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Amateur Astronomy & Telescopes
2/5/2014 Amateur Astronomy & Telescopes - Webster 1
Josh Webster
Outline
• What is amateur astronomy? • Misconceptions in visual astronomy - Rods & Cones - Magnification!!! • Common telescope types - Advantages/disadvantages • Eyepieces • Basic related equations • Tallahassee Astronomical Society (TAS)
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What is amateur astronomy?
Notable amateur astronomers: Thomas Bopp & Alan Hale (PhD) – comet Hale-Bopp George Alcock – several comets & novae David H. Levy – comet Shoemaker-Levy 9 and 22 others Grote Reber – pioneer of radio astronomy; first sky survey in radio freq.; worlds only radio astronomer for almost an entire decade.
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• It’s casually observing the sky. • It is considered a hobby. • Many professional astronomers also enjoy
amateur astronomy. • Amateur astronomers can contribute to
science. • Two main branches: visual and photographic
http://www.celestronimages.com/details.php?image_id=7230
Common/Beginner Misconceptions in Amateur Astronomy
• Disadvantages of the Human Eye
• Magnification!!!
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http://www.abrutis.com/photo-probleme+de+telescope-29456.html
Disadvantages of the Human Eye Rods & Cones
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http://www.anatomybox.com/retina-sem/retina-rods-and-cones-sem/
• Two types of photo-receptors: “rods” and “cones”.
• Visual astronomy primarily deals with rods.
Rods: • Take over in low light • More numerous than cones • Much higher sensitivity • Little response to red light • Downfalls: - No color detection - Slow response - Best in peripherals - Low acuity
Disadvantages of the Human Eye
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Similar to Human Eye
http://en.wikipedia.org/wiki/File:Orion-Nebula_A_A_Common.jpg
DSLR Camera
Hubble Space Telescope
http://en.wikipedia.org/wiki/File:Orion_Nebula_-_Hubble_2006_mosaic_18000.jpg http://en.wikipedia.org/wiki/File:The_Orion_Nebula_M42.jpg Albeit Hubble is in space.
Not Magnification, AMPLIFICATION!
• Magnification is by far the most common misconception and usually the least important factor.
• A manufacturer marketing strategy • Average max resolvable magnification (on Earth with virtually no
atmospheric turbulence) ≈ 100x • The more you magnify the sky, the more you magnify the atmosphere. • High magnification is usually only useful for splitting double/binary stars.
Aperture: - The diameter of the primary mirror. Usually given in mm or in. - Almost always the most important factor of a telescope. - Amplifies the light “signal”. Commonly referred to as Light Gathering
Power (LGP). - Allows us to see dimmer objects.
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Common Telescope Types
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http://en.wikipedia.org/wiki/File:Newton-Teleskop.svg
http://en.wikipedia.org/wiki/File:Cassegrain-Telescope.svg
http://en.wikipedia.org/wiki/File:Maksutov_spot_cassegrain.png
Maksutov-Cassegrain
Schmidt-Cassegrain
Refractor (Keplerian)
Reflector (Newtonian)
http://en.wikipedia.org/wiki/File:Schmidt-Cassegrain-Telescope.svg
Telescope Types Continued…
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Newtonian Reflector
Refractor
Schmidt-Cassegrain
Maksutov-Cassegrain
http://img3.findthebest.com/sites/default/files/810/media/images/Orion_Apex_127mm_Maksutov-CassegrainZ_Telescope_75704.JPG
http://images1.opticsplanet.com/365-240-ffffff/opplanet-celestron-nexstar-130-telescope.jpg http://3.bp.blogspot.com/-QWbUB7lvqYE/TW4oB_dzUOI/
AAAAAAAAA6Q/qyH90ARmHbs/s400/11063_cge1400_mid.gif
http://www.novac.com/resources/reviews/images/80ed1.jpg
Advantages/Disadvantages of Telescope Types
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Type Advantages Disadvantages Cost Comparison
Refractor Simple design, little maintenance, no secondary mirror obstruction, great optics for Astrophotography (AP)
Highest cost for size, heavy, aperture limitations, not great for DSO’s
Orion ED80 (80 mm ≈ 3.14”) w/o mount - $500 LGP = 130x
Reflector Least expensive design, great for DSO’s due to no size limitations
Some light loss from secondary mirror, more maintenance due to exposed optics
Orion XT10 (254 mm ≈ 10”) - $600 LGP = 1317x
Catadioptric Combines advantages of the other types, little maintenance, great for AP
More expensive than reflectors, some light loss from secondary mirror
Celestron 127 Mak (127mm ≈ 5”) - $500 LGP = 329x
The focal ratio of a telescope also plays an important role. It is a measure of the speed of a telescopes optics. (Equation later)
Eyepieces (EPs)
• Many designs; ranging from single to eight element designs. • Focal length - 17mm – 40 mm for DSO’s - 6 – 17 mm for Stars, Planets and some Clusters • Eye-relief may be important to you.
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http://www.optcorp.com/pdf/OPT/EDU/eyepiece-580x393.gif
http://www.vikdhillon.staff.shef.ac.uk/teaching/phy217/telescopes/eyepieces.jpg
Basic Related Equations
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𝐸𝑥𝑖𝑡 𝑃𝑢𝑝𝑖𝑙 =𝑓𝑜𝑐𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ𝐸𝑃
𝑓𝑜𝑐𝑎𝑙 𝑟𝑎𝑡𝑖𝑜𝑠𝑐𝑜𝑝𝑒
Diameter of image projected out of the eyepiece.
𝑭𝒐𝒄𝒂𝒍 𝑹𝒂𝒕𝒊𝒐 =𝒇𝒐𝒄𝒂𝒍 𝒍𝒆𝒏𝒈𝒕𝒉𝒔𝒄𝒐𝒑𝒆
𝑨𝒑𝒆𝒓𝒕𝒖𝒓𝒆
A measure of a telescopes speed. Also called f/stop or f-ratio.
𝑴𝒂𝒈𝒏𝒊𝒇𝒊𝒄𝒂𝒕𝒊𝒐𝒏 =𝒇𝒐𝒄𝒂𝒍 𝒍𝒆𝒏𝒈𝒕𝒉𝒔𝒄𝒐𝒑𝒆
𝒇𝒐𝒄𝒂𝒍 𝒍𝒆𝒏𝒈𝒕𝒉𝑬𝑷
The magnification of a telescope using a specific focal length eyepiece.
𝑀𝑎𝑥 𝑈𝑠𝑒𝑓𝑢𝑙 𝑀𝑎𝑔𝑛𝑖𝑓𝑖𝑐𝑎𝑡𝑖𝑜𝑛 = 60 × 𝐴𝑝𝑒𝑟𝑡𝑢𝑟𝑒
A measure of how far you can push the limits of your telescope. A more realistic approximation is to use 30 instead of 60. A good way to see if you can split certain binary stars.
𝑀𝑖𝑛𝑖𝑚𝑢𝑚 𝑈𝑠𝑒𝑓𝑢𝑙 𝑀𝑎𝑔𝑛𝑖𝑓𝑖𝑐𝑎𝑡𝑖𝑜𝑛 = 3 × 𝐴𝑝𝑒𝑟𝑡𝑢𝑟𝑒 Minimum magnification is good for viewing large areas of the sky.
𝑳𝒊𝒎𝒊𝒕𝒊𝒏𝒈 𝑴𝒂𝒈𝒏𝒊𝒕𝒖𝒅𝒆 = 𝟐 + 𝟓𝐥𝐨𝐠 𝑨𝒑𝒆𝒓𝒕𝒖𝒓𝒆
Tells you the highest magnitude object that can be resolved for a given telescope aperture.
𝑳𝒊𝒈𝒉𝒕 𝑮𝒂𝒕𝒉𝒆𝒓𝒊𝒏𝒈 𝑷𝒐𝒘𝒆𝒓 =𝑨𝒑𝒆𝒓𝒕𝒖𝒓𝒆 (𝒎𝒎)
𝟕 𝒎𝒎
𝟐
A measure of the ability of a telescope to gather light. 7mm is the average diameter of the dark-adapted pupil.
𝑅𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝑎𝑟𝑐𝑠𝑒𝑐𝑜𝑛𝑑𝑠 =138
𝐴𝑝𝑒𝑟𝑡𝑢𝑟𝑒 (𝑚𝑚)
The Rayleigh limit. A general measure of the ability of a telescope to distinctly resolve two stars of equal magnitude.
𝑅𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝑎𝑟𝑐𝑠𝑒𝑐𝑜𝑛𝑑𝑠 =116
𝐴𝑝𝑒𝑟𝑡𝑢𝑟𝑒(𝑚𝑚)
The Dawes limit. Reached when the airy disk of one star is in the diffraction ring of another, and the stars are seen as just barely touching.
Conclusion
• Amateur astronomy can be casual or serious and scientific. • There are physical phenomena associated with the human
eye that effect what you see. • Size matters! Amplification >> Magnification • Large reflectors are cheaper to build than comparables. • A good eyepiece even on a cheap telescope can make a
world of difference, and so can a cheap eyepiece on a good telescope.
• Most important equations: focal ratio, and magnification.
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Tallahassee Astronomical Society (TAS)
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If anyone is interested in coming out on an observing session let me know!
• Meetings: 3rd Saturday of the month Challenger Center (IMAX).
• Planetarium show: the 1st Saturday of every month at the Challenger, Free!
• Observatory! • Mag lab open house Feb. 22nd.
References
Nave, R. (2000). Rods and Cones. Retrieved January 24, 2014, from Hyperphysics: http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html Sharma, G. (n.d.). Understanding Telescopes. Retrieved January 24, 2014, from Telescopes.com: http://www.telescopes.com/telescopes/helpunderstandingtelescopesarticle.cfm Wikipedia. (2014, January 17). Amateur Astronomy. Retrieved January 24, 2014, from Wikipedia: http://en.wikipedia.org/wiki/Amateur_astronomy Culp, R. (2012, November 16). Telescope Equations. Retrieved January 24, 2014, from RocketMime: http://www.rocketmime.com/astronomy/Telescope/telescope_eqn.html 2/5/2014 Amateur Astronomy & Telescopes - Webster 15
Questions?
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