The objective of this chapter is to show you how to get the very best out of your visual observing. I think that it is somewhat of a pity that many amateurs have almost given up using their eyes to view the heavens to concentrate on astro-imaging. It’s nice to think that the photons being detected by your retina from M31, the Andromeda Galaxy, left the galaxy some 2.5 million years ago – you are literally looking back in time! There are many images that remain in my memory: the scars on the face of Jupiter as fragments of Showmaker-Levy 9 impacted its surface, the globular cluster M13 appearing almost three dimensional when I first viewed it with my newly constructed 10-inch Dobsonian telescope, the Double Cluster in Perseus when observed with my 12-inch Maksutov and the iridescent green of the Dumbbell Nebula when seen through a 16-inch Schmidt-Cassegrain. But telescopes do not have to be so large to observe some memorable sights: when testing a 5-inch Newtonian from a dark site I was delighted to observe a supernova that had recently occurred in the galaxy M101.

The Eye

We should not forget that the eye is an important part of the imaging chain (see Figure 9.1). Understanding its strengths and weaknesses can help make the best use of its (impressive) abilities. The aspheric lens has a focal length of around 24 mm but, by use of the ciliary muscles, its focal length can be changed to enable the eye to accommodate both near and far distances (at least in young people). The lens has an ‘aperture stop’, the pupil, to help the eye accommodate a range of brightness. Increasing in aperture from ~2 to 7 mm, this can increase the eye’s sensitivity by about 12 times. But, sadly, as we grow older the pupil no longer increases in diameter so much, and 5–5.5 mm may be its maximum. As the collecting area is proportional to the area of the pupil’s diameter, the difference between a 7-mm pupil and a 5-mm pupil is a factor of 2 in area; thus young people may be able to see stars with their unaided eyes which are about two-thirds of a magnitude fainter than those that can be seen by people in their 60s.

Collimation is the process that is used to align the optics of a telescope in order to provide images of the highest possible quality, whilst star testing is a way of testing the optics of a telescope. These two topics are combined in this chapter because star testing can show that a telescope needs collimation and also provides the way of making the final collimation adjustments. Star testing can also highlight other optical problems and in some cases allow them to be corrected but, be warned, it is exceedingly sensitive and very few telescopes, though providing excellent images, will give perfect star tests. In more complex optical designs, star testing might well give somewhat odd results and will not be a good indicator of the telescope’s optical performance.

Refracting telescopes rarely, if ever, need any maintenance but, after a year or two, the primary mirror of a Newtonian or the corrector plate of a Schmidt-Cassegrain may need cleaning. This is not too frightening a prospect, provided that it is done carefully, so let me first describe how best to do it. If the mirror surface is in really bad condition, with the reflecting surface beginning to erode, this might be the time to have it re-coated. If so, it is worth having one of the new multi-coated, high-reflectivity coatings applied, such as the Hi-Lux coatings provided by Orion Optics in the UK. Not only should such coatings last for many years, the overall contrast of the telescope will be significantly improved.

As the cost of large-sensor cooled CCD imaging systems has, until recently, been very high, many astro-imagers wanting to make wide-field images of the sky have taken to using DSLR cameras. With their inbuilt colour filters forming a Bayer matrix above the sensor, colour images are easily produced without the cost of the filters and filter wheels that are used with the majority of CCD cameras − not to mention the increased image processing time that is then required afterwards.

There is no doubt that Canon DSLRs have, up to now, been the choice of virtually all astro-imagers, with Canon providing free software to allow computers to remotely control their cameras and download the resulting images. Two more sophisticated programs to control Canon cameras are ‘Astro Photographic Tool’ (APT), which is freeware, though a small payment is (rightly) requested, and ‘Backyard EOS’, which is somewhat more expensive. Both allow sequences of exposures to be made so that the imaging system can be left unattended. They also record the sensor temperature for each exposure, which can be very useful if ‘dark frames’ are to be subtracted in the image processing software rather than in the camera, as will be discussed later. For example, APT can add the sensor temperature to the file name as in ‘L_3745_29C’, where L indicates a light frame.

No telescope can be used sensibly unless it is supported by a mount which is sufficiently sturdy to hold it steady. This is an obvious statement, but many mounts supplied with less expensive telescopes are not really up to the task. Often the tripod supports are so light that, when one attempts to move the telescope, the tripod moves! (The solution to this problem is to fill the tripod legs with sand.) As telescope apertures increase, it may well be that the cost of a suitable mount will exceed that of the telescope tube assembly, but this is a price worth paying as a poor mount will cause one endless frustration when observing. A really solid mount is of prime importance for astro-imaging, and some authors state that one should halve the nominal load capacity of a mount for this use. Many telescopes at the lower end of the price range are sold only as a package with an included mount, but for more expensive refractors or reflectors, the tube assembly can usually be bought separately and the user can chose a suitable mount.

Mounts come in two basic types: altitude/azimuth (Alt/Az) or equatorial. Before the advent of computer-controlled drive systems, most mounts were equatorial. The reason was simple. Once an equatorial mount has been aligned on a star, drive − at a fixed sidereal rate − need be applied to only one axis to track it across the sky. Thus a simple electronic controller, based on a crystal oscillator to give an accurate time base, could be used. In contrast, the Alt/Az mounts have to be driven in two axes at variable rates in order to track. For example, when a star is rising in the east, the altitude drive rate will be quite high whilst the azimuth drive rate will be fairly low, but as the star crosses the meridian, due south, the azimuth rate will be high but the altitude rate will be zero and will change from a positive rate whilst the star was rising to a negative one whilst it is setting.

When Mars was closest to the Earth in August 2003, the Macclesfield Astronomical Society held a star party at Jodrell Bank Observatory with quite a number of telescopes set up to observe it. As the evening progressed a consensus arose that two scopes were giving particularly good images: my own FS102 4-inch Takahashi Fluorite Refractor (at around £3500, or $5000, with its mount) and an 8-inch Newtonian on a simple Dobsonian mount newly bought for just £200 ($300). I personally preferred the view through the f6 Newtonian but others thought that the f8 FS102 gave a slightly better image, so we will call it a draw. It is worth discussing why these performed so well and, just as importantly, why perhaps the others did not.

The majority of scopes had been set up on a large concrete patio outside our visitor centre, but the FS102 and Dobsonian were on grass and not observing over the patio. This, I believe, was the major reason these two scopes had performed so well. During the day (remember it was August) the concrete would have absorbed heat, which was then released during the evening, causing localised air turbulence through which the scopes mounted on the patio were viewing Mars. It is not, therefore, surprising that the two mounted on grass performed better. One of the world’s top solar telescopes, the Big Bear Solar Observatory, rises out of a lake so that it is almost totally surrounded by water in order to minimise any local thermal effects. One of my friends went to Egypt to observe the transit of Venus in 2004. It was very hot in the holiday complex, and he said that it would have been nice to observe from the shallow end of the swimming pool. I suspect that, had he done so, he would have had steadier images too! An obvious piece of related advice is that when observing the planets, particularly in winter, one should not observe them over rooftops, as the turbulence caused by the escaping heat will severely degrade the image. Peter Shah, one of the country’s leading astro-imagers, whose beautiful image of M31 is shown in Plate 15.8, has recently lagged the concrete pier on which his telescope is mounted to improve its imaging quality!

The latest DSLR cameras can do a very good job of astro-imaging and can, of course, be used for general photography as well, so why go to the expense of buying a cooled CCD camera? The main reason lies in the word ‘cooled’. All imaging chips produce dark current noise which increases with exposure time and is also highly dependent on its temperature, that of a typical chip dropping by half for each drop of 6 degrees Celsius in temperature. So, if the chip is cooled by 30 degrees below ambient temperature, the dark current noise will have dropped by about 5 times, so allowing longer exposures to be taken before dark current noise becomes a problem. Given dark skies that do not suffer from light pollution, this can allow images to reveal faint nebulosity that would otherwise be lost in the noise. When significant light pollution is present, the exposure times, and hence the dark current contribution, have to be less, before the skylight becomes obtrusive, and so cooling does not confer as great an advantage. The latest chips have very low dark currents, and it is rarely worth cooling them down below about –20 C. This temperature can normally be reached with the single-stage Peltier cooling employed in CCD cameras aimed at the amateur market.

Chip Size

The dimensions of the CCD chip allied to the scope’s focal length dictate the field of view of the resulting images. The chip dimensions in millimetres divided by the focal length in millimetres give the field of view in radians. Multiplying this by 57.3 gives the field of view in degrees. So, obviously, a bigger chip will provide a bigger field of view. For example, my 80-mm refractor has a focal length of 550 mm, and this is used with a CCD camera whose sensor has dimensions of 18 × 13 mm. This gives a field of view of 1.9 × 1.4 degrees.

In the imaging examples given throughout the book, some of the processes that can be used to improve the quality of the resulting images have been described. In this chapter, these will be summarised along with other techniques that may be well be useful in producing the best results from your captured images.

Use Raw Whenever Possible but Preferably Raw Plus JPEG

RAW data is that which has been captured by the camera sensor without any processing and will be digitised to 12 or 14 bits per channel. In contrast, a JPEG will be digitised to only 8 bits – just 256 levels as opposed to 4,096 or even 16,384 – per channel. In addition, the raw conversion software used to provide the image in your computer may well be more sophisticated than that used in your camera. To keep all the inherent quality in the image, the raw files should be converted into 16-bit TIFF files. Not only will a JPEG have less depth, the compression will cause some artefacts within the image. One result of using raw and TIFF files is that they will be fairly large both when one is capturing the image and when processing them later. I use an external USB hard drive to store the images when imaging with my laptop. Very compact drives that are powered from the USB connection are available with capacities of 500 gigabytes or more.

This chapter covers the two major accessories that are used with virtually all telescopes – finders and eyepieces – along with some possible upgrades, such as an improved focuser. A finder and, usually, a pair of eyepieces are supplied with all telescope/mount packages, but if an optical tube assembly is being bought by itself these may not be provided, so allowing you to choose those that will fit your needs best. When coupled with a 2-inch wide-field eyepiece, the short-focal-length refractors that are now in common use have sufficiently large fields of view that a finder may well not be needed. With the increasing use of computerised ‘go-to’ mounts a finder will be used only during the initial alignment on bright stars. This has made it possible for finders without ‘magnitude gain’ (i.e., not employing a small telescope) to be commonly used.

Over the years, telescope eyepieces have become increasingly sophisticated – but also more expensive. It is now very easy to spend more on a high-quality eyepiece than the initial cost of a telescope, particularly if it is bought second-hand, and this rather goes against the grain. It was only after I had spent a large sum buying my Takahashi fluorite refractor that I felt justified in spending a significant amount on eyepieces. They did not then seem quite so expensive! One general point with respect to eyepieces: the simple eyepieces that are often provided with beginner’s telescopes, perhaps with just three elements, are not necessarily of low quality. The fact that fewer glass elements are used can be a good thing, in that there are fewer internal interfaces between the optical elements themselves and the air surrounding them, so minimising scatter and light absorption and thus giving higher-contrast images. ‘Simple’ eyepieces are much prized by planetary observers for whom contrast is so important. Their only real loss as compared with premium eyepieces is the fact that the available field of view will be smaller.

The classical Cassegrain telescope utilises a parabolic primary mirror and a small, convex, hyperbolic secondary mirror inside the focus of the primary which directs the light cone through a central aperture in the primary to a focal point beyond. Such a configuration is ideal for large telescopes, as the often-heavy instrumentation is located where the major weight of the telescope lies. As the light path is folded within the telescope tube, the instrument is quite short in relation to its focal length. The convex secondary mirror multiplies the focal length by what is termed the ‘secondary magnification’, M, which is the focal length of the system divided by the focal length of the primary. One result of a configuration with a high secondary magnification (to yield a compact system) is to give significant curvature of field.

A Planetary Cassegrain

Relatively few pure Cassegrain telescopes are in use by amateur astronomers; variants of the design, described later, are far more popular. However, one pure Cassegrain has recently come onto the market aimed specifically at planetary observing and imaging which could, perhaps, give these telescopes a little more prominence. The obvious design aim would be to make the secondary mirror as small as possible, and this is achieved by having a primary mirror of higher focal ratio than normal and hence a secondary mirror with a smaller magnification ratio. The telescope would thus be longer than a typical Cassegrain.

As its name implies, the Newtonian telescope was invented by Sir Isaac Newton in 1668. Newton suspected that white light was made up of a spectrum of colours and that the chromatic aberration seen in the singlet objectives used in refracting telescopes was due to the light being split into its constituent colours. He thus reasoned that if an image were made using a concave mirror, chromatic aberration would not occur. He chose to make a spherical mirror out of speculum metal, an alloy of tin and copper, and used a flat elliptical mirror to reflect the converging light path sideways to the eyepiece situated just outside the telescope tube – this being the hallmark of the Newtonian design. The theoretical surface of the concave mirror is parabolic. Usually the mirror blank is first made to have a spherical surface, which is then figured to a parabolic shape. Below about 100 mm in aperture, the difference in the two is sufficiently small for this second step not to be required.

Using a mirror of 33-mm aperture, Newton was able to see the moons of Jupiter and the crescent phases of Venus, but otherwise it is not thought that he carried out any astronomical observations. Over the years the quality of reflecting telescopes improved, reaching a first high point when, in 1781, an amateur astronomer, William Herschel, used a 160-mm, f13 telescope to discover the planet Uranus. His mirror was figured more accurately than those used by professional astronomers of the time, and so his images were clearer, enabling him to see that what had been thought by other observers to be a star had, in fact, a planetary disk.

In the nineteenth and early twentieth centuries, every ‘gentleman astronomer’ would have had a 3½-inch (90-mm) brass refractor with a focal length of 42 inches (1,080 mm) and so a focal ratio of 12. Indeed, I have one myself, though I would not claim to be a gentleman! They may have had a 6-inch (150-mm) Newtonian telescope as well. But as Newtonians with larger apertures became available and as more emphasis was put on deep-sky observing, refractors went out of fashion. Over the past 30 years, however, they have had a renaissance as improved glasses and computer-aided design have made available refractors that can give exquisite images of the planets, whilst those of shorter focal lengths give wonderful wide-field views of open clusters and the Milky Way. I really do feel that every amateur astronomer should have one.

The Dutchman Hans Lippershey, a spectacle maker, is generally credited with the design of the simple refracting telescope, which uses two lenses to create a magnified image of a distant object, although it is unclear if he actually invented it. He applied for a patent in 1608 but did not receive one, as there were several claims made by other spectacle makers. In Italy, Galileo Galilei heard of the device and carried out experiments to find the optimum design of the singlet objective lens. His empirical design of a biconvex lens with differing radii of curvature was almost exactly that which would be designed now by ray tracing methods. He used his telescopes, whose magnifications ranged up to 30, to observe the Moon and planets and discovered what are now known as the four Galilean moons of Jupiter. He observed that Venus could show almost full phases during part of its orbit and realised that this could happen only if Venus passed beyond the Sun, thus showing that Venus orbits the Sun, not the Earth, and so proving the Copernican model of the solar system.

This is an interesting, if not common, branch of amateur astronomy which bridges the gap between visual observation and imaging. One problem with imaging is that to produce an image that stands comparison with those seen in magazines and on the Web will require many hours of work, so that only one or two images might be taken during one observing session. Unless one lives where clear skies are common, the number of celestial objects that might be imaged in a year will be quite small. But, of course, imaging enables us to eke out faint nebulosity that our eyes cannot see unless we have access to a very large telescope. Could there be a middle way to allow us to ‘see’ faint details even with a relatively small telescope? An astro-video camera gives us that ability.

For some years, a number of companies, including Watec, have been making video cameras containing very sensitive CCD chips for surveillance purposes under conditions of low light. Watec realized that these might have a useful role in astronomy, and the Watec 120N video camera was the result. The camera uses a 752 × 582 pixel array to provide a completely standard video output stream that can be observed on any monochrome video monitor. It is provided with a 1.25-inch adaptor that fits in the telescope focuser in place of an eyepiece. The monitor display is a very useful feature that enables quite a number of people to ‘see’ what is being observed at one time − very useful at star parties. (Incidentally, some portable DVD players have an external jack input that allows them to be used as a monitor.) The video output can also be ‘frame-grabbed’ and so imported into a laptop for display (in place of a video monitor) and for storage if future processing if desired. I use an EasyCAP USB2 video capture dongle for this use.

The majority of amateur astronomers have probably not even considered making any spectroscopic observations, but it is an immensely rewarding part of the hobby; the cost of the required hardware (a diffraction grating) is no more than that of a reasonable eyepiece, whilst software is readily available to process one’s spectra. In the United States, Tom Field has produced an exceptionally user-friendly piece of software called ‘RSpec’, and his Web site includes a set of video tutorials to show one how to use it. It is possible to run a full specification trial version for 30 days. A freeware program, ‘Visual Spectrum’, is also available.

Almost any camera can be used to capture spectra, and the use of DSLRs, webcams and CCD cameras will be covered here. By simply screwing the grating into an eyepiece, one can even observe the spectra of some bright stars visually. I really do hope that this chapter encourages many more amateurs to have a try!

Why Use Binoculars?

In a nutshell, binoculars will give you the best view possible of objects, such as the Andromeda Galaxy and the Pleiades Cluster, that are simply too large to be seen in the field of view of most telescopes and act as a ‘rich-field telescope’ giving wonderful views of the Milky Way (see Figure 3.1). They are usually more compact and weigh less than a telescope system and so can be taken abroad when the luggage allowance precludes a telescope. They are great things to have in any case! The standard parameters of a pair of binoculars are pretty obvious, but there are a few less obvious features that can be quite important, as we will see.

Magnification

This is the first number given in the basic specification of a pair of binoculars. Typical numbers are 8 and 10. One might think that the greater the magnification the better – but this is generally not the case. The greater the magnification, the smaller the field of view will be (as described later) but, perhaps even more important, the more the image will appear to jump about. Unless the binoculars are to be mounted on a tripod or are image stabilized, a magnification greater than 10 is not to be recommended.