Why equatorial?

Table 4.1 sums up the advantages and disadvantages of equatorial and altazimuth mounts.

There are two main reasons for using an equatorial mount (such as the one in Figure 4.1): to eliminate field rotation (Figure 4.2) in long-exposure photography, and to establish which way is north in the sky so that you can use charts and measure double-star position angles. Apart from that, altazimuth mode is almost always preferable. Setup is simpler and quicker, and you don't need an equatorial wedge to tilt the base.

There is one situation in which an equatorial mount is easier to set up than an altazimuth one. That is when you have a permanent telescope stand that is accurately aligned with the Earth's axis. In that case, all you have to do is attach the telescope and sync on one star. That's all – the telescope is aligned, calibrated, and ready for both visual observing and photography.

Must field rotation be eliminated?

Equatorial mounts get rid of the field rotation illustrated in Figure 4.2. Celestial objects tilt as they rise, travel across the sky, and set. An equatorially mounted telescope tilts with them, so that everything remains stationary in the field of view, but altazimuth-mounted telescopes suffer field rotation. With an altazimuth mount, the object that you're tracking remains centered, but everything else rotates around it.

What eyepieces do you need?

The ease and comfort of using a telescope depend to a surprising degree on the eyepiece, which contains more optical elements than the rest of the instrument. Most importantly, the eyepiece determines the magnifying power.

The magnifications that work well with any telescope are proportional to its aperture. For example, a 4-inch (10-cm) telescope at 25× and an 8-inch (20-cm) at 50× are both at the low end of their power ranges. They are operating at 2.5 power per centimeter of aperture, or 2.5×/cm for short, equivalent to about 6 power per inch.

What this implies is that the eyepiece focal lengths that work well with any telescope depend on its f-ratio. For example, a 20-mm eyepiece gives low power with an f/5 telescope but medium power with an f/10 telescope.

With that in mind, Table 6.1 makes specific recommendations. You do not have to get the exact focal lengths in the table, of course, but the table will show you where any particular eyepiece stands. If you have an f/10 telescope and choose to get a 32-mm eyepiece, which is not in the table, you'll know that it's in the low- to very-low-power range.

You do not need a lot of eyepieces. Three eyepieces are enough for almost anybody; two (low and high power) are almost enough; and one eyepiece (low or medium power) will get you started. Go for quality, not quantity.

Sunset

Are there significant differences between sunrise and sunset? It is a question I often ask myself, and to which I have yet to find a definitive answer, if indeed there is one. Other than that events in one occur in the reverse order to events in the other, it seems to me that any physical differences are those of detail only. By and large, it seems the conditions that give rise to a splendid sunset are exactly the same as those that produce an equally memorable sunrise, although it has been said that these conditions occur more often at the end of the day. Most of us have seen the Sun set many more times than we have seen it rise, and so it's difficult to judge the truth of this.

Introduction

Although it came on the market slightly later than the Meade Autostar, the original model Celestron NexStar is a simpler design, so it is logical to describe it first.

This chapter is based on my experiences with a NexStar 5 purchased in 2001. This is a 5-inch (12.5-cm) Schmidt–Cassegrain and was initially the flagship of the NexStar fleet. It is practically identical to the original (one-armed) NexStar 8.

Related products

Celestron has subsequently introduced a wide range of NexStar telescopes, ranging from small refractors to an 11-inch Schmidt–Cassegrain. Some of the smaller NexStars have been sold under the Tasco Starguide label.

No two NexStar models have entirely the same firmware, and the product line is still evolving rapidly. Accordingly, this description of the NexStar 5 will serve only as a general guide to the rest of the product line; changes are ongoing. Many different versions of the NexStar 5 firmware are already extant.

The top-of-the-line NexStar GPS telescopes are radically different and are not covered here. They include GPS receivers to determine the latitude, longitude, and time; periodic-error correction; and other advanced features.

Evaluation of the NexStar 5

Compared with Meade LX200 and Autostar telescopes, the NexStar 5 is appreciably easier to use because it has fewer features; the overall design of the firmware is simpler. The optics are from the classic (and, in my opinion, underrated) Celestron 5.

General concepts

The first thing a telescopic observer of the planets notices is that they all look rather small and blurry. Some training of the eye is required in order to see planetary detail. Not only are planetary features faint, but they require constant attention in order to take advantage of brief moments of steady air.

Drawing the planets is one of the best ways to learn to observe them (Figure 4.1). You can always see, and therefore draw, more detail than you can photograph with the same telescope. A good scale is about 5 cm (2 inches) for the diameter of the planet. Always draw what you see, not what you think you ought to see.

Every planetary drawing should be labeled with the date and time, particulars of the telescope, and eyepiece, and quality of seeing (atmospheric steadiness, Table 2.2, p. 12). Colored filters are often helpful. Such drawings have considerable scientific value and are collected for research purposes by the B.A.A. and A.L.P.O. (see p. 30).

Good handbooks for planet observers include, among others, Thomas A. Dobbins, Donald C. Parker, and Charles F. Capen, Introduction to Observing and Photographing the Solar System (Willmann–Bell, 1992), and Fred W. Price, The Planet Observer's Handbook (Cambridge, 1994). For current planetary research, see J. Kelly Beatty et al., The New Solar System (Cambridge, fourth edition, 1999; revised regularly).

Ice halos

Take the advice of the engaging Edmund Halley (of comet fame), and ‘… always look out …’ for ice halos in the sky. Sooner or later you're bound to develop a nose for them, just as he did. Despite being an eminent astronomer and mathematician, and a leading member of the English scientific establishment of his day, Halley was never so busy that he hadn't time to glance up at the sky in search of halos and rainbows, and share his observations with others in his characteristically informal, easy-going manner.

Introduction

In 1999, Meade Instruments made the sensible decision to use the same firmware, as far as possible, in all their computerized telescopes. The result was the Autostar system, used in a wide range of instruments.

The first popular Autostar telescope was the ETX-90 EC, the computerized version of a 9-cm Maksutov–Cassegrain that already had a reputation for excellent optics. The Autostar line quickly extended downward to the Meade Digital Electronic Series (DS) refractors and Newtonians, and upward to the LX90 Schmidt–Cassegrains, which are a lower-cost alternative to the LX200 for those who are not doing advanced astrophotography.

This chapter is based on my experience with an ETX-90 EC (Figure 12.1), but virtually everything in it applies to all models of Autostar. The menu system is mapped on p. 211.

Unlike some of its competitors, Autostar is relatively well documented, and I assume that you have Meade's manuals available for reference. In particular, the LX90 manual is worth reading even if you are using an ETX or one of the other lower Autostar models. By the time you read this, it should be online at http://www.meade.com. It is already online at http://www.astronomics.com.

Related products

The Autostar II system used in the LX200 GPS is an enhanced version of Autostar with periodic-error correction and a keypad that makes more of the menus available at a single keystroke.

There are several models of the original Autostar controller.

Coronae

When a Moon that is full, or nearly full, is seen though a thin veil of altocumulus cloud, you may sometimes notice that it is surrounded by a series of more-or-less concentric, coloured rings. These are known as diffraction coronae. Corona is Latin for crown. Coronae also form around the Sun in the same circumstances, though you are unlikely to notice any colours in clouds that lie close to the Sun because of the brightness of the clouds.

What's inside a computerized telescope

A computerized (or “go to”) telescope is one that finds celestial objects by itself. Well, not exactly by itself – you have to show it the positions of two stars, and from there it can find everything else. Two stars are sufficient to determine the position of the whole celestial sphere.

The telescope takes two kinds of commands. You can tell it to go to a particular object, based on its current knowledge of the position of the sky, or you can tell it to sync (synchronize) on an object that you have identified and centered in the field. The latter is how you tell the telescope the exact position of the sky.

Besides all the parts of an ordinary telescope, a computerized telescope has a computer, motors, and encoders.

Computer

The computer translates right ascension and declination to the position of the telescope on its mount – a coordinate transformation that involves lots of spherical trigonometry (see p. 35).

The software built into the computer is called firmware and contains a built-in catalogue of stars and deep-sky objects, plus algorithms to compute the position of the Moon and planets, so that you can choose objects by name.

Computerized telescopes are a testimony to the low price of powerful computers. The Celestron NexStar 5, for example, has four CPUs in an internal network. Comparable computing power would have cost millions of dollars in the 1960s and would have filled several rooms.

Estimating distance

There are several ways in which we estimate how far something is from us. In most situations we rely on our acquired knowledge of the relative size of things and on the fact that objects that are close to us can block out parts of objects that are further away. However, these clues work only in circumstances and with objects with which we are familiar. Furthermore light, colour and shape can trick our eyes. An object may look nearer than it really is, if it is either uphill or downhill from us, if it is bright, if it is seen across water or snow or if the atmosphere is particularly clear (see section 1.5). On the other hand, in poor light or if the colour of the object blends with its surroundings, it may appear to be further away.

Even with knowledge of relative size, without binocular vision judging distance would be difficult in some situations. Binocular vision works by fusing two slightly different viewpoints of the same scene to give us a sense of depth. The spacing between our eyes, however, is small and so binocular vision is unreliable beyond a distance of some 30 m. You can check this for yourself by looking at an object that is more than this distance from you and then checking whether there is an apparent shift between object and background when you close one eye.

Daily motion

The universe is a mass of swirling motions, but most of the time, you can ignore all but one of them. That one is daily motion (diurnal motion), caused by the rotation of the Earth. You will see it immediately if you aim a 100× telescope at a star with the drive motor turned off.

As you know, celestial objects rise in the east, move across the sky, and set in the west. But as Figures 2.1 and 2.2 show, that is not the whole story. The motion is not directly from east to west; instead the whole sky rotates like a globe with Polaris at its north pole.

In the southern sky, each object rises somewhere on the eastern horizon (not necessarily due east!), passes across the sky, and sets somewhere in the west. Its path may be long or short. In the far southern sky, objects rise just east of south, climb only a short distance above the horizon, and set again a short time later, just west of south.

The most northerly celestial objects are circumpolar; that is, they do not rise or set at all.