Telescopes Flashcards
o To investigate the construction and properties of telescopes. o Understand the meaning of the term “afocal” o Know and be familiar with the equation for the power of two thin lenses.
What are the two classifications for telescopes?
Refracting telescopes - the objective is a lens (e.g. Keplarian and Galilean)
Reflecting telescope - the objective is a mirror
Which type of telescope is more common for low and high magnifications?
Refracting telescopes - low magnification and compact
Reflecting telescope - high magnification >100 in astronomy
What are the similarities between a Keplerian and Galilean Telescope?
Both telescopes allow the magnification of objects from point infinity and are afocal (i.e they do not alter the relative vergence of the beam and all beam exit the optical system in a concurrent manner).
Both have a positive power objective lens that serve to collect bundles of rays from point infinity to a closer point (i.e. at the back focal point of the objective lens), where it is then magnified by the eye lens.
What are the differences between a Keplerian and Galilean telescope.
Keplerian telescope has a positive power eye lens whilst the Galilean telescope has a negative power eye lens.
The Galilean telescope is shorter than the Keplerian telescope.
The objective lens of the Keplerian telescope is the aperture stop and the entrance pupil whilst the aperture stop and exit pupil of the Galilean telescope is the entrance pupil of the observer’s eye.
The Keplerian telescope projects an inverted image whilst the Galilean telescope projects an upright image.
The overall magnification is smaller in the Galilean telescope than the Keplerian telescope.
How can we construct a Keplerian telescope?
A positive power object lens is placed to the right of the object plane. A positive power eye lens is placed at a position where the back focal point of the objective lens and the front focal point of the eye lens coincide. This ensures that the system is afocal.
Where are the images formed in a Keplerian telescope?
A real intermediate image is formed at the back focal plane of the objective lens. which is coincident with the front focal plane of the eye lens. The final image is magnified by the eye lens and is located at infinity.
What are the characteristics of afocal telescopes?
Because the object and image are at infinity, the system has zero power. Hence the ray will not appear to cross the axes anywhere and there are no cardinal points (i.e. no effective power, no focal points, no principle points and no nodal points).
What determines the afocal condition of the Keplerian telescope?
Having the back focal point of the objective lens coincident with the front focal point of the eye lens.
How can we calculate the telescope length?
The sum of the focal lengths (objective + eye lens)
How is the magnification of a Keplerian telescope defined?
The magnification is defined as the angular magnification and is calculated as: angular size of image seen through the telescope/angular size of object seen unaided. This gives rise to (-)Fe/Fo; negative sign indicating the image is inverted.
What happens if the object is not at infinity using a Keplerian telescope?
The image size is smaller than the object size but the image appears to be closer. The transverse magnification of the image is 1/M. Hence the image appears to be closer by a factor of M^2 and is still magnified in angular size by a factor of ~M.
Where is the aperture stop and entrance pupil of the Keplerian telescope?
The objective lens.
How can we locate the exit pupil of a Keplerian telescope.
The exit pupil is the image of the aperture stop in image space and can be found by tracing the paraxial pupil ray.
Is the final image of a Keplerian telescope real or inverted?
Real.
What is the eye relief?
The eye relief is the distance from the eye lens to the exit pupil.
