Astronomical Note Taking
When you first start to observe, the differences that make each object unique may seem to be lost in a sea of similarities: they all seem to be just so many faint smudges. However, as you observe more and more, you will find yourself noticing details that you were not able to see before. This special kind of visual acuity will steadily improve as you continue to observe and your observing skills develop.
Here is a brief outline of what you should try to notice as you observe at the telescope.
Date, Time, Location
First of all, for each observation, fill in the date, time, location, and the telescope you used.
Seeing and Transparency
Note the general observing conditions at your site in terms of transparency and seeing.
A good way to rate the transparency is to estimate the limiting visual magnitude of stars near the zenith, either with the naked eye or with the telescope.
To determine the seeing, estimate the actual diameter of stellar images in arc seconds, using double stars as a gauge. Look for double star pairs that are merged, or others that are resolved, but whose known separation permits an estimate of the size of the images. A list of close, equal double stars to help gauge seeing can be found in a good list of double stars. Find a favorite and then use it each time you observe.
If all of this seems too complicated, use the Seeing and Transparancy Tables as a guide.
When taking notes on a deep-sky object, be specific. General observations such as “great,” “beautiful,” or “dull” may be a major part of your impression at the time, but these are least useful in comparing what you see to what others see. These words should not be the major part of your written observation.
Begin by noting the relative difficulty of the object. Objects easily visible at low power may be rated “bright” or “easy,” those visible only at high-optimum powers are “difficult” or “pretty faint.”
At the best high power for your telescope, estimate the size, either a circular diameter, or the lengths of the longest and shortest dimensions. Do this from knowledge of the true angular field of view of the eyepiece you are using, and estimate to what portion of that field of view the object covers or extends. Doing this accurately takes practice. Be careful. It is easy to overestimate the size of objects that are small in comparison to the size of the eyepiece field.
If an object is elongated, note the approximate orientation (“elongated NE-SW”), or make an accurate measurement of the position angle (PA).
Position Angle is a compass angle measured from north, counterclockwise on the sky. Thus due north is PA 0 degrees; an object elongated exactly northeast to southwest will have a PA of 45 degrees; elongation southeast to northwest is PA 135 degrees. To eliminate redundancy for deep-sky objects all angles are referred to the first half of the circle: no position angles exceed 179 degrees. Positions relative to a deep-sky object and double star orientations can naturally cover the full 360 degrees. Double star position angles are always with reference to the brighter star. If you using an equatorial mount, with practice you will be able to estimate position angles to about 15 degrees of accuracy. With equatorially mounted Cassegrainian telescopes, you can do even better, since the orientation of the sky in the eyepiece (if you don’t use a star diagonal) is always the same relative to the telescope: marking the cardinal directions on the tailpiece of the telescope could be useful here.
Brightness and Density
Note any zones of brightness in galaxies or nebulae, and the relative concentration of stars in clusters. Apply the following terms consistently to specific observable phenomena in deep-sky objects.
A “stellaring” is any faint star-like manifestation appearing on the surface of a nebulous object. In galaxies, these may be true stars of the Milky Way superposed on it, or a bright star cloud or HII region within the galaxy that appears stellar due to the modest telescopic power in use.
Use the terms “halo,” “core,” and “nucleus” to refer to more-or-less well-defined zones of brightness from the edge to the center of an object.
The general terms “concentration” or “condensation” refer to any brightening in a nebula, or to the rise in apparent density of stars toward the center of a cluster. How the brightness changes with radius varies from object to object. Brightness profiles can be described as “broad,” “even,” and “sharp” concentrations. Globular clusters, more than any other group, exhibit most clearly the idealized concentration types. Note if the stars in the globular cluster can be resolved.
Count the Stars
In star clusters, try to count the stars. Do not merely guess at the number. If there are many stars, count only half or a quarter of them and multiply accordingly. In the case of open clusters, restrict star counts to some area that seems to be the natural size of the group, and then note this size. Doing so permits a better comparison between observations made on different dates or with other instruments.
Note the location of nearby stars or other interesting aspects of the field around an object. Estimated magnitudes, directions, and distances of stars are often included, sometimes using cardinal point or PA and angular measure, sometimes only relative to the object, i.e., “a prominent red mag. 9 star at the west edge of the cluster;” or “a mag. 13.5 star just off the NE flank of the halo”.
If you use a filter, compare the appearance of the object with and without the filter.
Sketches are a helpful supplement to verbal notes, though making them can be very time consuming. Sketches are most effective with interesting, detailed objects and when indicating nearby stars or features difficult to describe verbally. Sketches can also serve to show the relationships among several objects in the same field, and provide a means of later identifying unknown objects.
Observing programs offer a structured and fun way to pursue your study of the night sky. At first, you will find yourself learning the constellations and what time of year they are visible in order to locate your targets. As you progress, you will come to enjoy a deeper understanding of the universe and the world we live in.
The American Association of Amateur Astronomers, through its affiliation with the Astronomical League, provides a wide range of observing programs for amateur astronomers, from beginners all the way to advanced observers. The Astronomical League offers certification for its binocular, planetary and lunar programs. The most famous of these is the Messier program, a tradition of observing the galaxies, clusters and nebulae on the list compiled by French astronomer Charles Messier in 1781. Click on the links below for more information.