Optical Instrumentation

Question 1

What are some examples of third order/monochromatic aberrations?

Answer 1

• spherical aberration
• coma
• astigmatism
• curvature of field
• distortion
• chromatic aberration (refracting elements)

Question 2

What causes aberrations?

Answer 2

Imperfections in optical elements and rays hitting the elements beyond the paraxial approximation.

Question 3

What is a spherical aberration?

Answer 3

Image points formed by rays from the same object point will appear to come from different positions due to reflection/refraction from different points of the imaging element.

Question 4

How can you rectify spherical aberrations?

Answer 4

Stopping down the system to restrict the cone of rays or use a combination of positive and negative lenses to cancel out the aberration.

Question 5

What is chromatic aberration?

Answer 5

Due to different wavelengths of light experiencing different refractive indices passing through an element, the focal length becomes wavelength dependent.

Question 6

How can we reduce chromatic aberration?

Answer 6

Use mirrors or multiple refractive elements of opposite powers.

Question 7

Reflecting telescopes use …

Answer 7

mirrors

Question 8

Refracting telescopes use…

Answer 8

lenses

Question 9

What type of telescope uses both mirrors and lenses?

Answer 9

catadioptric

Question 10

What type of telescope is the Keplerian/astronomical and what type of image does it produce (how)?

Answer 10

• refracting telescope
• uses two lenses and produces inverted image

Question 11

What type of telescope is the Galilean and what sort of image does it produce (how)?

Answer 11

• refracting telescope
• uses a negative focal length eyepiece lens to produce an upright image

Question 12

What allows a real image in the object plane to be produces by Keplerian and Galilean telescopes?

Answer 12

Large objective lens collecting ~ parallel rays from distant objects

Question 13

How do telescopes often correct for chromatic aberration?

Answer 13

Making the objective lens a doublet

Question 14

The intermediate image for the eyepiece is the image formed by the objective. What type of image is it for Keplerian and Galilean?

Answer 14

Real for Keplerian
Virtual for Galilean

Question 15

What is the formula for angular magnification for a telescope?

Answer 15

M = fo/fe

Question 16

What sign of M (angular magnification) gives an upright image?

Answer 16

negative

Question 17

What is the formula for telescope length?

Answer 17

L = fo + fe

Question 18

What is the exit pupil for telescopes and where should it be?

Answer 18

The image of the entrance pupil (objective lens) in the ocular)
It should be far enough from eyepiece for comfortable eye relief

Question 19

Where is the exit pupil Galilean

Answer 19

behind eyepiece

Question 20

Where is the exit pupil Keplerian

Answer 20

just outside ocular

Question 21

Where would a reticule be placed for an astronomical telescope?

Answer 21

in the real intermediate image

Question 22

What is the formula for linear magnification of a telescope?

Answer 22

me = Dex/Dobj = -fe/fo = 1/M

Question 23

Why are reflecting telescopes favoured for larger installations?

Answer 23

• don’t require large heavy lenses
• maintain optical quality
• remove chromatic aberration

Question 24

Name the 3 basic reflection telescopes

Answer 24

Newtonian, Cassegrain, Gregorian

Question 25

How does a Newtonian telescope work?

Answer 25

- parabolic focusing mirror, all rays to focal point - plane mirror intercepts rays, diverting them to a point near the eyepiece - parabolic mirror removes spherical and chromatic aberrations

Question 26

How does the Cassegrain telescope differ from the Newtonian?

Answer 26

- it uses a hyperbolodial mirror instead of a parabolic mirror - this focusses light through a hole in the primary mirror - allows perfect imaging between primary and secondary mirrors

Question 27

What type of secondary mirror does the gregorian telescope use?

Answer 27

concave ellipsoid

Question 28

Describe how the Schmidt telescope works

Answer 28

- catadioptric - refractive component is a correcting plate ensuring spherical aberration from the concave mirror used to form the image is minimised - this modifies the optical path - it is a modification to conventional reflection telescope

Question 29

What differs about the rays coming into a telescope and a microscope?

Answer 29

the rays coming into a microscope are coming from nowhere near infinity

Question 30

where does a microscope form real and virtual images?

Answer 30

a real image is formed within the microscope tube intermediate image is just inside the eyepiece focal length the observer sees an inverted, magnified, virtual image

Question 31

How can you give a hard edge to the field of view of a microscope?

Answer 31

Place an aperture by the intermediate image to act as a field stop

Question 32

What is the formula for the angular magnification of a microscope?

Answer 32

M = 25(cm)/feff(cm) = 25(fe + fo - d)/fofe

Question 33

What is the image to object distance ratio for a microscope?

Answer 33

so'/so = (d-fe-fo)/fo where so'=d-fe (d is dist between lenses)

Question 34

(in words) what is the total magnification for a microscope?

Answer 34

the product of angular magnification of eyepiece at infinity and linear magnification of objective

Question 35

What is the formula for total magnification of a microscope?

Answer 35

M = (so'/so)(25/fe) = -(25/fe)(L/fo) standardised L=16c,

Question 36

What is the lateral magnification?

Answer 36

m = hi/ho = L/fo where L is dist between objective image and second focal length

Question 37

How are modern microscopes infinity corrected?

Answer 37

They incorporate a tube lens between the objective and eyepiece to create infinity space between tube and objective

Question 38

What happens to the focal length and diameter of the objective as the magnification increases?

Answer 38

They decrease

Question 39

(in words) what is the numerical aperture

Answer 39

The ability of a lens to collect rays

Question 40

What is the formula for NA

Answer 40

NA = nsinθ where θ is the half angle of the cone of rays from an object to the edge of the lens

Question 41

What is the maximum value for NA in air

Answer 41

1

Question 42

How does a microscope system overcome the limit of the numerical aperture?

Answer 42

using an immersion oil improves the NA, matching to the refractive index of the coverslip to increase NA by noil

Question 43

What is total internal reflection fluorescence microscopy?

Answer 43

images a very thin section of a sample above coverslip, requires very high NA