6. 2. Telescope Aberration
Refracting Telescope
• Chromatic aberration. A
lens bends blue light waves
the most and bring them to
a focus closer to the lens
than red light waves.
Reflecting Telescope
• Spherical Aberration. An
improperly curved mirror
does not reflect light waves
to a single focus.
7. Telescope Aberration Solution
Refracting Telescope
• Achromatic lens (double lens)
instead of a single lens.
Reflecting Telescope
• Parabolic Mirror instead of
Spherical Mirror.
8. Biconvex lens
f=focal length
r=radius of lens
a=radius of curvature
t=thickness of lens
n=refractive index of
lens
9. lensmaker’s equation
This is the ‘lensMaker’s equation’ for a thin biconvex lens whose
surfaces have the same radii of curvature.
10. Consider a biconvex lens made of crown glass, taking radius of curvature:
(a=1000 mm), which gives a focal length of and astronomical refracting
telescope:
Blue
486.1 nm
Green-Yellow
589.3 nm
Red
656.3 nm
Crown 1.524 1.517 1.515
Flint 1.639 1.627 1.622
white light
11. Achromatic Doublet
f=focal length
r=radius of lens
a=radius of curvature
t=thickness of lens
n=refractive index of
lens
12. The calculation shows that focal length is now depends on the term .
We can choose a value of a=393.6 mm, to give the same focal length for green-yellow
light as for the biconvex lens.
Color Wavelength
Blue 0.409 962.3 mm
Green-Yellow 0.407 967.0 mm
Red 0.408 964.6 mm
14. Geometry of parabola
The distance from any point
perpendicular to the optical axis to
the point on the surface below.
The distance from the point on the
mirror surface to the focus.
15.
16. 3. Use of Telescope
We can see fainter objects with telescopes
• The lens of your eye size is typically 2.5-3 mm in day light and 5-7
mm under dark conditions.
• A telescope with aperture 150 mm will collect square of 150/7
times more light than human eye.
• This enables a person to see 460 times fainter star than human eye
alone.
• This can be converted into a magnitude difference:
• Assuming that our eye can see a star of 6.5 magnitude, then with a
150 mm telescope we could be able to see a star of 6.5+6.65=13.15
magnitude. This is called the limiting magnitude for that telescope.
17. Image of Andromeda Galaxy
Image of Andromeda Galaxy
twice the diameter of
telescope.
18. 4. Angular Resolution
To see more detail in an image
• There is always a fundamental limit to the detail in the image
produced by a telescope which is caused by effect of
diffraction.
• The image formed by a source is a central disc surrounded by
a number of concentric rings rapidly decreasing in brightness.
• The angular size of this pattern , ,
(Airy disc)is the function of both the
wavelength, , and diameter of the
telescope objective, D:
19. If one consider a 150 mm telescope observing in green light of m,
wavelength. One gets the size of an Airy disc of:
Larger aperture telescope will theoretically gives higher resolution.
22. Cassegrain Telescope
• Majority of professional telescope are of this
design including the Hubble Space telescope.
• Secondary mirror is hyperboloid which reflects
light down through a central hole through the
primary mirror to focal plane.
• Heavy equipments such as spectrometer can
be placed.
23. Catadioptric Telescope
• Combination of mirror and lens to produce
image.
• Spherical mirrors are used to produce the
image.
• To avoid spherical aberration a lens is used
normally called a “corrector lens”.
24. Schmidt Camera
• It was invented in 1930 by Bernhard Schmidt.
• Spherical mirror was used as primary, and to
avoid spherical aberration a corrector plate
was placed at the radius of curvature.
• Ideal to photograph large star fields in the
Milky Way, showing 10,000 stars on one
negative.
• Highly valuable sky surveys have been done
using such cameras.
27. ISPA Telescope
• Refracting Telescope
• Main lens: 225 mm
• Eye Piece: 60 mm to 2.5 mm
• How fainter can we see
• We can see a star of magnitude 7.5+6.5= ‘14’ magnitude
• Resolution: 0.61 Arc second.
