Refractors

•‘first’ telescopes

Refractors (Dioptric)

Problems:

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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.

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Special types:

Achromatic: telescope has been color-corrected with the use of multiple lenses and/or coated lensesApochromatic: corrected for both chromatic and spherical aberration

http://www.tjmes.com 铜球阀截止阀蝶阀http://www.u51688.com http://www.qiwhy.comReflectors (Catoptric)

•Use mirrors

Problems:

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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:cassegrainDobsonian:

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

Basic definitions

af

a

fa

secondaryprimaryfaf

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

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 lightand D is objective diameterR 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 aidIn optics, it is also expressed as M = F/fMmax = 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, thenLGP = D2/49 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= 15arcmin24 h1 h4min4min1min

•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!!

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

http://members.aol.com/bemusabord/cleaning.htmlhttp://www.company7.com/library/clean.html

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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=8&page=34•http://zebu.uoregon.edu/~mbartels/kolli/kolli.html•http://ngc1514.com/Celestron/collimate.htm

Polar Aligning

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

http://www.aa6g.org/Astronomy/Articles/drift_align.htmlhttp://www.darkskyimages.com/gpolar.html

http://www.minorplanetobserver.com/htms/Drift_Alignment_Made_Simple.htm

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Balancing your scope

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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.

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http://www.skywatchertelescope.net/EducationST.htmlhttp://hometown.aol.com/billferris/startest.htmlhttp://www.astunit.com/tutorials/startest.htm

Focusing

When my students complain that they can’t see anything in the telescope, I check1.the dust cover, make sure it’s off2.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!

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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!

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