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Deepsky Observer's Companion tutorial
Part 4: Observing techniques

In this section: star hopping, directions in the sky, measuring the field of view, observing logbook, recording descriptions, an observing checklist.

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 eyes 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.


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).

"One tends to 'discover' a lot of interesting things while star-hopping"

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 star chart. 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.

Clark (1990) feels strongly about this basic technique, as do I:

"Star hopping should be learned by all amateur astronomers. It is the method of starting at a known naked-eye star and using a chart to move carefully from star to star until the desired object is reached."

Garfinkle (1995) notes:

"Getting there is half the fun, they say. This is certainly true when travelling between the stars in search of star clusters, nebulae, galaxies and such. Using a technique called star-hopping, you step your way across the heavens until you arrive at your ultimate destination. Its easy, its fun, and its a great way to learn the landscape of the night sky. Star hopping is like crossing a wide stream on stepping stones. To get from point A to point B, you step, or hop, from star to star using a finder scope's (or eyepiece's) field of view as a guide. Your hops often lead to a wonderful object while taking you past a multitude of interesting sights along the way.

MacRobert (1988) offers the following star-hopping tips:

"Double- and triple-check directions in the eyepiece. Always think in terms of celestial north, south, east and west, never up, down, left or right. If you have an equatorial mount, polar align it at least roughly. Now the mounts motions follow the four cardinal directions. Turn the finder's eyepiece to make its cross-hairs line up with these directions too."

"Remember that a map is not exactly like the sky." The magnitude-symbols used to show the stars do not accurately reflect the way we perceive stellar brightness, so don't be caught by brightness discrepancies when using a starchart.

"Pay particular attention to star's positions. Look for patterns of at least three stars that fix the point you're after: little triangle, rectangles, kite shapes, and so forth. Triangles are the most basic units of star-hops. Pay close attention to their shapes."

"Know your scale." Figure out how big your finder's field of view is on the chart, and also for the eyepieces you use to star-hop with.

Veteran observer Steve Coe described his technique to the IAAC mailing list as follows:

"I have a method of star-hopping that I use for faint or difficult to find objects. It involves the fact that I have four aids that allow me to easily and confidently move to fairly exact location in the sky. Those four things are: Tel-rad, University 11X80 finder with an Amici prism, Bright Star Atlas and Uranometria 2000 star atlas. I get the general idea of where I want to go in the sky with the Telrad and Bright Star Atlas. Using these together I can with certainty point the scope at a "jump off star", generally naked eye, to start from. The Bright Star Atlas lets me answer the question "OK, which one of you guys is Sigma" or whatever. Now that I am on the starting location I am ready to move up to a deeper magnitude."

One of the first accomplished starhoppers

Sir James South claimed that William Herschel was able, from a cold start, to find any object in the sky in under five minutes with the 20-foot telescope.

"I turn to the page in Uranometria that has the bright star on it, and hopefully it also has the deep sky object I seek. Then I note an asterism that is directy north, south, east or west of the object I am looking to find. This works because I have a Bigfoot Mount, which is essentially a German Equatorial. Then once I am directly even with the object in a cardinal direction, I move the scope to the object, moving in only one direction at a time. I have found two helpful things: the large finder shows all the stars marked on Uranometria and the Amici Prism will show the sky in the same orientation as the charts."

"I used to spend a lot of time looking for a faint object, not certain if I just wasn't seeing it, or am I in the wrong location. This method allows me to be quite certain that I am at the location marked on the chart. Now I can move to the right spot and if the object isn't there, it isn't there and I can move on."

Lew Gramer, IAAC listowner, wrote:

"By the way, it may be worth mentioning something else about star-hopping: the hopping techniques you use probably will depend on your sky conditions! For instance, under extremely bright skies (or with a smaller scope), I find that "aiming" the scope with a Telrad (i.e., a "gun-sight" pointer), then doing wide-field eyepiece sweeps usually picks up any star pattern I'm trying to hop to, But under darker skies (and/or larger apertures), having a finder as well as the Telrad and Panoptic eyepiece can be very handy!"

"In the end, I guess I use a combination of finder, Telrad, and eyepiece hopping techniques with my dobsonian - and a completely different set of techniques (involving distinct north-south and east-west motions) when I'm using an equatorial mount!"

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 to 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°. To specify directions more accurately, the position angle (PA) is used. A PA of 0° is North and 90° is East. Northeast would be PA = 45° while PA = 260° points a little south of west.

Easy doubles to test your PA

StarmagsSep "PA °
alpha Cru1.1, 5.190202
alpha Leo1.4, 7.6177307
gamma Lep3.6, 6.296350
delta Ori2.2, 6.8530
sigma Tau4.7, 5.143014
nu Sco4.0, 6.341337

To help you get to grips with position angles, you might like to try and estimate the PA for some easy double stars. The adjacent short list gives 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.

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 +90° and the south celestial pole at –90°. 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.

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 one degree (1°) = 60 arc minutes (60') = 3600 arc seconds (3600").

Binoculars typically have fields larger than four degrees, and telescopes normally give a view smaller than one degree.

Handy angular size estimates

solar / lunar diameter½°
width of index nail at arms length
Orion's Belt
short arm of Crux4½°
long arm of Crux
width of clenched fist at arm's length10°
long arm of Diamond Cross10°
span of open hand at arms length20°

It is essential to be able to judge angular distances in the sky. The adjacent table lists some handy angular estimates.

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°.

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 (taken from a star catalogue, or estimated 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 –52.7°. 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.

Keeping observing notes: a logbook

"Anyone who observes the sky should keep a logbook," is the emphatic opinion of Roger Clark. David Levy (1989) writes: "If the hours we spend under the stars are precious, an observing log helps us remember them. Relying on memory alone just isn't good enough; as years pass, details fade away until events might as well not have happened ... So many people have told me that they would like to start an observing log but haven't gotten around to it. Yet it's easy and fun to do, and our observations will mean so much more when they're recorded accurately."

"The palest ink is clearer than the best memory."

Chinese proverb

"The format of your log is not important; the content is. So any system that works is fine. Some observers prefer to draw in their logs, while others would rather compute. Some keep a diary. And some prefer the discipline of forms. Although forms make sure that you remember what to put down, I find them confining. Blank paper lets you record the unexpected. ... The free-form approach leaves unlimited room for variable star estimates, planetary drawing, times for photographic exposures and notes from other observers."

"If you observe certain types of objects systematically, you might consider keeping separate observing logs ... Why not take rough notes in the field and then transfer the data to files on your home computer?"

Clark points out that even the most casual celestial sight-seeing becomes more meaningful is a few notes are jotted down in a permanent record. Thorpe (1988) says that "keeping a notebook makes a more satisfied observer. It will remind you of all the things you've seen, the way you felt while observing, and the frustration and successes you've had with the hobby. And, most importantly, it will prompt you to observe more carefully and to see more when you observe.

"A notebook is your private record of the universe. Although at first you may have to adjust to updating it, the notebook will eventually contain unique records of your journey through the universe. Astronomy offers great personal satisfaction. It would be a shame if all those wonderful memories of your involvement with astronomy simply faded away with time."

Clark laments the fact that he did not always keep good notes; reading through his recollections, I am much reminded of my own early attempts at note-taking: "Back then I also made many drawing a half inch to two inches wide, but rarely included field stars. Thus there is no way to tell the scale of a drawng or the size of what was seen. Is that fuzzy patch the full extent of a galaxy including the spiral arms, or only the bright central region?"

Further reading

  1. Eicher, D. J. (1989) "Don't Forget the Bug Spray" [Be prepared for anything under the stars with these ten essential items. Don't leave home without them!] Astronomy, August, 67.
  2. Levy, D. H. (1989) Star Trails, Sky & Telescope, 77(6, June), 659.
  3. Thorpe, A. M. (1988) "Should You Keep an Astronomical Notebook?" Astronomy, January, 50.
  4. Zentz, G. L. (1992) "A Personal and Permanent Journey through the Universe" [Time spent beneath the stars with a journal will richly reward astronomical memories] Astronomy, February, 80.

Keeping observing notes: recording descriptions

Sooner or later, the novice deep sky observer comes across an apparently non-sensical string of characters, like: vB. cL. mE. mbM. r, used to describe an object.

These abbreviations were popularised by Sir William Herschel, and first described by him in his paper "Catalogue of One Thousand new Nebulae and Clusters of Stars" published in 1786. He wrote: ". . . and that I might describe all these objects in as small a compass as could well be done, I have used single letters to express whole words. . ."

Over the years these abbreviations were refined and added to; they were used by John Herschel, and Dreyer used them when he published the "New General Catalogue" or NGC. Some years later, a German astronomer published an enhanced list of abbreviations, reproduced below in Appendix 5.

Modern observers, too, can benefit from such abbreviations. It makes note-taking at the eyepiece a cinch, reducing the writing time and increasing accuracy. In my case, my normal handwriting approximates graphological schizophrenia, and trying to decipher notes from last week is agonising. Using a combination of abbreviations and normal text makes for concise descriptions and goes a long way in solving the legibility problem. Some folk maintain that abbreviations restrict your thinking and that your descriptions then all turn out more or less the same; this simply isn't true, in the same way that taking dictation doesn't reduce the vocabulary of the speaker.

Some examples of my note-taking for open clusters:

(1) "Cl D = 1/6 K18. Cl is constrn by 3 * W & 2 E"
(2) "S: 2 9m * (no col) rough NS with neb. Co Cl L&S *, F*s to W of 2 B *; best seen avv"
(3) "1 * 9m on edge eRi glow vF-eF *s. Of these, the Ber * are in S groups iR scatt abt."

The translation of these is quite straight-forward:

(1) "Diameter of cluster is one-sixth of the field of view of the 18mm eyepiece. The cluster is constrained by three stars to the west and two to the east."

(2) "The first impression, in the sweeping eyepiece [lowest power]: the cluster shows as two 9th magnitude stars (of no distinct colour), arranged roughly north-south, within a nebulous haze. Closer examination shows a coarse cluster of large and small [i.e. bright and faint] stars, with the fainter stars to the west of the two brighter ones; cluster needs averted vision to show well."

(3) "A 9th mag star on the edge of an extremely rich glow of very faint to extremely faint stars. Of these, the brighter ones are gathered in small groups irregularly scattered about."

The abbreviated version is 73% shorter, and consequently much quicker to write down. Use the time saved in this way to sketch the object.

Which brings us back to William again. He wrote: "By going into the light so often as was necessary to write down my observations, the eye could never return soon enough to that full dilation of the iris which is absolutely required for delicate observations. The difficulty also of keeping a proper memorandum of the parts of the heavens which had been examined ... intermixed with many short and long stops while I was writing ...." lead him to modify the design of his telescope and his observing methods.

The changes were successful: "Soon after I removed also the only then remaining obstacle to seeing well, by having recourse to an assistant, whose care it was to write down, and at the same time loudly to repeat after me, every thing I required to be written down. In this manner all the descriptions of nebulae and other observations were recorded; by which I obtained the singular advantage that the descriptions were actually writing and repeating to me while I had the object before my eye, and could at pleasure correct them, whenever they disagreed with the picture before me without looking from it."

Few of us have the luxury of a night assistant, willing to takes notes for us. But technology can step in and offer a willing surrogate: the dictaphone or portable tape recorder. For obvious reasons, mine is christened Caroline.

Often, however, the fluidity of ideas is best expressed by a free-form diagram, making a combination of note-taking and electronic recording necessary. A final note: Don't forget to add details about yourself, your telescope, etc. to your descriptions. Herschel wrote: "It should be observed that all estimations of brightness and size must be referred to the instrument with which the nebulae and clusters of stars were seen; the clearness and transparency of the atmosphere, the degree of attention, and many more particular circumstances, should also be taken into consideration."

A checklist for creating a deep sky description

Having a checklist of features to look out for when observing an object is not meant as a rigid thought-constraining framework, but rather as a tool to make sure you don't forget to note a particular aspect. Browse the current version of the Deep Sky Observing Checklist.

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