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Do you remember the content of A-level Physics regarding lenses? 👩🏫
Reflecting telescopes
Focal point of concave mirror
- Focal point = in-front of mirror
- Set up = highly polished curved glass mirror
- Primary mirror
- Mounted at bottom.
- When starlight shines on this mirror, it is reflected back up the tube to form an image at the primary focus
- You can place photographic film or electronic devices at the prime focus to record the image
- Or use additional mirrors to reflect the light to another spot for viewing.
- Cassegrain arrangement using a parabolic concave primary mirror and convex secondary mirror, ray diagram to show path of rays through the telescope as far as the eyepiece
- Schmidt-cassegrain telescope combines an extremely short focus primary mirror at the back end of a sealed tube with a thin lens at the front.
- Small convex mirror, a secondary mirror, to reflect light back through a hole cut in the primary mirror at the bottom end of the tube
- More compact than a refractor or Newtonian reflector of the same aperture.
- Relative merits of reflectors and refractors including a qualitive treatment of spherical and chromatic aberration.
Reflectors
Advantage:
Suffer no chromatic aberration
- Only one optical surface
- Cheaper to produce
- Telescopes over about 200mm are all reflectors
Disadvantages:
Central obstruction due to secondary mirror
- Scattering of light and loss of contrast
All Newtonian reflectors suffer from coma
- Smaller the focal ratio the greater the coma
- Incorrected diffraction limited field becomes smaller.
Maintenance
- Mirrors require recoating after several years
- Sensitive to jostle, bumped and transported
- The shorter the focal length, the shorter the collimation tolerances for achieving diffraction limited performance, so accurate collimation becomes much more important.
Large reflectors with thick primary mirrors have a difficult time colling to ambient temp
- Fans sometimes used
Refractors
Advantages:
Totally clear aperture
- No central obstruction causing light to be scattered from bright to darker areas
- Better contrast than reflector.
Low maintenance
- Lenses don’t require recoating
- Optical tube assemble does not require collimation (accurate adjustment of line of sight)
- Lenses are fixed in tube and usually do not become misaligned.
Disadvantages:
Refractor is a closed tube assembly
- Requires longer amount of time to cool to ambient temp
Some chromatic aberration in bright images
- Commonly manifests itself as faint fringes of colour around objects like mars or Jupiter.
Expense of producing a large achromatic lens
- A chromatic triplet lens has 6 surfaces which must be figured
- Lost ratio between a 150mm apochromatic lens and high quality 150mm mirror is at least 10-to-1.
Spherical aberration
no clear focal point
- Mirror defect that blurs a stars image
- Defect of spherical surfaces
- Parabolic mirror avoids mirror defect
- Parts of the mirror at different distances from the optical axis reflect starlight to slightly different focal points.
Chromatic aberration
- Lens defect
- Starlight consists of all the colours of the spectrum
- Light focuses different colours at different distances
- Blurs image
- To counter defect an achromatic lens is used
- Combination of two or more lenses made of different glass
- A properly curved mirror reflects all the starlight at the same point
- Image formed by a reflector had no blurred colours.
Resolving power of a telescope, Rayleigh criterion
- A telescopes resolving power is its ability to produce sharp, detailed images under ideal observing conditions.
- Rayleigh Criterion
- Depends directly on the size of the aperture and inversely on the wavelength of the incoming light
- Determines the smallest angle between two stars for which separate, recognisable images are produced
- Two light sources can just be distinguished if the centre of the airy disk from one source is at least as far away as the first minimum as the other source.
Reference:
https://getrevising.co.uk/resources/lenses_and_optical_telescopes
This is the end of the topic!
Drafted by Cherry (Chemistry)