Download Introduction to Telescopes

Lab 2 – Introduction to Telescopes.
(updated 1/11/2007)
I. Introduction
In this lab we will give you a general introduction to telescopes and what can be seen
through them. At future labs, such as CCD imaging, you will have a chance to explore
the universe in greater depth.
Astronomers use a wide range of instruments to gather and focus electromagnetic
radiation (light, radio waves, X-rays, etc.) into a form, such as an image or a spectrum,
that is usable for astronomical research. On other evenings in this class you will use a
CCD camera to capture an image, or you might use the light to measure the properties of
a star, such as its brightness or composition. For now, we will take the time to instruct
you on some of the basic operations of our telescopes, mountings, eyepieces (visual
observations) and a CCD camera (imaging and photometry).
II. Telescopes
We have several different types of telescopes that we use in our astronomy labs, so let’s
have a brief overview of the three basic optical designs.
1) Refracting telescopes are the type of telescope that most everyone is familiar with.
This is the oldest design, and the one that Galileo used to make the first telescopic
Ladd Observatory’s 12” refractor
observations. Refractors use two or more lenses at the front end of the tube to bend light
(refract) to a focal point, where the primary image is magnified by an eyepiece. Although
the images produced by a refractor can appear very sharp and clear when used visually,
there is always some spurious light left that does not completely focus, showing up as a
faint halo of false color surrounding the object viewed. This false color (chromatic
aberration) may appear nearly invisible to the eye, but CCD cameras easily record this
artifact, which effectively blurs the image to some degree. Therefore, the telescopes that
we will be using for the more advanced labs are of the reflecting type listed below.
2) The basic reflecting telescope was invented by Sir Isaac Newton,
and today is referred to as a Newtonian reflector. It uses a concave,
parabolic mirror, at the bottom of the telescope tube, to gather and
bend the light to a focal point. This focal point is located on the side of
the tube, near the front end of the telescope. The light from the primary
mirror is diverted to this point by means of a small, elliptical, flat
mirror – centered in the light path of the telescope. For some of our labs, you may be
using a small “Astroscan” telescope, which is a 4.25” Newtonian reflector.
3) Catadioptric telescopes combine both mirrors with corrective lenses
to form a compact, essentially color-free optical system. The 16”
telescope mounted in our roof-top observatory is a catadioptric design
known as a Schmidt Cassegrain. All of our advanced labs will use this
telescope. In addition to the 16” telescope, we have several 3.5” telescopes that will be
used for some of the labs, including tonight (weather permitting).
Regardless of the optical design: magnification, light-gathering power, and resolving
power are the three essential powers of a telescope.
The TA/Instructor can help you with these formulae:
I) Magnification = F.L. Telescope/ F.L. Eyepiece (F.L. = focal length)
II) Light-Gathering Power = Area of Objective/ Area of your pupil
Question 1
Determine the light gathering power of the following telescopes:
A. Astroscan 4.25” Newtonian reflector
B. Meade ETX 3.5” Maksutov - Cassegrain
C. B&H Observatory’s 16” Schmidt Cassegrain
III. Eyepieces
There are many different eyepiece designs – some offer a very wide FOV(field of view the area of sky visible in the eyepiece) for observing extended objects; others might offer
a smaller, but highly corrected FOV for observing subtle detail on the planets. We will
not concern ourselves with the differences of these optical designs, but will instead
concentrate on just two common properties of eyepieces in general.
Eyepieces come in different focal lengths (measured in mm), typically ranging from 4mm
up to 56mm. The longer the focal length of an eyepiece, the lower the magnification it
will produce. For example, if you are using one of our small Meade ETX telescopes, with
a focal length of 1250mm, then a 32mm eyepiece will produce a magnification of 38.8X.
In addition to wanting to know how much magnification we are using, it is often useful to
know how much of the sky we can actually see with a particular eyepiece. The formula to
determine the FOV with any telescope/ eyepiece combination is:
Actual FOV(degrees) = Apparent FOV of eyepiece/ magnification.
The apparent FOV of any eyepiece is dependent on its optical design, and is usually listed
by the manufacturer. Many of our eyepieces have an apparent FOV of 52 degrees, while
some over slightly less than this; some a little more.
Question 2
Using the same 28mm eyepiece with a 52 apparent FOV, determine the magnifying
power and true FOV for the following telescopes:
A) Astroscan 4.25” (445mm f.l.) Newtonian reflector
B) Meade ETX 3.5” (1250mm f.l.) Maksutov - Cassegrain
C) B&H Observatory’s 16” (4,072mm f.l.) Schmidt Cassegrain
IV Telescope Mountings
There are two families of telescope mounts. The alta-azimuth mount allows the telescope
to move from horizon to the zenith (altitude axis) and in a circle parallel to the horizon
(azimuth). This is an extremely simple mount.
The sky doesn’t work that way, however. It rotates (or appears to rotate because the Earth
rotates) around the axis that connects the north and south celestial poles. At the latitude of
Brown University, the pole star is at an altitude of approximately 41 degrees. Suppose
that we would like our telescope to be able to track the motions of the stars as trace giant
circles around the pole star (Polaris). If we tilt our telescope mount so that one axis is
pointed at the pole star, then we have solved part of the problem. The telescope mount at
this stage is called an ‘equatorial mount’. At the North Pole in the Artic the ‘alt-azimuth’
telescope is the same thing as the ‘equatorial’ telescope.
To compensate for the rotation of the Earth and the apparent motion of the sky it causes,
the mount rotates on its polar axis by means of a motorized drive system.
Alt-Azimuth Configuration
Equatorial Configuration
V Celestial Coordinate System
Early in the history of
astronomy it became necessary to
devise a system for describing
the positions of the stars on the
celestial sphere. The most
obvious system was one based on
the local horizon and named
simply the Horizon System. It
gives the star’s position in terms
of the observer’s horizon plane,
with one coordinate (azimuth)
measured in terms of compass directions along this plane. However, this system of
coordinates has disadvantages in that the azimuth and altitude of a star change both with
the time of day and the location of the observer.
It is desirable to have a system of coordinates permanently attached to the celestial
sphere. Such a system is the Celestial Equatorial System. The polar axis of the earth is
projected outwards, defining the poles of the celestial sphere. The equatorial plane of the
earth is projected outward and it defines the celestial equator. The coordinates of this
system are right ascension (analogous to longitude on the earth), measured along the
celestial equator and declination (analogous to latitude), measured north and south from
the celestial equator. The coordinates of stars in this system are constant rather than
dependent on the observer’s time of day and place.
VI CCD Imaging
As the introduction of photography about a century ago revolutionized observational
astronomy by allowing scientists to get images of celestial objects too faint to be seen
visually even with the biggest telescopes, so did the introduction of ‘charge-coupled
devices’ (or CCDs) two decades ago. A CCD is a small electronic chip divided into a
large number of little squares, so-called ‘pixels’. When light hits one of these pixels, the
material of the CCD emits electrons which are trapped within that pixel, so that charge
accumulates on each pixel proportional to the light intensity. When such a chip is
attached to an astronomical telescope and exposed for a certain period of time, the pattern
of the charges on it represents an image of the region of the sky the telescope is point
towards. That pattern can be read out by a computer, so that on its screen a picture of the
corresponding celestial object will be displayed.
The big advantage of taking CCD images over photography is the much higher sensitivity
of a CCD. Whereas exposure times of 10 or 15 minutes are necessary with standard film
material in order to get an image of a star cluster or a nebula, the CCD has to be exposed
for only 10 to 20 seconds to get the same result. Therefore all of the modern research
telescopes are equipped with CCD cameras, which have greatly pushed out the limits of
detecting faint celestial objects, similarly to the introduction of photography 100 years
ago. Another advantage of the CCD is that the image is available in a format allowing
computer aided processing which can bring out more and fainter details.
The chip in our CCD camera holds about 1 million pixels in 1024 columns by 1024 rows.
Each pixel has a physical size of 24 μm (1 μm – 1/1000 mm). The information on the
charge (and therefore the intensity) on each pixel is transformed into a scale of integer
numbers from 0 to 65535 before it is transmitted to the computer operating the CCD. ) 0
stands for zero charge, and 65535 for the highest possible value. This is usually set to be
above the saturation charge on a pixel, which is the maximum charge that can be
collected.
Additional questions to be answered in your lab book:
3) Telescopes designed for amateur observations of the Moon and planets frequently
advertise the magnification power of the telescopes. Professional telescopes
designed for studying distant stars and galaxies don’t even bother to list the
magnification, but instead focus almost exclusively on the area of the primary
mirror. Why do you think this is so?
4) The equatorial coordinate system is convenient because stars always have the same
coordinates in this system. Do the Sun and Moon always keep the same
coordinates? Why or why not?
5) The Quantum Efficiency (QE) of a detector is the fraction of the light shining on it
that can be detected. For example, 1-2% of the optical photons entering the pupil of
your eye react with the rods and cones in your retina to produce the impression of
light. CCD detectors now have QEs of around 80%, compared to 1% for
photographic film. How long would it take a telescope equipped with
photographic film to equal the detecting power of an identical telescope with a CCD
that is capable of taking a 20 minute exposure of the sky? Compare this to the
length of the day.