ASTR 130 (O'Connell) Lecture Notes

3. OBSERVING TECHNIQUES

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:

www.cleardarksky.com: Gives predictions of local observing conditions for next 48 hours, based on forecast maps from the Canadian Meteorological Center. An example chart is shown below. In the first four rows, dark blue coding indicates the best conditions. You can click on any bin to see the large-scale map on which the forecast is based. The site also includes links to Sun/Moon rise/set information, star maps, and other items.
7 Day US Weather Service Forecast for Charlottesville
Enhanced Infrared Satellite Images (UNISYS): shows past twelve hours of cloud cover over US.
Sky Watch Forecast (Intellicast)
Night Sky Live: connects to continuous skycams around the world.

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:

Residual Sunlight: The sun illuminates the sky for a considerable period after sunset. The end of astronomical twilight is defined to be the first time after sunset (or last time before sunrise) when there is no trace of sunlight measurable in the sky. (This is a more restrictive definition than "civil twilight.") The sun must be 18 degrees below the horizon for full darkness. You can find tables of the times of astronomical twilight on the Web. Here is an annual table of evening/morning twilight for Charlottesville.
Transparency: means an absence of clouds, haze, or fog that would absorb or scatter starlight. Although low-altitude clouds are the most obvious, high-altitude cirrus clouds are more commonly a problem. Transparency predictions for Charlottesville are included on the cleardarksky.com website.
Seeing: "Seeing" is defined to be the diameter of star images (measured in seconds of arc) caused by turbulence in the atmosphere. See the discussion in Lecture 2. "Twinkling" of stars is a sign of an unstable atmosphere, which will probably produce bad seeing. But you can only actually determine the seeing through a telescope. One technique you will use in Lab 3 is to measure the diameter of the components of a binary star of known separation.
Other Atmospheric Effects: See these pages for additional background on effects of the atmosphere:
Moonlight: If the moon is in the sky and more than "half-full" (astronomers would say "between first quarter and last quarter"), moonlight scattered by the atmosphere can easily obscure fainter telescopic targets (like nebulae and galaxies). Astronomers divide each month into "bright time" and "dark time" according to the phase of the moon. Of course, only near its full phase is the moon in the sky all night long, so parts of "bright time" nights can still be dark. Judge effects by the faintest stars you can see.
Light Pollution: a growing problem everywhere and certainly here in Charlottesville. Terrible when the Stadium lights are on. Impact will vary with the amount of low-level dust or mist in the atmosphere. Again, judge effects by the faintest stars you can see.

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.

coordinates

next section

magnitudes

Here are some useful websites for obtaining information on potential targets for small telescopes:

Heavens Above: very useful reference giving current positions for solar system objects (including comets & asteroids), star charts, bright star lists for each constellation, predictions of Earth satellite passages (including Iridium flares and the International Space Station), and other info. This link provides listings specific to Charlottesville.
US Naval Observatory: detailed information on solar system objects and bright stars for any date
Sky & Telescope Almanac: sun, moon, and planet information for any date.
The Messier Catalog of Deep-Sky Objects (SEDS) The Messier Catalog (compiled by Charles Messier in 1781) is the primary list of (110) brighter northern hemisphere non-stellar objects: star clusters, nebulae, and galaxies.
The Caldwell Catalog of Deep-Sky Objects (Wikipedia): updated list of brighter non-stellar objects, including the southern hemisphere.

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:

Heavens Above
Your Sky: an "interactive planetarium" which can produce standard wide-angle bright star charts or "virtual telescope" charts near given targets to a range of magnitude. A YourSky chart for the region near the Andromeda and Triangulum galaxies (M31 and M33) is shown at the right.
Skyview: the "Internet's Virtual Telescope," which can produce images of any part of the sky to high resolution from a large database. Includes the optical-band Digital Sky Survey and data in other bands from radio to gamma ray. Is a research tool as well as a key resource for planning observations.

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:

At what times of night is a target above the horizon?

At a given time, approximately where is a target (e.g. which quadrant: SE, SW, NE, NW)?

What is the maximum altitude a given target can have from the horizon?

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:

Astronomical objects are located in the sky by their Right Ascension (RA) and Declination (RA and DEC), which correspond to longitude and latitude, respectively, on the celestial sphere.
Your zenith will always fall at a DEC equal to your terrestrial latitude. That is 38 degrees for Charlottesville. Stars with DECs smaller than 38 degrees will always cross your meridian south of the zenith, while stars with larger DECs will always cross north of the zenith.
The celestial sphere rotates around the Earth in 23 hours 56 minutes, not 24 hours. This means that a given point on the sphere returns to the same location in your local sky 4 minutes earlier each night. You must take this systematic shift in the sphere's orientation into account in locating targets.
The sidereal time describes the location of the "zero-point" of the RA system on the celestial sphere. ST is numerically equal to the amount of time that has elapsed since the point where RA = 0 (which is the vernal equinox) last crossed your local meridian.
By knowing the ST and the RA for any object, you can determine how far east or west it is from your local meridian (in units of time). That is, how long it has been since it crossed your meridian (if it is now west) or how long it will be until is crossed (if it is now east). This time is called the hour angle (HA) of an object. It is, of course, continuously changing.
By knowing both the HA and the DEC of an object, you can locate it precisely in the sky. This requires using the special geometry of the celestial sphere.
By knowing the DEC of a target and your latitude (38 degrees) for Charlottesville, you can easily determine the maximum altitude (angular distance from the nearest point on the horizon) of any star. This occurs when it crosses your local meridian ("transits"). For Charlottesville, the altitude of a transiting object from the southern horizon is equal to DEC + 52 degrees. This implies, for instance, that the celestial equator crosses the meridian at an altitude of 52 degrees.

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.

The eye operates like a camera, with a lens, an iris, and a retina.

The refractive lens is filled with fluid, and its shape can be adjusted by the surrounding muscles so that a sharp ("in focus") image is produced on the retina over a range of distances to objects being viewed. Limitations in the ability to adjust the lens produce the phenomena of "near-sightedness" and "far-sightedness." The resolution of the lens-plus-detector system of the eye is typically 1-2 minutes of arc.
The iris automatically adjusts the eye's input aperture ("pupil") diameter according to the prevailing light level. This provides a focal-ratio for the eye in the range f/3 to f/8 (approximately). The largest aperture size, under very dark conditions, is about 5-7mm.
The retina is a complex photo-detection system. Light falling on retinal cells triggers chemical changes that produce an electrical signal which is sent down the optic nerve to the brain. There are two kinds of photo-receptor cells in the retina: rods and cones. The cones are less numerous and operate only at high light levels, where they provide information on color as well as the brightness of scenes. The rods are more sensitive, so operate best at low light levels, but they produce only a "grey-scale" image without elements of color.
Under the very low light conditions you will typically encounter when observing faint sources through a telescope, the rods slowly become more sensitive. It takes over 20 minutes to achieve highest sensitivity, or "full dark adaptation," but reasonable sensitivity occurs in about 3 minutes. You must avoid looking at bright lights to become dark-adapted. Using a red flashlight helps preserve night vision.
The rods are preferentially distributed toward the edge of the retina, which is therefore more sensitive to faint sources. This is the basis for the "averted vision" observing technique, where you stare at a point about 20 degrees away from the object of interest but focus your attention on the target.
Dynamic range: in a given scene, the eye can distinguish levels of illumination that have a brightness range of about 100:1, so it is said to have a dynamic range of 100. The maximum range of brightnesses detectable by the eye, after adjustments such as dark adaptation, is a remarkable 1,000,000:1.
Integration time: the eye automatically adjusts the time interval during which it accumulates light energy before sending an image to the brain. This interval is called the "integration time" and is typically 0.1 sec; it increases to perhaps 0.2 sec for low-light levels. The reason that astronomical images show so much more detail than you can see with your eye is mainly due to the fact that cameras can have very long integration times, up to hours if necessary.

F. LAB REPORTS & OBSERVING FORMS

Appendices D and E in the ASTR 130 Lab Manual describe how you are expected to use the standard observing forms to record observations with binoculars or telescopes and how to write up your lab reports. Read these sections carefully.
You are expected to have filled out the "prep" part of each form before going to the Observatory.
To keep forms neat, fill out all sections in pencil and erase as necessary.
Blank observing forms and a sample, filled-out form can be found here.

Assignment

Download, print, and read the notes for Lecture 3.
Take the Review Quiz for Week 4 on the Collab site.
Read Appendices A and B in the ASTR 130 Manual
Review the material in the "motions and coordinates" supplement to this lecture (link given below).
Complete Labs 1 and 2 at the earliest opportunities.