observational astro

Law of Reflection

when light hits a reflecting surface, the light ray will be reflected from the surface at an angle that is equal to the angle of the incident ray.

Law of Refraction

when light passes from one medium to another, the angle that the light beam makes with the normal to the interface between the two media is always less in the medium of higher index of refraction (i.e. density).

Snell's Law

What shape is a perfect lens?

parabolic

Focal Length

distance between the lens and the point where parallel incoming rays are brought to a focus.

Objective Lens

a lens that forms the image of an object extremely far away.

Focal Ratio

FOCAL RATIO = "f/#" = Fo / Do

also known as "speed"

"Fast" Optic

brings light to a focus near that optic (i.e., "more quickly")

"Slow" Optic

brings light to a focus farther away (i.e., "more slowly").

Plate scale

the angular size on the sky projected to each millimeter.

scale = α/y = α/(Fo α) = 1 / Fo (in arcseconds / mm)

small plate scale means more sky is projected into one mm.

The bigger the focal length, the (larger/smaller) things appear in the focal plane.

larger

The bigger the focal length, the (brighter/fainter) things appear in the focal plane.

fainter

What are the characteristics of a telescope used for faint imaging?

small Fo, big plate scale, brighter images (because imaged sources are more concentrated).

What are the characteristics of a telescope used for accurate positions?

larger Fo, small plate scale, easier to measure positions accurately because sources are more spread out.

How many arcseconds per radian?

206265 arcsec / radian

Pixel Scale

pixel scale = (pixel size)(plate scale)

What coordinates are used to describe an observer's coordinate system?

altitude, azimuth

What coordinates are used to describe the equatorial or celestial coordinate system?

right ascension (α), declination (δ)

What coordinates are used to describe the ecliptic coordinate system?

ecliptic longitude (λ), ecliptic latitude (β)

What coordinates are used to describe the galactic coordinate system?

Galactic longitude (l), Galactic latitude (b)

Meridian

vertical circle that contains the zenith, nadir, the north celestial pole, and the due north and south points on the horizon. Your meridian, which is a secondary great circle, also divides your sky in half.

Altitude

how far above the horizon to look for an object, from zero degrees at the horizon to 90 degrees at the zenith.

Azimuth

the direction towards the horizon one must face to look up from the horizon to the object.

(90° is East, 270° is West)

Where on Earth is 0° longitude?

Greenwich, England

Declination

δ: the angular distance north or south of the celestial equator.

equivalent of the "latitude" small circles on Earth.

(+/- 90° at poles)

Right Ascension

the coordinate for measuring the east-west positions of celestial bodies

equivalent of the "lines of longitude", except are fixed in space regardless of the earth's spin

(0-24 hours)

write with one more significant digit (one more word in RA than dec)

Altitude-azimuth mount

based on the horizon system

moves up, down, left, right

we must always drive the telescope continuously in both axes to follow objects in the sky as the Earth turns, and the rate of motion is constantly changing -- requires a computer.

Equatorial Mount

a mount for instruments that compensates for Earth's rotation by having one rotational axis, called polar axis, parallel to the Earth's axis of rotation

Equation to account RA lines converging with declination

1 hour = 15°cos(δ)

altitude of a star above the southern horizon (SALT) equation

SALT = (90° - LAT) + DEC

Transit

When a celestial object moving from East to West due to the diurnal motion of the Earth crosses your meridian

Solar day

Earth's rotation period as defined by the position of the Sun in the sky

24 hours

Earth must turn about 361° every solar day

Sidereal day

The time for one complete rotation of the earth relative to a particular star, about 4 minutes shorter than a mean solar day

23 hours 56 minutes

Hour angle definition and equation

the amount of time before or after transit for any given object.

HA = LST - RA

A star to the west of the meridian has a (positive/negative) hour angle, while a star to the east of the meridian has a (positive/negative)

West: RA < LST and therefore HA > 0

East: RA > LST and therefore HA < 0

Ecliptic

The apparent path that the Sun takes through the sky over the course of a year.

The ecliptic is inclined by 23.5° from the celestial equator

Vernal equinox

March 21

Sun rises directly in the east and sets directly in the west

Autumnal Equinox

September 22

Sun rises directly in the east and sets directly in the west

How many rotations about Earth's axis in a year?

366.25

off-axis mount

very big scopes, tube is counterweighted by a weight

german if supported by a cantilever

english if supported on north and south ends

german mount

supported by a cantilever

fork/yoke mount

fork if supported by one side, yoke if supported by both

horseshoe mount

variation of yoke that allows access to the north celestial pole

advantages of alt-az mounts:

stable platforms for instruments

structurally sturdier

less massive and expensive

more compact -> smaller enclosures

refractor

A telescope that uses a convex lens to concentrate light from distant objects

problem of refractors

suffer with chromatic aberration. need more power for blue light than red

achromatic doublet

concave flint glass and convex crown glass lens that counteract each other - used to reduce (not eliminate) chromatic aberration

reflector

a telescope that uses a concave mirror to gather and focus light from distant objects

• no chromatic aberration

newtonian focus

The optical design in which a diagonal mirror reflects light out the side of the telescope tube for easier access

cassegrain focus

the optical design of a reflecting telescope in which the secondary mirror reflects light back down the tube through a hole in the center of the objective mirror

nasmyth focus

added 2nd and 3rd mirror allows for remote equipment.

puts beam of light through the altitude bearing of an alt-az mounted telescope

coude focus

light is reflected from a secondary mirror and brought to a focus in an adjacent room. focus removed from tube.

puts the beam of light through the polar axis of an equatorially mounted telescope

lens

serves as the objective and focuses light to the back of the eye

focal length around 24 mm

iris

acts as an adjusting pupil that dilates and contracts to adjust the f/ratio (speed) of the eye

squeezes the lens to different shapes

retina

image detector, consists of light-sensitive neurons that lie at the focal plane of the lens of the eye

lies on a curved surface (back of the eye) which allows it to collect rays coming into the lens at extreme angles (~180° FOV)

fovea

center of the retina, place with the most visual acuity

optic nerve

electronic cabling that carries info from the retinal neurons to the brain

cornea

protective front window covering the lens

sclera

tough flexible shell that has the soft parts of the eye

includes the cornea (only transparent part of the sclera)

aqueous humor

fluid that fills the corneal chamber

vitreous humor

transparent jellylike fluid that gives the eyeball strength and rigidity

has floaters

choroid

layer inside the sclerotic wall with blood vessels and melanin

absorbs stray light

rods

(2 micron diameter)

majority of cells in the retina, centered around the outer parts

panchromatic (sensitive to lots of wavelengths)

shades of gray

use averted vision because there are no rods in the fovea centralis (macula)