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
https://www.doczj.com/doc/554193068.html, 铜球阀截止阀蝶阀https://www.doczj.com/doc/554193068.html, https://www.doczj.com/doc/554193068.html,
Reflectors (Catoptric)
?Use mirrors Array
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:
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 f
a
f =focal length –the distance it takes for light to come to a focus after refractin
g throug
h 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 ×(λ) / (D)
where λis wavelength of light
and D is objective diameter
R will be in arcseconds
If you substitute 550nm for λ, 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 = 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= 15arcmin
24 h 1 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!!
?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.
?https://www.doczj.com/doc/554193068.html,/telescope-optics.html
?https://www.doczj.com/doc/554193068.html,/faq/aa01faq8.htm
?https://www.doczj.com/doc/554193068.html,/bemusabord/cleaning.html
?https://www.doczj.com/doc/554193068.html,/library/clean.html
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.
?https://www.doczj.com/doc/554193068.html,/telescope-collimation.html
?https://www.doczj.com/doc/554193068.html,/main.asp?section=8&page=34?https://www.doczj.com/doc/554193068.html,/~mbartels/kolli/kolli.html
?https://www.doczj.com/doc/554193068.html,/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.
?https://www.doczj.com/doc/554193068.html,/polar.htm
?https://www.doczj.com/doc/554193068.html,/polarnew.htm
?https://www.doczj.com/doc/554193068.html,/polalign.htm
?https://www.doczj.com/doc/554193068.html,/Astronomy/Articles/drift_align.html
?https://www.doczj.com/doc/554193068.html,/gpolar.html
?https://www.doczj.com/doc/554193068.html,/htms/Drift_Alignment_Made_Simple.htm
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.
?https://www.doczj.com/doc/554193068.html,/basics/balance.cfm
?https://www.doczj.com/doc/554193068.html,.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.
?https://www.doczj.com/doc/554193068.html,/EducationST.html
?https://www.doczj.com/doc/554193068.html,/billferris/startest.html
?https://www.doczj.com/doc/554193068.html,/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!