Telescope Fundamentals

Transcription

1 Telescope Fundamentals The focus of this presentation is to provide an overview of popular equipment available to the amateur astronomy community, as well as the equipment s applicability to differing aspects of astronomy

2 Different Optical Designs There are several different telescope designs. Each has its own advantages and limitations The differing designs include: Refracting Reflecting Catadioptric 2

3 Different Optical Designs: Refracting Telescopes 3

4 Refracting Telescopes A Refractor uses lenses at the front of the tube to gather light, which is then sent (refracted) towards the back of the telescope - here, the light is intercepted by an eyepiece and brought into focus source: Sky & Telescope 4

5 Modern Refractors Today s refracting telescopes fall into two categories: Achromatic Apochromatic 5

6 Achromatic Refractors Objective lens consists of two elements, made of two types of glass The two different ty 6

7 Modern Refractors Apochromatic Refractors Objective lens may consist of several elements, made of exotic glasses, which greatly reduce or eliminate false colour 7

8 Achromatic & Apochromatic Refractors ADVANTAGES of Achromatic Refractors Inexpensive in smaller sizes Low maintenance Portable in smaller sizes Sturdy Provide very good contrast and image sharpness 8

9 Achromatic & Apochromatic Refractors DISADVANTAGES of Achromatic Refractors False colour can become severe Limited light-gathering capability in small sizes Larger achromatic refractors require big, heavy mounts 9

10 Achromatic & Apochromatic Refractors ADVANTAGES of Apochromatic Refractors Totally free of false colour (no chromatic aberration) Extremely high-contrast and sharply-defined images Can be made very compact without sacrificing image quality - portable 10

11 Achromatic & Apochromatic Refractors DISADVANTAGES of Apochromatic Refractors Very expensive, even in smaller sizes Can take several months, even years, to obtain higher end models long waiting lists 11

12 Different Optical Designs: Reflecting Telescopes 12

13 Reflecting Telescopes The Newtonian Reflector uses a concave mirror at the back of the tube, and a flat secondary mirror placed at a 45-degree angle at the front of the tube - the secondary mirror reflects the light out the side of the tube, where it is intercepted by an eyepiece and brought to a focus source: Sky & Telescope 13

14 Newtonian Reflectors ADVANTAGES of Newtonian Reflectors Least expensive per inch of aperture Completely free of false colour Good light gathering power in 6 or larger Eyepiece location allows for comfortable viewing position 14

15 Newtonian Reflectors DISADVANTAGES of Newtonian Reflectors Light loss is greater than that of refractors due to multiple reflections Both mirrors require optical alignment (collimation) Open tube design subjects mirror to dust & other elements 15

16 Different Optical Designs: Catadioptric Telescopes 16

17 Catadioptric Telescopes Modern commercially available catadioptric (compound) telescopes incorporate two designs: Schmidt-Cassegrain Maksutov-Cassegrain 17

18 Catadioptric Schmidt-Cassegrain The Schmidt-Cassegrain passes light through a gentlycurved glass corrector lens, which then strikes the primary mirror, then reflected back onto a curved secondary mirror, and finally out to the eyepiece. source: Sky & Telescope 18

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20 Schmidt- & Maksutov-Cassegrains ADVANTAGES of Schmidt-Cassegrains Moderate cost per inch of aperture Compact size renders it portable Convenient eyepiece position Highly adaptable to astrophotography 20

21 Schmidt- & Maksutov-Cassegrains DISADVANTAGES of Schmidt-Cassegrains Higher cost when compared to Newtonian reflectors Image quality may not be comparable to that of refractors and reflectors Corrector lens susceptible to dew 21

22 Schmidt- & Maksutov-Cassegrains ADVANTAGES of Maksutov-Cassegrains Very sharp, high contrast lunar and planetary images Excellent for solar system object photography Compact and rugged design makes them excellent for traveling 22

23 Schmidt- & Maksutov-Cassegrains DISADVANTAGES of Maksutov-Cassegrains Cost can be very high compared to other optical designs Optical alignment may require instrument be returned to factory Corrector lens susceptible to dew 23

24 Telescope Eyepieces Two size standards; American and Japanese Adhere to the American Eyepiece Size Standard! Eyepieces built to the Japanese standard have a barrel diameter of 0.965". Those built to the American standard have a barrel diameter of 1.25 or 2 24

25 Telescope Eyepieces Generally, inexpensive beginners' telescopes are usually outfitted with Japanese standard eyepieces - telescopes in this category have 0.965" focusers, or eyepiece holders Much more desirable are telescopes that are designed to accept American standard eyepieces. These scopes are generally built to better standards, and are able to utilize much better quality eyepieces. 25

26 Telescope Mounts Two categories of mounting systems: Alt-Azimuth (AZ) Equatorial (EQ) Each type has several designs associated with it 26

27 Mounts Simple Alt-Azimuth (AZ) Alt-Azimuth mount is the most rudimentary type Simple up-down (Altitude) & left-right (Azimuth) motions Better AZ mounts have slowmotion controls on each axis, permitting fine adjustment of the telescope s aiming, by twisting knobs or cables 27

28 Mounts Dobsonian (AZ) Dobsonian mount utilizes a simple, box-like design Newtonian reflectors are the most common telescope type used on Dobsonian mount The solid, squat design, coupled with smooth movements, allow for mounting of very large reflectors Mainly used for visual observing (not photography) 28

29 Mounts Equatorial (EQ) Consist of two perpendicular axes; Right Ascension (RA) & Declination (Dec) Design allows for manual or electronic tracking of objects (compensating for the earth s rotation) 29

30 Mounts German Equatorial (EQ) Most common equatorial mount design Shape resembles a letter T Generally, a very solid design Allows free access to virtually any part of the sky 30

31 Mounts - Computerized A computerized database can be integrated with virtually any type or design of mount Computer systems fall into two main categories: Manual Aiming (Digital Setting Circles) a digital readout indicates which direction & how much to manually move the telescope to find selected object Fully-Automated Electronic Go- To systems high-speed slewing motors automatically move the telescope to selected object 31

32 Aiming Devices Main telescope s field of view is generally too narrow to allow for easy aiming Aiming devices have low/no power & wide fields of view to accurately point the main telescope toward its target Fall into two main categories: Magnified Optical Finderscopes Zero-Magnification Reflex Sights 32

33 Aiming Devices Optical Finders Are small, low-power, widefield refractors Most common is straightthrough design, as pictured here Models with angled eyepiece positions (45 deg. or 90 deg.) are also available, allowing for more comfortable aiming source: Orion Telescopes 33

34 Aiming Devices Reflex Sights Simple electronic device A bull s eye target or dot is projected onto a clear piece of glass attached to the body of the finder Comfortable & simple to aim gives more context than a magnified finder source: Telrad 34

35 Aiming Devices Reflex Sights Simulated View - Telrad Target appears to be superimposed on the night sky and is aimed at the desired object 35

36 Choosing a Telescope Aperture Size and Light-Gathering A telescope is rated by its aperture (the size of its primary lens or mirror), not by how much it magnifies The larger the aperture, the greater its light-gathering capability which is the key element 36

37 Choosing a Telescope Resolving Power A telescope s ability to show fine details is dependent upon aperture size and optical quality Its resolving power is based on the diameter of the lens or mirror 37

38 Choosing a Telescope Focal Length Simply, the length of the path of light from the primary mirror or lens to the focal point (eyepiece location) The longer the focal length, the higher the magnifying power of any given eyepiece 38

39 Choosing a Telescope Magnification The measure that indicates how many times larger than the naked eye an object appears through a scope To vary the magnification of a given telescope, only the eyepiece has to be changed There are magnification limits to every telescope 39

40 Choosing a Telescope Field of View The measure that indicates the amount of sky that is seen through the eyepiece/telescope combination, generally measured in degrees (a.k.a. actual field of view ) Actual field of view depends on the design and specifications of the eyepiece, and focal length of the given telescope 40

41 Choosing a Telescope Where is your Observing Site Located? City or Suburbs Highly light-polluted skies: Portability often an issue for city-dwellers Well-suited are refractors up to about 4 inches in diameter, or medium-sized Schmidt- and Maksutov- Cassegrains Newtonians with longer focal lengths are quite acceptable Computerization/Go-To can be beneficial in pin-pointing objects that may be more elusive due to sky glow 41

42 Choosing a Telescope Where is your Observing Site Located? cont d Rural Area Dark skies with little or no light pollution: Virtually any type & size of telescope will work well Best-suited are larger Newtonians or Catadioptrics Any type of sturdy mount will suffice 42

43 Choosing a Telescope What Objects do you want to Observe? Mainly Moon and Planets: Large aperture not required quality optics are most important. Well-suited are: Apochromatic refractors (3-7 ) Smaller (2.4-4 ) achromats 4-8 Newtonians with longer focal lengths 4 + Schmidt- and Maksutov- Cassegrains Tracking ability an asset 43

44 Choosing a Telescope What Objects do you want to Observe? Mainly Deep Sky: Aperture size is most important Best suited are 6 or larger Newtonians Larger Schmidt-Cassegrains are also very good Dobsonian mounts highly preferred for larger Newtonian reflectors 44

45 Choosing a Telescope How do you want to Locate Objects? Two means of locating objects are: Manually Computer Locating Objects Manually Star-hopping method is most widely used A larger finderscope is preferred for fainter objects Reflex sights are useful for locating brighter objects, and to bring fainter objects into proximity 45

46 Choosing a Telescope How do you want to Locate Objects? cont d Locate Objects by Computer No need for large finderscope - a reflex sight will suffice Little knowledge of night sky required Modern computerized systems are highly interactive and user-friendly Particularly attractive to those whose recreation time is restricted 46

47 Choosing a Telescope Is Portability an issue? If the telescope is too large, heavy and complicated, it will become a burden to set up and use!!! The most popular portable telescopes range from 2.4 to 5 refractors, up to 8 SCTs, up to 7 Maksutov-Cassegrains, and up to 8 reflectors 47

48 Choosing a Telescope What is your Budget? Wide range of prices - several hundred to several tens of thousands of dollars Trends for the true novice to serious beginner (based on sales & customer queries) telescopes up to approximately $ For astronomical and terrestrial use, most popular models are 70mm to 100mm AZ mounted achromatic refractors ~$300 to $700 48

49 Choosing a Telescope your Budget? cont d For those who need/want portable telescopes (e.g. those who live in hi-rises/city, have no vehicle, like to travel, etc.), most popular are computerized/go-to 90mm to 125mm Maksutov- and Schmidt-Cassegrains ~$1,000 to $2,000 49

50 Choosing a Telescope your Budget? cont d For those who do not have such an issue with portability, and would like the best value for the money, most popular are 6 to 10 Dobsonian Newtonians (8 being most popular, by far) ~ $400 - $1,000 For those with a slightly higher budget, have less free time (unable to devote much time learning the sky), and are less concerned with portability, the most popular models are 6 to 10 go-to Schmidt-Newtonians on German equatorial mounts, 5 & 6 go-to refractors, and 8 electronically driven Schmidt- Cassegrains ~ $1,300 to $2,000 50

51 Conclusion This information will help you get the most out of your night time observing If you are passionate about the night sky, a good quality optical instrument will increase your chances of success and satisfaction 51

52 Thank You 52