ASTR 1230 (Whittle) 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
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:
- 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
A quantitative measure of transparency is given by the
magnitude of the faintest star you can see with the naked eye.
The bowl of the "Little Dipper" provides a convenient set of standard
stars, with approximate magnitudes of 2,3,4,5. See this image.
More elaborate techniques for estimating limiting magnitude are given
- 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
- 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
Familiarity with the constellations and use of your sky wheels will
help locate the brighter targets (e.g. stars) that have names associated
with constellations. However, for many targets (e.g. Messier objects),
you will need to use their coordinates to locate them.
These are discussed in the next section.
Brightnesses are measured in magnitudes. Stars
up to magnitude 11-12 are visible in the 8-in telescopes. However,
more diffuse objects of a given magnitude will be more difficult
to see than stars.
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:
- 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.
Caldwell Catalog of Deep-Sky Objects (Wikipedia): updated list
of brighter non-stellar objects, including the southern hemisphere.
- Heavens Above
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
- 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
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:
- 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?
- 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
- 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.
In using HA, you must take account of the fact that because of the
convergence of lines of constant RA toward the poles, a given E-W
distance in time converts to different distances in angle depending on
the DEC of the object.
In using DEC you must take account of the fact that the celestial
sphere is tilted with respect to your local horizon (unless you
are observing from the N or S pole). That implies that stars with
certain declinations (below -52 degrees) will never be visible
from Charlottesville. Other stars, however, (above +52 degrees) will
always be above your horizon; these are called
circumpolar, and they never set.
You can quickly determine which stars are circumpolar by rotating
your sky wheels through 24 hours. Circumpolar stars never "set".
Once calibrated, the Celestron automated target finding software
is able to determine the sidereal time, look up the RA and DEC of each target,
and then point at the correct HA and DEC to find your target. It will
work either with or without the equatorial wedge. Without the wedge,
it computes the altitude and azimuth of the target.
- 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
- 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
- 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 1230 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
- Blank observing forms and a sample, filled-out form can be found
- Download, print, and read the notes for Lecture 3.
- Read Appendices A and B in the ASTR 1230 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.
Related Web links
April 2009 by rwo
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.