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.

The colour of the daytime sky

Our atmosphere is a thin, transparent layer of air, not much more than 100 km deep, that separates us from the dark and starry void beyond. On cloudless nights we look out at this fathomless darkness, and only the twinkling of stars betrays the presence of the atmosphere through which we gaze. But when the Sun is up, the whole sky glows brightly, and the dark abyss is concealed from us by an almost tangible blue dome that seems forever out of reach, beyond even the highest clouds and the furthest horizon.

There is no blue dome, of course. Instead during daylight hours we are immersed in what is known as airlight, which is the glow of the atmosphere when it is illuminated by the Sun.

Unweaving the rainbow

How many rainbows have you seen recently, say in the last six months? Not many, I'll wager. The fact is that, unless you live somewhere where the conditions that favour their formation occur on an almost daily basis, such as an oceanic island like Hawaii, you probably don't often get the chance to see a rainbow.

Darkness and night vision

Dark adaptation

The human eye takes time to adapt to dim light. Although the pupil opens up almost immediately, that's not the whole story. Dark adaptation involves the release of the light-sensitive chemical rhodopsin (visual purple) in the retina. For astronomy, useful dark adaptation takes about ten minutes, and substantial improvement continues for half an hour or more.

The central part of the retina does not function in dim light; faint objects disappear if you look straight at them. Experienced observers use averted vision, which means that they view the faintest stars, nebulae, and galaxies by looking slightly to one side of the object rather than directly at it.

Visual perception of faint objects is not continuous. A star near the limit of vision may be evident only a third of the time; it will seem to pop into and out of view. As long as you keep seeing it in the same place, you can be sure that it's real even though you can't see it continuously.

Even a small amount of bright light prevents complete dark adaptation; that's why a distant streetlight or even an illuminated doorbell button can be so annoying. Red light does not do this as much as other colors, which is why astronomers use red flashlights. The light must be red (or orange), not just reddish; what matters is the absence of blue wavelengths, not the red color.