Spotting Scopes - An Introduction

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What exactly is a spotting scope?

In simple terms, a spotting scope is a telescope for (but not only) terrestrial use.

In other words, usage that requires correct image orientation such as bird watching
nature study
aircraft observation
watching shipping
or simply enjoying a view from your home.

A modern spotting scope is usually a small compact prismatic telescope, often with internal focusing and commonly with a zoom eyepiece, or else interchangeable eyepieces, to increase or decrease magnification.  Spotting scopes are normally designed around a common principle, that is a compact, small and if possible, lightweight optical telescope, containing prisms that correct the orientation of the image so that what we see through the eyepiece is the right way up and correct left to right.

Any optical telescope can be used as a terrestrial telescope by simply using prisms that correct the image orientation, it’s just that a spotting scope is designed specifically for this purpose, and its features have evolved over several decades to suit the demands from hobbyists and professionals alike, that require the use of a small portable telescope. Early spotting scopes were drawtube types (like a captain’s telescope) with twist focus eyepieces. Today we see modern compact designs with bayonet fitting rapid removal eyepieces, internal focusing using moving prisms or focus lens, gas pressurised waterproofing, rubber armouring, dual speed focusing for course and fine focus, wide field eyepieces and camera attachments for prime focus photography using the scope as a telephoto, or attachments that permit compact cameras to be used in the modern activity of digiscoping.

The important point to realise is that a spotting scope is just a telescope, but tailored to a range of specific uses. Birdwatchers, nature enthusiasts and hunters are often on the move, so require a lightweight compact instrument, quick and easy to focus, the availability of magnification change, preferably waterproof, as it rains…………….often………………particularly in the UK. It needs to be rapidly attached to a camera tripod for support when required, and survive rough handling. Aircraft watchers (plane spotters) don’t necessarily need a scope to be waterproof, but the other features are also useful for their hobby.


Lets take the familiar refracting telescope model, (the typical long white astronomical refractor tube), and turn it using our imagination, into a spotting scope.

We reduce the focal length of the objective lens so that the instrument is shorter and lighter, we add prisms to correct the orientation of the image, and to increase compactness further. We remove the rack and pinion focuser from the back and build in an internal focusing lens with a focus knob on the area of the scope that houses the prisms. Add an eyepiece (zoom or fixed), cover the scope in rubber armouring……….or not if you prefer, seal all joints to ensure water resistance, or gas pressurise the inside to make completely waterproof. Finally provide the base of the scope with a photo standard tripod fitting of a ¼ Whitworth threaded hole, and we are done.There is nothing more to it.

Why then do we sometimes see these instruments described as Fieldscopes?  
No real reason apart from marketing is the short answer. Fieldscope arguably sounds more high tech than Spotting scope. A telescope is a telescope by any other name.


Lets look a little more closely at spotting scope designs.
Spotting scope objective types.
By objective, we mean the main image-forming optic.

Refracting type.
By far the most common design of spotting scope is the refracting type, those that have a front convex objective lens. 99% of modern spotting scopes are refracting. The reasons for this are manufacturing mass production simplicity and economics, robustness, suitability for the task, optics that fit the design aesthetics and ergonomics important to consumers, plus the best optical design for telescope apertures of 60mm to 100mm (apertures most commonly used because of acceptable size and weight). Objective lenses in spotting scopes vary from doublet (two lens element) pairings, to three or four element objectives found in some more esoteric designs. Whatever the design or quality of the objective lens type, the function is the same; to bring an image to focus at the eyepiece. Light passes through the objective lens group, converges in a cone, passes through the prisms and comes to focus at the eyepiece position.

There are three general configurations of prism types we see in modern refracting spotting scopes.

1.  Porro prisms. These are familiar because of their use in binoculars. Porro prisms result in the eyepiece having a parallel axis to the objective lens. In other words the eyepiece points in the same direction as the objective. Many users find this intuitively to be the most natural configuration for a spotting scope. This type of spotting scope is known as Straight. Hunters and gamekeepers prefer straight spotting scopes as quite often they hand hold the instrument. A straight spotting scope is also the ideal instrument for bird watching from a hide.

2.  Roof prisms. Again, familiar from their use in binoculars. Roof prisms are usually only used in more traditional designs such as retractable telescopic spotting scopes.  As these spotting scopes collapse down to a very short length, they are still preferred by hunters, gamekeepers and a few die-hard birdwatchers.

3.  Angled prisms. Angled prisms are a combination of prisms that result in the image exiting the scope at an angle of 45 degrees to the straight position. This means that the eyepiece is also at 45 degrees from the straight position.  This type of spotting scope is known as Angled. Many spotting scope users prefer this configuration. There are two main advantages. First, if tripod mounted, the tripod does not need to be elevated as much, in order to bring the eyepiece up to a usable level. This means the entire scope/tripod assembly is more stable, as it is lower. Second, birdwatchers often need to follow a bird in flight with their spotting scopes, this is much easier with an angled viewing position. No neck craning is required. The angled refracting spotting scope is now the most popular type in use.


Focusing a refracting spotting scope.
As the optimum design of a spotting scope is one of a self-contained and sealed instrument, we don’t want to see focusing achieved by moving the eyepiece in and out. Modern spotting scopes use an internal focus mechanism; either a small moving internal lens, or one of the prisms moves. In the case of a focus lens, the scope either has a large ring or band around the scope body, which when twisted round, moves the internal lens backward or forwards, or a small knob positioned near the prism housing, which performs the same task. A scope with prism focusing uses a knob near or on the prism housing.

Why is internal focusing important?
Two main reasons. 1. With internal focusing we can have a sealed unit with no external moving eyepiece position. 2. Focusing from an object at or near infinity to an object at close focus (say 5 metres) requires only a centimetre or two of travel for the internal focus lens or prism. Focusing by drawing the eyepiece backward and forward (such as with an astronomical refractor), from infinity to around 5 metres would require the eyepiece to move many inches. The compactness of the instrument would be lost. Internal focusing is thus more convenient, besides we have all been using internal focusing in our camera lenses for many decades without even thinking about it.


As refracting spotting scopes are usually offered as complete instruments, this means they will normally include an eyepiece. It is possible to purchase just the body of a spotting scope, but usually this is only the case with higher priced instruments. The price tag is less without an eyepiece!
For most hobby use and terrestrial applications, magnifications of between 20X and 40X are ideal. This is because objects are easier to find and follow at these lower magnifications, and because images are brighter when lower magnifications are used. Sometimes magnifications of 50X or 60X are required, when identifying small details on a bird or aircraft, or when an object is at a great distance. Generally though, magnifications of around 30X are ideal. It is common to find 60mm, 70mm and 80mm spotting scopes offered with 30X eyepieces, the alternative is usually a zoom eyepiece, with magnifications ranging between 20x - 60X. Most modern fixed magnification eyepieces of 20X to 40X are wide-field eyepieces. This simply means that when we look into the eyepiece, the image we see is within a wide viewing window so to speak. A 30X eyepiece with a wide field is preferable to a narrow field 30X eyepiece for obvious reasons.  It is important to realise that the aperture of the spotting scope has no bearing on the width of field of the image, only the focal length of the objective lens and the eyepiece controls this aspect of the image. 

Now we need to quickly dispel a myth. Eyepieces do not have magnifications. Even though spotting scopes have eyepieces that are announced as 20X or 30X, this is largely incorrect. What it really meant is that this particular eyepiece used with this particular model spotting scope gives a magnification of 30X. Using the same eyepiece on another model scope would likely give a different magnification.

Why is this?

The objective lens of a spotting scope has a focal length, just as all lenses do. An eyepiece is also a lens.............with a focal length. If you divide the focal length of an eyepiece into the focal length of the objective lens, the figure reached is the magnification. 

Example. A 60mm spotting scope has an objective lens with an aperture of 60mm and lets give it a focal length of 420mm. We have an eyepiece that we know has a focal length of 20mm. 420/20 = 21. The magnification with this combination is then 21X. If we know that this eyepiece will be used only with this particular spotting scope model, then we can label it as a 21X eyepiece, instead of a 20mm eyepiece, because the user is more interested in the magnification than the focal length of the eyepiece. If this eyepiece can be fitted to a spotting scope with a longer focal length, e.g. 480mm, then the magnification with this combination is 24X. We can think about spotting scope eyepieces as giving a particular magnification, as it is likely we will only use it on one model scope, however, we now know that in reality, eyepieces have focal lengths, not magnifications.
Manufacturers tend to prefer their own eyepieces to be used only with their own scopes. This is achieved by engineering a fitting from eyepiece to scope body. Just as lenses used for SLR cameras have different bayonet or screw type fittings that enable them to be fitted only to the same brand camera, the same is true of spotting scope eyepieces. There are some that have a general thread size that is shared by others, but these tend to be more generic spotting scopes, mass produced in China, and branded by several different suppliers/distributors.


Catadioptric types.
A catadioptric telescope (or spotting scope) is one that uses refraction and reflection, in other words, lenses and mirrors to form the image. In catadioptrics the main objective optic is usually a concave internal mirror. The refracting lens element is there in an aberration correcting role. A typical catadioptric telescope has an optical layout that folds the light path by internal reflections. The outcome is that the length of the tube assembly is much shorter than its own focal length. This is a tremendous advantage for catadioptrics, as a powerful compact instrument. Larger aperture instruments with longer focal lengths can then be used successfully as spotting scopes, and as powerful telephoto lenses for photography. There are two main catadioptric telescope types used as spotting scopes. 

The first is the Maksutov Cassegrain. The common form of the Maksutov consists of a concave correcting lens at the front, a concave primary mirror inside toward the rear, and a small convex reflecting mirror surface at the centre of the inside of the front correcting lens. The front corrector is called a Negative Meniscus, the main mirror is termed the Primary, and the small convex mirror is termed the Secondary.

The second popular catadioptric is the Schmidt Cassegrain. Instead of a meniscus lens as a corrector, the Schmidt Cassegrain (abbrev. SCT) uses a thin glass plate, flat on the outward facing surface, and with a very subtle Schmidt curved internal surface. The SCT also has a primary mirror, and a convex secondary, although the secondary is a separate mirror held in a small cell at the centre of the Schmidt Plate

The terms Maksutov, Schmidt and Cassegrain are the names of the Russian, German and French originators of the optical designs.


Why do we need these full aperture correcting lenses or plates anyway?
The primary mirrors in Maksutov and SCT spotting scopes are concave, and the concave surface is spherical in curvature. This creates an optical aberration (optical error) known asSpherical aberration that must be corrected, or the focused image will not be sharp and clear. The meniscus lens and Schmidt plate correct for a specific level of Spherical aberration prior to the light hitting the primary mirror that creates the aberration to begin with.

In both types of catadioptric spotting scopes the path of light is essentially the same. Light passes through the front correcting lens or plate, reflects off the concave primary mirror, has a second reflection off the convex secondary mirror, and then passes through a hole in the centre of the primary where the image comes to focus outside the rear of the telescope tube. This is where the eyepiece is positioned. 
This folding of the light path is a clever and convenient way of creating, for example, a 90mm aperture Maksutov that is shorter in length than most 60mm refracting spotting scopes and lighter in weight than many 70mm spotting scopes. A 125mm aperture SCT spotting scope is shorter in length than any 80mm refracting spotting scope and lighter in weight than many. 


Catadioptric spotting scopes are like refracting telescopes in that the image without prisms is upside down and reversed right to left. As no prisms are incorporated into the design of these instruments, they must be added at the rear where the eyepiece fits. Fortunately a 45 degree terrestrial or erecting prism is included by the manufacturer with the spotting scope package. These fit onto the rear of the scope, and the eyepiece fits into an opening in the prism. There are even some catadioptric spotting scopes that have a sealed rear section and an eyepiece position on top of the scope at 90 degrees to the main tube. Inside, there is a small flat mirror that reflects the light upward into the eyepiece. As there is only one reflection, this means that although the image is the right way up, it is still reversed right to left. This is not as disastrous as it may sound. A bird looks the same whether it is pointing left or right, so does an aircraft. The only difficulty experienced by right-left reversed images, is following a fast moving object, or reading words. Even then it is surprising how quickly the brain adapts. Using a reversed image spotting scope becomes second nature in only minutes.


Focusing a catadioptric spotting scope.
Similar to modern refracting spotting scopes that have internal focusing, the Maksutov and SCT spotting scopes also make use of an internal moving optical component. In the case of the Maksutov and SCT though, there is no small lens or prism that we move, it is the primary mirror itself. Remember, the primary mirror has a hole at its centre where the light cone passes on its way to focus. The centre of the mirror sits on a small short tube, which itself sits around a longer snug fitting smaller diameter tube (called the Primary Baffle), with a little lubricant between the two. One tube slides slowly backward and forwards over the baffle tube, thereby moving the primary mirror. The eyepiece remains in the same position and does not need to move. The moving primary thus brings images of objects at or near infinity to focus at one end of its travel, and images of close focus objects at the other end of its travel. A simple and very convenient way to focus. This focus system works equally well whether we are using an eyepiece to view the image, or a camera in place of the eyepiece, to photograph the image. 


The eyepieces used with catadioptric spotting scopes are astronomical types. That is to say they have a standardised fitting which is simply a 1.25" diameter  barrel that simply pushes into position, and is held in place in the erecting prism by a small thumbscrew. 1.25" astronomical eyepieces are all like this, and it doesn't take long to realise that this means the range of eyepieces that can be used with catadioptrics is almost limitless. There are hundreds of different brands and types that will fit, although for most users, a small handful is adequate. 
All astronomical eyepieces have a stated focal length on the eyepiece body. It is a simple task (as we did previously), to divide the focal length into the scope focal length to arrive at the magnification. Lets use for example, the same 20mm focal length eyepiece we used in our original example. On our 60mm scope with a focal length of 420mm our 20mm eyepiece gave us 21X. Now lets take a 125mm SCT spotting scope with a focal length of 1250mm and insert a 20mm eyepiece into the prism. The magnification we get is 1250/20 = 62.5X.  

One of the benefits of using astronomical eyepieces is that most are threaded to accept small filters. This means that small filters such as polarizing, neutral density  or contrast boosting filters can be used, which are beneficial in certain conditions. This is not possible with the vast majority of conventional refracting spotting scopes as the eyepieces are not threaded to accept filters.

As Maksutov and SCT spotting scopes are usually larger aperture instruments with longer focal lengths than conventional refracting spotting scopes, this means that higher magnifications are possible. There are occasions when magnifications much greater than the standard 30X are required, and as long as the tripod keeps the telescope steady, and the outside air and seeing conditions are stable and fairly still (warm moving air currents blur image detail and are generally destructive to image quality), magnifications of over 100X are possible. The occasions when this high magnification level is required may be limited, however, when it is required, larger aperture catadioptrics are the only spotting scopes that can deliver good image quality at very high magnifications.


Comparison of refracting spotting scope types and comparison of refracting against catadioptric spotting scopes for terrestrial use.
The comparison between fundamental designs, and usefulness of different types and sizes of spotting scopes must be judged in two categories. 1. Optical performance. 2. Suitability for purpose.

Optical performance.
Spotting scopes are optical instruments. It follows then that the quality of the optical performance should be the over-riding factor when choosing one.

To cover the basics first.
Image brightness is affected by aperture, magnification, the optical quality (accuracy) of the lens surfaces, the optical accuracy of the mirror surfaces, the optical quality of the prisms, the design of the objective lens/prism combination and the mirror/corrector combination, optical transmission coatings and reflection coating quality, and not forgetting collimation (alignment of the optics). Contrast is reduced by poor optical coatings or no coatings, dirty or stained optics, inadequate internal baffling of the light path, light scatter from internal surfaces and veiling glare.

Resolving power is a function of aperture. However, the resolving power of a lens is concerned with the ability to split closely positioned double stars. The resolution of objects that are not stars (termed collectively extended detail) i.e. moon, planets, birds, aircraft, insects, mountain views...............and just about everything that is not a point of light, are not just subject to the resolving power of the lens aperture. When we discuss resolution in the context of spotting scopes, we generally mean the sharpness of the image, the resolution of fine detail. All the factors that affect contrast and brightness of the image can also affect the telescope's ability to resolve fine detail. We know that a large aperture objective captures more light than a small aperture objective. Therefore the greater amount of light can result in brighter images, higher contrast images and also higher magnifications can be used. It is fair to assume that care taken by the manufacturer to ensure that the lenses, mirrors and prisms are produced to a high accuracy and with good quality control, will result in the consistent high quality images required to give us our contrast and resolution. It is also fair to assume that the manufacturer has taken care to ensure that the optics are collimated correctly. Poorly collimated optics result in lower resolution images with lower contrast. We also know that fully multi-coated optical lens and prism surfaces results in higher transmission, and that high quality aluminium coatings and other metal coatings to increase the reflectivity of the mirrors will both give rise to brighter images with higher contrast. All of this applies equally to catadioptric scopes as it does to refracting scopes. 

Let us accept that all of the above criteria are met and consider our comparison to be purely concerned with the important differences between modern refracting and catadioptric spotting scopes. Before we do that, we need to discover an important aspect of modern refracting spotting scopes that we have not yet discovered. This is important because it changes the game somewhat when we come to compare refracting and catadioptric spotting scopes.

Lets first line up two 80mm refracting spotting scopes with the same focal length, side by side for the purpose of highlighting one important difference between two versions of refracting spotting scopes. Lets place the same 30X eyepiece in each scope, swing them round till they point at the branches of a tree with the background of a bright sky. We focus the scopes so that both are looking at the same spot on the same branch. We can see the bark and the subtle detail of moss and perhaps tiny cracks in the wood. We can also see the bright sky behind the branch. We are also aware of a difference between the images. In one image the edge of the branch is bordered by bright colour. A fringe of strong purple/violet light clings to the edge of the branch, all along its length. It is clearest against the bright background sky, but also encroaches onto the edge of the branch. The result is that some of the edge detail and contrast is lost in a violet haze. In the other image there is no violet colour affecting the image. This image looks perfectly natural. What's going on?

What we have just seen is the difference between an achromatic spotting scope, and anapochromatic spotting scope. Remember the old school experiment of passing white light at a particular angle through a prism? The light spreads out after exiting the prism and splits into the colours of the spectrum. The different colours represent different wavelengths of light, and demonstrates that glass has a subtle effect on different wavelengths of light. Our achromatic objective lens is not managing to focus the shorter wavelengths (violet) to the exact same position as the other wavelengths (the other colours). What our eyes see is the image of the branch in violet light, and out of focus. Just a violet blur superimposed on our branch. This light is then lost to the focused image and if there is lost light, there is reduced image brightness. This is in addition to the lost resolution of some of the fine detail. Our apochromatic scope is not losing light in the same way, because there is no colour fringe, the image has higher visible resolution, and the image looks like it would if we were standing close to the branch and inspecting it with our naked eyes. Although the physics involved here is more complicated than this brief explanation appears to present, it is sufficient to make a point. Lenses transmit different wavelengths of light so that the focus positions of the wavelengths are separate from each other, and as white light (polychromatic light) is made up of all visible wavelengths, we can see where the problem comes from. A lens has a different focal length for each colour (wavelength). We need to find a way of using glass in our objective lenses that does not split light up at focus in this way, or at least so that our eyes are not aware of it.

Fortunately our other scope demonstrates that it is possible.
Lens designers know that a glass lens creates this colour error (termed Chromatic aberration), and this is why refracting spotting scopes (as well as astronomical refractors and binoculars), use at least two lens elements in the objective lens. One lens element corrects for some colour (wavelength) error created by the first lens, but can't correct for all wavelengths, hence why we see unfocused light of certain colours in the image created by an achromatic lens pair. Chromatic aberration as its name suggests, is an optical error, and all aberrations reduce image quality to some extent.

This splitting of light wavelengths is termed dispersion. What we need is another lens that creates a re-combination. There are many different glass types with which to make a lens, all with subtly different properties. A small number of glass types are low dispersion, and a lens made from this low dispersion glass can be matched and paired with a standard lens to combat the dispersion error so that the image is free from noticeable chromatic aberration . As well as glass, a low dispersion lens can also be crafted from a large single crystal of Calcium Fluoride. These lens pairs (or sometimes triplets - two standard glass lenses plus one low dispersion lens), are grouped collectively as ED lenses. ED (Extra-low dispersion) lenses are used routinely in modern spotting scopes. ED spotting scopes (included under the umbrella term of apochromatic scopes), are usually recognisable as terms such as ED, HD or FL are included in the name of the scope. ED scopes are, as you would expect, more expensive than achromatic scopes, however there are now several Chinese-made ED scopes that are less expensive than most European and Japanese achromatic scopes.


Lets turn to our virtual comparison of refracting spotting scopes and catadioptrics.

It would have been simpler and less time consuming to just state the facts about spotting scopes. After all, nearly all guides like this one are simple lists of "specifications" that don't really mean that much to someone wishing to choose a spotting scope or binocular. Just lists of figures. Many guides go further and are simply misleading.  What we need is meaning. 
This guide is a kind of test-drive. First we had explanation of the relevant and important aspects of these instruments, but now we take them out and use them on a virtual bird watch. Hopefully we can show how the technical data fits with our viewing experience. The technical specifications then become meaningful.

The next step is to visit us here in Northallerton, or contact us for advice and assistance.

The best way of highlighting any meaningful differences in image quality between these types of spotting scope, is to place them in a typical observing scenario and see how they get on.
Lets take five scopes with which to provide a feel of what these different scopes can offer.
1. 60mm ED scope
2. 80mm achromatic scope
3. 80mm ED scope
4. 90mm Maksutov
5. 125mm SCT
Lets use eyepieces in each case that give us the same magnifications with each scope. Lets choose 30X and 60X. We can say that the two 80mm scopes share the same focal length and that the 90mm and 125mm catadioptrics also have the same focal length as each other. The 60mm scope is the odd one out. So we need six different focal length eyepieces to give us our 30X and 60X with all five scopes. We will assume that all refracting surfaces are multi-coated and that all mirror surfaces are aluminised (covered in a coating of highly reflective aluminium), and over-coated with other metals for longevity of the aluminium surface. All the refracting scopes have angled eyepiece positions. 
For the sake of argument, we will set up our scopes in a hide overlooking a bird reserve. The reserve has some water in the form of a small lake, reeds, some mud and sand spits and some dark woodland across the other side of the small lake. It doesn't really matter what we look at, the outcome would be the same if we were watching aircraft at an airbase, or ships out at sea.

We are facing West, the Sun is in the south-east and although it is not particularly warm, we can see there is a little heat haze across the lake against the trees.
We focus our scopes on a single gull standing on one of the sand spits. The bird looks clear and sharp in all scopes. We examine the gull properly using the 60mm ED scope first and then compare the image with the other scopes. We can clearly see the small white gull has a grey back and a black head with a dark or black bill, it also sports a small white eye-ring around its eyes. Its tail feathers are black with small white markings. The legs appear to be pinky-red against the background water, although this is not quite as clear in the 80mm achromatic scope. Some violet fringing makes it more difficult to establish this, even though we know from the other scopes what the colour is. The gull swivels its head to rest it onto its back and narrows its eyes.
Another gull of the same type lands on the spit a few metres away from the first gull and a little closer to the hide. This one is a bit noisier. We move the 80mm achromatic scope onto it. It looks the same although maybe larger. Through the ED scopes and the SCT and Mak, we are able to see that the black head is actually a dark chocolate brown. This wasn't immediately obvious with the achromatic scope. We also notice that the bill appears to be a dull red, again, not immediately obvious in the achromatic scope. This gull also has an eye-ring and reddish legs. 

From this first test we have discovered two important points about these scopes. First, the colour fringing in the achromatic scope is making the areas of subtle colour on the bird harder to detect or slower to become obvious. The image of the bird is bright and sharp, but the ability to distinguish some small contrast differences is diminished. Second, the catadioptrics are every bit as good as the ED scopes at showing true colour. In other words, the catadioptrics are not exhibiting chromatic aberration in the way the achromatic scope does, and are considered apochromatic telescopes. 
We turn our attention to the far side of the lake. We can see with the 60mm ED scope, a smaller brown wading bird is making its way steadily along the edge of the lake. Although at a distance, we can still make out it has maybe dark reddish legs and a lightly marked generally brown body feathers, perhaps a little streaky in appearance. The bill is slim and quite long. The 80mm ED scope shows, arguably, a slightly more contrasty image, but not enough to show any more identifying features.  The 80mm achromatic scope shows less colour and it becomes clear that with a small distant object, the loss of some fine detail because of colour fringing is just getting in the way of clearly seeing identifying features.  In the 90mm Mak the image is clear and sharp, although you could argue that the 80mm ED scope is still showing a touch more resolution and general clarity. Its not chalk and cheese, but the 80mm ED gets the nod so far. The 125mm SCT is similar to the Mak, although we notice that the heat shimmer in the image is slightly worse in the SCT. 

Lets increase magnification. We place our 60X eyepieces in the scopes. Now we can see the destructive effects of heat shimmer on the image in all the scopes. The SCT is the worst, followed by the Mak and the 80mm achromatic scope. The 60mm ED appears to fare a little better in this respect. The image is duller in the 60mm scope although still clear enough to identify our wader as a Redshank, during moments of steady air. The 80mm achromatic scope is no better, a little dull with more obvious violet fringing. A little brighter  in the 80mm ED, although the heat shimmer is more troublesome than the 60mm ED. With the catadioptrics, the heat shimmer is preventing us from achieving a sharp focus. Its as though we need to keep changing focus slightly to keep the image at its sharpest.  We kind of get the idea that the image quality is there, were it not for the heat haze. This provides us with other useful pieces of information. The larger the aperture, the more the unsteadiness of the air, and general seeing conditions will affect the image quality. We also learn that the smallest apertures lose image brightness, more obviously than larger apertures do with the same higher magnifications. Even larger apertures, if they have an optical aberration, demonstrate the destructive effect of the aberration more clearly at higher magnifications.

We catch sight of another bird as it flies through the top of the field of view of the eyepiece and lands on a branch of a tree in amongst the dark background of the wood. We train our scopes on this bird. The 80mm achromatic scope can see the bird at both 30X and 60X magnifications, but due to lower resolution, chromatic aberration and heat shimmer, we can't identify it. We switch to the 60mm ED scope. At 30X during moments of steady air, we get the feeling its a raptor. At 60X, the image is duller, but during our steady moments, when the bird turns its head, we identify it as a Hobby. The 80mm ED at 60X, gives a slightly brighter image. The 90mm Mak at 60X, fares as well as the 80mm ED. The SCT is a little better than the Mak at 60X, it shows a little more brightness but the image is not as steady. What we need is better air stability.

Right on cue the Sun disappears behind a cloud. The brightness of the entire scene drops markedly. After a few seconds, we can see the heat shimmer has reduced. In another minute it has almost gone completely. Now the image of the Hobby is clearer in all scopes at both magnifications. We do notice however, that the 80mm ED still has a slight image impact advantage over the 90mm Mak at 30X, but there is less of a difference at 60X. The SCT is faring the best at the higher magnification.

We also happen to have with us a 10mm eyepiece that fits the catadioptrics. This gives us a magnification of 125X. There are no high power eyepieces for the refracting spotting scope as the manufacturers have decided to stop at 60X. At 125X, the 90mm Mak can still focus to a fairly sharp image, although brightness has dropped noticeably. It is surprisingly easy to observe with this magnification using the 90mm Mak as the eyepiece is no more difficult to use than the 60X eyepieces on the ED scopes. With the SCT at 125X, the image is brighter still. At this image scale, we become aware that there is a band around one of the bird's legs, possibly a pale green or a pale blue colour. This Hobby has been caught and ringed at some point in its past.

The Mak at 125X can also now show the ring. The image is not as bright as the SCT, and the colour not quite as clear, but the image is steady. During split second moments of clear motionless air, the Mak image sharpens even more and we are treated to a clear view of the bird's plumage. 

At 60X with the 80mm ED scope, the image brightness appears to be greater than that shown in the SCT image at 125X, but we can't clearly make out that the bird is ringed, although we tell ourselves we can see it because we now know that it is. Do we actually mean that the image is visibly brighter in the 80mm ED at 60X than the SCT at 125X or do we mean we can just see more of the view because the magnification is lower and the field of view wider?  We also get the impression that if it were at 125X, the 80mm ED scope would not be performing as well as the SCT or the Mak.  The larger image scale at 125X with our 125mm SCT and Mak permits us to see small detail that is simply unnoticed at 60X. The perceived clarity of the image at 60X in the 80mm ED being better than the SCT at 125X appears than to be a perception of something we assume is greater clarity, because can we actually compare clarity (whatever we mean by clarity) between telescopes at different magnifications? Surely we mean it the other way round, that the telescope delivering the greater amount of detail is revealing greater clarity. We have an interesting position here. One telescope with a smaller aperture and lower magnification, is providing us with an image that appears to be brighter than the same image viewed through a larger aperture telescope but at twice the magnification. One image is showing us more detail, one is showing us a wider field of view and a perceived brighter image. Which do we say is giving the more useful image? 

Lets pause and look at what these scopes are delivering. The 80mm ED scope has a magnification of 60X. This means an exit pupil of 1.33mm. The 125mm SCT is using a magnification of 125X. This means an exit pupil of 1mm. One exit pupil area is 78% greater than the other. This could well explain why one image looks a little brighter. Is this what we mean by clarity? Well, only if it is revealing more detail.  The bird's leg ring is obvious at 125X but not at 60X. The detail is clearer at 125X. However, once the Sun comes out again, the advantage is lost because of unsteady air.

The lessons from this point of our comparison are that -

a.)  A larger aperture provides brighter images at higher magnifications than a smaller aperture. At low magnifications we don't need the large aperture scope in daylight.

b.)  A larger aperture can resolve more detail, but we sometimes require more magnification to resolve that detail. Luckily, larger apertures permit greater magnification, because they also provide the necessary brightness and contrast. Without higher magnification, the extra detail remains hidden. We can demonstrate this because we first identified the Hobby with our 60mm ED. It did not require the 80mm ED. This tells us something useful, that at magnifications of around 30X (i.e. 20X to 40X, the most popular magnifications used during daylight), we don't generally need more than a good quality 60mm scope to identify what we wish to see. The larger 80mm scope is usually only required for lower light situations at the same 20X - 40X magnifications, and for magnifications of around 60X in daylight. Greater image brightness is realised in a larger aperture scope, but this benefit is only seen in either lower light or with increased magnification. Increased visibility of greater resolution of detail is seen only at higher magnifications.

c.)  Telescopes with long focal lengths allow more comfortable viewing at higher magnifications, because the eyepieces required for the higher magnifications have lens sizes that do not require the user to squint. Our 80mm ED scope (lets say it has a focal length of 480mm), would require an eyepiece of only 3.8mm focal length to give 125X. Not only would the image be dark and unusable, but the observer would have to squint though a tiny eyepiece lens. If he also wears spectacles, he would have no chance at all.

d.)  Between the ED scopes, the 80mm ED scope gave us as bright an image as we would normally need during daylight. The 60mm ED scope was equally useful during bright daylight, but as a lot of bird activity occurs in poor light, the 80mm scope will offer the best performance over the range of daylight conditions, and at standard magnification ranges (20X - 60X).
We carry on enjoying the birdlife for a few more hours. Eventually, the Sun begins to set and the light level drops way down. The reserve becomes more active as more birds drop in to roost for the night. The Sun has been in front of us for the last couple of hours, making life difficult somewhat because many of the birds have been backlit and against brightly reflecting water. Now that the Sun is setting, that problem has lessened considerably, but now the light level and contrast have dropped, the birds and water take on an unusual appearance because the remaining light from the western sky gives everything a different colour tint.

Now our scopes face a different challenge. The dominant wavelengths of light that are illuminating our scene have changed. On top of that the light level is much lower and contrast is thus reduced. On the plus side, now that the Sun has gone, the air is steadier.

The first thing we notice is that the 60mm ED and 80mm achromatic scopes, even at 30X are not that useful anymore. We can still see birds through them, but the images lack the bite that brightness and contrast bring. Perhaps if we dropped the magnification further to 20X, this would help. Indeed it would, but only if we can still identify the birds at this low magnification. Experienced birdwatchers can use their knowledge of flight type, bird movement and bird calls to help identify them, but what if they are not moving much or calling? Suppose we need to identify markings on gulls in a roost of hundreds, in low light, and at a distance? We need magnification for this, and to use the normal 30X magnifications in low light requires larger apertures, and scopes without image-degrading aberrations. 

The 80mm ED scope is performing OK as are the 90mm Mak and 125mm SCT. At 30X, SCT is the stand-out performer in this low light. The light level falls further. The Mak and the 80mm ED are both usable, and the brightness between the two is comparable. Eventually as it gets darker, the SCT is showing the brightest image, and becomes the choice scope for continued use in this poor light.

What can we take from this evening session?

1.  In our introduction to binoculars section, we briefly covered the area of exit pupils and the pupil of the eye. We could see that in a fixed aperture instrument, decreasing the magnification increases exit pupil diameter, or increasing aperture whilst keeping magnification the same has the same effect. As the light level drops, the pupils in our eyes widen. Our brains require more light so it increases the aperture of the eye. Lets say that our pupils reach 5 mm in the evening low light at our reserve. Our 80mm ED scope has an eyepiece that gives 30X.  From our binocular introduction article, we know that aperture divided by magnification gives the exit pupil diameter. So, 80/30 = 2.67. Exit pupil is 2.67mm. The SCT has an aperture of 125mm and an eyepiece that also gives 30X. 125/30 = 4.17. Exit pupil is 4.17mm. This is closer to the 5mm that our eye's pupil is currently at and so will deliver more light to our retina. Is it worth increasing magnification a little if we have a bit more light to play with?  Sometimes it is, but only if we can't see the detail at the lower power. With poor illumination, it is often better to keep the magnification where the detail visible is at its best. With brightly lit subjects, or good illumination, we can push magnification. In practise it is found that the balance between aperture and magnification in good illumination is not quite the same as the balance required in poor illumination. The controlling factor is once again the pupil of the eye. The brightest image we can experience is with the naked eye. So, if our naked eye pupil is 5mm, why not fill that pupil with as much light as we can? 80/5 = 16X. A bright image but too low magnification to be really useful, unless we require a low magnification and wide field to see more birds in the field of view. But don't we have binoculars for that?  125/5 = 25X. A bright image with more useful magnification. 
It may have been noticed that the last two simple bits of arithmetic were dividing aperture by eye pupil diameter instead of aperture by magnification. Doing so reveals the magnification required to fill our eye's pupil if it is 5mm in diameter, one with the 80mm scope, the other with the 125mm SCT. This demonstrates that in low light, we can still employ useful magnification levels as well as filling the pupils of our eyes with light, as long as we use a larger aperture scope.

2. Shouldn't the 90mm Mak perform better than the 80mm ED when it comes to visible detail in low light? It has a larger aperture than the 80mm ED refractor, but the level of detail visible at 30X and 60X was not seen to improve over that of the 80mm ED as the light faded.  Why does the extra 10mm of aperture not provide the Mak image at 30X in low light, and 60X in good light, with an advantage over the 80mm ED? The answer lies in an important difference between refracting and catadioptric scopes. 
Is this answer to do with the fact that catadioptrics use mirrors and that that this somehow drops the level of light reaching the eyepiece? Well, it is possible to construct an experiment to show how much light is lost due to light encountering each refracting and reflecting surface in each telescope, but the differences this makes for the use we are making of these scopes is not really that relevant. Remember that we have already said earlier that all the scopes have multi-coatings on all optical surfaces. Well OK, even though all optical surfaces are multi-coated, there is still a tiny percentage light loss at each glass/air, air/glass and glass/glass surface. Lets say 0.5% per surface. The objective lens in our 80mm ED is an air-spaced doublet. Two surfaces per lens, four in all. The focusing lens is a cemented doublet (two small glass lenses of different types and densities that are cemented together so no air separates them). Lets be generous and say that is only two surfaces. Lets also be generous and say that we need only consider the entrance and exit face of the prisms as air/glass and glass/air. That's another two surfaces. That's a total of eight surfaces each losing 0.5% of the light to reflection. A total of 4% loss by the time the image reaches the eyepiece.
Our Maksutov has two aluminised mirrors with modern high reflectivity aluminium and over-coated by protective metal coatings. Lets say we are losing 5% per mirror (in other words the mirrors are 95% reflective), plus 0.5% for each face of the erecting prism. A total of 11% loss.

Does this 7% loss difference in transmission make up for the extra 10mm aperture? Certainly not that we would notice in the field. Its contributory but not really noticeable. There must be an additional reason.

Catadioptric spotting scopes all have secondary mirrors. These secondary mirrors are small compared to the aperture, but nevertheless sit in the centre of the light path at the centre of the correcting lens or plate. The secondary is also referred to as the central obstruction and it is because it is an obstruction or part obstruction to the incoming light that causes a problem. The smaller the aperture of the catadioptric scope, the greater the problem of the secondary obstruction appears to be in low light. Larger aperture catadioptrics  collect more light and deliver brighter images with higher resolution, so for terrestrial use in lower light, a slight loss of contrast due to the secondary is not really noticeable. In bright sunlight, our 90mm Mak was performing well, better than the 80mm achromatic scope and similar to the 80mm ED.

So, although there may be a very small loss of light due to a central obstruction, this is not really the issue. The area of transmission of the front meniscus lens, even if we minus the central area taken up by the central obstruction, is still slightly greater than the area of transmission of our unobstructed 80mm ED lens.

It is the change this obstruction makes to the detectable detail that is more important. If the secondary obstruction is 30mm in diameter, then the diameter of the obstruction is 33% that of the 90mm diameter aperture. A 33% central obstruction is quite large isn't it? In what way should we view it? A lost central area or just as an obstruction? We have already seen that the extra 10mm in aperture of the 90mm Mak over the 80mm ED refractor means that the clear area of aperture (even when we minus the area of the central obstruction) is slightly higher than the unobstructed 80mm aperture. So, it must be the fact that it is an obstruction.

It can't be that much of a deal, as we preferred the image in the Mak against the 80mm achromatic scope, and that has the same aperture as the 80mm ED. So, whatever the issue is, it is not as big a deal as an aberration like chromatic aberration.

There are several aberrations that can occur in optical instruments. Some are more destructive to image detail than others, particularly when an aberration is severe. In a nutshell, we can consider the perfect image to simply not exist in nature. All major and minor optical errors affect the purity of the formed image to some degree. Manufacturers have to decide on what errors they can get away with for a certain priced instrument, and what level of quality control the product warrants. Its all a matter of economics. It always is. Aberrations modify the image at a fundamental level. They can reduce image brightness, reduce contrast, reduce resolution and distort the shape of the objects in the field of view. Happily, most of the time we don't notice this as the aberration levels are very low, particularly when we use low magnifications like 20X and 40X. A trained eye can spot aberrations in virtually any image, but unless they start to affect the image being useful for its intended task, they are not usually a problem and tend to go unnoticed.

A central obstruction of the size we find in catadioptric spotting scopes is not an aberration, but the obstruction is still modifying the image a little. At a fundamental level, the resolution of some detail in extended objects (i.e. not stars) is reduced to the equivalent of a smaller unobstructed aperture. This is a simplified picture of what is going on, but enough to make the point that the greater the central obstruction in a telescope aperture, the more its resolution of some detail is reduced. At this point it starts to get complicated with talk of MTF graphs to demonstrate losses at certain spatial frequencies. Lets not go there. All we need to know is that the way a large central obstruction modifies the image is in a small loss of some (not all) detectable detail. The strange thing is, that this loss does not affect the finest detail. The limit of resolution of fine detail is still greater with an obstructed 90mm telescope than an unobstructed 80mm telescope. It is slightly courser detail that we find is blurred a little, and then only enough to notice a slight dulling of the image in low light. It is as though the obstructed telescope has lost a touch of image impact in a way that we can't quite put our finger on. 
Is this actually an important consideration when choosing a spotting scope? No, not really. If it made a notable difference, then a certain expensive 90mm Maksutov by the name ofQuestar would not have been the ultimate birding spotting scope from 1960 to about 2000. 40 years at the top. In no way should we then abandon the idea of using catadioptrics as spotting scopes. We have seen how they handle high magnifications better than shorter focal length refracting spotting scopes, and we have seen how the larger aperture SCT outperformed everything else in low light. The good news is, that these larger catadioptrics are not expensive anymore. Refracting spotting scopes have slowly increased in price over the decades, catadioptrics generally have not.

Did anyone pick up on our first gull on the sand spit? Someone needs to text that one in to a local bird group. Its a Bonaparte's Gull..............a bit of a rarity.  


Lets make a few sensible points about the choice of spotting scope, and then briefly look at the relative plus and minus points for each type of scope.        

1. If you know that your hobby requires you to observe objects or wildlife in dull or low light conditions, or will require magnifications greater than about 30X, then it makes more sense to choose an 80mm scope over a 60mm. If it doesn't, there is nothing gained by choosing an 80mm scope.

2. If your budget stretches to the price of an achromatic scope only, then for normal use it will perform well. Just be aware that there are circumstances in certain hobby uses where chromatic aberration can mask some fine detail and subtle colour. For keen birdwatchers this is an important consideration. For plane spotters, this is not so much of a handicap. 

3. If you need or enjoy very high magnification images during daylight, and/or require a spotting scope that will provide the brightest images in the dullest low light situations at normal magnification levels (30X), then a 125mm SCT is the best choice.
Note. If you require a powerful spotting scope for stationary use only, and do not require portability, then you can consider the 100mm ED refracting spotting scopes or 150mm SCT optical tube assemblies listed below.

Popular spotting scope types. General comments. Low light refers to light levels at dawn and sunset. Maximum magnifications are recommendations for daylight use.

50mm ED. 
Useful at lower magnifications in daylight. Not useful in low light unless with low magnifications. Extremely lightweight and very compact. Tend to be expensive for the performance. Usually gas filled waterproof. Maximum magnification for normal use - 40X. Can be used for digiscoping.
60mm achromatic. Useful in daylight. Very limited in low light unless with low magnifications. Will show chromatic aberration which may mask some fine detail at magnifications of 30X and greater. Lightweight and very compact. Low cost. Usually rainproof, some models gas filled waterproof. Maximum magnification for normal use - 40X. Can be used for digiscoping.

60mm ED.
 Useful in daylight. Useful but with some limitations in low light. Will provide the brightest and sharpest images for a 60mm aperture. Lightweight and very compact. Prices are usually above £400 and can be much higher for some German, Austrian and Japanese brands. Usually gas filled waterproof. Some models rainproof only. Maximum magnification for normal use - 60X. Can be used for digiscoping. Some expensive models can be used for prime focus photography.

80mm achromatic.
 Useful in daylight. Useful but with limitations in low light. Will show chromatic aberration which may mask some fine detail at magnifications of 30X and greater. Medium weight and longer length (in some cases 500mm). Low to medium cost. Usually rainproof, some models gas filled waterproof. maximum magnification for normal use - 60X. Can be used for digiscoping.

80mm ED.
 Useful in daylight. Useful in low light. Will provide the brightest and sharpest images for an 80mm aperture. Medium weight and longer length (in some cases 500mm). Prices are usually above £500 and can be much higher for some German, Austrian and Japanese brands. Usually gas filled waterproof. Some models rainproof only. Maximum magnification for normal use - 80X. Can be used for digiscoping. Some expensive models can be used for prime focus photography.

90mm Maksutov Cassegrain.
 Useful in daylight. Useful in low light. Offers a greater range of magnification use due to eyepiece choice and longer focal length. Very good performance at higher magnifications. Lightweight and very compact. Prices are usually below £500. Modern Questar over £3500. Usually not waterproof or rainproof.  Maximum magnification for normal use - 90X. Can be stretched to around 125X in ideal stable viewing conditions. Can be used for digiscoping. Can be used for prime focus photography. Example - 1250mm f/13.9.

125mm Schmidt Cassegrain. Useful in daylight. Very useful in low light. Offers a greater range of magnification use due to eyepiece choice and longer focal length. Very good performance at higher magnifications. Medium weight and compact. Prices are usually below £600. Usually not waterproof or rainproof.  Maximum magnification for normal use - 125X where stable viewing conditions permit. Can be stretched to around 150X in perfect stable daylight conditions. Has higher resolving limit than any other spotting scope. Can be used for digiscoping. Can be used for prime focus photography. Examples - 1250mm f/10.  Also 787.5mm f/6.3 by addition of f/6.3 focal reducer. Ideal for surveillance.

There are other apertures of both types of scopes available. For example.
65mm and 66mm ED scopes. Performance slightly better in low light, but essentially the same as 60mm ED scopes.

75mm and 78mm ED scopes. Performance essentially the same as 80mm ED scopes.

85mm and 88mm ED scopes. Performance slightly better in low light, but essentially the same as 80mm ED scopes.

100mm ED scopes. Better performance in low light than 80mm ED scopes and 90mm Maksutov scopes. Very long, bulky and heavy. Requires a heavy tripod. Better suited to stationary observing.

102mm Maksutov Cassegrain Optical tube assembly. Similar weight and length to 125mm SCT and not as useful in low light.

125mm Maksutov Cassegrain Optical tube assembly. Longer and heavier than 125mm SCT with no gain in performance. Very long focal length and focal ratio with no facility to accept high quality focal reducer, means that it has limited use for prime focus photography. Requires a heavy tripod. Better suited to stationary observing.

150mm Schmidt Cassegrain Optical tube assembly. Compact for its aperture but bulky and heavy. Not suited for portability. Can offer useful magnifications from 37.5X (with 40mm eyepiece) to around 180X under perfect stable air conditions. Excellent powerful telephoto for prime focus photography at 1500mm f/10 and  945mm f/6.3. by addition of f/6.3 focal reducer. Requires a specialist heavy tripod. Better suited to stationary observing. Ideal for surveillance.