# An Introduction to Telescopes

Urban and Civil

Nov 15, 2013 (4 years and 5 months ago)

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Refractors (Dioptric)

Use lenses

‘first’ telescopes

Problems:

chromatic aberration: A lens will not focus different colors in exactly the same place because the
focal length depends on refraction and the index of refraction for blue light (short wavelengths) is
larger than that of red light (long wavelengths). The amount of chromatic aberration depends on the
dispersion of the glass.

spherical aberration: For lenses made with spherical surfaces, rays which are parallel to the optic
axis but at different distances from the optic axis fail to converge to the same point.

Special types:

Achromatic: telescope has been color
-
corrected with the use of multiple lenses and/or coated lenses

Apochromatic: corrected for both chromatic and spherical aberration

Reflectors (Catoptric)

Use mirrors

Problems:

spherical aberration: For mirrors made with spherical surfaces, rays which are parallel to the optic
axis but at different distances from the optic axis fail to converge to the same point.

collimation: alignment of the optics

Special types:

newtonian:

Herschelian:

cassegrain

Dobsonian:

Use mirrors and lenses

Problems:

spherical aberration: For mirrors made with spherical surfaces, rays which are parallel to the optic
axis but at different distances from the optic axis fail to converge to the same point.

collimation: alignment of the optics

Special types:

Schmidt
-
cassegrain

Maksutov
-
cassegrain

Basic definitions

f

a

f =

focal length

the distance it takes
for light to come to a focus after
refracting through a lens or reflecting
off a mirror

a =

aperature

the diameter of the main
(objective) lens or mirror (primary)

magnification =
f
scope
/f
eyepiece

F/ratio

=
f / a

f

a

f

a

primary

secondary

f

a

Properties: Resolving Power

The minimum angular separation two stars can have and still
appear as two stars.

R = 252,000
×

(

) / (D)

where

is wavelength of light

and D is objective diameter

R will be in arcseconds

If you substitute 550nm for

, and D is in cm, then you have Dawes Equation.

R = 12/D

Very subjective

Depends on seeing (atmospheric condition)

Properties: Magnification

Measure of how big something appears…

M = angular size with aid/angular size without aid

In optics, it is also expressed as M = F/f

M
max

= 20 x D where D is in cm

Properties: Light Gathering Power

Measure of how much light can enter a telescope to be
brought to focus

LGP = area of objective/area of pupil (eye)

If the human eye opening with faint light is about 7mm, then

LGP = D
2
/49 where D is the objective diameter in mm

So a bigger light bucket is better!

Properties: Field of View

The region of sky that can be seen through the instrument.

(While there are formal methods to calculate the field of view, a much simpler
method is to watch a star drift across your view.

Select a star near the zenith

turn off any tracking motors so that the star drifts across the view

adjust the telescope so that the star drifts directly across

Place the star just outside the view and when it first drifts into view start a
stopwatch or other timer

Stop the timer when the star leaves the view

The time may be several seconds to several minutes depending on the size of
the telescope and eyepiece used

Knowing that the earth spins on its axis once every 24 hours or sees
360degrees of sky/24hours

360deg

=
15deg

=

1deg

=
60arcmin

=
15arcmin

24 h

1 h

4min

4min

1min

So if it takes 150 seconds, that’s 2.5minutes and a FOV of 37.5arcmin.)

Mounts

Telescopes must be supported by some type of stand, or mount
--

otherwise you would have to hold it all of the time. The telescope mount
allows you to:

keep the telescope steady

point the telescope at the stars or other object (birds)

adjust the telescope for the movement of the stars caused by the Earth's
rotation

free your hands for other activities (focusing, changing eyepieces, note
-
taking, drawing)

Alt
-
azimuth

basic camera tripod

dobsonian

Equatorial

German equatorial

fork

Cleaning the Optics

DON’T!!

You should only clean your telescope optics twice a year (and only if
needed!), remember less is more. To help keep optics clean always replace the
telescope cover when not in use and put your eyepieces back in their
containers or plastic sandwich bags.

Never cover your telescope optics or eyepieces if they have dew(or frost) or
condensation on them, instead use a hairdryer on low heat until they are dry
then cover them.

http://www.telescopehome.com/telescope
-
optics.html

http://www.corvus.com/faq/aa01faq8.htm

http://members.aol.com/bemusabord/cleaning.html

http://www.company7.com/library/clean.html

Collimating your telescope is not hard but it does require some practice.
Here are several sites that have details for different telescopes.

http://www.telescopehome.com/telescope
-
collimation.html

http://www.rochesterastronomy.org/main.asp?section=8&page=34

http://zebu.uoregon.edu/~mbartels/kolli/kolli.html

http://ngc1514.com/Celestron/collimate.htm

Polar Aligning

For most things, rough alignment is sufficient since most people only observe
an object for a few minutes and it doesn’t matter that the object drifts out after
10 minutes. But if you plan on doing any astrophotography then this is a
critical procedure to learn.

There are several sites that give good descriptions of the rough alignment as
well as the more accurate star
-
drift method.

http://www.celestron.com/polar.htm

http://www.astrocruise.com/polarnew.htm

http://www.tucsonastronomy.org/polalign.htm

http://www.aa6g.org/Astronomy/Articles/drift_align.html

http://www.darkskyimages.com/gpolar.html

Not often mentioned or covered, is how to balance your telescope. A small
refractor on an al
-
az tripod does not need to be balanced; however, a refractor
or newtonian on an equatorial mount will have a counterweight shaft and need
to be balanced for you to be able to use the telescope and for the tracking to
operate optimally.

http://www.starizona.com/basics/balance.cfm

http://www.telescopes
-
astronomy.com.au/telescopes019.htm

Star Testing Your Optics

One way to find out the quality of your
optics is to do a star test. The patterns can
also reveal other problems.

http://www.skywatchertelescope.net/EducationST.html

http://hometown.aol.com/billferris/startest.html

http://www.astunit.com/tutorials/startest.htm

Focusing

When my students complain that they can’t see anything in the telescope, I check

1.
the dust cover, make sure it’s off

2.
the focus

3.
where they are pointing

Usually, they are out of focus and not pointing at anything. Learning how to focus is
simpler than learning where to look in the sky!

During the day, point the telescope at a very distant tree or lightpost.

As you watch through the low
-
power eyepiece, turn the focus knob first in one direction,
and if nothing happens and it stops turning, turn it in the other direction. (You should end
up halfway between the two extremes so if you count the number of turns…) Eventually,
you should see your target appear blurry then sharper as you improve the focus. Changing
eyepieces will require that you change the focus but it will only be a few turns.

That night, point the telescope at a bright star. Hopefully, you’ll see a big blob (out of
focus star), but you may either have to adjust the pointing or really turn the knob. If you
have a newtonian or cassegrain the blob will actually look like a donut when it is way out
of focus. Very dim stars pretty much disappear when they are out of focus so be sure you
are looking at a bright star!