Physics 101 - Astronomy - Spring 2019
Class notes for day 8, Feb. 7, 2019
We covered most of Ch. 6 about Astronomical Instruments.
The main points are summarized here, and you should look at my PowerPoint, which has many pictures not in the book.
Why do we need telescopes?
Humans can only see 6000 stars at night and cannot resolve (discern) the detail
of objects in the sky if they are too close together.
Humans also cannot see anything but visible light.
We also need some way to record images, like in photography.
To make a telescope we can use lenses or mirrors, or a combination of both.
A Refracting Lens can form an image by using the principle of the refraction
of light. Refraction of light occurs when light passes from one type of
material into another, in which the speed of light is different. The light is
then bent from its original path.
Image Formation can also occur with a concave mirror, like a makeup mirror (the
guys would call this a shaving mirror).
There are two basic types of Telescope: Reflectors and Refractors
Refractors are no longer used for much research. The Yerkes observatory
(near Chicago) has the largest-ever refractor, with a 40 inch diameter lens.
The main types of reflector are
Prime focus
Newtonian
Cassegrain
See the PowerPoint for diagrams of these.
I also showed the Coude reflector, which is a variation of the Newton and
Cassegrain combined together. This is only used in a few very large telescopes.
This is not mentioned in the OpenStax book. See the powerpoint.
Telescopes should be large to give lots of sensitivity to dim light, so the only
way to do this is with reflectors, since glass lenses cannot be made too large
or they lose their shape and don't work well. Hence in general, reflectors
can have larger apertures than Refractors. Longer exposures also produce
better images. But you have to quit before dawn, so the exposure time of
the photo is obviously limited.
Resolution is limited by Diffraction, the spreading out of light when it goes
through a small opening (a small aperture).
One cause of poor images is Atmospheric Turbulence, which is reduced at high
altitudes. So many large telescopes are placed on mountains. This also puts them
above most of the clouds, humidity, and turbulence of the lower atmosphere.
Adaptive Optics (also called active optics) use a laser beam as a “guide star”
and continually change the shape of the mirror to compensate for atmospheric
turbulence and distortion.
See the power point for some examples of large telescopes. I also talked about space telescopes for looking at infrared, ultraviolet, X-rays, and gamma rays.
Suggested reading for this material is in Ch. 6, sections 6.1 - 6.6, pp. 189-224. However, there are long lists of various telescopes, etc., which you can just skim over.
In case you want to explore more, all the major telescopes have web sites
that you can browse.
However, this is way more than you need to know for the class. If
you are interested, look at my
List of
Observatories.
I suggest that you read through Ch. 7 when you can. This is just a summary of the solar system, and we will study these topics in detail later in the course.