Announcements No lab tonight due to Dark Sky Observing Night last night Homework: Chapter 6 # 1, 2,...
-
Upload
ruth-jennings -
Category
Documents
-
view
216 -
download
0
Transcript of Announcements No lab tonight due to Dark Sky Observing Night last night Homework: Chapter 6 # 1, 2,...
Announcements•No lab tonight due to Dark Sky Observing Night last night
•Homework: Chapter 6 # 1, 2, 3, 4, 5 & 6
•First Quarter Observing Night next Wednesday. Set-up starts around 6:45pm
•Due in two weeks: Telescopic Observations of the Moon. If you want to check out a Dobs for the weekend, you can get it tonight or tomorrow before 4:30pm.
Telescopes
Galileo’s early telescope and lens
Newton’s original telescope with mirror
Telescope basics
Telescopes either use refraction or reflection to focus light to a point. For refraction, the basic law is Snell’s Law: n1sinq1 = n2sinq2. The law of reflection is a much simpler: ri
If the surfaces of a piece of glass are curved, they will
focus light to a point
The focal length, f, is the distance from the lens axis to the focal point or focal plane. The focal length of a lens depends on the material of the lens and the curvature of the surfaces
2121
1111
1
RnR
dn
RRn
f
R1 and R2 are the radii of curvature of the two faces, n is the index of refraction of the glass and d is the center thickness of the lens
A concave mirror will also focus light to a point
For a spherical mirror the focal length is just half the radius of curvature of the mirror. For other shapes the formula is somewhat more complicated
2
rf
For any telescope, the most important property is the
Light Gathering Power (LGP)
49
20dLGP
do is the diameter of the objective in mm.
Compares the light gathering
power to that of the human eye
Telescopes are often referred to by their f-ratio
d
ffratiof / f is the focal length of the
objective and d is its diameter
Magnification is determined by the focal lengths
primary
eyepiece
fM
f
When using an eyepiece
When using a CCD camera, the image scale determines the magnification
206.265I
primary
Sf
m is the size of a
single pixel in micrometers
The ability of a telescope to resolve fine detail is given by
the Rayleigh Criterion
d
22.1sin
l is the wavelength of the light being used and d is the diameter of the aperture. qis the smallest resolvable angle of the telescope
AberrationsSpherical aberration can be corrected by using parabolic or hyperbolic surfaces
Coma is worse for parabolic surfaces than spherical ones. Correction is to use hyperbolic surfaces
Astigmatisms are the result of a non-axially symmetrical lens.
All three of these aberrations apply to both lenses and mirrors
The Refracting Telescope
Examples of Refractors
Problems With Refractors:Chromatic Aberration
Achromatic Doublet
Provides some color correction but doesn’t completely remove all chromatic aberration.
Apochromatic Design
The best apochromats use a three element design with low dispersion glass to reduce chromatic aberration to a minimum. Of course, the more glass the light goes through, the greater the loss.
Newtonian Reflector
Still a popular design among amateurs but not widely used by professionals. The focal plane is not large and there are lots of off-axis distortions.
Examples of Newtonian Reflectors
Invented by Isaac Newton in 1668
Cassegrain Reflector
Classic Cassegrain…Parabolic MirrorsRitchey-Chretien (RC)…Hyperbolic Mirrors
Invented by Laurent Cassegrain in 1672
Examples of Cassegrain’s
Gemini North
Keck 1Most professional telescopes use a Cassegrain focus with hyperbolic mirrors: an RC
Schmidt-Cassegrain Design
Primary mirror is spherical instead of parabolic. The “correcting lens” corrects for spherical aberrations.
Examples of Schmidt-Cassegrain telescopes
Maksutov-Cassegrain Design
The correcting lens is a meniscus shape. The Maksutov-Newtonian is also a popular design
Examples of Maksutov Telescopes
Mounts
Alt-Az Mounts
The Dobsonian Mount is an Alt-Az mount
Equatorial Mounts
German Equatorial Mount
Fork Equatorial Mounts
Other Types of Equatorial Mounts
Cross-Axis Equatorial English Yoke Equatorial
Example of a Yoke mount
Hooker Telescope
Eyepieces come in a variety of different optical designs
The magnification of a telescope is just the ratio of the focal length of the objective to the focal length of the eyepiece
eyepiece
objective
f
fmag
Since the light is passing through glass, eyepieces suffer from chromatic aberration
My personal favorites are Tele Vue eyepieces
Field of View depends on the eyepiece
objective
eyepieceeyepiece
eyepiece
objective
eyepieceeyepiece
f
fovf
ff
fov
mag
fovfov
82 ° fov
68 ° fov
60 ° fov