OP - Telescopes - Week 4 Flashcards
What are the two classifications for telescopes? Describe what composes each system.
Refracting - made completely of lenses
Reflecting - the objective is a mirror or mirror system
Which type of telescope is more common for low and high magnifications?
Refracting for low
Reflecting for high
What is the similarities and differences between a keplerian and galilean telescope?
The both have a positive power objective, but the galilean has a negative power eye lens, where the keplerian has a positive one.
What kind of overall power does a keplerian telescope have, and why?
The object and image are both at optical infinity, and so the system has zero power.
Define an afocal system.
A system in which no net divergence or convergence occurs, such a system has objects and images both at optical infinity.
Define Fo, Fe, and d. Describe the relationship these values have that eventually results in the overall power of the keplerian telescope system.
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
Define the equation for magnification.
Define the equation in terms of F.
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
What does a negative sign on magnification indicate?
The image is inverted.
Define the aperture stop and entrance pupils of a keplerian telescope.
The objective lens is both the aperture stop and the entrance pupil.
What is the exit pupil of a keplerian telescope, and how can it be found?
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.
Is the image of a keplerian telescope real or virtual?
It is real.
Define the eye relief.
Distance from the eye lens to the exit pupil.
Define the equation for eye relief, and exit pupil diameter.
Eye relief = -d/M
Define the equation for magnification in terms of pupil diameters.
M = entrance pupil diameter / exit pupil diameter
Are field lenses always used? What can a field lens be used to mitigate?
Not always used.
It can be used to reduce the effects of vignetting.
What happens to the entrance and exit pupils if a field lens is used?
Entrance pupil is unaffected.
The exit pupil will change position.
What happens to the exit pupil if the field lens is placed exactly at the intermediate image plane?
Exit pupil size will not change.
Can a field lens be used with a galilean telescope? Explain why.
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.
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.
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
Does a galilean telescope have an aperture stop?
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.
