Introduction to Telescope

What is a Telescope? How many types of telescopes are there?  An instrument designed by an arrangement of lenses or mirrors or both that gathers visible light and bring it to a focus, where the radiation can be analysed. The primary purpose of most astronomical telescopes is to provide the brightest images.  There are two types of telescopes: Refracting Telescope: Reflecting Telescope:

Refracting Telescope: A telescope in which light from an object is gathered and focused by lenses, with the resulting image magnified by the eyepiece. A lens bends the light as it passes through the glass and brings it to a focus to form a small inverted image

Design of Refracting Telescope Internal Design: External Look:

Drawbacks of Refracting Telescope Chromatic Aberration: When light is refracted through glass then shorter wavelengths bend more than longer wavelengths and blue light comes more closer than the red light. For example, if we focus the eyepiece on the blue light then red light is out of focus which produces a red blur around the image and vise a versa. Solution: This can be minimized by using achromatic Lens!

Only peripheral Support: The lens can be supported only on its periphery and sags to gravity. The problem is more severe for larger lenses. Design is bulky: Optical path (focal length) can be large and hence pipe size goes on increasing. The maximum refracting telescope can be of 1 m lens diameter. Difficult to use for photography: Since the size of telescope is bulky, chance of image vibrations is higher and photographs tend to get blurred. Cost of the telescope becomes higher: Since this type of telescopes uses only lenses therefore as we go for higher size of the aperture the cost goes on increasing as large lenses are expensive to make.

Reflecting Telescope: A telescope in which light from the object is gathered and focused by a concave mirror, with the resulting image magnified by the eyepiece. A concave mirror reflect the light as it passes through the glass and brings it to a focus to form a small inverted image In both, refracting and reflecting telescopes, image formed will be inverted, but this can be reinverted by using extra lens. But this is an unnecessary expense, and leads to loss of some light!

Design of Reflecting Telescope Internal Design: Light gets reflected from primary concave mirror. After reaching the secondary mirror, light again gets reflected to the eyepiece assembly. Whole optical design can be assembled in a smaller mechanical design. Hence, telescope becomes compact. Depending on how light is taken out of the tube assembly, there are two types of reflecting telescopes which we will see soon!

External Appearance:

Types of reflecting telescope Newtonian Reflector: In the optical design, where light is taken out of the tube by placing a plane mirror near the focus point of the primary mirror. The plane mirror reflects the light by 45 0 angle. The telescope having this type of arrangement is known as Newtonian Reflector Telescope. These types of telescopes are easy to use and easy to make. These types of telescopes are widely used for planetary and moon viewing. Most amateurs build and use this type of telescope.

Types of reflecting telescope Schmidt-Cassegrain Reflector: In these, light is taken out of the tube through a hole made in the primary mirror while a thin correcting lens (called the Schmidt lens) is placed above the secondary mirror. These types of telescopes are easy to use but not easy to make. They are usually made by professional telescope manufacturers These are costly and can be used for astrophotography and photometry.

Parameters of Telescope Aperture/Objective of telescope (D): The size of the first concave mirror/lens from where light gets reflected or refracted is known as aperture or objective size of the telescope.  This defines the “size” of the telescope  Measured in mm, cm, inches and meters  This can either be a lens or a concave mirror Light Gathering Power: This defines how much light does telescope can gather as compared to the human eye!  It depends on the size of aperture  This can be calculated as:- LGP =

Focal length of the objective (f or λ): The distance between refracting lens or reflecting mirror and the point where all incoming light is focused is known as a focal length of the objective or telescope.

f /ratio of the telescope: The f/ratio of a telescope is determined by dividing the focal length of the primary lens or mirror by the aperture size lens or mirror.  The lower the f/ratio the faster the telescope is said to be; that basically means that it provides a brighter image than a similar sized telescope with a higher f/ratio.  Remember that there is always a tradeoff that by getting brighter images you loose some magnifying power.  Telescopes with low f/ratios are good for viewing star clusters and faint nebulae.  Telescopes with high f/ratios aren't quite as bright, but yield higher magnifications with narrower fields of view. They are ideal for planet viewing and splitting binary stars.  The f/ratio can be calculated as: f/ratio = focal length / aperture = f / D

Magnification: There is no fixed magnification for any telescope! But can be changed! To calculate the magnification of any telescope you simply divide the focal length of the objective lens or mirror by the focal length of the eyepiece. Remember to use the same measuring units!!!! Eyepieces are generally specified by their focal length in millimeters while Telescopes are often sold by their focal lengths in centimeters or inches. Therefore don't forget to convert first! Magnification = Focal length of the Objective/Focal length of the Eyepiece

Resolving Power of the telescope: The ability to resolve the two near by objects is known as Resolving Power of the telescope.  The resolving power or angular resolution of a telescope depends on the diameter of the primary mirror or lens and on the wavelength of the light begin used.  If the diameter (denoted D) of telescope is given in m and the wavelength of the light, λ, is given in μ m, the resolving power,α, in seconds of arc can be found from the formula: For example, a Telescope of a diameter of 40 cm = 0.4 m and is operated at the wavelength of λ = 500 nm = 0.5 μ m then the resolving power of the telescope is:

Field of View of Telescope: It is defined as the region of sky that can be seen through the telescope. There are formal methods to calculate the field of view, a much simpler method is to watch a star drift across your view as below: Select a star near the zenith Turn off any tracking motors so that the star drifts across the view Adjust the telescope so that the star drifts directly across Place the star just outside the view and when it first drifts into view start a stopwatch or other timer Stop the timer when the star leaves the view The time may be several seconds to several minutes depending on the size of the telescope and eyepiece used Knowing that the earth spins on its axis once every 24 hours or sees 360degrees of sky/24hours 360deg = 15deg = 1deg = 60arcmin = 15arcmin 24 h 1 h 4min 4min 1min For example, if a telescope takes 150 seconds to drift from the objective then the FOV of a telescope is about 37.5arcmin.

Limiting Magnitude: The faintest star you can see with a telescope (under excellent seeing conditions) is referred to as the limiting magnitude.  A rough formula for calculating visual limiting magnitude is: 7.5+5LOG (aperture in cm). How it is defined? An object is said to have a certain numerical magnitudes. Astronomers use a system of magnitudes to indicate how bright a stellar object is. The larger the magnitude number, the fainter the object is. Each object with an increased number (next larger magnitude number) is approximately 2.5 times fainter. The faintest star you can see with your unaided eye, with no telescope and dark skies, is about sixth magnitude, whereas the brightest stars are magnitude zero, or even a negative number.  For example, the limiting magnitude of a 70mm aperture telescope will be 7.5+5LOG7=11.7  Atmospheric conditions and the visual ability of the observer will often reduce limiting magnitude.

Mounting:  Mounting is one of the important aspect of the telescope making.  Mounting provides a smooth and controlled movement of the telescope to point and guide the object.  It also support the telescope firmly so that you can view and photograph the objects without having the image disturbed by movement.  There are two main types of mountings # Altazimuth Mounting: # Equatorial Mounting: Altazimuth Mounts: An altazimuth mount is a design to hold the Optical Telescope Assembly and allow the user to guide the scope by using two motions.  The up and down motion is called as altitude, and  The back and forth or round motion is called azimuth. Between these two motions you can point a telescope to view any position above the horizon. It's a simple system and easy to use and leads itself well to casual stargazing.

Design and example of Altazimuth Mount

Equatorial Mounts Because the earth spins, the stars and planets moves in heaven above us. Tracking a star while viewing requires a constant repositioning of the telescope to compensate for the motion of the earth. A second type of mount called the equatorial mount was designed to make tracking match the apparent motion of the stars. The basic idea of this mount is a telescope, aligned along an axis that runs parallel to the earth's spin. The two axes in this mount are:  The up and down motion is called as Declination axis, and  The round and round motion is called Right Accession axis. This type of mounting requires polar aligning of telescope.  This type of mounting is available with most professional telescopes.  This type of mounting allows for long exposure photography.

Design and example of Equatorial Mount:

Optical telescopes in India Asia’s largest telescope of 2.34 meter diameter It is located at 175 km away from Bangalore, at Kavalur in TN Vainu Bappu Telescope It is operated by Indian Institute of Astrophysics

Himalayan Chandra Telescope World’s highest telescope(4.5km) located at Hanle in J & K Telescope works in optical and Near Infrared bands It is remotely operated by Indian Institute of Astrophysics Aperture is 2 meter with Alt- azimuth mount

Located at the height of about 1000 m and located at Girawali near Pune Telescope works in optical and Near Infrared bands. It is operated by Inter University Center for Astronomy and Astrophysics Aperture is 2 meter. Operating since IUCAA Telescope

Sampurnanand Telescope Located at the height of about 2000 m and located at Manora Peal in Nainital Telescope works in optical only. It is operated by Arryabhatta Research Institute of Observational Science (ARIES) Aperture is 1.04 meter. Operating since 1972.

Located at the height of about 1680 m and located at Mt Abu (Rajasthan) Telescope works largely in near infrared and optical. It is operated by Physical Research Laboratory, Ahmedabad Aperture is 1.2 meter. Operating since Gurushikhar Telescope

Udaipur Solar Observatory It is Solar observatory and works only for the observations of the SUN. It is located inside the lake (Fatehsagar) at Udaipur (Rajasthan). It is operated by Physical Research Laboratory, Ahmedabad Aperture is from 15cm to 30 cm.

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