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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)


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.


• 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!!!


http://www.abrutis.com/photo-probleme+de+telescope-29456.html

Disadvantages of the Human Eye Rods & Cones

2/5/2014 Amateur Astronomy & Telescopes -

Webster 5

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


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.


Common Telescope Types


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…


Webster 9

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

http://3.bp.blogspot.com/-QWbUB7lvqYE/TW4oB_dzUOI/




Advantages/Disadvantages of Telescope Types


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.


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


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.


Tallahassee Astronomical Society (TAS)


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?


Webster 16

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