An Introduction to Optical Telescopes

Telescopes are our eyes over the cosmos! These instruments help us to observe some of

the most brilliant and beautiful phenomena going on in several regions of the Universe,

although the telescope is a term used for instruments that are some amazing combinations

of glass lenses or mirrors and more devices as per requirements so that distant objects can

be observed closely.

Tracing back to the early 17th Century, they were invented in The Netherlands for the first

time. Though their exact inventor is not known to date, it is speculated that they were the

creation of a spectacle maker (yes they are way more talented than you think haha ;) Hans

Lippershey. 

The name Galileo often strikes our mind as soon as we hear about telescopes (probably that's how our

 minds have been wired after hearing facts). He was one of the first to have observed Astronomical

 Phenomena via these instruments and is known to

have noted several of his prime observations, most famously the 4 Jupiter Moons:

Ganymede, Io, Calisto, and Europa, craters on Moon and the pulchritudinous rings around Saturn

(she looks amazing with those rings ;)

Something you would like to show when asked 'What are the greatest things Humans have made so far'.

From your left, the legendary Hubble Space Telescope.

Carrying the legacy of Hubble into the future and the past , The James Webb Telescope.

Telescopes are a perfect demonstration of the beauty of optics and engineering. They have

evolved, but the paradigm on which they were built remains the same. With the

advancement in technology, these eyes have transformed and have become capable of

seeing far in the universe, probing the unknown regions of the universe. From manually

operated to self-operated, telescopes have now become capable of working on their own.

With the ability to collect, store and transmit data, their functionality has increased i.e. they

no longer are just an arrangement of lenses.

Optical Telescopes:

Of the simplest types, these telescopes gather light from the visible range of the

electromagnetic spectrum (4000 angstrom- 7000 angstrom). These telescopes also enhance

apparent brightness. They are mainly of two types: Refractive and Reflective:

Refractive Telescopes:

The first telescopes were Refractive Telescopes. They were based on the laws of

refraction of light via glass. Their structure (generally) is a combination of Objective and

Eyepiece. An Objective is a term used in optical engineering for any device which gathers

light from an object being observed and focuses the light rays to produce a real image. The

eyepiece is the lens you use to look through a telescope. It is placed near the focal point of

the former to augment the image. 

Galilean Telescopes, Keplerian, Achromatic, and Apochromatic telescopes are types of

reflective telescopes.

Galilean Telescopes used a Plano-convex lens as the objective lens and a Plano-concave

for the eyepiece. The diagram clearly shows it.

A Galilean Telescope Ray Diagram

Keplerian Telescopes were an improvement over Galilean telescopes. As you can see, they

use a convex lens instead of the Plano-concave lens of Galilean telescopes, allowing greater

eye relief.

A Keplerian Telescope

To understand the next two telescopes, let me first introduce the aberration concept.

Spherical aberration is the property of spherical lenses in which they are unable to

focus all light rays at a single point. The light rays passing away from the centre tend to

refract more or less than the general trend of refraction expected. This causes different light

rays to meet at different points and leads to the production of a blurred image. This happens

in optical systems with a spherical lens. To eliminate them, a combination of the lens should

be used.

Spherical Aberration

(look towards the center and then towards the corners, notice the blur)

A similar type of property is chromatic aberration. This can be understood as the

consequence of Snell’s law, i.e. refractive index is different for lights of different

wavelengths.

Due to this, white light, which is composed of different wavelengths doesn’t refract and focus

at a point as expected. To remove this, specially designed lenses called achromatic lenses

are used.

Chromatic Aberration

(notice how you see purple color. This defect may be corrected during image processing)

As the name suggests, Achromatic telescopes use achromatic lenses to overcome

chromatic aberration. They also use a combination of lenses, which helps them avoid

spherical aberration. This was a breakthrough in the development of refractive telescopes.

Apo-chromatic Telescopes use extremely low dispersion glasses to eliminate the first cause

behind chromatic aberration. Some even use traces of CaF2 (fluorite) in an objective to

obtain a very crisp image.

Reflective Telescopes

Another class of telescopes ( which are infact a gift by Newton, he turned all stones trust me), are

Reflective Telescopes (also called catoptric telescopes). They are based on the laws of reflection of

 light from the mirrors as the name suggests. They use curved mirrors to form an image, unlike their

 refractive counterparts. They were not made because Newton wanted to invent another thing, but

because the refractive telescopes at that time suffered from severe chromatic aberration and

the scopes didn’t pave way for large diameter objectives. (Remember larger the objective more light it 

is able to collect and better becomes the image). Thanks to the achromatic lens.

Almost all telescopes used in Astronomy research are reflective.

Talking about their design, their basic optical element is a curved Primary Mirror. This

primary mirror is generally a solid cylinder made of glass that is rounded to a spherical

shape or a parabolic shape. Aluminium is now carefully, atom by atom deposited on the

glass to obtain a highly reflective surface.

These telescopes don’t produce the perfect images; you have to compromise some things

when it comes to imaging objects at infinity. Also, as the primary curved mirror forms the

image in front of its reflecting surface (at focus or in focus plane), a secondary mirror, film or

detector has to be kept to obtain the image formed. This obstructs the amount of light

received by the primary mirror. Sometimes, diffraction also plays a game in the case of very

small detectors. This can be seen in form of spikes produced in the image: these spikes are

commonly known as diffraction spikes. Images from Hubble show 4 spikes (and 4 smaller

fainter spikes sometimes), whereas images from James Webb show 6 spikes and 2 small

spikes. By counting the number of spikes, we can figure out which telescope the image was

likely captured by.

On your left: Image clicked by Hubble Space Telescope

On your right: Same Image clicked by James Webb Telescope

Spherical Aberration is also observed in the case of spherical mirrors; the light rays reflected

from the corners of mirrors don’t seem to converge with light rays that are reflected from the

centre of the mirror. To eliminate this, mirrors in the shape of the parabola are used (yes

conic sections are everywhere don’t try to run away from them).

Gregorian, Newtonian, and Cassegrain are a few types of reflective telescopes.

Gregorian Telescope uses a concave mirror as a secondary mirror which reflects the light

into a hole in the primary mirror.

A Gregorian Telescope

Newtonian design uses a parabolic primary mirror and a flat secondary mirror which

reflects the image out to be viewed easily.

A Newtonian Telescope

Cassegrain Telescope seems to be obsessed with conic sections. It uses a parabolic

primary mirror and a hyperbolic secondary mirror.

A Cassegrain Telescope

Besides these three designs, several other designs have been incorporated where the

secondary mirror has been adjusted to avoid obstructions. These designs are collectively

known as Off-Axis Designs.

I wish this blog article apprised you with the optics of telescopes and optical telescopes!

Article by-

Ojjas (School Student, Astrophile)

Published by - Kirti Tomar (Amateur Astronomer)