OP - Telescopes - Week 4

Question 1

What are the two classifications for telescopes? Describe what composes each system.

Answer 1

Refracting - made completely of lenses

Reflecting - the objective is a mirror or mirror system

Question 2

Which type of telescope is more common for low and high magnifications?

Answer 2

Refracting for low

Reflecting for high

Question 3

What is the similarities and differences between a keplerian and galilean telescope?

Answer 3

The both have a positive power objective, but the galilean has a negative power eye lens, where the keplerian has a positive one.

Question 4

What kind of overall power does a keplerian telescope have, and why?

Answer 4

The object and image are both at optical infinity, and so the system has zero power.

Question 5

Define an afocal system.

Answer 5

A system in which no net divergence or convergence occurs, such a system has objects and images both at optical infinity.

Question 6

Define Fo, Fe, and d. Describe the relationship these values have that eventually results in the overall power of the keplerian telescope system.

Answer 6

Fo - focusing power of the objective lens
Fe - focusing power of the eye lens
d - distance between the objective lens and the eye lens.
Fo is the distance between the objective lens and the field lens.
Fe is the distance between the field lens and the eye lens.
F = Fo + Fe - dFoFe
d = Fo + Fe
Therefore F = 0

Question 7

Define the equation for magnification.

Define the equation in terms of F.

Answer 7

M = angluar size of the image seen through the telescope / angular size of the object seen unaided
or
M = θ’ / θ
The final equation in terms of F is:
M= -Fe / Fo

Question 8

What does a negative sign on magnification indicate?

Answer 8

The image is inverted.

Question 9

Define the aperture stop and entrance pupils of a keplerian telescope.

Answer 9

The objective lens is both the aperture stop and the entrance pupil.

Question 10

What is the exit pupil of a keplerian telescope, and how can it be found?

Answer 10

A ray can be traced to show it is the image of the aperture stop. It is found in front of the eye lens.
It is the focal point of the eye lens.

Question 11

Is the image of a keplerian telescope real or virtual?

Answer 11

It is real.

Question 12

Define the eye relief.

Answer 12

Distance from the eye lens to the exit pupil.

Question 13

Define the equation for eye relief, and exit pupil diameter.

Answer 13

Eye relief = -d/M

Question 14

Define the equation for magnification in terms of pupil diameters.

Answer 14

M = entrance pupil diameter / exit pupil diameter

Question 15

Are field lenses always used? What can a field lens be used to mitigate?

Answer 15

Not always used.

It can be used to reduce the effects of vignetting.

Question 16

What happens to the entrance and exit pupils if a field lens is used?

Answer 16

Entrance pupil is unaffected.

The exit pupil will change position.

Question 17

What happens to the exit pupil if the field lens is placed exactly at the intermediate image plane?

Answer 17

Exit pupil size will not change.

Question 18

Can a field lens be used with a galilean telescope? Explain why.

Answer 18

No, because a negative power lens is placed in front of the objective lens, before the beam converges to a point. Thus the objective lens forms a virtual image to the right of the eye lens, and there is no suitable intermediate image plane.

Question 19

Describe how keplerian and galilean telescopes give inverted or erect images, using the magnification equation (in terms of F), and the sign of the lens.

Answer 19

M = -Fe/Fo

Keplerian
Has a positive eye lens, and so Fe remains negative, and the image is therefore inverted

Galilean
Has a negative eye lens, and so Fe becomes positive, and the image is therefore erect

Question 20

Does a galilean telescope have an aperture stop?

Answer 20

The iris of the eye acts as an aperture stop, only as a combined system.
The galilean telescope has no intrinsic aperture stop.
The effective aperture stop is therefore the entrance pupil of the eye.