Dr. E M MohammedReader in Physics Maharaja’s CollegeErnakulamKeralaIndia - 682011

Objectives

Reflective and Refractive Telescopes

Telescope Optics

Optical Aberrations

Telescope Configurations

Telescope Mount

Galileo's telescope

•The earliest known telescope.

Giovanpattista della Porta included thissketch in a letter in August 1609

Three Main FunctionsLight Collectors : Simple telescopes focus light to a smaller areaResolve light so that finer details can be seenMagnification Making objects look bigger (closer)

As we are observing at very large distances, light can be assumed to come from infinity Represented as parallel rays

Reflective and Refractive telescopes

Basic Refractor Type

Basic Reflector Type

Real Image formed here

For research telescopes or for astrophotography we will place the analyzer (e.g. photometer, spectrometer) at the location of the focal plane

For direct visualization we need an eyepiece to properly collect the light through the objective lens or primary mirrorinto the eye

Focal point of objective and eyepiece coincide

Optics Review

Lens formula: 1/i +1/o =1/f

Telescope Optics

One of the important characteristic of a telescope

is its Focal ratio (or f / number)

Telescope’s aperture diameter = 5 cm

Focal length of primary = 25 cm

Focal ratio = 25/5 = 5 ( a f/5 telescope)

Fast Telescope (or fast scope < ~6)

Small focal ratio (or small f/ number)

So short telescope for fixed aperture

Wide Field of view (with same eyepiece)

Excel at low power (low magnification) views of deep sky objects, e.g. galaxies, nebula, or open clusters

Slow Telescope ( or Slow scope, >~8)Large focal number

Narrow field of view (with same eye piece)

Good for high power (magnification), small field

observing, e.g. planets, double stars

Most large research telescopes are slow

e.g. Hubble Space Telescope (f/24)

Negative sign in formula dropped

The size of image of objective lens as seen by the eyepiece is called the Exit Pupil

All light passing through the objective must also pass through the Exit Pupil

Exit Pupil

Eye Relief : distance between eyepiece and exit pupil

Usually fe>> fo and we have

Practical Information:

Diameter of Human pupil ~ 7 mm

Comfortable eye relief ~ 6 – 10 mm

Let D = diameter of the Primary mirror or objective lens

And d = diameter of the exit pupil

Any deviation from perfection of an image not due to diffraction are known as aberrations

There are six primary aberrations:Spherical, Coma, Astigmatism, Distortion, Field curvature and Chromatic aberration

All except the Chromatic aberration affect both refractive and reflective telescopes. Chromatic affects only refractive telescopes

Optical Aberration

Spherical aberration depends on shape factor (R2+R1/R2-R1)Aplantic lens is free from spherical aberration and coma

Spherical Aberration

Usually seen in thick double convex lenses

Differential transverse magnification for different distances of image away from optical axis

Explain why there is a practical limitation in the magnification achievable from a simple magnifier

Ortho-scopic lens is free from distortion

Distortion

The focal plane is not a plane, but a curved surface Flat detectors e.g. CCD will not be in focus over its entire regionPlano objectives eliminates this curvature effect

Field Curvature

Where n1, n2 are the index of refraction of the two media

Snell’s law

Chromatic Aberration

Chromatic Aberration

Good

Misaligned Astigmatic

Spherical Aberration Poor Seeing

Hetero-chromatic (affect multiple wavelength light, on and off axis

Chromatic

Mono-chromatic, off axis onlyField curvature

Mono-chromatic, off axis onlyDistortion

Mono-chromatic, off axis onlyAstigmatism

Mono-chromatic, off axis onlyComa

Mono-chromatic (affects single wavelength light), on and off axis only

Spherical

Summery of Aberrations

Different types of TelescopesNewtonian

The first working reflector (1668, 1 inch diameter)Eye piece moved to the side of the telescope

Basic configuration for most large research telescopes

(eg. Hubble, Keck 10 m, VLT 8.2 m)

Secondary mirror produces narrow cone of light

Can have large focal length compared to the physical length of telescope (telephoto advantage)

Small field of sharp focus (few arc minutes)

Cassegrain

Variation of the Cassegrain

Removes coma aberration

Good quality images over a larger field of view (10 – 20 arc minutes)

Ritchey-Chretien

Wide Field of view (6 – 10 degree) good for sky surveying work

Polomar, Siding Spring (1.2 m) are these types

Telephoto is the disadvantage (long telescope length versus focal length)

Combined Reflector and Refractor Schmidt Camera

Compact design: compromise of large field of view and long focal length

Popular design for small telescopes, especially for astro-photography

Schmidt-Cassegrain

36 mirror segments (1.8 m) equivalent of a single 10 m mirror

New developments of optical telescope

Two axes: polar axis (parallel to Earth’s rotation axis) and declination axis (perpendicular to polar axis) Star tracking can be done with only one constant speed motor rotating in opposite direction of Earth’s rotation along polar axis.

Expensive to build and asymmetry gravity effects

Equatorial Mounting

Used by all big (>4m) telescopes recently built. Symmetric gravity effects, cheaper to build

To track objects in the sky need two axes rotating at different speed.

Rotation of image during tracking need to be overcome (by rotatingcameras) the Dead-zone near zenith.

Alt-Azimuth Mounting

Eight Inch refracting telescope

Thank You