Download Issues with Telescopes

Issues with the use of telescopes
Magnification
Magnification determines how much larger the image is as compared to
the size of the source of the light (the object)
Magnification =
fo
fe
Where
fo is the focal length of the objective
fe is the focal length of the eyepiece
Issues with the use of telescopes
Magnification
From the group exercises, most of the object the average observer would
look at at actually relatively large.
Magnification is not the most important characteristic of a telescope for a
backyard astronomer.
Professional astronomy also requires the examination of deep sky objects,
which require a high magnification.
Issues with the use of telescopes
Magnification
Further problems – if the image produced by the telescope does not
accurately represent the source, and high magnification might only magnify
the inaccuracies.
The crucial issue - RESOLUTION
Issues with the use of telescopes
Resolution
More important (possibly more important) than magnification is resolution.
Resolution – the property of an instrument to identify (resolve) small details.
The smallest angular size identifiable by an instrument is given by
Θmin = .25
Where
λ
D
λ is the wavelength of the EM waves being collected in μm (1 μm = 10-6 m)
D is the diameter of the aperture (the opening which collects the wave) in
meters
The calculated value of Θ will be in seconds of arc (arc seconds)
Issues with the use of telescopes
Resolution
Θmin is called the diffraction limited
resolution of the telescope
Issues with the use of telescopes
Resolution
Θmin (in arc sec) = .25
λ (in μm )
D (in m)
The ability to resolve small details is determined by the wavelength fo the EM
wave and the diameter of the aperture.
For high resolution, one would observe as the shortest wavelength
possible and use the largest diameter aperture possible.
For a telescope, the diameter of the aperture is determined by the
diameter of the objective.
Issues with the use of telescopes
Resolution
Θmin (in arc sec) = .25
λ (in μm )
D (in m)
For the naked eye,
Shortest visible wavelength ≈400 x 10-9 m = .4 μm
Diameter of the aperture (the pupil) ≈ 3 mm = 3 x 10-3 m
Θmin ≈ 33” = .55’ = .0093o
The average human eye can resolve object with an angular diameter of about
a half a minute.
Compare with your estimate from Group Exercise 2
Issues with the use of telescopes
Resolution
Θmin (in arc sec) = .25
λ (in μm )
D (in m)
For the Mount Palomar 200 inch optical telescope,
Shortest visible wavelength ≈400 x 10-9 m = .4 μm
Diameter of the aperture (the objective) = 200 in = 5.08 m
Θmin ≈ 1.96 x 10-2 “ = 3.2 x 10-5 ‘ = 5.5 x 10-7 degrees
The Mount Palomar telescope can resolve objects about 1700 times
smaller than the naked eye
Issues with the use of telescopes
Resolution – The Hubble Space Telescope
Hubble works on the same principle as the first reflecting telescope built in the 1600s by Isaac
Newton. Light enters the telescope and strikes a concave primary mirror, which acts like a lens
to focus the light. The bigger the mirror, the better the image.
In Hubble, light from the primary mirror is reflected to a smaller secondary mirror in front of the
primary mirror, then back through a hole in the primary to instruments clustered behind the focal
plane (where the image is in focus).
Mirror size
Primary mirror: 2.4 m – (94.5 inches) in diameter
Secondary mirror: 0.3 m - (12 inches) in diameter
Angular resolution
Hubble's angular resolution is 0.05 arcsecond. This is the "sharpness" of Hubble's vision. If
you could see as well as Hubble, you could stand in New York City and distinguish two
fireflies, 1 m (3.3 feet) apart, in San Francisco.
Issues with the use of telescopes
Resolution
Θmin (in arc sec) = .25
λ (in μm )
D (in m)
If the Mount Palomar 200 inch optical telescope recorded radio waves of
wavelength 1 meter,
wavelength ≈ 1 m = 1 x 106 μm
Diameter of the aperture (the objective) = 200 in = 5.08 m
Θmin ≈ 4.9 x 104 “ = 820’ = 13.7o
The angular diameter of the moon = 30’
The angular diameter of the Andromeda Galaxy ≈ 178’
The Mount Palomar telescope would not be able to resolve these objects
It would not be able to “see” the moon !
Issues with the use of telescopes
Resolution
Θmin (in arc sec) = .25
λ (in μm )
D (in m)
For the National Radio Astronomical Observatory Robert C. Byrd Radio
Telescope,
wavelength ≈ 1 m = 1 x 106 μm
Diameter of the aperture (the objective) = 100 m
Θmin ≈ 2500” = 41’ = .69o
The angular diameter of the moon = 30’
The angular diameter of the Andromeda Galaxy ≈ 178’
The NRAO telescope would be able (roughly) to resolve radio sources of
these angular diameters
This is the worlds largest fully steerable radio telescope
Issues with the use of telescopes
Resolution
Θmin (in arc sec) = .25
λ (in μm )
D (in m)
For the Arecibo Radio telescope,
wavelength ≈ 1 m 1 x 106 μm
Diameter of the aperture (the objective) = 305 m
Θmin ≈ 819” = 13.7’ = .22o
The angular diameter of the moon = 30’
The angular diameter of the Andromeda Galaxy ≈ 178’
The Arecibo telescope would easily be able to resolve radio sources of
these angular diameters
However, the Arecibo telescope can only “see” object directly above it
Issues with the use of telescopes
Resolution
Naked Eye
Mount Palomar
Hubble
Palomar (using
radio)
NRAO Radio
Telescope
Arecibo
400 nm = .4µm
400 nm = .4µm
400 nm = .4µm
1 m = 1 x 106
µm
1 m = 1 x 106
µm
1 m = 1 x 106
µm
3 mm = 0.003
m
200 inches =
5.08 m
94.5 inches =
2.4 m
200 inches =
5.08 m
100 m
305 m
Θmin = 33” = .55’
= .0093o
Θmin = 1.6 x 10-2
“ = 3.2 x 10-5 ‘ =
5.5 x 10-7
degrees
Θmin = 5 x 10-2 “
= 10 x 10-5 ‘ =
17.2 x 10-7
degrees
Θmin = 4.9 x 104
“ = 820 ‘ = 13.7
degrees
Θmin = 2500 “ =
41‘ = 0.69
degrees
Θmin = 819 “ =
13.7‘ = 0.22
degrees
Moon: 30’ = .5 degrees
Sea of Tranquility: 3.3’ = 0.056 degrees
Andromeda Galaxy: 178 ‘ = 2.9 degrees
Issues with the use of telescopes
Resolution
Naked Eye
Mount Palomar
Hubble
Palomar (using
radio)
NRAO Radio
Telescope
Arecibo
400 nm = .4µm
400 nm = .4µm
400 nm = .4µm
1 m = 1 x 106
µm
1 m = 1 x 106
µm
1 m = 1 x 106
µm
3 mm = 0.003
m
200 inches =
5.08 m
94.5 inches =
2.4 m
200 inches =
5.08 m
100 m
305 m
Θmin = 33” =
.55’ = .0093o
Θmin = 1.6 x 102 “ = 3.2 x 10-5 ‘
= 5.5 x 10-7
degrees
Θmin = 5 x 10-2
“ = 10 x 10-5 ‘ =
17.2 x 10-7
degrees
Θmin = 4.9 x 104
“ = 820 ‘ = 13.7
degrees
Θmin = 2500 “ =
41‘ = 0.69
degrees
Θmin = 819 “ =
13.7‘ = 0.22
degrees
Moon: 30’ = .5 degrees
Sea of Tranquility: 3.3’ = 0.056 degrees
Andromeda Galaxy: 178 ‘ = 2.9 degrees
Issues with the use of telescopes
Resolution
Naked Eye
Mount Palomar
Hubble
Palomar (using
radio)
NRAO Radio
Telescope
Arecibo
400 nm = .4µm
400 nm = .4µm
400 nm = .4µm
1 m = 1 x 106
µm
1 m = 1 x 106
µm
1 m = 1 x 106
µm
3 mm = 0.003
m
200 inches =
5.08 m
94.5 inches =
2.4 m
200 inches =
5.08 m
100 m
305 m
Θmin = 33” =
.55’ = .0093o
Θmin = 1.6 x 102 “ = 3.2 x 10-5 ‘
= 5.5 x 10-7
degrees
Θmin = 5 x 10-2
“ = 10 x 10-5 ‘ =
17.2 x 10-7
degrees
Θmin = 4.9 x 104
“ = 820 ‘ = 13.7
degrees
Θmin = 2500 “ =
41‘ = 0.69
degrees
Θmin = 819 “ =
13.7‘ = 0.22
degrees
Moon: 30’ = .5 degrees
Sea of Tranquility: 3.3’ = 0.056 degrees
Andromeda Galaxy: 178 ‘ = 2.9 degrees
Issues with the use of telescopes
Resolution
Naked Eye
Mount Palomar
Hubble
Palomar (using
radio)
NRAO Radio
Telescope
Arecibo
400 nm = .4µm
400 nm = .4µm
400 nm = .4µm
1 m = 1 x 106
µm
1 m = 1 x 106
µm
1 m = 1 x 106
µm
3 mm = 0.003
m
200 inches =
5.08 m
94.5 inches =
2.4 m
200 inches =
5.08 m
100 m
305 m
Θmin = 33” =
.55’ = .0093o
Θmin = 1.6 x 102 “ = 3.2 x 10-5 ‘
= 5.5 x 10-7
degrees
Θmin = 5 x 10-2
“ = 10 x 10-5 ‘ =
17.2 x 10-7
degrees
Θmin = 4.9 x 104
“ = 820 ‘ = 13.7
degrees
Θmin = 2500 “ =
41‘ = 0.69
degrees
Θmin = 819 “ =
13.7‘ = 0.22
degrees
Moon: 30’ = .5 degrees
Sea of Tranquility: 3.3’ = 0.056 degrees
Andromeda Galaxy: 178 ‘ = 2.9 degrees
Issues with the use of telescopes
Refractors
Advantages:
No path obstructions – Good image
Disadvantage:
*
*
*
*
*
VERY EXPENSIVE to make large lenses
Physical size of long focal length objectives
Absorption of light by the lenses
Grinding two precision surfaces
Chromatic and spherical aberrations
Issues with the use of telescopes
Reflectors
Advantages:
* Compound telescopes with long focal length
objectives can be relatively compact
* Single ground surface
* “Relatively” cheap to make a large diameter
objective
* Does not suffer from Chromatic Aberrations
Disadvantage:
* MUST have an obstruction in the path
* Spherical Aberrations
Issues with the use of telescopes
Wide Field Telescopes
Designed in 1930 by Bernhard Schmidt, the Schmidt Camera
was the forerunner of the modern Schmidt-Cassegrain
telescope. Notwithstanding all of the advances in optical and
electromechanical technology over the intervening years,
however, the classical Schmidt Camera to this day
accomplishes feats of astrophotography that are simply
unattainable with any other telescopic lens, telescope, or
electronic imager. No other photo-optical instrument permits
such extremely wide-field photography at such fast photographic
speed and with such an amazingly flat imaging area to the field
edge. Unlike the Schmidt-Cassegrain telescope, however, the
Schmidt Camera is strictly a photographic instrument and can
not be used visually.
Issues with the use of telescopes
Chromatic Aberrations
Associated with lenses.
The Problem: A lens acts like a prism.
The lens will break visible light into a rainbow, resulting in a different
focal length for each wavelength in the incoming wave.
Issues with the use of telescopes
Chromatic Aberrations
Good Lens
Issues with the use of telescopes
Chromatic Aberrations
The fix:
Use a second lens to refract the wave in the exact opposite direction as
the refraction by the objective. A carefully designed second lens will
correct the problem.
Computer aided correction for digital images.
Issues with the use of telescopes
Chromatic Aberrations
Good Lens
Better Lens
Issues with the use of telescopes
Spherical Aberrations
Associated with BOTH lenses and mirrors.
The Problem: Light hitting different parts of the lens or mirror will
refract or reflect differently than other at other locations.
The lens or mirror will have different focal lengths, depending upon
where the incoming wave strike the objective.
Issues with the use of telescopes
Spherical Aberrations
The fix:
Grind the lens or mirror with more precision to eliminate the multiple
focal lengths
Use a second lens to refract the wave in the exact opposite direction as
the refraction by the objective. A carefully designed second lens will
correct the problem.
Computer aided correction for digital images.
Issues with the use of telescopes
Spherical Aberrations - HST
When the Hubble Space Telescope first send back images, the results were
highly disappointing. It was determined that the telescope suffered from
spherical aberrations. The belief is that the mirror was ground, polished,
and tested here on earth, but that the constructors did not take adequate
provisions for the change is shape of the objective mirror in the lower
gravity environment of earth orbit.
Issues with the use of telescopes
Spherical Aberrations - HST
Manipulations of the digital images were carried out with computers, fixing
the problems with spherical aberrations
Issues with the use of telescopes
Atmospheric Effects
The atmosphere is, in of itself, a lens. As a result, it refracts EM waves.
The refraction becomes complicated by motions or turbulence in the
atmosphere, which causes VERY complicated refraction characteristics.
A ground based telescope will simply magnify these effects, causing
distorted or blurred images.
Issues with the use of telescopes
Atmospheric Effects
This variation in refraction as a result of atmospheric motions is what
causes starts to twinkle.
The fix: GET THEE INTO SPACE
OR
Issues with the use of telescopes
Adaptive Optics
What is Adaptive Optics?
Rays of light from a distant star or galaxy are blurred as they pass through
the turbulent air of the earth's atmosphere, preventing a telescope on the
surface of the earth from forming sharp images. Instruments using a new
method called adaptive optics, can eliminate the blurring effect of the
atmosphere. This images formed with the 100-inch telescope plus adaptive
optics are as sharp as the images from NASA's Hubble Space Telescope.
This is the most revolutionary development in astronomy since Galileo
invented the telescope in 1609.
From the Mount Wilson Observatiory
Issues with the use of telescopes
Adaptive Optics
How Adaptive Optics Works
Adaptive optics measures the atmospheric distortions in the incoming light
from a star or other object and sends electronic signals to a deformable
mirror that can change its shape rapidly to correct for the distortions. In the
system built for the 100-inch telescope, the light reflected from the telescope
mirror is divided into several hundred smaller beams or areas. Looking at the
beam of light from a star, the system sees hundreds of separate beams,
some of which have been deviated because of atmospheric turbulence. The
electronic circuits in the system compute the altered shape of a mirror
surface that would realign the separate beams so that they are all going in
the same direction. Then a signal is sent to the deformable mirror to change
its shape in accordance with these electronic signals, resulting in an
undistorted beam.
Issues with the use of telescopes
Adaptive Optics
Kinda sorta like this
Issues with the use of telescopes
Adaptive Optics
Kinda sorta like this
Issues with the use of telescopes
Adaptive Optics
Neptune
With AO
Without AO
Issues with the use of telescopes
Adaptive Optics
Upper Left:
Io image taken with Keck
adaptive optics; K-band,
2.2micron.
Upper Right:
Io image based on visible light
taken with Galileo spacecraft
orbiter.
Lower Left:
Io image taken with Keck
adaptive optics; L-band,
3.5micron.
Lower Right:
Io image taken without Keck
adaptive optics.
Issues with the use of telescopes
Rotation of the Earth
Problem: When you are looking at an object, the telescope will
rotate with the earth. This will cause the object to move out of the
field of view.
For high magnifications, this can happen amazingly fast.
Correction: Motor Drives and computer controlled motor drive.
Can be made to fix on a point on the object, and move the
telescope to keep the position of the object in the eyepeice fixed.
GPS Guided motor drives
Issues with the use of telescopes
Rotation of the Earth
Issues with the use of telescopes
Rotation of the Earth
Issues with the use of telescopes
Rotation of the Earth