Astronomical Telescopes - A Beginner's Guide
The most common question that is asked regarding the choosing of an astronomical telescope is "Which is the best type of telescope I can buy for looking at the stars"? The answer to this is quite simple - The one that you will use the most often.
There are many types and qualities of telescopes available. Some for more professional usage and academic research, some specifically for astrophotography, some for dedicated deep-sky observing and imaging, some for dedicated planetary observing and imaging, some for casual astronomy that can also be used as spotting scopes, and some for all-round astronomical use designed and built for the major sector of the market, populated by keen amateurs that like to experience a small portion of all aspects of the hobby. Each individual fits into one or more of the above categories, and the telescope that suits his/her needs the best is the one that will be used more often and hence the "best telescope".
Let's dispel a few myths.
1. A beginner's telescope is one that is in the lower quality bracket and lower price bracket. Let's start by dispelling this myth. There is no such thing as a beginner's telescope. Choosing a first telescope based on low price or perceived lower performance because of price, is a mistake. It is a myth perpetuated by all sections of the industry that suggests that beginners are somehow incapable of appreciating or undeserving of higher performance instruments, and as such should serve a form of apprenticeship by progressing through a series of instruments from low to high quality. Of course this is good for the telescope sellers, as a regular upgrade means more telescope sales. It doesn't mean however that this is necessarily in the interest of the user. Upgrading over time is fun, but not absolutely necessary if the correct choice is made initially.
The beginner's telescope idea is based on the erroneous notion that beginners are not skilled enough to be able to detect optical quality differences between telescope types and standards of image quality within types. Further, it is assumed that more stable, higher engineering quality mounts are unnecessary for the beginner. It is often thought that a low priced lower performance instrument is ideal for a beginner, because a beginner is someone who is simply dipping the toe into the water, and being more discerning is a waste of time if there is a chance that the beginner will become bored and give up their hobby. The reality is that once the astronomy bug is caught, it is rarely if ever given up. This author has never experienced a beginner or youngster who, after expressing an interest initially, has given up astronomy after observing deep-sky objects or the planets with telescopes of higher optical quality. There are a great many occasions however, when a poor quality telescope has led to a disappointing experience, and a sense of "is this all there is to it"? A high quality image stimulates the mind and the enthusiasm, and this accent on image quality should be the goal of the beginner, not a low quality trial experience. This does not mean that a beginner should be expecting to pay for an expensive telescope, just that a discerning choice is not the sole ownership of those that wish to own an expensive telescope. Let's dispose of the term "Beginner's telescope" and replace it with just "Telescope".
2. One type of telescope is better than others for observing particular objects in the night sky. "If you like the moon and planets, buy a refractor. If you like deep-sky observing buy a Newtonian", is something that is and has been advised by countless people, albeit with good intentions, and it all sounds quite neat and simple. We like to categorise and place products into convenience boxes. We do this with most of the products we buy. It somehow makes it easier to choose, or takes the work and the thought out of making informed decisions. It is no different with optical instruments, it becomes engrained in our thought processes and because of that, it is perpetuated by manufacturers and those that sell us our telescopes. The reality is that all objects in the night sky (or even the Sun during the day), are best seen with telescopes of higher optical quality, and that all objects in the night sky are best viewed with larger aperture telescopes. It is optical quality and aperture of instrument that matter with astronomical observing, not type. Some instruments, because of certain aspects of their design, lend themselves to being more practically useful or optically rewarding when used to observe or image specific objects. However, this design advantage is only beneficial if the particular instrument model has been manufactured to a high standard. Telescope type means very little. Telescope optical quality and aperture means everything.
3. Some telescopes are advertised as being "powerful" with claimed high magnification figures such as 500X! Isn't it one of these I should be aiming for? Some telescope distributors and manufacturers make all sorts of marketing claims about their telescopes, some of them rather optimistic and at worse completely misleading. Although the prime purpose of a telescope is to collect light and provide a magnified image so we can clearly see detail, the limit of high magnification (as we shall see later) is set by aperture and optical quality. Add to that the stability of the air and seeing condition, and these high magnification claims are shown for what they are...............meaningless. A telescope that has a high magnification figure stated as a major feature of its performance, should be largely ignored.
If you are a beginner to astronomy, a simple but effective philosophy regarding how we look at telescopes should be adopted and retained. It will always be useful. Think of a telescope as a black box. A device which collects light and forms an image. We can see the box but not what is going on inside. It is not necessarily important for us to know what the internal optical design is. What matters is the quality and brightness of the image when it emerges.
Before we look briefly at the different design and types of telescopes, let's first note the optical aspects that are fundamental to the success of an astronomical telescope.
By aperture, we mean the diameter of the objective lens or primary mirror. What does a larger aperture give us for visual observation?
a.) Greater resolving power and resolution of extended detail. b.) Facility for higher magnification. c.) Brighter images. d.) Detection of fainter objects. e.) Greater visibility of colour.
Greater resolving power and resolution of extended detail.
How is the resolving power of a telescope determined by aperture? When we look at the stars we are looking at points of light we term point sources, that is to say the smallest point of information we can see. A point source such as a star is not actually a true point of light. A true point of light would have an infinitely small diameter. The image of a star in a telescope that we describe as a point image, is given a rather distinctive shape by the aperture that forms the image. At higher magnifications the star is a tiny bright disc with a faint ring of light surrounding and nearly touching it. In some telescope images an even fainter ring sits around the first ring. This tiny disc and rings is not the image of the resolved star at all, but pure diffraction. All telescope apertures are finite apertures, and it is the diameter of the aperture that determines the angular diameter of this tiny disc. The larger the aperture, the smaller the disc (termed Airy Disc). If the Airy disc represents the smallest bit of information we can resolve, then it makes sense to increase the telescope aperture to reduce the disc size. Diffraction is the unavoidable result of image formation by a finite aperture. We cannot escape diffraction and indeed it is the diffraction pattern of disc and rings the creates all images we see resolved..............whether we can see the individual discs or not.
This last statement leads to the second part of our sub-title, the resolution of extended detail. What do we mean by extended detail? Anything that is not a point image or a field of discrete point images is a single sentence answer. Images of the moon, Sun and planets, nebula, galaxies, birds in the trees, ships at sea and even a tiny Thunderfly caught in a web 20 metres away, are all examples of extended detail. All extended detail (whether course detail or fine detail) is fundamentally a field of indiscrete Airy discs, a mesh of overlapping diffraction patterns (disc and rings) making up every line, curve, dot, edge, plane and irregularity that together form the extended detail image. So, if extended detail is made up fundamentally of overlapping diffraction patterns, we can begin to see how a larger aperture telescope can provide the resolution of finer detail as well as resolving closely positioned point sources (so-called splitting close binary stars), both by the reduction of the angular size of the Airy disc and diffraction rings.
Facility for higher magnification.
If a larger aperture telescope is able to split close binary or double stars easier than a smaller aperture because of the reduction in angular diameter of the Airy discs and diffraction pattern, then it follows that as the Airy discs are smaller, we will need greater magnification to see them as separated individual discs. It turns out that although every aperture has a resolving power limit, the Rayleigh Resolution Criterion or Rayleigh Limit, (1.22 λ/D) set by dividing the wavelength of light (expressed in millimetres) by the telescope aperture (expressed in millimetres) and multiplying the result by 1.22, this doesn't mean we can see the stars as resolved. The image is much too small. The pupil of the eye is much smaller than a telescope aperture and hence its resolving power is much less. What we need to do is to magnify the resolved telescope image to an angular size where the eye can see the stars clearly split. If the Airy disc diameters of the stars are smaller in a larger aperture telescope than in a smaller aperture telescope, we need the greater amount of magnification to see those resolved stars. Once we can see the stars split at the resolving limit for a telescope aperture by adding magnification, then further magnification is pointless, as the stars will not appear to be further split. In other words, we have reached top speed so to speak, for that telescope aperture, and further magnification is termed Empty Magnification. It yields no further visible information. This then is why we can use higher magnifications for larger aperture telescopes.
The above is also true for resolution of extended detail, with the caveat that although 1.22 λ/D is still determining the angular diameter of all the millions of Airy discs making up the extended detail field, the resolution of this extended detail is also affected by the spatial frequencies of all the low and high contrast shapes and sizes of the lighter and darker markings that make up the extended detail, that are themselves fundamentally made up of diffraction patterns. So although larger apertures can resolve finer detail, it is more chaotic, and thus complicated to express as a single equation because of the presence of different spatial frequencies within the detail. It is true to say however, that an upper limit of magnification also applies to the resolution of extended detail.
Image brightness and Detection of fainter objects.
It may not be surprising to learn that larger aperture telescopes can provide brighter images than a smaller aperture telescope, but we need to look briefly into what this actually means for astronomy. Whereas resolving power of a telescope objective lens or primary mirror is determined by its diameter, the brightness of an astronomical image is determined by area of the objective. E.g. A 100mm aperture telescope has an angular resolving limit (with a wavelength of 555 nanometres) of 1.4 arc seconds, a 200mm aperture telescope has an angular resolving limit of 0.7 arc seconds. The area of the 100mm aperture is 7855mm2. The area of the 200mm aperture is 31420mm2. The 200mm has four times the light grasp of the 100mm. This means that deep-sky objects such as planetary nebulae and galaxies that are visible in the 200mm telescope will be brighter and with more visible detail than they show in the 100mm telescope. It also means that a faint galaxy just beyond the grasp of a 100mm aperture would be easily visible in the 200mm. So, for an aperture twice the diameter of another aperture, the images in the eyepiece are much brighter and more resolved. What this also means is that we can use more magnification on deep-sky objects and still see them clearly with even more resolved detail than the same image at lower magnification.
So, a larger aperture resolves closer double stars, has higher resolution of extended detail, can facilitate higher magnification, shows deep-sky objects (galaxies, globular clusters, planetary nebulae and emission nebulae), brighter with more structure, and makes fainter deep-sky objects visible. There is one other benefit to choosing a larger aperture telescope.
Greater visibility of colour.
When we use a small aperture telescope (e.g. 80mm to 150mm), we are conscious that certain bright astronomical objects (stars, planets) have some colour. Colourful double stars are beautiful in any telescope, and subtle colours on planets such as Jupiter and Saturn are visible with many smaller telescopes. With deep-sky objects it is more difficult to see colour as most of the deep-sky objects are relatively faint and diffuse. Most appear as little more than grey blobs, some lighter and with structure, but not showing colour. Once we get to apertures of around 200mm and above, things start to get interesting. As we go up in aperture we become aware that some of the brighter emission nebula and some planetary nebula begin to show subtle colour. The first colour we usually become aware of is blue/green. Why is this?
Our eyes are sensitive to all wavelengths in the visible spectrum, but have peak sensitivity to different wavelengths in different light levels. During daylight, our light-adapted eyes are termed Photopic, and although we can see all colours, the peak sensitivity is green/yellow at 555 nanometres wavelength (0.000555mm). The dark adapted eye is termed Scotopicand the peak sensitivity drops to the peacock blue/green part of the spectrum. In other words our peak sensitivity drops to shorter wavelength light. The problem is, we can't really see different colours in darkness because the colour sensitive cells (cones) in our retina do not operate efficiently until they are flooded with enough light energy. The light sensitive cells in the retina (rods), are responsible for our sight in very low light or near darkness (such as astronomical observation of deep-sky objects or walking along a moonlit road). So the rods are light sensitive and responsible for detection of faint objects. The cones for colour contrast in brighter light, plus the resolution of detail. They require much more light. When the brightness of a faint object such as an emission nebula reaches a critical level, the cones become activated and register our peak sensitivity colour (green/blue) first. Larger aperture telescopes collect more light and produce brighter images, so it is the larger aperture telescopes that can collect enough light to switch the cones on. The result is that, for example, the Great Orion Nebula (Messier 42) can show a faint green tint in the eyepiece of a larger aperture telescope.
Larger aperture telescopes also show brighter colours on the surface of the planets, and are able to detect more subtle contrasts in planetary colour.
All of the benefits that increasing aperture brings are important in order for us to see the detail and subtle colours of the moon and planets, the brightness and structure of deep-sky objects and the detection of fainter galaxies. If choosing a larger aperture telescope is all it takes to reach our goal, it would be easy for us. Unfortunately its not quite as straight forward. The other aspect of a successful astronomical telescope is the precision of the optics.
Manufacturing optics to astronomical standard is difficult and to keep up the manufacturing consistency is even more difficult. As we would expect, the most consistent high quality telescopes tend to be the most expensive, as producing the highest quality optical surfaces is very time consuming and costly in terms of materials and testing equipment. However, the major Chinese and Taiwanese manufacturers are now producing more consistent quality without the very expensive price tags, and this is the main reason why astronomy is now more accessible than ever before in terms of budgeting for equipment. Telescopes that produce high image quality are now available within most budgets.
What do we mean by high image quality? We mean that the optical components of the telescope, whether refracting, reflecting or both, are free of optical aberrations, or rather, low enough in their severity, so that their presence does not noticeably degrade the image. Such an instrument would have high image quality.
Let's look briefly at optical aberrations to see how they can impact on the image in the eyepiece.
Aberrations are errors in the geometry of an optical surface, e.g. spherical aberration, astigmatism, errors that can occur on-axis and off-axis even if the geometry is perfect, e.g.coma, field curvature, errors that create subtle magnification variance across the field, e.g.coma, distortion, and errors that are wavelength dependent, e.g. chromatic aberration.
Some aberrations reduce image resolution of fine detail, some reduce image quality at the edge of the field of view, and one distorts the overall field shape.
Those that reduce the ability of the telescope to resolve close double stars, or to resolve fine detail, or even to reduce the telescope's ability to detect the faintest of deep-sky objects, are the aberrations that are the most destructive in astronomical observing. Of course we don't like to see the image degrading and losing sharpness toward the edge of the field, but it is vital that the central region of the image should show the detail the telescope is capable of. The central region of the field of view where the image is showing the highest resolution image is termed the diffraction limited field. This means essentially that the level of resolution is limited only by diffraction, (the unavoidable result of light waves encountering a finite aperture, that we met earlier). The aberrations that reduce image quality toward the edge of the field are lessened as we move toward the field centre, and eventually are masked by diffraction. The perfect Airy disc, as we saw, is a particular angular diameter for a given aperture. If we add optical aberrations, the Airy disc shape or intensity is modified along with the rest of the diffraction pattern, which results in reducing resolution. Once the Airy disc and diffraction rings are changed detrimentally, the image is no longer limited by diffraction and such a telescope is generally considered not to be of astronomical quality.
When we say that the Airy disc intensity is modified we usually mean that some of the light focused into the disc is lost into the diffraction rings. This means that the first diffraction ring brightens in comparison to the disc, and the outer rings brighten slightly in sequence. This may not seem like such a big deal, but it actually means that the resolution of some detail is reduced in an extended detail field, such as a planet or the moon. Not exactly what we want! For maximum resolution we need to see the maximum amount of light focused into the Airy disc that diffraction permits. If the disc loses light and the diffraction rings gain brightness, the ring diffraction pattern appears to (for want of more suitable words), "fatten with light" which has the detrimental impact on the resolution of fine detail. An aberration that causes this problem is spherical aberration.
Aberrations that change the shape of the Airy disc and rings also reduce the resolution of extended detail, as well as the resolving of close double stars. Astigmatism is caused by light from perpendicular planes from asymmetric cross sections of a lens or mirror surface coming to focus at slightly different positions. A star imaged by one plane focuses to a line instead of a disc and rings. From the other plane a star focuses to a line perpendicular to each other. So the same star imaged by both planes focuses as a tiny cross instead of the disc and rings.
On-axis coma is produced when the optical components of the telescope are not aligned with each other, and hence the telescope is said to be out of collimation. If the optics are skewed in one particular direction, then we would expect the fundamental image to be skewed in one direction. If the skew is slight, the diffraction pattern has a shift of light distribution so that one half of the rings and disc receives more light than the other half. If the skew is severe the Airy disc becomes more elongated, with the rest of the pattern losing its ring shape and spreading out into a fan, resembling a comet (hence the namecoma). When a telescope is collimated correctly, we can still see coma in the field of view of some telescopes, but restricted to the area of the field outside the diffraction limited field. This is normal off-axis coma.
When the aberration is wavelength dependent, it is known as a chromatic aberration.This aberration is only usually present with images from achromatic refracting instruments, (or sometimes poorly designed catadioptrics), because lenses have a different focal length for each wavelength (colour). This means that some colours are not brought to focus at the same point as others, and manifests itself in the image as a focused star image surrounded by bright colour e.g. violet or red. The colour is the unfocused star image in that wavelength. We can also see chromatic aberration when we view extended detail such as the moon and planets. On the moon the colour is most obvious on the limb, and where there are obvious brightness contrasts between the edges of adjacent features on the surface. On the planets, chromatic aberration can mask subtle detail on the surface of the planets as well as surrounding and colouring the edge features of the planets.
It must also be noted that the aberrations covered briefly above can also reduce a telescope's ability to show the faintest deep-sky objects its aperture should be capable of. Any aberration that reduces the focusing of light into the Airy disc, or changes the shape of the diffraction pattern, modifies all diffraction patterns in the field. This means that the very faintest stars that the aperture should be able to record are rendered too faint, and also extended detail such as galaxies on the edge of visibility.
The two remaining aberrations do not affect the resolution of the central diffraction limited field. Field curvature is an aberration where the image at the edge of the field is not in focus at the same time as the image in the diffraction limited field. A shift in focus is required to sharpen the edge image, at which time the central diffraction limited field becomes out of focus.
Distortion is an aberration that subtly changes the shape of the field, due to a small magnification change from edge of field to centre. If there is greater magnification at the field edge, the image centre appears slightly squeezed, and this is termed Pincushion distortion. If there is greater magnification at the centre of the field, the field of view appears to bulge out a little toward the centre, and this is known as Barrel distortion.
It should be noted that distortion in the image of an astronomical telescope is commonly formed by the eyepiece. This may also be true of other aberrations. Both the objective optics of the telescope and the lenses of an eyepiece can introduce aberrations that are visible in the image. Luckily, most modern eyepieces designed for use with astronomical telescopes tend to be corrected for on-axis aberrations that would deteriorate the diffraction limited field of the image.
Two further optical surface errors exist that can greatly reduce the resolution of fine detail on lunar, planetary and extended deep-sky objects. Both are created during the grinding and polishing of the optical surfaces prior to the optical coating process.
If the geometry of the curvature of the surface of a mirror or lens is not perfect, it can be described as smoothly under-correct or smoothly over-correct. The result is Lower Order Spherical Aberration. If the imperfect geometry occurs in circular zones, either at the centre of the mirror, or around the edge area, or even somewhere in between, then the profile of the optical surface has an overall identifiable curvature geometry but with one or more flatter or raised circular zones. These zones mean that the optical surface suffers from aHigher Order Spherical Aberration.
Zones can be very destructive to a telescope's ability to show fine detail, and the severity of the image destruction depends on the position and width of the zone. If the zone is at the centre of a lens its contribution to image degradation is less than if the centre of a mirror has a zone, because the centre of the primary mirror of a Newtonian reflector or a Cassegrain telescope is masked or partly masked by the central position of the secondary mirror. If the edge area of the lens or mirror is slightly flatter or slightly raised compared to the rest of the optic, it is often described as having a turned edge. A turned edge is the worst form of zone to have on an optical surface, simply because the area of the zone around the edge of an optic is often equal to or close to the amount of area of the optic with the correct surface. A severe zone is similar to severe lower order spherical aberration in that there is a fundamental change of the light distribution into the Airy disc and diffraction rings. The result is much lower resolution. The image lacks the sharpness and contrast we expect from a good quality astronomical telescope.
When optical surfaces are polished there are three main errors that can occur. One is that a zone is polished into the surface, another is that the surface is accidentally marked by scratches or sleeks during the polishing process, and the third is that the fine polishing is not completed to a satisfactory level, and the smoothness of the surface is not what is required for the sharpest images with the highest contrast. All of these problems can occur because of economics. That is to say, that astronomical standard optics are difficult and time consuming to produce, and sometimes the economics of mass production mean that the highest quality is not always achievable within a set manufacturing price. The finest astronomical telescopes all have one thing in common, they all have exceptionally smooth optical surfaces. Even those that have optics that have a small amount of lower order spherical aberration but have been polished very smooth, will still give excellent lunar and planetary images and will produce high quality deep-sky images.
Modern optics manufacturing
As little as twelve years ago, modern astronomical telescopes were relatively expensive instruments compared to similar instruments today. An aspect of modern mass production techniques is that more advanced manufacturing and testing equipment, particularly in China and Taiwan, has led to an entire catalogue of higher quality affordable astronomical equipment. This has been led by the Chinese telescope giant Synta, who manufactures telescopes for the Skywatcher and Celestron brands, amongst others. Manufacturing costs in China have permitted astronomical quality refractors, Maksutov Cassegrains, Schmidt Cassegrains and Newtonians to be offered at high street prices that are much more affordable than they ever were. This has led to an explosion in the number of people taking up astronomy as a hobby. Popular and respected brand names such as Skywatcher, Celestron, Meade, Vixen, Long Perng and Guan Sheng now manufacture instruments and equipment for all budgets - £99 to >£10,000. The revolution in computerised GOTO mounts and CCD/CMOS digital cameras for astronomy has accelerated the momentum we now experience. Amateur astronomy is now more approachable than at any time in its history.