Telescope Basics Telescopes_v4.ppt

1
Telescope BasicsTelescopes_v4.ppt

• Elizabeth Warner
• UM Observatory
• 5 October 2005
• For a copy of this presentation, email
  ewarner_at_umd.edu

2
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

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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

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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

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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
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Properties Resolving Power

• The minimum angular separation two stars (or
  other features) 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 Mmax
20 x D where D is in cm Unfortunately, most
department store telescopes will advertise that
the telescope is capable of 900x! With the
addition of some other optics (like a barlow
lens) that may be true. But what is the quality
of the view through that 2.5 scope? Well, a
scope of 2.5 aperture has a maximum
magnification of about 125x. Thats a far cry
from 900x! Think of it this way just because
your cars speedometer has 200mph on it doesnt
mean that the car can go that fast!!
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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!
Seeing and Transparency
Most people understand that to get the best
views, most astronomers like clear and dark
skies. What does that mean exactly? Well, dark
skies mean skies that are free from light
pollution street lights, house lights, business
lights any light that is aimed at the sky rather
than at the appropriate target (street, your own
house or business). Basically, the contrast
improves if you have darker skies. But more
specifically, astronomers are concerned with
Seeing and Transparency which can be loosely
translated as how steady the air is and how clear
it is respectively. Weather wise, it might be
clear (no clouds) but it might be too hazy to
view some objects. And then sometimes, a slightly
hazy sky is an indicator of steady skies which is
very important for observing planets.
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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 celestial equator
• 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)
  then the FOV is
• 2.5min x 15arcmin 37.5arcmin
• 1min

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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

• First, read the instructions that came with your
  telescope, but in general.
•  
• If you have a telescope on an altaz mount (ie,
  small refractor mounted on a camera tripod), then
  theres not much to do except to plop the tripod
  down and aim your telescope. You will need to
  learn how to read starcharts and to starhop.
• If you have a telescope on an (german) equatorial
  mount, then things get a little more complicated!
• 1.   Level the tripod.
• 2.   Adjust the angle of the mount head to your
  latitude
• 3.   Point the RA axis (stem of T) to the north
  (and the dec axis or top of T runs east-west)
•  
• If you have a telescope on a fork equatorial
• 1.  Level the tripod and attach the wedge.
• 2.   Angle the wedge for your latitude.
• 3.   Turn the tripod so that if you stuck your
  arm through the wedge from underneath, youd be
  pointing North.
• 4. Attach the telescope.

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Cleaning the Optics

• DONT!!
• You should only clean your telescope optics when
  needed but not much more than twice a year,
  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. You can be creative here eyepieces
  can be stored in plastic sandwich bags and the
  telescope can be covered with a shower cap or
  those new food covers.
• 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.
• If you must clean your optics, research what is
  the best method for your scope. Some telescopes
  require that you remove the main lens or mirror
  to clean them. Others can be cleaned in place.
  Never scrub optics as the abrasions will scratch
  your lens and mirror and can remove any special
  coatings.
• Only the outer surface of the optics usually need
  to be cleaned so dont take apart eyepieces!
• Eyeglass lens cleaners and general glass cleaners
  are NOT designed for the delicate optics of your
  scope. Find out what the manufacturer recommends
  but sometimes simpler is better. A very diluted
  mix of water and a mild dish detergent followed
  with a rinse with distilled water and isopropyl
  alcohol (99-97).
• http//www.company7.com/library/clean.html
• http//www.televue.com/engine/page.asp?ID143
• http//www.astro-tom.com/tips_and_advice/cleaning_
  optics.htm

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Collimating your scope

• Over time the alignment of the optical components
  will no longer be perfect and the image will
  appear fuzzy. Making minor tweaks (and sometimes
  major tweaks) to the alignment of the optics in
  relation to each other can drastically improve
  the performance of the telescope.
• Collimating your telescope is not hard but it
  does require some practice. Collimating is
  important for newtonians, dobsonians and
  cassegrains. Refractors rarely need to be
  collimated because of the way the lenses are
  mounted in the tube. Here are several sites that
  have details for different telescopes.
• Collimation procedures can vary slightly between
  the different types of reflectors. Be sure to
  find the right instructions for your telescope.
• http//legault.club.fr/collim.html
• http//www.fpi-protostar.com/collim.htm
• http//www.amateurastronomy.com/collimate.html
• http//ngc1514.com/Celestron/collimate.htm

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Polar Aligning

• As the world turns the stars will drift through
  your field of view. If you have your telescope
  polar aligned, you might just need to push your
  telescope in the RA axis to catch up (assuming
  you are on an equatorial mount). If your
  telescope is powered and polar aligned, then
  objects should stay centered in the field of view
  for quite a while. 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.
• If you are on an alt-az mount and unpowered no
  GOTO then polar aligning is pretty much
  irrelevant for your situation.
• There are several sites that give good
  descriptions of the rough alignment procedure as
  well as the more accurate star-drift method of
  aligning telescopes.
• http//www.celestron.com/polar.htm
• http//www.astrocruise.com/polarnew.htm
• http//www.tucsonastronomy.org/polalign.htm
• http//astrophotography.aa6g.org/Astronomy/article
  s.html
• http//www.darkskyimages.com/gpolar.html
• http//www.minorplanetobserver.com/htms/Drift_Alig
  nment_Made_Simple.htm

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Balancing your scope

• Not often mentioned or covered, is how to balance
  your telescope. A small refractor on an alt-az
  tripod does not need to be balanced however, a
  refractor or reflector (newtonian or SCT) 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. Some fork-mounted scopes might also
  need to be balanced if you piggyback other
  equipment or attach heavy cameras.
• http//www.starizona.com/basics/balance.cfm
• http//www.telescopes-astronomy.com.au/telescopes0
  19.htm

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Star Testing Your Optics

• One way to find out the quality of your optics is
  to do a star test. The resultant image might show
  that you need to collimate your scope. Or it
  could reveal more fundamental problems such as a
  bad figure to your mirror. Interpreting the
  results takes some practice and experience. Dont
  be afraid to ask others for help!
• http//www.skywatchertelescope.net/EducationST.htm
  l
• http//hometown.aol.com/billferris/startest.html
• http//www.astunit.com/tutorials/startest.htm

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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 should only be a few
  turns at most.
• 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!

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Actually Finding Objects

• The telescope is aligned, balanced, collimated
  and focused, what should you look for??
• The Sun (with proper filters!)
• Moon
• Planets
• Asteroids and comets
• Stars, binaries, variables, open and globular
  clusters
• Nebulae
• and Galaxies, oh my!
• Learn to read starcharts and to starhop. Even
  with an automated GOTO scope, you should have a
  rudimentary awareness of where certain
  constellations and stars are in the sky so that
  you can confirm the telescope is heading in the
  right direction.
• Make sure your finder scope is co-aligned with
  your main scope. During the day, point at a
  distant pole top and center in the telescope
  field of view. Then adjust the screws holding the
  finder scope so that the object becomes centered
  in the finder. You should not be moving the scope
  to get the object in the center of the finder!
  Assuming that you dont knock the alignment
  putting the scope back into its case or carrying
  it out to the yard, if you point towards a star
  and get it centered (by moving the scope) in the
  finder, then it should be centered in the main
  telescope.

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Actually Finding Objects, pt 2

• So what can you look for? If you have a GOTO
  telescope, you can let it give you a tour. I
  personally dont find that very satisfying
  because for me, part of the fun is finding my
  target. Another common mistake of newbies, is
  that they look but they dont observe. In other
  words, theyll find an object, glance at it, then
  move on. Oftentimes, they miss the exciting but
  subtle details this way. To help slow yourself
  down when observing, try sketching the object.
  Once youve looked at the common objects like the
  Messiers and planets, check out the webpages for
  the Astronomical League. They have several
  Observing Clubs for which you can earn a
  certificate. Some of the clubs are easier and can
  be completed over a few weeks. But there are some
  that could take you years to finish!
• You might also want to think about observing
  projects like tracking comets and asteroids,
  occultations, variable and binary star
  observations These are just a few of the areas
  where there are already professional-amateur
  collaborations. You the amateur can observe an
  object for a long time and the professional
  combines your data with data from big scopes (on
  which he might only have a few data points).
• http//www.astroleague.org/
• http//www.aavso.org/
• http//www.lunar-occultations.com/iota/iotandx.htm
  
• http//cfa-www.harvard.edu/icq/icq.html
• For a copy of this presentation, email
  ewarner_at_umd.edu