The term ‘deep sky’ has been used to cover a wide variety of dissimilar objects. Generally it means objects which are neither Solar system members nor stars. Deep sky can be broken out into two broad classes; non-stellar objects which are members of our own galaxy (nebulae) and external galaxies. The study of these faint, fuzzy patches of light actually has it roots in the search for comets. While Herschel and others noted the presence of whispy images, it was Charles Messier who catalogued them. In actuality, he was searching for comets and occasionally came upon small glowing clouds which looked like comets. In order to eliminate the false comets, which do not move with respect to the stars, he made a list of non-cometary extended glowing patches. In his day it was unclear that some of the objects were internal to our galaxy while others were external galaxies in their own right. Indeed, at the time, the true structure of our galaxy was only beginning to dawn on Herschel. Thus, the Messier Catalogue has external galaxies and internal galactic objects mixed together.

Messier's list of non-comets is shown in Appendix 10. There is some question about several of the objects. At least one, M1 02, is an accidental repetition of M1 01. M40 is just a double star. The last five or so objects were actually reported by Messier's colleague, Pierre Méchain.

Books

These are the volumes on my bookshelf that are dog-eared and worn. They are right beside my desk so I don't have to get up when I need one. This is by no means an exhaustive list but rather a set of references which I've found handy. You may find others which are perfectly good too. I have given the ISBN identification of books wherever possible. The reader should note that often these standard reference works are updated with later editions or are made available in soft cover. Such changes generally alter the ISBN but the contents remain essentially the same.

Allen's Astrophysical Quantities, C. W. Allen, The Athlone Press, 1973, ISBN 0 485 11150 0

Astronomy, Robert H. Baker, Van Nostrand Press, 1964

Astrophotography for the Amateur, Michael Covington, Cambridge University Press, ISBN 0 521 40984 5

Burnham's Celestial Handbook, Robert Burnham, Dover Publications, Inc. ISBNs for the clothbound series are: 0-486-24063-0, 0-486-24064-9 and 0-486-24065-7

The Cambridge Astronomy Guide, Bill Liller and Ben Mayer, Cambridge University Press, ISBN 0 521 39915 7

The Cambridge Deep-Sky Album, Jack Newton and Phillip Teece, Cambridge University Press, 1983

Introduction to Asteroids, Clifford Cunningham, Willmann-Bell, Inc., 1988, ISBN 0 943396 16 6

The Sky: A User's Guide, David H. Levy, Cambridge University Press, 1991, ISBN 0 521 391121

One set of books stands out, however, and that is the Harvard Books on Astronomy.

Before you set up your telescope you might ask where the Moon is. After a few months of observing you'll have a sense of where the Moon is just as you know, at any time, roughly where the Sun is in your sky. For now, you may have to look it up in one of the popular astronomy magazines. Most newspapers also list the times of Sunrise, Sunset, Moonrise and Moonset in the section with local weather predictions.

Select a site for your telescope which doesn't have trees or streetlamps in the way. After reading your telescope manual, set up the telescope. Now you must align the polar axis to the Pole Star. There are many methods for accomplishing this and your telescope manual probably describes at least one. Another method is described in Appendix 2. Don't let anybody tell you that there is only one ‘proper’ procedure for polar alignment. The varying procedures differ in accuracy of alignment and time to accomplish.

If you are going to casually visually observe the Moon and bright planets then you don't need to have the polar axis aligned any closer than about three to five degrees. If you are going to look for faint galaxies and nebulae, especially ones which you've never seen before then you need an accuracy of about one degree. This is crucial if you intend to use the setting circles on your mount.

Photometers

As the name implies, photometers measure light intensity. Modern instruments are much more sensitive than the light meter in your snap-shot camera. Indeed, some of the better photometers actually count individual photons. In astronomy, the photometer has become an invaluable professional tool which allows precise quantitative measurements to be made. Thus, the eye is no longer used for estimates of star brightness, eliminating all of the physiological problems of eyesight. This does not mean, however, that the problems such as variation of sensitivity with wavelength have been eliminated. The photometer has its own set of unique problems but they are much more amenable to analysis and calibration than the human eye.

There are two basic types of photometers, the photomultiplier tube and the solid-state photodiode. The photomultiplier, which has been around for half a century, is gradually being replaced by solidstate devices which are smaller, require less exotic high-voltage power supplies and are now more sensitive. You may elect to buy a complete photometer or build one yourself, as shown in Fig. 10.3.

While a photometer measures all of the light falling on its detector, we generally don't want all of the photons entering the telescope tube to hit the light-sensitive element. A photometer placed at the image plane of your 20-cm S–C should be adjusted so that it sees the light from only a small area, typically the light from just one star.

The Cassegrain telescope was invented in about 1672 by Sieur Guillaume Cassegrain, a sculptor and metal founder employed by the King of France. The mirrors were probably made of speculum or some similar metal which could take a polish. Cassegrain communicated his design to Monsieur de Bercèe who took it to the French Academy and by May of that year the idea had appeared in Journal des Sçavans and Philosophical Transactions. Almost immediately a dispute erupted concerning the relative qualities of the Cassegrain and two English telescope types, the newly invented Newtonian telescope and the older Gregorian design. Battle lines among scholars and their scientific academies were drawn along national boundaries – in this case down the middle of the English Channel. Even Sir Isaac Newton himself joined the fray and published dissertations establishing his claims.

The Gregorian telescope is composed of a concave parabolic primary mirror with a concave elliptical secondary. The Cassegrain uses a similar concave parabolic primary with a convex hyperbolic secondary. The Newtonian has a concave parabolic primary with a plane secondary. All three designs had the advantage of no color aberration since they used mirrors instead of lenses. The invention of the achromatic lens was a half century in the future at the time. The Gregorian design was popular because it produced an erect image and it was free from spherical aberration – theoretically.

Asteroids

Asteroids are more properly called Minor Planets, for they orbit the Sun just as the Earth, Jupiter or the other planets do. They are, however, much smaller with the largest being only about 834 km in diameter. They are too tiny to resolve surface details with Earth-based telescopes and thus they appear as points, just like the stars. This does not, however, make them uninteresting, for they move amongst the stars with complex motions just as the major planets do. Occasionally they will even pass in front of a more-distant star.

The first asteroid was discovered by G. Piazzi from the island of Sicily on the first evening of the nineteenth century. He noticed it because over a period of several minutes it moved with respect to the stars. The object was named Ceres and found to be in an orbit about 2.8 times as large as the Earth's orbit. This placed it roughly between the orbits of Mars and Jupiter. The following year a second asteroid, Pallas, was discovered and by 1891 some 322 asteroids had been catalogued. Most orbited between Mars and Jupiter. Up to this point all discoveries had been via visual methods but then Max Wolf at Heidelberg applied the new technology of photography to the hunt and since then hundreds have been found each year. Astronomers keep track of the brightest 5000 or so objects regularly.

The science of astronomy evolved from the practice of astrology and that, in turn, came from mythology and theology. Thus, we do not have a rigorous and logical set of names for the planets, stars and nebulae. We have a rag-tag collection of appellations derived from a score of cultures and epochs. At times during the last few centuries, astronomers have tried to rename everything to some sort of standardized system but this has met with only limited success, primarily because the newer naming systems are complex and many people still refer to the more popular objects with their older, more easily recognized names. Thus, the star Polaris (the North Star) is also known as Alpha Ursæ Minoris. This second name for the star reflects the system of designating stars by constellation and brightness. Polaris is the brightest star in the constellation Ursa Minor (Latin for the Little Bear). The second brightest star in that constellation is Beta Ursæ Minoris, the next brightest is Gamma Ursæ Minoris and so on.

This presents two immediate problems. First, the lettering is in a foreign language, Greek. Second, the spelling of the constellation is in yet another foreign language, Latin. If you go looking for it on a star chart, you will not find anything labeled ‘Ursæ Minoris’, for that is the genitive Latin version of ‘Ursa Minor’, which is used on star charts.

The S–C telescope is a mechanical system with moving parts. As such, it is subject to vibrations induced by observers bumping the telescope, cars and trucks rumbling by nearby and even the workings of the telescope motors and accessories such as camera shutters. As the telescope vibrates it shifts its pointing angle slightly, resulting in the eyepiece image moving back and forth. The first rule in vibration reduction is that you can never totally eliminate vibrations. You can minimize them to such a low level that they no longer bother you but they will always be there.

Given that a telescope is ‘shaky’ there are several cures but first you must determine where the vibration originates. If you can grab the telescope at the eyepiece end, shake it gently and you hear the bearings rattling then moving parts of your telescope are too loosely coupled together. First look for loose screws securing the tube to the declination bearing plates and the fork arms to the base. Some makes and model types have adjustable bearings which can be tightened. Care should be exercised in tightening bearings since excessive tightness will make the telescope hard to move, especially in cold weather. Furthermore, overly tight bearings are merely high-efficiency transmitters of higher-frequency vibrations.

Next, look at the mount. If a tripod is used, ensure that all of the adjustment screws are tight.

The Flammarion Domes

The coordinates of this walled-plain are 3° S 4° W and it is, therefore, placed quite near to the centre of the mean disc. Formerly known as Herschel F, Flammarion is a large, rather open formation, irregular in shape with broken ramparts and having a mean diameter of 47 miles.

At the time of the observation, only a portion of the floor was visible, the remainder being heavily shadowed both to the north and south, but at this stage the extremely oblique illumination shows to advantage the low dome-like features on the interior. Both grouping and alignment seems preferential – the latter being in a NNE–SSW direction. Doubts have been expressed in some quarters as to the real nature of these low-relief features and whether they meet with the characteristics of categorised domes, but it is not clear upon what grounds these objections are based bearing in mind what was seen and depicted on this occasion under a solar altitude of only 1½°–2°.

It seems rather remarkable that there is no mention in early literature, or even as recently as the mid-1950s, of the Flammarion domes, as they are not difficult to descry under the right circumstances. Some, if not all, have been charted and incorporated in the ALPO dome surveys carried out in more recent years. The most likely explanation for their omission from earlier maps may lie in their essentially evanescent appearance.

The evening terminator from Messier A to Taruntius

Though not on my programme, the appearance of the topographical features in the neighbourhood of Secchi and the larger formation Taruntius on the morning of 1987 February 17 prompted me to attempt a representation which is shown opposite.

The diversity in the shape of lunar craters of medium size is sometimes quite remarkable and the formation Secchi is noticeably of irregular form, being open to the north and south with sensibly linear sections of wall on the western side. This deformation from circularity shows well under low evening light, being emphasised by the growing shadows as indicated in the drawing.

The marial ridges form an interesting subject although the advance of sunset at this stage of lighting demands rapid attention if they are to be shown correctly. It will be noted that these ridges display smooth winding configurations which are well defined and their linkage to features south of Taruntius might be worth closer study.

The outer western ramparts of Taruntius and environs were depicted during the second observing session; the low Sun brings out well the characteristics of the broad glacis which are scored with narrow valleys and enlargements or ‘pockets’.

The Mare Spumans under advancing evening light

The Mare Spumans or ‘Foaming Sea’ is a relatively little known region and one that is rarely, if ever, the subject of representation by the amateur lunarian.

The region SW of Encke

The ruined walled-plain called Maestlin R presents a somewhat brief but dramatic scene soon after sunrise by reason of the fine spires of shadow thrown by its eastern rampart. It would seem that the region is comparatively little known, but there are a number of interesting minor features in the form of crater chains and delicate rilles on the eastern side.

The failure of an observer to find delicate detail shown on maps is sometimes to be explained by the time at which the observation is made. The two observations shown are morning aspects made 4 hours apart and quite independently of each other. They show the rapid retreat of shadow and the emergence of fine features not visible under the lower light, even though it was not necessarily hidden – for instance, the afore-mentioned crater chains and rilles which are not revealed in their true character until surface brightening enhances the contrast. Such features are often best looked for some distance from the terminator when the Sun's local altitude is about 4° or 5°.

The Delisle–Diophantus Region

Whenever there is a good prospect of a long session at the telescope together with excellent seeing conditions, then undivided attention paid to a selected region resulting in the making of sequential drawings can very often yield a good deal of useful information not otherwise obtainable.