Dioptrics, catoptrics, and more! TELESCOPES

BINOCULARS VERSUS TELESCOPES There are two main types of image magnification devices Binoculars Telescopes Technically, telescopes are “monoculars” but no one calls them that Each carry their own specific advantages

BINOCULARS VERSUS TELESCOPES Binoculars are cheap A good pair of astronomical binoculars will be 1/4 th the cost of a good telescope Binoculars are light Their twin barrels are more natural to look through, and give a 3D effect The image in binoculars is always right side up (unlike in telescopes)

BINOCULAR MEASUREMENTS Binoculars have two main measurements Magnification For example, a pair of 7x50 binoculars magnifies images 7 times. 9x63 would magnify 9 times. 7x50 is ideal for a starters pair (higher magnification usually only shows the flaws of the lenses) Aperture The size of the front lens in each barrel (in mm) 7x50 has a 50mm sized front lens.

BINOCULAR MEASUREMENTS Binoculars have two main measurements Exit Pupil This is the diameter of the light beam leaving the eye piece and entering the eye The human eye (in dark conditions) has a pupil size of 7 mm To find the exit pupil of a pair of binoculars, divide aperture by magnification. A 7x50 binoc would have 50/7= 7.1mm exit pupil. A 10x50 binoc would have a 50/10= 5mm exit pupil You want to have an exit pupil as close to your eye’s actual pupil as possible If the exit pupil is too large, (like 8 or 9 mm) then that light is wasted, and the binocular isn't as powerful as it should be.

BINOCULARS IN ASTRONOMY Yuji Hyakutake used a pair of 25x150 binoculars to discover the brightest comet of the 90s What is the exit pupil on his binoculars?

A BASIC TELESCOPE Telescopes come in one of three categories: Refracting (or dioptric) Reflecting (or catoptric) Hybrid (or catadioptric)

REFRACTORY TELESCOPES Refracting (dioptric) telescopes use lenses to gather light Galileo built this type of telescope 2.4” (60 mm) is a good starter telescope. Above 4”, the cost is too great to be value for most amateurs.

REFRACTORY TELESCOPES Advantages: Very crisp views of the sky. No parts to align or focus. For their aperture, provides lots of light Disadvantages: Lenses over 4” in size are very, very, very expensive This limits aperture size, which limits light collection, which limits how far you can look into space.

REFLECTING TELESCOPES Reflecting (or catoptric…or Newtonian….) telescopes used a highly polished concave mirror to collect light, bounce it off another mirror, then up into the eye piece.

REFLECTING TELESCOPES Advantages: Cheap! A 20” aperture reflector costs the same as a 5” refractor Due to larger aperture, you can see further into space, and greater detail Disadvantages: Images are less crisp…some light is lost in the reflecting Mirrors can drift out of alignment over time and require calibration Can be very heavy

APERTURE SIZE COMPARISON The following are a series of pictures of M13, a star cluster, through different aperture scopes

HYBRID TELESCOPES Hybrid (or catadioptric) telescopes use a combination of combination of mirrors and lenses. There are two kinds: Schmidt-CassegrainsandMaksutovs

HYBRID TELESCOPES Advantages: Small size (about a third the length of comparable normal telescopes) Good for astrophotography Share the advantages of crisper images with wider apertures Disadvantages Not specialized…no huge apertures (max is about 8”), nor super crisp images Cost 50% more than a similar aperture reflector (but still far less than a refractor!!!)

TYPES OF TELESCOPE SUMMARY Let’s review: 1 = refractor. 2 = reflector3, 4 = hybrid/catadioptric 3 = maksutov 4 = schmidt-cassegrain

APERTURE NOT POWER When buying or using a scope, aperture is the key specification Magnification (or power) is altered by changing the eye piece, thus any telescope can be adjusted to higher or lower powers of magnification Aperture limits the maximum effective magnification 50x its aperture in inches. 2.4” scope x 50 = 120 power (120 times magnification) maximum 4.5 scope = ???? maximum power?

APERTURE NOT POWER Doubling the size of aperture increases the light gathering capacity by a factor of four 6” scope will collect four times the light of a 3” scope Doubling the aperture also doubles the resolving power A 4” scope can resolve things that are degrees apart An 8” scope can resolve things that are degrees apart. The human eye can only resolves things that are degrees apart.

SCOPE SPEED The focal length is given in millimeters. This is the length that light travels from the lens to the focal point. For refractors and reflectors it is roughly the length of the tube. For hybrids, though, the light is folded back and forth several times This allows for the overall length of the scope to be much shorter than a comparable reflector or refractor.

SCOPE SPEED Dividing the focal length by the aperture in millimeters gives its f-ratio A scope with a 2,000 mm focal length and a 200 mm aperture has an f-ratio of: f/10 (aka it is an f/10 telescope) A scope has a focal length of 1,000 mm, and a 200 mm aperture, what is the f-ratio?

SCOPE SPEED The smaller the f-ratio, the faster the scope is. This is useful primarily in astrophotography An f/5 scope will take a picture in a quarter the time of an f/10 scope But an f/10 scope will have slightly sharper images (but the tube will be twice as long)

TYPES OF TELESCOPE MOUNTS There are two types of telescope mounts: Alt-azimuth Equatorial

ALT-AZIMUTH Altazimuth (or alt-azimuth) are essentially identical to the mounts used for cannons. One axis controls x-plane rotation The other axis controls y-plane declination Many reflectors use a special type of altazimuth mount called a “Dobsonian” mount This mount is at the end of the telescope barrel

EQUATORIAL MOUNT One of its axis is pointed at the north celestial pole The other axis is used to track an object as it rotates around the north celestial pole This allows the scope to be used in astrophotography This mount is expensive, complex, and heavy.

EYEPIECES The Eyepiece magnify the images that the mirrors and lenses collect from the sky Eyepieces are sold by focal lengths 9 mm, 12 mm, and 25 mm are common sizes.

EYEPIECES To calculate how much power (or magnification) an eyepiece gives you: Divide the focal length of the scope by the focal length of the eyepiece A 20 mm eyepiece in a 2,000 mm scope would give you 100x power A 20 mm eyepiece in a 1,000 mm scope would give you 50x power. If you had a 1,000 mm scope, and three eyepieces, what power would each give? 25 mm 12 mm 6 mm

EYEPIECES Magnifications over 180x are not effective…they are too blurry to see clearly Avoid zoom eyepieces…too likely to be blurry Eyepieces have three diameters, or barrel sizes inches, 1.25 inches, and 2 inches 1.25 inches has the widest range of options for eyepieces Hyugens and Ramsden are the style of eyepieces found in cheap telescopes. Not good quality, distorts images. Kellner style produces crisp images in a narrow arc (40 degrees) Erfle style produce images (mildly distorted) that cover a wide angle (60 degrees) Plossl style are a good compromise between the two (50 degrees)