PRACTICAL ASTRONOMY Dr Adrian Jannetta BSc(Hons) MSc PhD PGCE MIMA CMath FRAS Berwick Educational Association Practical Astronomy 1 / 33 Adrian Jannetta
Objectives The three main tasks of any telescope. Telescope types Telescope mounts Eyepieces, focal ratio and magnification. Filters and other accessories. Considerations when buying... Binoculars for astronomy Practical Astronomy 2 / 33 Adrian Jannetta
Light collection One of the primary functions of a telescope is to collect light from faint sources. Larger apertures collect more light. This means fainter stars can be seen with bigger telescopes! Practical Astronomy 3 / 33 Adrian Jannetta
Magnification Telescopes are designed to magnify the apparent size of distant objects. The magnification of a telescope can be varied by changing the eyepiece used. Practical Astronomy 4 / 33 Adrian Jannetta
Resolution For small objects such as planets, or small craters on the moon then the resolving power of a telescope is just as important as the magnification. Larger aperture telescopes are able to resolve finer detail on astronomical objects. Practical Astronomy 5 / 33 Adrian Jannetta
Refracting telescope Objective Lens Eyepiece Refractors use an objective lens to collect and focus light. Single lenses suffer severe optical distortions and so astronomical refractors employ two or more lenses to improve the quality of the image. Glass lenses are expensive especially for larger apertures. Large lenses are also heavy and difficult to support so most refractors available to amateur astronomers have lens diameters of between 2 inches (50mm) and 5 inches (130mm). Practical Astronomy 6 / 33 Adrian Jannetta
Refractor with a diagonal Refractors often come with an accessory called a diagonal to make viewing more comfortable. Eyepiece Objective Lens Mirror Diagonal The light path is folded through 90 to make viewing objects more comfortable. The additional mirror also makes terrestrial use possible; objects appear the right way up with a diagonal in place. Practical Astronomy 7 / 33 Adrian Jannetta
Chromatic aberration B G R White light is a mixture of colours. Lenses bend the colours by different amounts, bringing them to focus at different places. This chromatic (colour) aberration can create colourful fringes around bright objects. It is usually minimised by constructing the objective lens from two or more components. Practical Astronomy 8 / 33 Adrian Jannetta
Achromatic and Apochromatic G R B An achromatic doublet focuses blue and red light to the same place. This is accomplished by using component glass with different refractive properties. R G B An apochromatic triplet focuses red, green and blue colours to the same place. Three different glass lenses with varying refractive properties achieve this.a telescope with this lens system is referred to as an apo. Practical Astronomy 9 / 33 Adrian Jannetta
Reflecting telescope Eyepiece Primary Mirror Secondary Mirror Reflectors use a primary mirror to gather and focus light. The secondary mirror directs the light out of the side of the tube near the top. Mirrors are cheaper than lenses of the same size and large mirrors are easily constructed and supported, but there is some loss in contrast because the secondary casts a shadow on the primary. This telescope was designed by Sir Isaac Newton and they are commonly called Newtonians. Practical Astronomy 10 / 33 Adrian Jannetta
Aligning the optics: collimation The primary and secondary mirrors must be precisely aligned within a reflecting telescope. The process of making adjustments is called collimation. Collimating a reflecting telescope is necessary to obtain the best possible view through it. Laser collimators are useful tools for quickly aligning the primary and secondary mirrors. Practical Astronomy 11 / 33 Adrian Jannetta
Catadioptric (compound) telescope Corrector Plate Eyepiece Primary Mirror Secondary Mirror This design uses lenses and mirrors to collect and focus the light. The corrector plate is a lens which compensates for distortions introduced by the primary mirror. The tube is short because the light path is folded by a secondary mirror on the correcting plate. The diagram shows a design called Schmidt-Cassegrain. Another common variation on this theme is the Maksutov-Cassegrain. Collimation may sometimes be necessary for this type of telescope. Practical Astronomy 12 / 33 Adrian Jannetta
Comparison of telescope designs Refractor Bright, high contrast images (so great views of moon and planets). Few collimation issues (fixed components). Small and portable. Large apertures are expensive. Reflector Large apertures are relatively cheap Good views of faint objects (nebulas, galaxies, deepsky objects) May require frequent collimation if transported. Catadioptric Short tubes because of folded light path Some collimation issues if portable. Closed tube reduces air currents (improves seeing). Practical Astronomy 13 / 33 Adrian Jannetta
Telescope mounts: alt-az Alt-az mounts are the simplest telescope mounts. Advantages: Cheap and simple to operate. Telescope moves up/down (altitude) and left/right (azimuth). Reflecting telescopes using this mount are known as Dobsonians Disadvantages: Unable to follow the the natural motion of the stars as the Earth turns. Imaging is restricted to short exposures; the telescope does not rotate to compensate for the motion of objects in the sky. Practical Astronomy 14 / 33 Adrian Jannetta
Telescope mounts: GoTo GoTo mounts are a type of motorised Alt-az mount controlled by an onboard computer. Advantages: Intitial alignment procedure is simple (1,2 or 3 stars). Some models use GPS for location. Huge database of celestial objects to choose from. More time looking - less time searching. Object tracking. Imaging may be possible (but field rotation is a problem like with Alt-az). Disadvantages: Less motivation to learn how to find objects unassisted? Practical Astronomy 15 / 33 Adrian Jannetta
Telescope mounts: Equatorial Equatorial mounts come in two basic types: German and Fork designs. They both work on the same principles. Advantages of an equatorial mount: Follows the natural movement of stars. Easy to motorise for imaging. Setting circles allow the positions of objects to be found via their Right Ascension and Declination coordinates. Disadvantages: Good quality equatorial mounts can be expensive. Takes longer to set up must be polar-aligned before use. Practical Astronomy 16 / 33 Adrian Jannetta
Telescope focal ratio The focal ratio is a useful number for thinking about the type of objects the telescope will excel at viewing. Larger f-numbers are more capable of producing higher magnifications; better views of small objects like planets. Small f-numbers give bigger telescopic fields of view; ideal for large objects such as nebulas, star clusters and galaxies. The focal ratio of a telescope is defined by: Focal length Focal ratio= Aperture The same units must be used for the two measurements on the RHS! Practical Astronomy 17 / 33 Adrian Jannetta
Eyepieces Change the eyepiece to adjust the magnification of a telescope. Standard fit (1.25 inch or 2 inch barrel). Special coatings to reduce internal reflections. Some eyepieces have wide fields of view...more soon. Zoom eyepieces are popular. Choice may depend f-ratio of scope. Useful magnification: about 50x per inch of aperture... Barlow lenses work with existing eyepieces to double or treble the magnification. There are a huge variety of different eyepiece designs! Practical Astronomy 18 / 33 Adrian Jannetta
Ramsden Kellner Orthoscopic Plössl Erfle Nagler Barlow Practical Astronomy 19 / 33 Adrian Jannetta
Barlow lens Barlow The Barlow lens must be used in conjunction with other eyepieces; they plug into the Barlow and the Barlow plugs into the telescope. A Barlow extends the focal length of the telescope. A Barlow rated as 2 will double the focal length of the telescope. Therefore: 2 Barlow will double the magnification of the eyepiece it is used with. Barlow lenses are a cost effective way of increasing the range of magnifications available on a telescope. Practical Astronomy 20 / 33 Adrian Jannetta
Magnification Some formulas for calculating telescope magnification: Telescope focal length Magnification= Eyepiece focal length = (f-number) Aperture Eyepiece focal length For example, consider a 9mm eyepiece used with the following telescopes: 1 Celestron NexStar 102 SLT. Focal length 660mm. Magnification= 660 9 73 2 Meade Lightbridge. Aperture 16 inches (406 mm), f/4.5. Magnification= 4.5 406 9 3 Meade LX200. Aperture 14 inches (356 mm), f/10 = 203 Magnification= 10 356 284 9 The magnification of a given eyepiece will vary with the telescope. For a visual representation of an object with a particular telescope-eyepiece combination see ØØÔ»»ÛÛÛº Ý ØÒ ØÑ Þ Ò ºÓÑ» Ð ¹Ú Û¹ ÐÙÐ ØÓÖ. Practical Astronomy 21 / 33 Adrian Jannetta
Field of view The apparent field of view (FOV) quantifies how much of the sky is visible through the eyepiece alone. Standard eyepieces usually have FOVs in the 50 range. Some eyepieces (e.g. Nagler) are designed to achieve a bigger apparent FOV. Such ultra-widefield eyepieces give spectacular, rich views star clusters, nebulae and galaxies. Note that the magnification doesn t change but the amount of sky visible (the true FOV) changes. There are numerous online tools which simulate the view through a great number of telescope/eyepiece combinations. See ØØÔ»»ÛÛÛº Ý ØÒ ØÑ Þ Ò ºÓÑ» Ð ¹Ú Û¹ ÐÙÐ ØÓÖ for example. Practical Astronomy 22 / 33 Adrian Jannetta
Finderscopes and red-dot finders Small telescope. Image is upside down and left-right inverted. Magnification: 6 30, or 8 50 typically. Faint objects are located and centred in the finderscope. Places a faint red marker in your field of vision. View of the sky is not magnified Not upside down, not inverted! Point the red dot at the location of the object; it should then be visible through the telescope. Practical Astronomy 23 / 33 Adrian Jannetta
Planetary (colour) filters Colour filters are mostly used to enhance the colour contrast differences between surface features on the moon or planets. The filters screw into a standard eyepiece barrel. The contrast differences are subtle and some observers report little or no observable differences. It may take patience and experience to recognise the differences. Practical Astronomy 24 / 33 Adrian Jannetta
Broadband and Narrowband filters UHC Ultra High Contrast filter. Increases the contrast of planetary and emission nebulas. OIII Oxygen-III filter. Similar to UHC, but higher contrast on certain nebulas. Hβ Hydrogen Beta filter passes a particular colour of hydrogen light found in certain nebulas. Polariser Darkening the moon and enhancing contrast on the planets. IR pass Infrared pass filter is useful for getting steady views of the planets because IR is less disturbed by the atmosphere. Light pollution A filter to increase contrast of objects when observing from urban areas. Darkens the sky by blocking street-lights whilst passing light from other colours. Practical Astronomy 25 / 33 Adrian Jannetta
Spectroscopy Spectroscopy is the analysis of light. Light can be separated into a spectrum of colours. The presence of bright or dark bands can provide information about an object - such as the composition - that cannot be seen through a telescope. Practical Astronomy 26 / 33 Adrian Jannetta
Spectra of Vega (left) and P Cygni (right) obtained with the SA100 and an 8 SCT. Practical Astronomy 27 / 33 Adrian Jannetta
Solar filters Home made white-light solar filters can be constructed at fairly low cost. The filter reduces the amount of sunlight entering the telescope by a factor of about 100,000. This type of filter allows sunspots, eclipses and transits of Mercury and Venus to be viewed safely. Practical Astronomy 28 / 33 Adrian Jannetta
Solar telescopes Solar telescopes are specialised, small refractors with an filter system designed to isolate a particular wavelength of sunlight, such as hydrogen-alpha or calcium. They can only be used to study the Sun. Practical Astronomy 29 / 33 Adrian Jannetta
Comparison of a white-light and hydrogen alpha image of the Sun at the same time. Practical Astronomy 30 / 33 Adrian Jannetta
Binocular Astronomy A binocular is a pair of refractor telescopes for each eye. The light path is folded using prisms. The light is reflected and refracted many times so glass surfaces should be fully coated to prevent internal reflections and loss of contrast. Practical Astronomy 31 / 33 Adrian Jannetta
Binocular considerations Binoculars are usually described by a pair of numbers: 15 50 Here, the magnification is 15 and the objectives are 50mm in diameter. Great views of the moon, Milky Way, star clusters, nebulas, galaxies and bright comets. Planets and bright asteroids can be seen. Useful aid for aligning a telescope. Choose binoculars that are comfortable to hold for prolonged periods. Large binoculars may need to be tripod mounted for comfort. Practical Astronomy 32 / 33 Adrian Jannetta
Further reading P S Harrington. Star Ware: The Amateur Astronomer s Guide to Choosing, Buying, and Using Telescopes and Accessories. Wiley, 4th edition, 2007. C R Kitchen. Telescopes and Techniques: an Introduction to Practical Astronomy. Patrick Moore s Practical Astronomy Series. Springer, 2nd edition, 2003. M Mobberley. Astronomical equipment for amateurs. Patrick Moore s Practical Astronomy Series. Springer, 1998. P C Moore. Exploring the Night Sky with Binoculars. Cambridge University Press, 4th edition, 2000. R Scagell. Stargazing with a Telescope. Philips, 2004. R B Thompson and B R Thompson. Astronomy Hacks: Tips and Tools for Observing the Night Sky. O Reilly, 2005. Practical Astronomy 33 / 33 Adrian Jannetta