ASTR 1230 (O'Connell) Lecture Notes


Tomb Raider

Lara Croft, the Tomb Raider, at her telescope

In this lecture, we cover a number of aspects of preparing for and making observations with small telescopes, oriented toward the main telescope labs (3 & 4). The more difficult or complicated your telescope is to use, the more important is good planning. For a famous historical example of a telescope that demanded really good planning to use successfully, see this picture.


Astronomers need good weather. Ideal conditions are cloudless, windless, low humidity, and stable. Even high altitude, thin "cirrus" clouds (see picture below) that TV weather forecasters would ignore can seriously hamper many kinds of astronomical observations. On the other hand, less critical observations can be made through gaps in lower level clouds as they pass over.

Thanks to weather satellites, it is possible to track weather conditions and make fairly accurate predictions of observing weather for the next several days at any place in the US. Satellites identify water vapor over a given location by using infrared-sensitive cameras. Here are some of the more useful websites related to weather:

Weather maps often quote time of day using Greenwich Mean Time (GMT), which is 5 hours ahead of Eastern Standard Time (4 hours ahead of Eastern Daylight Time). The notation "Z" ("zulu") denotes GMT.

This is why professional astronomers are cautious about observing even in good weather after a snowstorm.


Even when weather is reasonably good, it is important to learn how to evaluate prevailing conditions at the telescope. The main determinants are the following:


Before you come to the observatory, you need to know what objects you intend to observe and how to find them. In some cases, the target list for a lab is specified in advance; in others, you are free to choose from many possibilities.

Primary lists of potential targets, with brief descriptions and sometimes observing hints, can be found in the ASTR 1230 Lab Manual (see semesterly and all-sky lists in Lab 4) and the Mag 5 Star Atlas. It is probably best to choose targets first by astrophysical category (e.g. star cluster, nebula, galaxy) and then rank candidates in order of location in the sky and brightness.

Here are some useful websites for obtaining information on potential targets for small telescopes: For fainter targets, you may want to make finding charts, which show their immediate vicinity, as an aid to locating them. Here are some sites providing sky or finding charts:


For best viewing, objects should be as high in the sky (as far from the horizon) as possible during the time you will be observing. Sky location is determined by an object's astronomical coordinates, the time of night, and the date. Unless there is no alternative, you should avoid observing any object at an altitude of less than 30 degrees above the horizon.

The most important questions you need to answer in planning observations are:

For brighter objects in Labs 2 through 4, you can usually answer these questions satisfactorily by using your Sky Wheels. For other objects, you will be able to use the automated target finding software in the Celestron telescopes, which is capable of placing any target into the telescope field of view (once you have calibrated the pointing control system for that particular night).

The basic considerations in locating targets in the sky are described in detail in the
Supplement on Astronomical Motions and Coordinates. You should skim this material, half of which is also covered in ASTR 1210, but you aren't required to know it in detail. Here is a brief summary:



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Last modified December 2020 by rwo

Text copyright © 1998-2020 Robert W. O'Connell. All rights reserved. Noctilucent cloud image copyright © P-M. Heden. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1230 at the University of Virginia.