Telescope Technology and Selection

An amateur telescope is an optical cylinder. It is attached to a mount that allows moving it around with precision: most mounts are meant to be attached over a tripod. One can buy telescope, mount and tripod separately, but for the beginner it is much more convenient—both in terms of hassle and money—to buy a complete telescope set, which includes telescope, mount, tripod and a few basic accessories such as eyepieces. Let us now discuss the many different alternatives we have to choose from when selecting a telescope set.

APERTURE. This is the diameter of the main lens or mirror in the telescope's cylinder. For a beginner I prefer an aperture not far from 4in = 102mm. My personal opinion is that anything significantly smaller is only convenient for kids, while more than this size is more expensive and bulky. This aperture is said to allow for a maximum "practical" magnification (with respect to the average human eye) of about 200 times, but actually any magnification over 150 times will be at the expense of sharpness: see our Resolution/Magnification webpage for more details.

TECHNOLOGY. Three different type of telescopes are available, all of them very popular among present-day amateurs, though differing in performance and cost.

• Refractor: This is the telescope invented by the Dutch in the 16th century as a toy and ship navigation aid, and famously perfected in the first years of the 17th century by Galileo for astronomical observation. It consists of a large lens (actually a group of lenses assembled together) as “objective” on the far side of the telescope, and another group of small lenses as “eyepiece” at the other end of the cylinder. Compared with similar sizes of the main lens or mirror, this technology is very good for some uses and, being a sealed unit, no dirt can go inside. Problem is, a refractor of reasonably good quality (avoiding aberrations in the objective lens) is inevitably expensive, about twice as much as similar-magnification telescopes with alternative technologies. A good Galileo Refractor 102mm-aperture telescope set can be had in Europe in the region of €500. When we add a minimum of additional accessories, this technology is out of the reach of the amateur with a limited budget.

                

• Reflector: This system also originates in the 17th century: initially it was just a cylinder having at its bottom a polished concave mirror: its reflection was captured slightly offset by an eyepiece. The first practical design is attributed to Newton: a small mirror is placed near to the tube's top at 45º angle, reflecting towards an eyepiece. This design provides “maximum magnification at the lowest cost”, and is therefore today preferred by many amateurs: a good beginner's Newton Reflector 102mm-aperture telescope set costs in Europe in the region of €200. The main issues of Newton's Reflector are: (1) the telescope has a cylinder often over 1/2 metre in length thus not easy to move around in small quarters, (2) the mirrors get frequently out of alignment and need a calibration called "collimation" and (3) the position of the eyepiece near to the top end is counter-intuitive, and not too comfortable when one turns the telescope around.

             

• Catadioptric: This system was created in the 18th century by Cassegrain. It consists of a reflector telescope, but at the top end, instead of Newton's flat mirror at 45º, a convex mirror is placed reflecting the light back into the main mirror: this has a hole in the middle and it is there where the eyepiece is inserted. The cylinder is about half as long as an equivalent Newton Reflector. Particularly popular is the modern Maksutov variant, where the top of the telescope carries a spherical lens that in its centre doubles as the convex central mirror. With respect to Newton's Reflector the Maksutov has the following advantages: (1) the tube is less than half as long and therefore much easier to move around, (2) collimation is rarely if ever needed and (3) watching from the bottom end of the tube is both intuitive and comfortable. The Maksutov has only one minor issue in its cost: in Europe a good beginner's Maksutov Catadioptric 102mm-aperture telescope set is in the region of €300.

                                       

Whichever the model you choose, to the costs mentioned above for telescope set (telescope, mount, tripod and basic accessories) shortly afterwards you will have to add a few hundred Euro worth of some essential additional accessories and cases. There are further differences among the three technologies above: each one has different types of optical distortions and different values of luminosity and contrast. These make the different models more or less suitable for nebulae (any amateur telescope can show the large solar system planets and the moon). Having read countless websites and reviews, I decided that I would go for a Catadioptric Maksutov, which not surprisingly is perhaps the most popular model lately.

MOUNT. Here also there are different technologies.

• Azimuthal. You can control the telescope's horizontal direction or "Azimuth" and the vertical direction or "Elevation", as in a photographic camera tripod. This simple and low-cost solution, typical of telescopes for kids, makes it easiest to point to an object in the sky. Unfortunately, as the earth rotates, with up to 200x magnification, you can see through the telescope how your object moves and within a few seconds falls out of the eyepiece's image. From most locations on Earth the sky moves “diagonally” and to follow the sky object you have to move both Azimuth and Elevation controls to different extents, something really less than practical.
• Equatorial. Here the mount has a tilting base, and instead of two controls you have four. You first point the mount's base to the Celestial Pole, using the "Horizontal" control to turn the base around and the "Latitude" control to turn it up. You then block this "Polar Alignment", and proceed to your star-watching using the other two controls: the "Right Ascension" that turns the telescope around in the sense of the equator and the "Declination" that moves it perpendicular to it. Now to centre the telescope on a sky object is more difficult, but the great advantage is that afterwards you can follow the object by moving only the Right Ascension control. Equatorial mounts have been the common fare of astronomers for almost two centuries now, and in recent decades have become available to amateurs as well.
• GoTo. This is a computer-controlled mount, in most cases consisting of a simple Azimuthal one connected to a small electric motor and to a numerical pad or a computer. Many “Dobson” mounts begin in this group. You enter coordinates or just select a sky object from a menu and “presto!” the telescope points at it and follows it as the Earth rotates. Impressive! The “cons”:
  • You need an electricity power source for the GoTo mount movement and electronics.
  • The GoTo movement is not infallible especially in low-cost ones.
  • Telescope and/or computer do everything for you and accordingly you fail to learn the basics of astronomy, telescope technology and sky-object searching, which is why you went into this hobby to begin with (!).
  • Price is significantly higher than for an Equatorial mount.

Although ideas vary, for most amateurs my recommendation is to begin with an Equatorial mount.

FINAL SELECTION

At this point I was looking for a good-quality 102mm-aperture Maksutov telescope with an Equatorial mount, plus a tripod and basic accessories. Having read many reviews and comparative comments, I decided to buy a Skywatcher Maksutov 102/1300 Avant AZ-EQ. This consists of a Skywatcher Maksutov 102/1300 telescope (aperture 102mm, focal length 1300mm, also abbreviated as “SkyMax-102”), plus an Avant mount, a tripod and a few basic accessories. The details:

• SkyMax-102. This telescope has received very good reviews for the quality of its image and its value for money. Within its tiny cylinder size (280mm long and 120mm diameter) it manages to house a 102mm mirror and a focus length of 1300mm. Once received I found it quite solid in its external metal parts. Tests also showed an excellent collimation. (See our Resolution/Magnification webpage for further tests.)
• Avant. An Equatorial mount that can also be conveniently used as Azimuthal for daylight nature watching. Its movement is manual [2020 update: a small motor is now available, useful for astrophotography]. With its simplicity and low cost it is ideal for the beginner or occasional amateur user. The beautiful white-painted parts appear made of durable cast iron.
• Tripod. A simple one, sturdier than the average amateur-photo one but still lightweight. Unfortunately the central struts that keep the legs open are made of plastic: the user should be careful to guarantee a long duration. (If after some years a replacement is needed, the original tripod can be had for €69, and anyway, with a standard photo-head 3/8" screw, any solid photo tripod will do.)
• Accessories. This telescope included two eyepieces, a diagonal and a red-dot finder.
  (See our Telescope Accessories webpage for full details.)

The main limitation of the SkyMax-102 AV set is the Avant mount: it is sturdy and its movement is accurate, but lacking coordinate scales and pointing electronics (actually a bonus for the beginner, who is forced to learn the basics of astronomy), finding a sky object is not easy: you have to use nearby constellations, or else compass and inclinometer.
(See our Star Watching webpage).

The SkyMax-102 AV set cost me slightly over €300. Of course, within a few weeks I also bought a few—well ... ehm ... I mean actually quite a few!—hundred Euro worth of additional accessories that in my opinion are really required (see the Eyepieces, Accessories and Budgets webpages). One thing I like of this telescope (and others in its class) is that it is entirely optical and mechanical: it includes no electrical or electronic items (except of course when you attach to it the recent small motor drive, or if you take pictures attaching to it either a CMOS camera or a smartphone). Except for the materials and maker tools, essentially you have the technology that was in use from the middle of the 18th century well into the 20th century.