ASTR 1230 (Whittle) Lecture Notes


3. OBSERVING TECHNIQUES

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, 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, see this picture.


A. PREDICTING GOOD WEATHER

Astronomers need good weather. Ideal conditions are cloudless, windless, low humidity, and stable. Even high altitude thin "cirrus" clouds (see picture below) that television weather forecasters would ignore can seriously hamper some kinds of quantitative 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. There is also now a network of sky webcams at various observing sites around the world that can provide additional information. Here are some of the more useful sites:

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


B. JUDGING SKY CONDITIONS

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:


C. OBSERVING LISTS AND FINDING CHARTS

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 130 Lab Manual 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:


D. TARGET COORDINATES AND SKY LOCATION

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. You should avoid trying to observe 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 should be able to place 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
Lec 3 Supplement on Astronomical Motions and Coordinates. You should skim this material, half of which is also covered in ASTR 121, but you aren't required to know it in detail. Here is a brief summary:


E. THE HUMAN EYE AS A DETECTOR

With or without optical aid, the only light detector most of you will use in this course is the human eye. Although we take its operation for granted, the eye is, in fact, a remarkably capable optical instrument, and it is important to understand some aspects of its behavior.


F. LAB REPORTS & OBSERVING FORMS



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Last modified April 2009 by rwo


Text copyright © 1998-2009 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 130 at the University of Virginia.