Refractors (Dioptric)

1
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

2
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

3
Catadioptric

• 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

4
Basic definitions
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
fscope/feyepiece F/ratio f / a
f
a
f
secondary
a
primary
f
a
f
5
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)

6
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 Mmax 20 x D where D is in cm
7
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
D2/49 where D is the objective diameter in mm So
a bigger light bucket is better!
8
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, thats 2.5minutes and
  a FOV of 37.5arcmin.)

9
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

10
Cleaning the Optics

• DONT!!
• 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

11
Collimating your scope

• 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
  8page34
• http//zebu.uoregon.edu/mbartels/kolli/kolli.html
  
• http//ngc1514.com/Celestron/collimate.htm

12
Polar Aligning

• For most things, rough alignment is sufficient
  since most people only observe an object for a
  few minutes and it doesnt 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
• http//www.minorplanetobserver.com/htms/Drift_Alig
  nment_Made_Simple.htm

13
Balancing your scope

• 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/telescopes0
  19.htm

14
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.htm
  l
• http//hometown.aol.com/billferris/startest.html
• http//www.astunit.com/tutorials/startest.htm

15
Focusing

• When my students complain that they cant see
  anything in the telescope, I check
• the dust cover, make sure its off
• the focus
• 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, youll 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!