UNSW Astrophysics Postgraduate Course

Module 3, Session 2

Optical and Infrared Techniques


Convener: Michael Burton

Module Syllabus

Lecture 1. Telescopes

Outline notes of the lecture. Figures hopefully to come later.

Table of Contents

  1. Introduction
  2. Designs
  3. Aberrations
  4. Mountings
  5. Recent Innovations
  6. Miscellaneous
  7. AAT
  8. Tutorial Questions
  9. Tutorial Solutions

1. Introduction
Unknown inventor, but applied to astronomy in 1610 by Galileo with enormous consequences!

Observing system requires: Telescope + Analyser + Detector

2. Designs

Refractors
Use two lenses, but necessitates long tubes (focal length), support around rim, equal surfaces and has chromatic aberration.

Large plate scales and thermally stable so best for astrometry.

Reflectors
Uses coated mirror.

Mirror shape is sensitive to temperature changes, so some thermal control required.

Regular re-aluminizing needed.

Several types of optical design in use:

image not found or accessible Optical designs for Prime, Newtonian, Cassegrain and Coude focii for reflecting telescopes.

Prime Focus
Least light loss, but awkward for access and limited room.

Newtonian Focus
Essentially a bent prime focus. Popular on amateur telescopes but awkward for instrumentation.

Cassegrain Focus
Convenient for mounting instruments.

Uses a folded beam so long focal lengths can be achieved with large plate scales.

Secondary a hyperboloid, placed so one focus at primary focus and instruments at other focus.

Coude Focus
For high dispersion spectroscopy needing large instrumentation.

Use two flat mirrors in light path.

Requires a field rotator.

No good for IR astronomy!

Nasmyth
Equivalent of Coude for Alt-Az telescopes. 2 large platforms available for instrumentation.

3. Aberrations

Types
Listed in relative order of importance

Chromatic
Occurs for refractors (lenses) but not reflectors (mirrors).

Refractive index is a function of wavelength, so blue light brought to focus nearer than red light.

image not found or accessible Chromatic aberration, occurring for lenses only. The refractive index of glass varies with wavelength, so violet rays are brought to a focus nearer the lens than are red rays. The distance between two foci is greatly exaggerated in the diagram, but it is several percent of the mean focal length. The best image for a point source is a filled circle (the circle of least confusion).


Spherical

image not found or accessible Spherical aberration, illustrated for a lens; the difference in the focal length for rays from different parts of the lens is greatly exaggerated, but is typically greater than 1%.

Rays from edge of lens (mirror) come to focus nearer to lens than those further away. Similar to chromatic aberration in this sense (though no wavelength dependence!) but effect is smaller.

Typically aberration ~1% of focal length.

Removed by parabolic mirror, but these are harder to make than spherical mirrors.

HST suffered from spherical aberration!

Coma

image not found or accessible Coma for a paraboloidal mirror; each annular zone of the mirror forms a circular image whose size decreases with the size of the zone. For the positive coma illustrated here, the smaller images are nearer the axis. Within each image, each point is formed by rays from diametrically opposite sides of the mirror. The image on right shows the overall image shape formed by comatic aberration; the overlapping circles form a 'tear-drop' shape whose size is proportional to the off-axis angle.

Major off-axis aberration.

Proportional to phi, the angle between incoming ray and axis.

Each annular zone on mirror forms a separate image circle, which is centred further away from axis the further the zone is from the centre of the mirror.

Produces a 'comet-like' image.

Astigmatism

image not found or accessible Astigmatism for a paraboloidal mirror. Rays in the plane defined by the incoming beam form a line-image in an orthogoonal plane. Rays in that plane form a second line-image at a different distance from the mirror. The distance between the line-images, and so the size of the circle of least confusion, is proportional to the square of the off-axis angle.

Proportional to phi2 so wide-angle imaging most affected.

Focal length of plane of light perpendicular to focal plane different to that from the mutually orthogonal plane.

Also these two planes form lines at their focal points which are orthogonal.

Thus produces elliptical images.

Not related to astigmatism in the human eye!

Applications
Dealing with these aberrations has lead to:

Ritchey-Chretien Designs
Hyperbolic conics used for primary (eliminates coma) and secondary (eliminates spherical aberration).

Schmidt Telescopes
Spherical primary with corrector plate placed across aperture, with smoothly varying radial thickness, to remove the aberration.

Stops limit the effective aperture so that angles of off-axis rays are small, minimising coma.

Thus aperture of telescope is smaller than primary mirror!

Distortion in plate scale proportional to phi3, so affects wide-field most.

image not found or accessible The principle of the Schmidt camera. All rays pass through the centre of curvature, C, because other rays are cut out by a stop at C. Thus there are no off-axis aberrations. If the spherical mirror has radius R then the focal surface, which also is spherical, is at radius R/2. The right hand image is a schematic of the Schmidt at Siding Spring, with the inset showing the shape of the correcting lens.

The lower diagram shows a Maksutov-Cassegrain camera in which the tube is made considerably shorter because light is reflected off an aluminized patch on the back of the meniscus correcting lens and is brought to a focus behind the mirror.


Both these designs create field curvature in the focal plane.

4. Mountings

image not found or accessible Left: Schematic of an equatorial telescope mounting. The polar axis points to the celestial pole and is parallel to the Earth's rotation axis. The declination axis is perpendicular to the polar axis and to the optical axis of the telescope. The telescope is moved around both axes until it is pointing to the object of interest, and then is clamped. Pointing is maintained by driving the entire mount about the polar axis at the same rate as, but in the opposite direction to, the Earth's rotation.

Right: Schematic of on alt-azimuth mounting. The telescope is mounted between two vertical supports and moves in a vertical plane about a horizontal axis (the altitude axis). The supports are themselves mounted on a platform, which can rotate about a vertical axis (the azimuth axis). To keep the telescope pointing at a given object it is necessary to drive simultaneously about both axes to compensate for the Earth's rotation.

Equatorial
To follow the Earth's rotation by driving in one axis.

Polar axis parallel to rotation axis.

Declination axis perpendicular to it.

Hard to mount due to large torques producing bending.

Horseshoe
AAT
Fork
INT
Yoke
UKIRT
German

image not found or accessible Mounting systems for (a) Horseshoe, (b) Fork, (c) Yoke and (d) German equatorial telescopes.


Alt-Azimuth
The two rotation axes are aligned with the zenith-nadir for azimuth, and the horizontal for altitude control.

Much easier to construct, but requires more complicated computer control.

Field rotation occurs, and objects at zenith cannot be followed (infinite slew rate!).

Transit Circle
For accurate positional work, by minimizing motions about an axis.

5. Recent Innovations
To see fainter objects we need both:
(a) larger apertures to gain more light,
(b) better, larger detectors to use it more efficiently.

Cost proportional to D3.

Engineering Approaches
Challenging designs. Two main approaches taken:

Monolithic Mirrors
which includes
(i) Spin casting

(ii) Thin, honeycomb mirrors.

(iii) Active support system to maintain primary shape.

Approach followed by Gemini.

Multi-Mirrors
Combining light from smaller telescopes by
(i) Binocular or multi-mirror telescopes.

(ii) Segmented mirrors (Keck).

Control of Light
Can be improved by:

Active Optics
Adjusting primary shape (timescales of minutes).

Adaptive Optics
Correcting for atmospheric turbulence (timescales of 10-100 msec).

6. Miscellaneous

Telescope Parameters
Resolving Power
f = lambda / D (radians) = 0.14 (1m/D) arcsec for V (5500Å).

Seeing
From microthermal temperature fluctuations ('turbulence').

Typically 0.5-2" in the visual, but varies as lambda-0.2.

f-ratio
fr = F / D

'Speed' increases as fr decreases (name from photographic origins). Fast means wide field but small pixel scale.

Infrared Telescopes
Some additional requirements to minimise background:

Slow
Large pixel scale, reducing background.

Low Emissivity
Clean mirrors with low emissivity coatings.

Low Background
Minimise stray radiation (from both sky and telescope) and make all obstructions small.

Cool the telescope and environment (eg Antarctica, space).

Often use a chopping secondary to make rapid differential measurements of the sky.

7. AAT
3.9m Telescope with multiple secondaries:

Prime focus f/3.3

Cassegrain focus f/8 (Ritchey Chretien), f/15, chopping f/36

Coude f/35

Cervit mirror (low expansion) with hyperboloid shape.

Aluminized yearly.

Horseshoe mount.

Active focus adjustment.

Some dome thermal control.

Superb tracking and pointing.

Impressive range of instrumentation!

image not found or accessible The telescope structure and mounting system for the AAT.

image not found or accessible Optical configurations available with the AAT, via top-end changes. Note this diagram pre-dates 2dF!!

Back to module course syllabus for Optical and Infrared techniques.

Michael Burton
August 6, 1997