sensitive, the parts of the image formed by blue and red light will not be in focus, so the object will seem to have a purple halo. As a result, such lenses are only seen in toy telescopes. This effect is called chromatic aberration and is the greatest technical difficulty refracting telescpopes face. The achromatic and apochromatic telescopes are variants which use multiple lenses on the front end made from different materials with different indexes of refraction to try to correct for this effect.
1. A- Achromatic Refractors
Achromatic refractors classically use two lenses in their objective elements to correct for chromatic aberration. The first variants used crown glass (the like the glass in windows) bonded to a second lens made from flint glass (lead crystal) to reduce chromatic aberration. In recent years there has been a lot of work done to make these relatively inexpensive telescopes perform better, and modern variants have had a resurgance of sorts as the pin-point sharp images refractors have always been known for have had less color fringing. These telescopes are still limited in light gathering ability compared to cost because of the expense of making large lenses.
1. A1 Long Focal Ratio
These are the long and skinny telescopes everyone thinks of in their mind's eye when they hear the word Telescope. Versions of this design with diameters of 80mm or more give respectable performance for viewing of planets and the moon. The high magnification associated with long focal ratios of f/11 or more robs these scopes of much of their light amplification ability in exchange for magnification. However, the long focal lengths also require less bending for incoming light rays and so chromatic aberration is less of a problem.
These are large and heavy telescopes for their aperture. The tiny 60mm aperture of the Tasco and other telescopes commonly seen in department stores is masked partially because the Optical Tube Assembly (OTA) has such large dimensions overall. In the final analysis, the long focal length refractor offers prospective observers the least for their telescope dollar. As a reusult, other designs have become quite common and have flourished. There are still some excellent Achromatics sold, such as the Celestron 150mm f/8 telescope, but the same amount of money could have bought far more viewing flexibility if it were spent on something else.
1. A2 Short Focal Ratio (Short Tubes)
The short tube achromatic refrator is a fairly recent phenomenon. In essence, these telescopes, such as the Wide View 80mm and variants up to 120mm diameter have focal rations down around f/5 and have skirted the edges of chromatic aberration to do it. Have no doubt- these telescopes will have noticable color in their images. However, they have superior glass to what used to be available and their geometries have been computer simulated to allow chromatic aberration to be controlled for a best compromise image. And, on a simple mount like the NexStar 80GT, these have been reasonably priced dream-scopes for kids.
In use, these little telescopes show wide fields of view in the night sky and have good light amplification characteristics. They will put violet halos around bright objects, but a trick has been used to make this less objectionable. Instead of a conventional design where the spectrum is balanced for a focus in the middle of the visual range, these are tuned to focus with yellow rather than green at the center. This makes red through green show up near focus, while far blue and violet are so badly focused, they look like a violet background in the image. So, on the Plieades, the blue color of the stars is gone, but they don't appear to have individual purple haloes. These scopes are quite economical and have gained a wide following thanks to their portability and practical usability for most astronomical viewing purposes. Orion, Celestron, Konus, SkyWater, and many other brand names can be found on the sides of these Trusty companion telescopes. As time has gone on, somewhat higher end versions of this geometry have started appearing with more exotic glasses such as ED (extra-low dispersion) glass or with triplet objective lenses which are NOT simple Achromatic scopes. Since there isn't a very clearly defined cutoff between the optical performance of achromatic and apochromatic telescopes, these designs have sometimes been referred to as semi-apochromatic. However, in practice they are mostly recognizable since the image will have true color compared to the Short Tubes, and cost 2 to 3 times as much.
1. B- Apochromatic Refractors (APO)
These telescopes are an evolution of the achromatic telescope. Where the term Achromatic means colorless, the term Apochromatic means without-color. Essentially, these telescopes can be identified by the special type of objective lenses which will be found in them. Apochromatic telescopes generally have three to four objective lens elements and at least one of them will be made from calcium flourite. Sometimes these telescopes will be referred to as Flourite scopes because of this. These telescopes have short focal lengths in the neighborhood of f/6 to f/7.5 and the most popular models have diameters from 70mm to 106mm, although examples also exist with diameters of 130mm, 155mm, and even larger. However, it should be noted these are the most expensive telescopes sold compared to their diameter.
The APO telescope has become popular for the incredibly sharp images it produces and several specialist manufacturers including Astro-Physics, Takahashi, Televue, and Thomas M Back have made their reputations making these instruments. If one is paying top dollar for a small telescope, it is most likely it will be one of these, and the quality is there. Many people feel there is no going back after they have looked through an APO telescope. On the other hand, at the price these have been sold for, it is usually certain something which would vastly outperform them in light gathering capability could easily have been procurred for the same cost. The price of an APO has been somewhat like the price of a new Dodge Viper or a Rolex in that regard: you are paying to say you own one.
Recently, though, much of this picture has been turned on its head by manufacturers such as Orion and Celestron and more recently Astronomy Technologies offering scopes such as the now-famous 80mm f/7.5 80ED and the superb 66mm f/6 AT66ED. In essence, these telescopes offer performance which really is equivalent to the expensive TeleVue, Takahashi, and Astro-Physics (which apparently has now abandoned telescope production in favor of mounts) telescopes. And, in comparison, I would have to say some of the historical APO scopes, such as the Televue 85, aren't even as good as this crop of less expensive scopes.
Even though the first parabolic mirrors capable of bringing parallel light rays to a common point were first made by the Greeks 2500 years ago, it took Isaac Newton to make their potential for imaging a reality. The reflecting telescope has several inherent advantages over the refractor for light handling. First, the reflector has no chromatic aberration because all colors of light are reflected the same way, unlike the process which happens in lenses. The other benefit of the reflector is light throughput efficiency. Unlike lenses, which can each take away up 10%-15% of the light hitting them for every lens, high performance mirrors routinely have reflectance levels of 96% or better.
However, many optical geometries using mirrors require complex curvatures which may be difficult to produce accurately. As a result, reflecting telescopes have a different set of difficulties from refractors generally called Spherical Aberration. The first surface the mirror grinding produces is a sphere. However, a spherical surface will not bring light rays to a common point. This causes bluring in the images a reflector produces. The most famous case of this defect appeared in the primary mirror of the Hubble Telescope, which was out of figure by only millions of an inch, but was degraded by this effect. In that case, an approach was employed which is also used on reflectors called Correction This refers to using different types of optics including large corrector plate lenses, interior corrector lenses, and specially shaped secondary mirrors to bring light rays to a common focus from a non-parabolic primary mirror. In commercially produced telescopes, thi allows simpler geometries for the primary mirror when this is designed into the telescope, but has the added complication of requiring a specially shaped corrector device which can potentiallly add its own errors into the machining process. Given the overall efficiency of having fewer surfaces to make, reflectors offer the greatest light gathering capability compared to cost.
2. A: Newtonian Reflectors
The Newtonian Reflector is the simplest and most common reflecting telescope design. This is the design Isaac Newton originated using a parabolic primary mirror with a flat secondary mirror held at the center of the light path to bounce the light rays to the side where an eyepiece can for a convenient view. This basic instrument layout is the foundation for all catadioptric telescopes with secondary mirrors used to move the focal point to a convenient location. Types with the focal point projected out the back through a hole in the center of the primary mirror are named for the inventor of this version, the Cassagrain. In general practice, reflecting telescopes with a secondary mirror in the center of the light path and an eyepiece on the side are referred to as Newtonian. Telescopes with a secondary in the center and an eyepiece at the back are called Cassagrain telescopes. Pure Cassagrain telescopes are very rare for small telescopes since the secondary mirror tends to be very large and interferes with the image quality.
Newtonian telescopes need maintenance which usually isn't required in Refractors in the form of periodic collimation. The primary and secondary mirrors are supported in cradles called cells which allow them to be precision aligned with each other and the eyepiece after the telescope is assembled. Newtonians are designed so this can easily be done periodically since the mirrors will get bumped slighly out of alignment as the telescope is moved around and used. Accurate collimation of the mirrors is essential for making quality images, so owners would be well advised to become familiar with how to do this. After gaining experience, a re-collimation can be done within a few minutes.
2. Ai Long Focal Ratio
The easiest Newtonians to make with high optical quality are those with long focal lengths. These require less curvature in the primary mirror and therefore spherical to parabolic geometry errors in machining are small since the curves are similar (this is why small 4 inch diameter f/10 telescopes can sometimes use spherical mirrors without much image degradation). These telescopes also have long slender cones of light going out to the secondary mirror. This improves the image because the secondary mirror causes a bright diffraction ring around everything in the image, which is brighter the larger the secondary mirror is. In the case of a long focal length Newtonian, a very small secondary mirror a long ways from the primary mirror will make the entire primary mirror visible from the eyepiece. As a result, high focal ratio newtonians can produce excellent images of planets and the moon. The image quality of such systems is limited only by the quality of their mirrors and the practical problem of having an obstruction in the tube for the secondary mirror.
Unlike refractors, the color seen through a Newtonian is a perfect representation of the real object. The Newtonian's image artifacts will become visible if one zooms in on a bright star, such as Sirius and carefully inspects the image. Instead of a bright spot (called an Airy Disk) with a faint bull's eye pattern of diffraction rings, the Newtonian will show a dimmer disk with the first diffraction ring much brighter, so some people suggest deducting the diameter of the secondary mirror from the primary's diameter for making resolution predictions for a catadioprtic telescope. This is an over-penalization, in practice. The Newtonian will also show Points on stars. For example, a Newtonian with four vanes holding its secondary mirror will have stars with four points oriented like a , while a telescope with three vanes will show stars having six points like an asterisk. There are different schools of thought on this last effect. Some people find the effect beautiful (to the point where they will put wires across the front of refractors to cause it on purpose), others contend it actually improves the image, and some wish all telescopes supported the secondary mirror on a glass plate to prevent this.
2. Aii Short Focal Ratio (Short Tubes)
One interesting aspect of Newtonians is the optical system is flexible enough to make mirrors with extremely short focal ratios- even down to f/3.6. These telescopes have incredible light amplification capability for their diameter and excel at illuminating galaxies, nebulas, and star clusters. One interesting effect of extremely short focal lengths is the images tend to gain fish-eye type curvature. At the same time, the blunter focusing path requires the secondary mirror in these telescopes to be far larger than the one used for a longer focal length. This intrinsically degrades high magnification performance for these telescopes, regardless of the mirror quality.
Extreme curvature of the primary makes out-of-figure errors more likely, and also produces a type of optical defect called coma. Coma is an effect where objects at the edges of the image get spread out. It is called coma because a star near the edge of the field of view will look like a miniature comet. These effects usually restrict these instruments to use as low power deep sky instruments rather than planet viewing instruments. However, one interesting effect of the extreme focal ratios is images through these telescopes appear to have magnified depth, where it is much more apparent what is in front or behind in an image. This effect is completely lost in photographs, but is present to some degree in all direct views through telescopes.
2. Aiii Dobsonians
The Dobsonian telescope is actually just a variation on the Newtonian which has become extremely popular over the years. John Dobson of San Francisco, realized in the 1970's most of the mounts, tracking devices, sophisicated telescope architectures, and machining methods of the day were producing great telescopes, but ones that had capabilities most amateur astronomers neither desired nor required, but added a lot of expense. His vision was to take all of the resources an amateur astronomer was putting into a telescope and use them to build (usually buy today) the largest primary mirror they could for a Newtonian telescope. The telescope body and mount would consist of the bare minimum of equipment to allow someone to point the telescope by hand at an object. The result was what is called a Dobsonian telescope. Now, 30 years on, John Dobson is still showing people how to build telescopes. However, commercial manufacturers have started making Dobsonians which, per inch of aperture, are the least expensive telescopes available.
The Dobsonian consists of the telescope mounted in a fork on top of a turntable. The telescope rests on felt, plastic, teflon, or other pads as bearings and is balanced to stay put in any orientation. The viewer simply chooses what they want to look at and then pushes the telescope to it. There is no electric drive, no slow motion knobs to turn, or anything else; it is as simple as can be. These telescopes are usually between f/4.5 to f/8 and work quite well in this mode. They may not produce the most fantastic images possible of many things, but they will produce brighter and perhaps sharper images than just about anything costing what they cost. Orion, Celestron, Meade, and many other manufacturers produce telescope along these lines. Given the ready availability of high quality large mirrors for relatively low prices in recent years, building your own telescope has become a more viable option than ever- especially if you build a Dobsonian.
2. B. Schmidt Cassagrain Reflectors (SCT)
The SCT telescope is a hybrid design which has, in many ways, revolutionized amateur astronomy. The model which kicked in the door for the rest of these instruments was the C8, which has been matched with computer drives in later years, as seen in the NexStar 8 GPS. These telescopes have an optical geometry with what is called a Schmidt corrector plate in the front of the telescope. This design first appeared in the Schmidt camera, a large observatory telescope camera. The principle behind this design is to use a complex curved piece of glass in the front end of the telescope to distort the incoming light into a path which will can be brought to a focus by a spherical primary mirror. In the most common versions of this telescope, the secondary mirror has an elliptical section and the secondary is mounted to the center of the corrector plate.
This telescope design makes intermediate telescopes with a focal ratio of f/10 fit into an incredibly small package, where a 5" diameter telescope with a 1250mm focal length such as the Celestron NexStar 5i may be only 9 inches in length overall. As a result, these telescopes have made large aperture instruments more mobile than they had ever been before. The largest manufacturers of SCT type telescopes are Celestron (www.celestron.com) and Meade (www.meade.com). Interestingly, although these manufacturers have been producing this design since the 1960's, there haven't been other significant entries to compete with them other than an abortive attempt by Bushnell in the 1970s.
2. Bi Class Performance
These telescopes are extremely economical for what they are capable of, although many people come to conclude they are masters of no type of viewing, I have finally decided that is not a fair criticism (more in a moment). If it is the only instrument an amateur astronomer owns, an SCT offers the ability to zoom in on planets as well as illuminate deep sky objects. However, the central obstruction and complex optics prevent the telescope from doing quite as well as a long focal length Newtonian or Refractor design. At the other extreme, the f/10 focal ratio means these telescopes can't get to the extremely wide angle views needed to take in large deep sky objects at once. But one reason SCTs are so popular is they have an answer to this is; the f/6.3 focal reducer, which essentially will convert any SCT into a scope with a much wider field of view with a brighter image.
The Celestron 8inch diameter C8 SCT changed amateur astronomy when it appeared in 1970. In essence, this design gave the home observer a quanum leap in performance from the relatively small aperture refractors and Newtonians available for the same price. The C8 was, in essence, a miniature observatory. At 45 lbs. with its tripod and fork mount, the telescope was portable by car.
Over time, several different variants have appeared and Meade and Celestron have largely converged on extremely similar product lines for SCT telescopes. There are no other major manufacturers of this type of telescope, and it is all but guarranteed any observing event will find many of these instruments taking to the field. The SCT has reasonable short-range focus capability and appears in one version as a spotting scope. Celestron has produced Diameters of 5, 6, 8, 9.25, 10, 11, and 14 inches. All but the ten inch are in production today. Meade has produced 4, 8, 10, 12, 14,and 16 inch variants and all but the 4 inch variant are currently in production. Between 5 and 8 inch diameters is a large size difference which takes systems from the neighborhood of twenty pound instruments to forty pound instruments. Another large step occurs between the 9.25 and larger sizes. Celestron apparently produced the 9.25 instead of a ten because it could still operate from the same mount as an 8 inch; whereas the ten inch and larger is an obviously larger telescope. These telescopes are a mainstay of astronomers everywhere, and if one is looking for an economical large scope with some planetarty capability, these have been the most popular answer for thirty years.
The reason I say much of the criticism of these scopes is unfair is the absolute performance compared to the amount of equipment. Looking around for a few minutes on the web will show supposed 1:1 comparisons of all sorts of telescopes. If someone takes an $800 SCT system and compares it against a $5900 APO refractor on a $3000 mount, and then declares the APO system had better performance, is it really a fair contest? For starters, we all know there is a certain amount of pressure to justify a large dollar value purchase. And secondly, the contest invariably takes the limitations of the much larger and more expensive APO scope as the governing parameter instead of the envelope of the SCT. But even though I have a couple of scopes which qualify as APOs, the small 5" SCT still gets more hours outside than any of the others. And the reason is it is packing more performance into a scope I can take out in one trip with its mount and everything. And no matter how much you spend, that's an impossible act to follow.
2. Bii Variations
Several different Schmidt variations have appeared over the years. the most common has been what is called a Schmidt Newtonian. This is actually a Newtonian layout with a Schmidt type corrector plate in front. In this case, the Schmidt corrector is used to overcome the problems Newtonians face at extremely low focal ratios, and thus these telescopes operate at very short focal ratios of around f/3.6. However, the corrector overcomes the coma and image quality problems which would otherwise plague the system.
Both Celestron and Meade introduced designs in the early 1980s for the last pass of Halley's comet. They had 5.5" or 140mm diameters with focal lengths around 500mm. They were capable of extremely wide fields of view. These telescopes also used an unusual focusing mechanism where the secondary mirror was mounted on a single structural support with the eyepiece in a sliding traversing mechanism. The telescope focused by moving the secondary mirror in relation to the primary, thus allowing a minimum-size secondary mirror. For some reason, Celestron's variant was able to use a smaller secondary and thus produces better images than the Meade. However, this telescope has problems with image ghosting which modern anti-glare coatings might be able to correct. In the meantime, these telescopes are still highly sought after for deep space viewing and sell used for approximately 80% of their new price, even though they have been out of production for 15 years.
Smaller manufacturers resurrect this design from time to time to deal with the short focal ratio problem. Interestingly, a careful inspection of the image through one at high magnification often shows planetary views are sharp, but ghosting has interferred with them. If effective countermeasures for this problem could be introduced, this type of design could be in a position to render APO refractors obsolete. If you happen to come across one of these telescopes in good working order, I suggest trying to get a viewing opportunity.
2. C Maksutov Cassagrain Reflectors (MAK)
The Maksutov Cassagrain Reflector (MAK) is a somewhat similar instrument to the SCT concept in that it uses a corrector plate at the front of the telescope to bend the light path to come to a focus from spherical secondary. However, this is where the similarity ends. The Maksutov was designed by a Moscow optician during WWII and is intended to produce high quality views by maximizing the use of easy to fabricate and verify shapes. The easiest shape to grind out of a piece of glass and verify is a spherical surface. Therefore a classical Maksutov has a corrector plate with two spherical surfaces, a spherical primary mirror, and a spherical secondary mirror. Due to the details of the geometry, Maksutovs have much thicker corrector plates than SCT telescopes and take longer to stabilize at new temperatures. The dished-in spherical meniscus; corrector plate is the most obvious distinguishing characteristic common to all Maksutovs.
Some designs are ratioed so the secondary mirror happens to coincide with the interior surface of the corrector plate. This mirror is produced by mirroring a dot on the center of the corrector and is visible from the outside, and yields an essentially collimation-free telescope.
2. Ci Class Performance:
Optically, Maksutovs are known for producing extremely high contrast views with perfect field flatness and color. In fact, some of the most highly prised telescopes produced, the Questar 3.5inch; and 7inch; telescopes are Maksutovs. These telescopes have been in production since the 1950's. MAK telescopes typically operate at long focal ratios of f/13 or higher. A few variants such as the Celestron C90 (G3) operate at f/11 and at larger diameters a few f/10 instruments have been produced. The main difficulty is the secondary mirror has to become larger for the telescope to operate at lower focal lengths, or at a distance which may be significantly separated from the corrector plate.
This aside, the MAK has a reputation for being an outstanding planetary telescope and at smaller diameters gives attractive views of the night sky. The MAK geometry, though far less expensive than an APO refractor, tends to be more expensive than SCT designs. Meade sells MAK telescopes with 90mm, 127, and 178mm diameters. All of these operate at higher focal ratios, but have developed a strong reputation for delivering APO performance at lower prices. Celestron currently only offers 90mm Maksutovs operating at f/11.
Maksutov telescopes have made a major resurgence in recent years as high quality MAK telescopes started being imported from Russia. The most famous Russian brands are Intes and Intes Micro, which offer MAK telescopes in 6, 7, 9, and sometimes other diameters. The Intes MK67 is credited by many for the recent increase of interest in this design. The Intes MK67 is a 150mm f/12 telescope sold for aroun $850 which is generally accepted as equalling the performance of 4" APO telescopes costing $3500 on planets. This telescope isn't optimized for wide fields of view, but can see a wider field of view than the 8inch; f/10 SCT telescopes can, and produces a very crisp high contrast image. This telescope has been previously sold by Orion, but currently availability in the US is limited. The telescope has gone through several generations of improvements over the last few years and currently has a a 153mm diameter, increased back-focus ability, and an improved finderscope. Larger telescopes appeared from such trusted names as Astro Physics (www.astro-physics.com), but are unavailable, now.
If I were to name a telescope I expected to be the immediate successor to the SCT, at this time I would have to say the MAK is very likely that telescope.
2 C ii Variations
Several different flavors of this design have been floated over the years. The Celestron C90 uses its own variation on the optics for this telescope and is a 90mm diameter instrument designed for use as a small general purpose telescoe. A complete explanation of this instrument and its history is available in my reviews of the C90 deluxe spotting scope and the G3 telescope.
Some of the most famous Maksutovs are the Questar 3.5inch diameter instruments. These telescopes are built to be works of art and they are indeed beautiful. The mount designed for them is made from aluminum and is beutifully cast, machined, and polished. The telescope tube is deep blue and has a map of the sky engraved on it. They cost $3500 each. On the other hand, people own one for a lifetime. There are pictures at www.questar-corp.com, but they aren't in color for some reason. In any case, these instruments where you pay top dollar and get top optics. The only real complaint I could level against them is they do have long focal lengths for their size and therefore have limited fields of view. However, once you've seen one, you'll be fascinated. Even the most expensive of APO telescopes cannot match the perfect field flatness and accurate color of these telescopes, and so they continue.
The Russian Maksutovs have finally come into their own after a somewhat difficult beginning where excellent optics were matched with flawed mechanical assemblies and so ended up being marginal products since they had problems such as poor fit, incompatibility with most western optical accessories, and the like.
The Russian telescopes have ebbed again, after what seemed to be a final move into the US once and for all. The most common brands have been Intes and Intes-Micro. The Intes telescopes have been itteratively improving over time, although some promised variants seemed to never appear. The fit and finish on these telescopes is actually quite good- they remind me of the old orange tube Celestrons, which were themselves excellent telescopes (these were the telescopes sold by Orion as the Argonaut brand). The Intes-Micro telescopes use Russian optics mounted in a western optical tube assembly and work prettymuch the same way as SCT assemblies.
Meade has introduced several new MAK telescoped over the last few years in 90mm, 125mm, and 7inch; diameters. These telescopes have fairly long focal ratios (all are over f/13) and though pretty, have odd quirks in their designs. For example, all of them have hard-to use (easy to cross-thread) screw-on dust caps. The other odd feature is the mounts available for the smaller 90mm and 125mm; variants (ETX-90 and ETX-125- see mounts).