My advice to anybody buying a telescope is to invest in quality rather than sheer size. This applies to the telescope's mounting as well as to its optics. One is able to do so much more with a telescope that is firmly mounted and has an accurate drive.

Nowadays fewer and fewer amateurs make their own telescope optics. However, an ever increasing trend is to install professionally made optics in a home-made mounting. Most telescope manufacturers supply telescope parts, as well as the completed instruments. The home constructor with limited workshop facilities can then make most of the telescope, apart from the optics and one or two of the more ‘difficult bits’.

This chapter details some of the major aspects of telescope hardware applicable to the advanced amateur astronomer. Basic knowledge is assumed. Owing to the variety of equipment design and materials in use, no specific contructional details can be given (this subject would require a large book of its own) but the reader is referred to the listing of books and articles given in Chapter 17. In addition, certain aspects of telescope equipment are considered in other relevant chapters of this book. For instance, guide-scopes for astrophotography are dealt with in Chapter 5.

Telescope tubes and baffles

A telescope's tube should maintain the optics in their correct spatial relationship whatever its orientation. It should also be difficult to induce vibrations within it.

For many people the Bohemian asteroids, ghostly comets, flaring meteors and ethereal aurorae are what astronomy is all about. These objects and phenomena rarely fail to strike up feelings of wonder in astronomers and lay persons alike. These are also areas where amateur endeavours are preeminent. The last time an amateur discovered a major planet was way back in 1781. I am probably not sticking my neck out too far in stating that the chances of a modern-day amateur doing the same are nil. However, there is every chance that you might discover a new comet or asteroid, or observe a particularly spectacular fireball-meteor, or a vivid aurora. Even if you do not make the actual discovery, you can still provide observations of real scientific value.

Observing asteroids

Known asteroids can be located from their ephemerides. Thousands have been discovered so far and many more remain yet to be found. They can be hunted down visually, though there is little to distinguish their appearances from stars, apart from their relative motions. Undoubtably the best way to discover new asteroids is to use wide-field photography. A Schmidt camera is the ideal tool to use but, failing that, use any camera capable of giving a fairly wide field of view (several degrees) and with the largest possible aperture. Figure 10.1 shows a photograph of the asteroid 5 Astraea as it passed through the Beehive star cluster.

This chapter, building on the last, extends to methods of quantitatively measuring the brightnesses of celestial bodies, rather than relying on eye-estimates alone.

Artificial star photometer

The general principle and construction of this device is illustrated in Figure 13.1. The idea is that an artificial star is created in part of the field of view of the telescope eyepiece. The real star is brought to a position nearby and the brightness of the artificial star is adjusted until it matches the real star. The eye is better at matching brightnesses than it is at gauging brightness differences.

In practice the bulb changes its colour together with its brightness. In any case, this colour might well be different to that of the star and so cause difficulties when making the comparison. Using a strong yellow filter to unify both the artificial star and the real star overcomes this problem.

Another difficulty is that the artificial star may look significantly different in size and brightness distribution to the real star. Defocusing the eyepiece slightly may help. The two small discs should now look much more similar. I haven't included the mechanical arrangement for the provision for altering the focus of the eyepiece in the diagram, for the sake of simplicity. A small rackmount, or a helical focuser, or a simple sliding tube should do the job.

If you use a 6V 3W bulb it will draw a current of 0.5 A and have a resistance of 12Ω when run normally.

It is true that the Sun is monitored daily by several solar observatories, using specialised equipment largely beyond the resources of the amateur. However, there are significant gaps in the professionals' monitoring programmes; especially so in the current climate of budget cuts. It could just be that you might watch the unfolding of an eruption on the Sun's surface at a time when no professional telescope is trained on our daytime star!

Methods of viewing the solar image

Sorry, I must say it: On no account look through any ordinary telescope, binoculars, camera viewfinder, or any other optical equipment not specially designed for the purpose, which is pointed at the Sun. Also, do not be tempted to use one of the dark filters which screw into the eyepiece barrel – they may well be supplied by the manufacturer of the telescope but they are NOT safe. I mean no disrespect to you, the reader, for throwing such an elementary warning in your direction. It is a sad fact that several people each year damage their eyesight by taking risks with the Sun. Why? I don't know. There are plenty of published warnings, when even common sense should be sufficient – and yet it still happens. If my warning rescues just one person from the temptation of having a peek, then I am sure you will agree that it is well worth including.

Spectroscopy is surely the most neglected of all the possible methods of observing by the amateur astronomer. This is not really very surprising. For one thing few amateurs have the necessary grounding in physics to make much headway with this analytical technique. Even if they have, a large telescope is required to obtain high dispersion spectra (explained later) of even quite bright celestial bodies and there is little that one can do to advance modern astronomy by taking low dispersion spectra of bright objects (with, perhaps, just one or two notable exceptions).

Nonetheless, I thought that I would include a few notes on spectroscopy in this book. Not every activity of the amateur astronomer necessarily has to advance science. There is also the enjoyment factor and the kinship created by pursuing a technique much used by his/her professional colleagues. Consequently I have restricted my treatment of this subject to just the areas likely to be of most interest and use to the backyard observer.

Fundamentals of electromagnetic radiation

Visible light is just one form of electromagnetic radiation. Figure 15.1(a) shows the classic ‘radio tuning dial’ representation of the electromagnetic spectrum. In some ways this radiation behaves as a stream of particles, or photons, but in other situations it behaves more like a stream of waves. In reality it is both. Figure 15.1(b) illustrates the concept of wavelength. Some people like to think of a photon as a ‘packet of wave energy’.

α Cygni variables

According to the nomenclature of the GCVS, luminous variable B and A supergiants are called α Cygni variables, and are classified among the pulsating variables. The class also includes massive O and late type stars, since these belong to the same evolutionary sequence. In the MK spectral-classification system, they have luminosity classes Ib, Iab, Ia and Ia+ (in increasing order of luminosity). The most luminous supergiants are also called ‘hypergiants’ - these are, in fact, Luminous Blue Variables (LBVs). Ia supergiants are pre-LBV objects, therefore we also refer to Section 2.1 for all details that are related to both groups of variables. All OBA supergiants are variable (Rosendhal & Snowden 1971, Maeder & Rufener 1972, Sterken 1977). The amplitudes of the most luminous supergiants resemble the microvariations observed in LBVs during quiescence, the level of variability increases towards higher luminosities for all spectral classes.

Pulsational instability accounts to some extent for the semi-regular variations (Leitherer et al. 1985, Wolf 1986) - it should also be noted that the β Cep instability strip widens into the supergiant region. The amplitudes of the variations seem to increase with the time scales at which they occur.

HD 57060 = UW CMa and HD 167971 are two interesting cases of microvariations. HD 57060 (Fig. 3.1) is a binary consisting of an O8 supergiant star and an O or B type main-sequence star in synchronous revolution with a period of 4d39.

Variable stars

All stars display variations of brightness and colour in the course of their passage through subsequent stages of stellar evolution. As a rule, however, a star is called variable when its brightness or colour variations are detectible on time scales of the order of the mean life time of man. The variations may be periodic, semi-periodic or irregular, with time scales ranging from a couple of minutes to over a century. It is this kind of variable star which is the topic of this book. The typical time scale, the amplitude of the brightness variations, and the shape of the light curve can be deduced from photometric observation, and those quantities place the star in the appropriate class. For example, a star of the UV Ceti type typically has brightness variations (the so-called flares) of several magnitudes in an interval of time as short as a few minutes, whereas a Cepheid shows periodic variations of about one magnitude in a time span of several days. However, spectral type, luminosity class and chemical composition are complementary important spectroscopic parameters that are needed for classifying variable stars according to the origin of their variations.

The classification of X-ray binaries is somewhat ambiguous. Some authors consider X-ray binaries to be any kind of interacting close binary with a compact degenerate object - that is, a white dwarf, a neutron star, or a black hole. A more specific definition is that X-ray binaries are only those interacting close binary systems which contain a neutron star or a black hole. In this chapter we shall restrict ourselves to the latter definition; interacting close binaries with a white dwarf are usually called cataclysmic variables which are described in Chapter 5 of this book.

The main (empirical) difference between the cataclysmic variable and the X-ray binaries as defined above is the X-ray luminosity: whereas X-ray binaries have X-ray luminosities of 1O∧35 - 10 ∧38 erg s which corresponds to 25 to 25000 times the total solar luminosity, cataclysmic variables have Lx ≦ 1034 erg s. Hence, X-ray binaries are discovered on the basis of their strong X-ray emission. The basic model of X-ray binaries is a close binary system with a ‘normal’ star (main sequence or giant, in exceptional cases a degenerate star too) filling its Roche lobe and transferring matter to the compact object, a neutron star or a black hole. Such a system is called a ‘semi-detached’ system. Due to the orbital angular momentum the matter cannot directly fall onto the compact object, and it forms an accretion disc around the latter (see also Section 5.4). Due to internal friction in the accretion disc (also called viscosity) the matter spirals inward until it eventually falls onto the compact object.

Supernovae

A supernova explosion is a rare type of stellar explosion which dramatically changes the structure of a star in an irreversible way. Large amounts of matter (one to several solar masses) are expelled at high velocities (several tens of thousands km s-1). The light curve in the declining part is powered by thermalized quanta, released by the radioactive decay of elements produced in the stellar collapse, mainly 56Co and 56Ni. The ejected shell interacts with the interstellar medium and forms a SN remnant, which can be observed long after the explosion in the radio, optical and X-ray regions.

Supernovae can be divided into two classes (and several subclasses), viz. SN I and SN II.

SN I have fairly similar light curves (see, for example, Fig. 5.1) and display a small spread in absolute magnitudes. Spectra around maximum show absorption lines of Ca II, Si II and He I, but lack lines of hydrogen. They occur in intermediate and old stellar populations. Their progenitor stars are not clearly identified, but massive white dwarfs (WDs) that accrete matter from a close companion and are pushed over the Chandrasekhar limit are good candidates. Another possibility is the hypothesis of the fusion of a binary consisting of two WDs. The collapse of the white dwarf leads in both cases to an explosive burning of its carbon, and the released energy is sufficient to trigger a disintegration of the complete object.

During the preparation of the observing programme of the TYCHO project on board the HIPPARCOS mission we started thinking about the large number of new variable stars that would be discovered. And since the TYCHO experiment yields only a scanty number of scattered measurements of each star during the life time of the satellite, it is immediately evident that one will encounter the problem of recognising the type or class of variability to which the variable star belongs. Such classification is - even with abundant data - not a trivial task, since many variable stars have light curves which, at first sight, look very similar. In addition, proper classification needs much more than a good-looking light curve, since luminosity and effective-temperature photometric indices also play a role, as well as miscellaneous data obtained with apparatus that are complementary to photometric instruments.

We thought to get some help by looking for standard light curves of typical variable stars that would be used as a template during the process of classification. We discovered then, with some surprise, that a compilation of typical photoelectric light curves of variable stars has never been published, nor does there exist a concise compendium of photometric properties of groups and classes of variables. What can be found, instead, is a large number of detailed morphological descriptions and numerous photometrically-incompatible photographic and visual light curves, scattered over many books and journals.

So, we decided to fill this gap and we started the compilation of typical light curves in a format that enables quick recognition of the pattern of variability.

Algol type eclipsing binaries

The Algol type eclipsing variables (EA) are a subgroup of the eclipsing binaries segregated according to light curve shape. The light remains rather constant between the eclipses, i.e., variability due to the ellipticity effect and/or the reflection effect is relatively insignificant. Consequently, the moments of the beginning and the end of the eclipses can be determined from the light curve.Eclipses can range from very shallow (0m01) if partial, to very deep (several magnitudes) if total. The two eclipses can be comparable in depth or can be unequal. In a few cases the secondary eclipse is too shallow to be measurable (when one star is very cool), or absent altogether (highly eccentric orbit).

Light curves of this shape are produced by an eclipsing binary in which both components are nearly spherical, or only slightly ellipsoidal in shape. Though not explained in the GCVS, one component may be highly distorted, even filling its Roche lobe, provided it contributes relatively little to the system's total light. This is, in fact, the case for at least half of the known EA variables.

Among the EAs one may find binaries of very different evolutionary status:

1. (i) binaries containing two main-sequence stars of any spectral type from O to M, with CM Lac an example

2. (ii) binaries in which one or both components are evolved but have not yet overflowed their Roche lobes, with AR Lac an example

3. (iii) binaries in which one star unevolved and the other overflowing its Roche lobe and causing mass transfer, with RZ Cas an example

4. […]