Contents
1. Introduction
2. Equipment
3. Planning
4. Techniques
5. Visual art
6. References

4. Observing Techniques

WHEN YOU ARRIVE at your observing site with your equipment and a list of objects to observe, you should ready the telescope for the night's work. Set up your telescope or binoculars using your red torch; this will allow your eye to start adjusting to the dark.

Both refractors and reflectors should be allowed to reach thermal equilibrium with the surrounding air, and reflectors should be collimated properly. Take the time to identify the constellations you will be observing in and look for useful asterisms or star patterns to help you find your way. You are ready to start the search for your first object.

4.1. Star-Hopping

ONE OF THE MOST basic, and valuable, observing skills is star-hopping, which is simply the following of a trail of stars from a place you know (eg. a bright star) to a place you don't know (eg. the galaxy you are hunting for). You can plan your star-hop at the eyepiece, but it is recommended that you plan your first star-hop in advance, while you are drawing up your evening's observing list. Find the deep-sky object in question, say a galaxy, on the starchart. Search around it for a bright star or asterism that you will be able to locate in the sky with ease. Now look for a trail of stars which will lead you from the star to the galaxy. It is rather like playing connect-the-dots. Sometimes you might have to go a roundabout way to get to your target, or you may choose a different bright star to start from. Bear in mind that once you have found the galaxy, you can use it as the beginning of a star-hop to the next object.

4.2 Directions In The Sky

TO PLAN YOUR STAR HOP, you will need to know how far each hop can be. This depends on the size of your field of view (the amount of sky you can see at one time). For binoculars, the diameter of the field remains constant, but each eyepiece used on a telescope has a different field diameter. Select your lowest-power eyepiece, which will have the widest field of view. Turn the telescope onto a bright star and centre the star in the field of view. If your telescope has a drive, turn it off. Now watch the star as it drifts across your field of view. The star will move towards the western (preceding) edge of your field and new stars will appear at the eastern (following) edge. North and south lie at right angles to the west-east line, south being the direction you have to move the telescope to find the south pole.

Thus, directions in the sky have east and west switched around; if north is to the top, then east will be to your left. Using these cardinal directions is really only useful to indicate angles which are multiples of 45 degrees. To specify directions more accurately, the position angle (PA) is used. A PA of 0degrees is North and 90degrees is East. Northeast would be PA = 45degrees while PA = 260degrees points a little south of west.

TO HELP YOU GET to grips with position angles, you might like to try and estimate the PA for some easy double stars. The following short list names six (out of many) double stars that can be split with binoculars. The magnitudes of the two components are given, along with their separation in arc seconds. The PA is measured from the brighter star, so it tells you where to look for the fainter star of each pair. Not only is it fun to estimate the PA of a double in the sky and look it up to see how close you came, it is also good practice for keeping your directions straight.

Alpha Cru, mags 1.1 & 5.1, separation 90 arcsec, PA 202 degrees
Alpha Leo, mags 1.4 & 7.6, separation 177 arcsec, PA 307 degrees
Gamma Lep, mags 3.6 & 6.2, separation 96 arcsec, PA 350 degrees
Delta Ori, mags 2.2 & 6.8, separation 53 arcsec, PA 0 degrees
Sigma Tau, mags 4.7 & 5.1, separation 430 arcsec, PA 14 degrees
Nu Sco, mags 4.0 & 6.3, separation 41 arcsec, 337 degrees.

ONCE YOU CAN IDENTIFY north-south and east-west in the sky, you will be able to read a star chart. Star atlases are calibrated according to the equatorial co-ordinate system which measures distances north and south of the celestial equator in degrees (declination), and west to east in hours (right ascension). The north celestial pole lies at +90degrees and the south celestial pole at -90degrees . Right ascension is measured west to east, going from 0h to 12h to 23h and back again to 0h. Right ascension thus increases to the east and declination increases to the north.

4.3 Measuring the Field of View

THE SIMPLEST METHOD of measuring the field of view relies on the use of a star chart. Knowing north and east in the sky, you can easily turn your star chart so that the image in the eyepiece corresponds to the chart. Look for two stars that just fit in your field of view, and locate these stars on the star chart. You can now measure this distance on the map and compare it with the scale on the margin of the map to convert your linear measurement to degrees or arc minutes. Remember that 1 degree = 60 arc minutes (60') = 3600 arc seconds (3600''). Binoculars typically have fields larger than 4degrees , and telescopes normally give a view smaller than 2degrees.

IT IS USEFUL TO be able to judge angular distances in the sky. The following table lists some angular estimates:

Solar / lunar diameter: 0.5 degree
Width of index nail at arms length: 1 degree
Orion's Belt: 3 degrees
Short arm of Crux: 4.5 degrees
Long arm of Crux: 6 degrees
Width of clenched fist at arm's length: 10 degrees
Long arm of Diamond Cross: 10 degrees
Span of open hand at arms length: 20 degrees

Everyday objects can also serve as angular gauges. To determine the apparent angular size of anything in degrees, divide its linear width by its distance from your eye, then multiply by 57. For example, a 30cm ruler held one metre from your eye measures 30 ÷ 100 x 57 = 17 degrees.

A MORE ACCURATE METHOD to determine the diameter of your field of view involves measuring the time it takes for a star to drift across your field along the east-west line. This method is only useful for telescopes, since a star will take ages to cross the large field offered by binoculars. Choose any bright star, preferably far from the south pole - a star in Orion's belt would be a good choice.
Centre the star in your field of view, turn off the drive, and place the star just outside the eastern edge of the field. As the star drifts into view, start your stop-watch. When the star disappears at the western edge, stop the watch and note down the elapsed time. Repeat this measurement several times and take the average. If this average time, T, is measured in minutes, then:

field of view in arc minutes = 15 x T x cosine( D ),

where D is the declination of the star, read off from a starmap. For example, suppose you measure several transits of Canopus and calculate the average time to be 3.5 minutes. Canopus' declination is roughly -52.7degrees degrees. The field of view is then 15 x 3.5 x cos(-52.7) = 15 x 3.5 x 0.6 = 31.5 arc minutes. Thus the field of view is roughly half a degree across.

Make a note of the size of each eyepiece in your logbook, since a given eyepiece used on a specific telescope has a fixed field of view.