DEEP-SKY OBSERVING
by
PETER BAXTER & ERIC BRINDEAU

Winter is here and there is no need to hibernate like our northern neighbours. As winters chill beckons, so does the promise of clear dark skies. Some of the most exciting deep sky targets can presently be found in constellations such as Centaurus, Scorpio, Ophiuchus and Sagittarius.

Your telescope may be collecting dust or waiting for the next Hale\Bopp discovery. Perhaps you have finished a telescope in the society’s telescope making classes and are wondering what to look for in the night sky. Now is the time to get out there for a new challenge, as there is always something new to see! We will be introducing you to the deep sky and show you by star hopping that getting there is half the fun, and the only way to learn the sky. Finding these lesser-known objects will enhance your knowledge of the night sky as it continuously changes throughout the year.

The deep sky is a class of objects such as globular and galactic clusters, multiple stars, galaxies and nebulae. These intriguing objects (often at the edge of visibility) offer the astronomer the challenge of discerning intricate detail, colour, distinctive shapes and delicate wisps millions of light years away. Sometimes just finding these faint celestial treasures is rewarding enough, the prize of a calculated search and an invaluable map of areas within the constellations. Many of the brighter deep sky objects can be enjoyed from our light polluted cities, and add a new dimension to your observing sessions. Finding these objects from truly dark skies allow you to enjoy them in all their splendour.

Before we begin our star hop around the familiar Southern Cross / Centaurus area, let’s discuss some useful tips to get you going. The objects we will be describing should be visible in a 6-inch scope with descriptions given for larger scopes in the 8,10 and 12-inch sizes. A smaller instrument could be used on some of the objects we will be describing.

From an equipment point of view a good finder scope is essential, as this optical aid will be our main guide. We also need to have a rough idea of where we are in the sky (a basic knowledge of the constellations and detailed star charts if available), plenty of patience (being astronomers we have plenty of this already!) and for young and old a good neck /back exercise for those awkward finder viewing positions (driving to work in reverse -say once a week does wonders!).

The finder scope should be at least a 6x30, preferably one with more light gathering power in a 50mm diameter objective lens This might be the time to upgrade an existing finder or consider building a new one. An old 7x50 pair of binoculars makes an excellent finder using one objective and the focussable eyepiece. A 7x50 has the added advantage of a 7mm exit pupil (formula: objective/power), the cone of light which closely matches a dark-adapted pupil (the MEADE standard on their good quality range is an 8x50). Although many a lengthy argument has surrounded the design of the optical alignment, don’t be too tempted in going for the comfort of a right-angled finder.

A straight through finder is very easy to aim at your target using the "both eyes open" method Although the image is upside down which our brains can accommodate easily, having the image reversed left to right in the right-angled design can be frustrating. An added feature to the finder design is a crosshair for accurate pointing. The crosshairs from a rifle sight (fine metal strands) or a hand drawn dot work well (keep in mind that the "eyepiece" of the optical arrangement magnifies the crosshair element and exaggerates it’s size and any defects).

A technique known as "averted vision" should be practised to improve the appearance of deep-sky objects. The retina at the back of our eyes consists of photoreceptor cells made up of rod and cone receptors. The rods are responsible for nighttime vision while the cones are responsible for daytime and colour vision. When looking directly at a faint star or deep sky object once dark adapted, it will seem to disappear. You need to use averted vision by looking slightly to one side, thereby using the rod receptors, to perceive it and study any details. This technique can also be used on brighter objects like studying the cloud bands on Jupiter.

THE DEEP- SKY STAR-HOP

The Southern Cross /Centaurus region is a pretty familiar starting point, well placed in the evening skies. So what deep sky objects lurk in and around this area? Choose a moonless night and spend the time to accurately align your finder. Remember to concentrate on the actual movement of the telescope so that the finder view won’t fool you. This star-hop will take us from the familiar to seeing magnificent colourful objects in the night sky. We begin our tour with two easy hops and you may find the third a little more challenging. The accompanying star chart details the constellations, while sketches will give you an idea of the star field as seen in a 7x30 and 7x50 finderscope. Try to imagine simple geometric shapes as you are faced with an array of stars to make the star-hop easier.

Our first object is a small globular cluster in Centaurus NGC 5286 (9.1 arcminute halo of 7.6^th mag.). Globular clusters are very old spherical associations of stars of which there are more than 150, which populate the halo of the Milky Way. This globular is easy to find and often missed as it is overshadowed by its close neighbour Omega Centaurus, the brightest and largest of the globulars.

Draw an imaginary line between Beta Centaurus and Beta Crux. Use this line as the base of an equilateral triangle. Epsilon will fall at the apex of your triangle. Epsilon Centaurus makes another clearly defined equilateral triangle, an easy way to find Omega. Using Epsilon and Zeta as the base of the next equilateral triangle, Omega Centaurus completes the triangle. In-between Epsilon and Zeta lie two stars clear to the naked eye. NGC 5286 lies next to M Centauri, the star closest to Epsilon. Appearing as a 9^th magnitude tailless comet at low power, the setting is beautiful, contrasting with the bright yellow M Centauri. Telescopes in the 10 to 12-inch range start to resolve the globular near the edge at high power. Now hop to Omega, a spectacular object visible to the naked eye. Although NGC 5286 can hardly be compared to Omega, it is a pretty object worth knowing.

The Next star-hop will take you to the famous Jewel Box (NGC4755), and try to find the jewel that has fallen out of the box! Start by aiming the finder at Beta Crucis, the bright star on the Western arm. In the finder view you should easily make out the seemingly tiny grouping of stars which lie a little away from Beta, moving in the in the direction towards Alpha Centaurus. Enjoy one of the youngest galactic or open clusters of about 50 stars, with several brilliant blue and red supergiants (some appear green as seen in telescopes of 8 inches and up). Note the fine chains of stars forming a "v" shape in-between the cluster.

Let’s move back to Beta and find the fallen jewel, a very red star that we are sure many of you might have missed. Centre Beta in your finder and see that Beta is actually a multiple with an 11^th magnitude companion, and a red star of 7^th magnitude close by. EsB 365 contrasts beautifully with its blue-white companions, appearing a ruddy red in a 6-inch and quite striking red glow in a 12-inch.

The last hop is a more challenging one, a planetary nebulae in Centaurus lurking next to Crux. NGC 3918 (12 arcsecond disk of 8^th mag.) is one of most spectacular southern sky examples of this deep-sky object. Described by Hartung as "vivid pale blue" and "very like Uranus" in Burnham’s Celestial Handbook. Planetary nebulae are expanding shells of gas, which are being expelled from their parent star in the final stages of its evolution. These objects are one of the most exciting of the deep-sky, offering the astronomer detail, shape, colour, smoke rings forming faint shells of gas and the possibility of tracking down the responsible central star. Most of these faint fuzzies have their faint light condensed into a tight ball, making them an easier target, even from our cities.

The concept of judging size when hunting any object will only come with practice. Deep sky objects are measured in arcminutes and arcseconds. Once you have successfully found a few, your experience will enable you to quickly estimate the size of the object that you are hoping to find. To give you an idea, an object of about 6 arcseconds is comparative to the size of a bright star and often described as "stellar" in articles. This is very small and close to our limit considering the size of telescopes we are using.

Planetary nebulae reveal themselves in two ways; colour and the fact that they seem to disappear as you try to look at them directly. Using the averted vision technique will allow you to see if indeed you have found a planetary nebulae. Many of these nebulas can be very colourful objects because of the ionisation of the different elements in the shells of ejected material. Ranging from grey puffs to luminous blues and greens, they are immediately recognisable as they pass your eyepiece.

Imagine a line extending from Beta Crux to Delta Crux. Continue to follow this direction about one finder diameter until you come across the first distinctive loose grouping of stars. Three pairs of stars forming an arc in ranging from small to large. Centre your finder on the bright star of the middle pair, NGC3918 lies very close by. In a medium power eyepiece, keep the mag. 5.7 star in the field of view, and look for an out of focus blue-like star. You might have to pan around carefully before you are on target. Once you have located it, use higher magnification to bring it into a definite disk. The luminous blue is truly magnificent in telescopes in the 8 to 12-inch size. A recent Hubble image shows two Saturn-like projections on either side, possibly out of reach to us amateurs unless you have access to a large light-bucket.