Outdoor activities form an integral part of school education. These include nature study, bird-watching, trekking, mountaineering, and so on. Astronomical observations are a natural extension of such activities. In particular, when students are taken for trips to remote and hilly places, they can be given some idea about the advantages of low levels of pollution (due to dust and light) in being able to watch the night sky. The conditions for night-time observation are very good in some places and it is possible to see, sometimes even without any optical instruments, objects in the sky like the Milky Way, the Andromeda galaxy, and the beehive star cluster in Cancer which are not visible easily from cities.
Introducing astronomy as a regular activity at school level has now become possible in India as a consequence of the availability of good optical instruments and components. Children are always excited to see cosmic objects starting from the planets to constellations, star clusters and galaxies. Our closest neighbour, the moon, offers clear views and the possibility of comparing your sketches with detailed moon maps.
Before putting your eye to a telescope, it is advisable to become familiar with the sky and the motion of stellar objects. An umbrella with diagrams of a few familiar constellations stuck on the inside at appropriate places can be a valuable tool for this purpose. Such an aid can be used also to explain the advantages of an equatorial mount for a telescope as compared to an altaz (altitude–azimuth) mount.
The first familiarization exercises are best carried out without optical aids. The large field of view of the naked eye is an advantage. The movement of the stars across the sky from sunset to sunrise and the annual variation in this pattern can be explained.
Many students these days have a pair of binoculars. The full potential of this instrument can be exploited. Binoculars have a comparatively wide field of view and are easier on the eyes. For viewing objects near the zenith, it is best to lie down on one’s back. It is worth noting that a large number of comets have been detected first by binocular observers working from their backyards. They can easily spot a new object straying in their field of view and track its motion night after night. Binoculars with objective diameter larger than 50 mm are too heavy to be held in hand. Also binoculars with magnification greater than 10 require a stand to get a steady image. Thus 10 × 50 seems to be the highest specification which can be used without a stand.
For those who prefer to keep their glasses on, binoculars with adequate accommodation are necessary. [Accommodation is the distance at which the eye must be placed behind the eyepiece to get a clear image.] Binoculars always give an erect image since they are designed for terrestrial use. Binoculars with individual focusing for each eyepiece are not convenient for bird-watching. However for astronomical observations they do not pose serious problems.
Selecting and Using A Telescope
The choice of a telescope for use in an educational institution is dictated by the following factors:
a) Number of students to be handled in one observation session.
b) Number of volunteers (teachers/students) available for adjusting the instrument from time to time.
c) How much portability is needed for the instrument.
d) Whether photography is intended.
e) Budget available.
On Building A Telescope
It is possible to build a Dob from a kit which includes the primary mirror, diagonal mirror, spider, two or three eyepieces, one Barlow lens and perhaps a solar filter. These components can be assembled using simple carpentry tools, plywood, and a few pieces of Teflon sheet. Such a homemade Dob may not have the sleek appearance of a ready made instrument but you can get valuable experience while making it and can ‘fix’ it if anything goes wrong. Also the total cost works about to about half of a ready made one. Dobsonian kits are available at $230 for a 6’ f/6 kit and $240 for a 8’ f/6 kit (www.e-scopes.com).
The major drawback of a home made Dob is that it needs to be adjusted repeatedly during observations not only because the object moves out of the field of view but also, more often, because new observers, while adjusting the eyepiece, disturb the alignment of the telescope. It is possible to construct a tracking mechanism on a Dob but this is a project involving stepper motors and computer programmes. This takes considerable expertise.
It is possible to mount a reflector telescope on an equatorial mount. However for mirror diameter larger than about 6’ the weight of the telescope makes it an unsatisfactory arrangement which tends to vibrate due to small disturbances. The portability of such a telescope is low. A smaller reflector of about 130 mm diameter mirror on an equatorial mount may be a good starting telescope for a school. This can be augmented later, by a larger telescope after some experience has been gained. A small telescope is always handy for beginners. A refractor with an equatorial mount is also a possible choice if the budget permits it because a refractor is more expensive than a reflector of the same size of objective.
Some models with an equatorial mount come with a drive motor. Often this motor does not make the instrument a self-tracking one. It only enables you to turn the appropriate knob by pressing switches. This is a help to avoid inadvertent disturbances of the alignment while turning knobs.
A basic choice that needs to be made is between a reflector telescope and a refractor telescope.
The position coordinates of an astronomical object in the sky is denoted by two numbers called the right ascension (abbreviated RA) and declination (dec). These are equivalent to the longitude and latitude respectively of a point on the surface of the earth. The RA is always indicated in time units using the equivalence of 360° in RA to a time interval of 24 hours. This is possible because we can imagine all the astronomical objects to be stuck on the inside of a large sphere with ourselves, that is, the earth, at its centre. This imaginary sphere is called the celestial sphere. The distance of all these objects from us is so large that the motion of the earth around the sun and of the sun in our galaxy has no significant effect on our view of the celestial sphere.1
A ‘Go To’ telescope refers to a telescope which is computer controlled and which has, in its memory, the coordinates of a large number (several thousand) of astronomical objects. Initially the location of the telescope (geographical longitude and latitude of the place of observation) and the local time have to be entered in the memory of the telescope. After that when the RA and dec coordinates of any astronomical object are entered on the keyboard, the telescope turns about the appropriate axes and points toward that object. Such an instrument, with the ability to track the given object, would be ideal for an educational institution. However, Go To telescopes are much more expensive than conventional instruments. Another factor to be kept in mind, particularly for educational institutions located far away from big cities (where the telescope dealers have their showrooms) is the possibility of electronic failure of the system. Even within the warranty period, it may take quite some time to get spare parts and repair services.
Building or selecting a refractor telescope also has its own special considerations that need to be kept in mind. Among these is the need to eliminate an optical phenomenon known as ‘chromatic aberration’. The focal length of an ordinary lens is slightly longer for red light than for violet light. This has an effect on the quality of the image which shows slight colours at its edges. This effect is known as chromatic aberration.2
Shifting a telescope from its storage room to the place for observation can be a time consuming and somewhat risky operation. One could keep the telescope on a wheeled platform which can be rolled out of the storage room onto an open space for observations. This arrangement will reduce the time required for shifting the instrument and for the initial setting.
World Wide Telescope (WWT)
Recently the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, in collaboration with Microsoft, have introduced an astronomy software called the World Wide Telescope (WWT) which is designed primarily for science and astronomy educators. This free software enables one to access the astronomical pictures recorded by the Hubble Space Telescope and also other major telescopes around the world. One can scan any part of the sky on one’s computer monitor with the high resolution that these instruments are capable of. Images taken at different wavelengths can be scanned. Even X-ray, gamma-ray and radio emission images (rendered visible) can be seen on the screen. Terra bytes of data are available waiting to be examined and interpreted by anyone who wants to explore this field. In the seminar on WWT held at Pune in September 2010 Prof. S George Djorgovski of the Centre for Advanced Computing Research, California Institute of Technology, USA, spoke about a paper on the optical discovery of an apparent galactic supernova remnant by Robert Fesen and Dan Milsisavljevic in The Journal of Astrophysics. Within weeks of the paper’s publication, someone used the WWT to scan the indicated part of the sky and saw the ring of smoke which looks like the signature of the supernova. This picture was projected on the screen during the seminar.3
Personally I believe that astronomy can trigger an expansion of awareness as no other subject can. To be able to see that a wisp of light waves, a mere disturbance in space, which left its source millions of years ago, was travelling for aeons through interstellar space, past galaxies and star clusters, to enter your eye tonight to give you information about its source, is nothing short of a miracle. A glance at the gigantic interplay of matter and energy going on perpetually in interstellar clouds of gas fills the heart with humility and gratitude—for having received the gift of a body and a mind which enables you to understand, in a small measure, the workings of the universe. It looks like a speck of creation getting enamoured with its own larger image.
- Patrick Moore and Wil Tirion, Cambridge Guide to Stars and Planets, Cambridge University Press, 2000.
- Sam Brown, Homebuilt Reflector Telescopes, Edmund Scientific Series no. 9066, Edmund Scientific Co., 1979.
- How to Use Your Telescope, Popular Optics Library, Edmund Scientific Co., 1975.
- Jack Newton, Philip Teece, Helen Sawyer, The Guide to Amateur Astronomy, Cambridge University Press, 1995.
- JM Pasachoff and ML Kutner, University Astronomy, WB Saunders Co., 1978.
- The position of the brightest star in the sky, Sirius, has the coordinates: RA= 6h 45min 8.9 sec; dec.= –16° 42’ 58’. The negative sign of the declination angle indicates that Sirius is in the southern (celestial) hemisphere. Catalogues containing the RA and dec coordinates of thousands of astronomical objects are available. Two of the best known are the Messier Catalogue and the New General Catalogue (NGC).
- The focal length of an ordinary lens is slightly longer for red light than for violet light. This has an effect on the quality of the image which shows slight colours at its edges. This effect is known as chromatic aberration. The objective of a refracting telescope is a convex lens which is made of two lenses often called an achromatic doublet. This is an arrangement of two lenses, one convex and the other concave, which are either cemented together with transparent glue or mounted on the same axis with a small separation. The combination acts as a converging lens with the desired focal length. The two lenses are made from different kinds of glasses in such a manner that the resultant focal length of the combination is the same for red and for violet rays. Thus chromatic aberration can be eliminated by using an achromatic doublet.
Just as the starting point for making a reflector is to acquire the primary mirror of the desired diameter and focal length, the starting element for a refractor is its achromatic objective. Unfortunately buying such a lens having a diameter greater than about 50 mm is not so easy in India. Some lens makers like Lawrence and Mayo do not seem to make these. If a search is made on the internet, though, one can find suppliers of these items (www.surplushed.com). For example one can get a 80 mm dia, 900 mm FL achromat for about USD 70.00 and a 127 mm dia. 700 mm FL achromat for about USD 140.00. Most of these objectives have an anti reflection coating. Apart from objectives, some of these dealers offer many other components like eyepieces, diagonal mirrors and eyepiece holders which can simplify the construction of a telescope. A suitable mount for a reflecting telescope can be purchased ready made or designed in a carpentry shop.
- Using WWT it is possible to make presentations in the form of ‘tours’ of different features in the sky like the solar system, different constellations, star clusters, variable stars, and so on. WWT is planned to be an evolving system. It was pointed out in the seminar that in the near future it should be possible to juxtapose images of the same object taken at regular intervals to demonstrate, for example, the intensity variation of Algol with time. The period of 2.7 days could then be compressed to 20 or 30 seconds. Such demonstrations, combined with sessions of telescopic observations could generate a lot of interest among students.