1.3 - reflecting telescopes

what is chromatic aberration?

lens diffracts coloured light differently, meaning the principal focus for each ray of light is at slightly different positions, causing coloured fringes around the image

DIAGRAM

why does chromatic aberration occur?

light of different colours have different wavelengths

what does chromatic aberration look like?

coloured fringes formed around the image

PICTURE EXAMPLE

what is a practical use of chromatic aberration?

3D glasses

what is spherical aberration?

focal point of light at different places due to the curvature of lens and mirrors, causing image blurring

DIAGRAM

why does spherical aberration occur?

a curved lens of a mirror / lens

what is the difference between chromatic aberration and spherical aberration?

• chromatic - due to coloured light of different wavelengths

• spherical - due to the curvature of lenses and mirrors

what is aberration?

when the principal focus of light is at slightly different points, causing the image to be unclear

how can spherical aberration be resolved in reflecting telescopes?

by using a parabolic mirror

which kind of aberration does a parabolic mirror solve?

spherical aberration

how can we fix aberration?

1. by using a reflecting lens rather than a refractive

2. by putting a concave lens after a convex lens (i.e., an achromatic doublet)

how does a chromatic doublet fix aberration?

DIAGRAM

it brings all rays of light into focus at the same focal point by using a convex and concave lens

what is an achromatic doublet?

a convex and concave lens of different types of glass cemented together

DIAGRAM

which circular mirror causes spherical aberration - a spherical mirror or a parabolic mirror?

spherical mirror

DIAGRAMS OF THE BOTH OF THEM

why do spherical mirrors cause spherical aberration?

DIAGRAM

each ray converges at different points, therefore creating a blurred image

why don’t parabolic mirrors cause spherical aberration?

DIAGRAM

each ray converges at the same point, creating a clear image

which spherical mirror is used in a telescope?

parabolic mirror

why are parabolic mirrors used in telescopes as opposed to spherical mirrors?

because parabolic mirrors do not suffer from spherical aberrations while circular mirrors do

what equation does a parabolic mirror fit?

y = ax² + bx + c

what.

‘think cuz it’s hard to have a curved piece of glass. cost is an issue’ what did i mean by this …

what are some examples of parabolic mirrors?

• newtorian reflector

• cassegrain reflector

what is a newtorian reflector?

DIAGRAM DESCRIPTION AFTER THIS

what is a cassegrain reflector?

DIAGRAM DESCRIPTION AFTER THIS

what is the difference between newtorian and cassegrain reflectors?

they follow the same principle (i.e., both parabolic mirrors) except with different lens composition

questions abt both reflectors

for parabolic lenses, where is the viewer placed?

at the lens

what are the benefits of reflectors?

• it can be made a lot larger (i.e., useful for telescopes)

what are the benefits of refractors?

• simple

why is it good, for telescopes, that cassegrain reflectors can be made larger?

because the focal length is increased, meaning more magnification

which telescopic lens benefits from being made bigger, reflective or refractive?

parabolic lenses (i.e., reflective lenses) as opposed to refractive lenses

why don’t refractive lenses benefit from being made bigger?

because aberration would also increase, resulting in an increasingly unclear image

why do refractive lenses suffer aberration more than reflective lenses?

lens number?

does aberration increase in reflective telescopes of increasing sizes?

yes, but at very small amounts that it’s negligible

what are the disadvantages of reflectors?

• they’re very heavy, and require lots of metal support and scaffolding

• spherical aberration

what are the disadvantages for refractors?

• chromatic aberration

• spherical aberration

what does the exterior of a refractive telescope look like?

a long lens

PICTURE

what does the exterior of the reflective telescope look like?

a dish

PICTURE

in a refractive telescope, does the eyepiece do much on it’s own? why?

no, because it’s not an objective lens

why does the eyepiece of a refractive telescope not do much on it’s own?

because it’s not an objective lens exactly why?

what is the scouting lens on a refractive telescope?

a less magnified lens on the top of the main telescope for scouting

what is significant about the image the scouting lens produces?

it is less magnified, and it is inverted

what factor should you not mention when discussing the advantages and disadvantages of parabolic lenses?

cost cuz they’re both super expensive sooooo