“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.” – David H. Levy
These guidelines will help you to get the most out of your observing sessions by providing a checklist of things to look out for when you examine a deep sky object. The checklist 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.
1. Identify the object
When you set out to describe an object, don’t forget to write down its catalogue number or name! If it’s a very well-known object, it should be sufficient to list its NGC number, or any other popular catalogue designation (e.g. Bennett, Messier, Caldwell etc.). It’s also a good idea to indicate the object’s approximate location within its constellation, just to give a general sense of where it is. Later, when you transcribe your observations from your observing notes to your logbook, look up its co-ordinates (right ascension and declination) and record these.
Sooner or later you’ll start observing more obscure objects – or perhaps even discovering your own. It then becomes particularly import to provide sufficient information so that others can also find the object, and that you can verify later what object was in fact being observed.
2. First things first
Once you’ve found your target, give a quick summary of what strikes you as most interesting about it. Look at it with an open mind and without preconceptions, and give an overall impression. At a glance, what does it look like? How easy is it to see? Also pay attention to the surrounding starfield within which the object lies.
3. Brightness
How bright is the object? You can perhaps use a verbal description, such as “extremely bright – very bright – bright – faint – very faint – extremely faint”.
If possible, estimate the magnitude. This will require experience and reference objects, but is a worthwhile step, particularly for advanced observers.
4. Size
How big is the object? Estimate its angular size (in degrees, arcminutes or arcseconds).
One way to estimate the size is to compare it to the size of the field of view of the eyepiece you are using.
A more accurate method is to compare the object’s size to the distance between two stars in the field. Note which stars you have used, and (with a star atlas or computer program) determine the distance between them.
5. Shape
What is the general shape of the object? How you answer this question will depend a lot on what type of object it is. Galaxies and planetary nebulae often have well-defined shapes, whereas open clusters and nebulae are more difficult to pin down and are often just “irregular”.
If the object has a regular shape, is it round or elongated (oval)? If it is elongated, to what extent? The degree of ellipticity can be indicated in words such as “very slightly oval – clearly elongated – extremely extended into a ray”. Preferably, estimate the size of the minor (shortest) and major (longest) axis. For an elongated object, you also need to specify its orientation, to indicate the direction in which the object lies. Usually, it is sufficient to use compass directions, for example: “the galaxy is elongated northwest by southeast”. More advanced observers may estimate the “position angle” (PA). A position angle of 0° means the object is positioned on the north-south axis; PA=45° is northeast by southwest; PA=135° is northwest by southeast, etc.
6. Brightness profile
Most “fuzzy” objects – galaxies, globular clusters, planetary nebulae, etc. – are not equally bright all over. Many have a central nucleus that is brighter than the outer regions. The brightness profile describes how the object’s brightness changes as you look towards the centre. Note by how much, and how quickly, it brightens. In some cases there is a sudden rise in brightness (sharp concentration) at the centre, and in other cases the brightness profile rises gradually. Some objects, galaxies in particular, grow brighter towards the centre at varying rates.
7. A checklist for observing the deep sky
| Nebulae | Star clusters |
| What are your first impressions? How easy is it to see? (visibility; brightness; magnitude) What shape is the nebula? How big is the nebula? How does the brightness change from edge to centre? (brightness profile) Is there a nuclear region? Are the edges sharp or diffuse? Are there darker parts or areas of uneven brightness? How well does the nebula stand out from the background field? What colour is the nebula? Are there stars very near, or within, the nebula? How does it relate to the surrounding star field? Rate your confidence in this observation. |
What are your first impressions? How easy is it to see? (visibility; brightness; magnitude) What shape is the cluster? How big is the cluster? Are individual stars seen? (unresolved … granular … partially resolved … well resolved, etc.) Are the stars concentrated towards the centre? (not at all … slightly … strongly, etc.) How does the brightness change from edge to centre? (brightness profile) How many stars can you see? (make an estimate; count the number within a specified diameter) What is the range of their brightness? (nearly the same … mixed; estimate magnitudes) Is there an obvious central or other prominent star? Do any of the stars have a particular colour? Are any of the stars double? Are there chains, rows, or clumps of stars? Are there prominent empty spaces or starless patches? Is there a background glow (unresolved stars/nebulosity)? How does the cluster relate to the surrounding star field? Rate your confidence in this observation. |
8. Sketches
“Good drawings do not require special artistic talent or experience, but they do demand close attention … and honesty in not recording details remembered from photographs but not positively seen. – Roger N. Clark
A sketch done with care rather than in a hurry can make a fine addition to your observing records.
Phil Harrington, veteran deep sky observer, wrote: “A picture is worth a thousand words, except in observational astronomy, when it is worth ten times that much! The drawing need not be extravagant; just a simple sketch giving a feel for scale and orientation. It is important that the astro-artist note the direction of the four cardinal points plus include at least a few of the field stars.”
9. Personal details and observing conditions
In your logbook, record your assessment of the quality/accuracy of the observation, and perhaps indicate a difficulty level: how much of a challenge the object was.
Record the location of your observing site and the date of the observation. Describe the instrument used: type (e.g. Binoculars, reflector, refractor) aperture and f/ratio. For each eyepiece you use, give its focal length and field size.
Note any relevant weather details, including the clarity of the sky. Give an indication of the seeing (stars are fuzzy – stars appear disk-like – images are sharp at medium power – sharp images at high magnification). Seeing is often measured as the size, in arcseconds, of a stellar image.
Rate the sky darkness by indicating the limiting magnitude, i.e. The brightness of the faintest star visible through the telescope.
10. Other techniques
If you have a selection of eyepieces, use each one. A variety of magnifications can bring out additional details and objects often look subtly different at different magnifications. Also, comparing objects as seen through different-sized telescopes can be instructive.
If you have access to filters (UHC, O-III, Light Pollution, etc.) note the visibility of the object with and without filters. If you have a spectroscope or diffraction grating, the spectral lines of bright nebulae and emission stars can be studied.
11. In conclusion
The bottom line: try to build a description of what you see so that – when you read it at some later date, or when it is read by someone else – it is vivid and complete enough so that you are able to picture it again in your mind’s eye.
Ignore all these guidelines. Or use some of them. The important thing is to enjoy what you are doing, and to capture the essence of each deep sky object in such a way that others, reading your description and looking at your sketch, can share in that enjoyment.
Acknowledgements
A long list of folk responded with insights and helpful suggestions to an earlier version of the list that I circulated. Thanks for sharing your expertise. In order of the number of characters in their name, they are: Steve Coe, David Levy, Dave Kratz, Lew Gramer, Dennis Webb, Bert Dekker, Brian Skiff, Tom Polakis, Doug Snyder, Marilyn Head, Tom Lorenzin, Owen Brazell, Alan McRobert, Murray Cragin, John Callender, Steve Gottlieb, Brent Archinal, Darren Bushnall, Phil Harrington, and Malcolm Thomson.