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telescopeѲptics.net
.......................................................................................... CONTENTS
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5.1. Seeing error, tube currents
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6. EFFECTS OF WAVEFRONT ABERRATIONS
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5.2. Alignment errors
 For
best performance, optical surfaces of a telescope have to be in their
optimum alignment. Any deviation from the optimum position - be it
tilt, decenter or despace - will negatively affect wavefront accuracy
(FIG. 37). Degree of sensitivity to misalignment vary with
the design and system properties. In general, the more optical surfaces and the more strongly curved they are,
the greater misalignment sensitivity. More specific consequences of
misalignment are addressed with each of the telescope types presented
afterward.
FIGURE 37:
Misaligned optical surface can be tilted, decentered, despaced, or any combination of the three in
respect to its proper position. Tilt is expressed in angular form, while
decenter and despace are linear quantities. Tilt and decenter result in one
side of
the optical surface becoming closer to the wavefront than the other
one - a recipe for coma, which is by far the dominant resulting
aberration. Astigmatism, which results from wavefront's inclination
relative to the surface is small in comparison. Despace causes
mainly spherical aberration with converging or diverging wavefronts
- as shown to the right, result of the wavefront-to-surface
respective curvatures at the location of
reflection/refraction being different from those at the optimum
position.
Probably the most common miscollimation
error is coma induced by misaligned primary and secondary mirror. While
with a flat secondary mirror (Newtonian) it simply shifts the axis of
the primary away from the eyepiece axis, bringing a portion of the image
affected by (existing) off-axis aberrations to the field center, in a
two-mirror system it creates coma where there was none before. As can be
grasped from the illustrations (FIG. 37, left and center), the
misaligned surface induces the same amount of error to the wavefront
regardless of its inclination. Hence this coma is invariant to field
angle; in a coma-free system, it affects the entire field equally, while
in a system with existing (inherent) coma, it is intact only in the
field center, while either lessening - or adding to - existing coma in
the rest of the field.
The enormously high surface accuracy
requirements for optical surfaces of a telescope results in high
sensitivity to even slight changes induced by external forces. Such
forces commonly are: (1) pressure by the mounting elements, (2) thermal
expansion and contraction, and (3) force of gravity.
Mounting elements
pressure
usually causes some form of astigmatism, due to a typically
radially symmetric
distribution of the points of support and/or retaining. Typical pinching
pressure, for instance, induces trefoil - a three-winged form of
astigmatism, quickly revealing itself in the appearance of diffraction
pattern. Mounting pressure can result from thermal expansion of
optical elements and/or mechanical structure, which is one more reason
why optical elements should be left slightly lose within mechanical
structures holding them.
Thermal expansion and contraction
causes surface deformities due to their uneven rate within the body of
an optical element. Given material homogeneity and thermal properties,
it becomes more of a problem as the volume of an element increases, and
as the mass distribution gets more uneven. Relatively small differences
in the temperature can cause significant surface deformations and
resulting wavefront error. The dominant aberration induced is
usually spherical and/or edge defect error. The only cure to it is to get optical elements to a
thermal near-equilibrium with the surrounding air.
Gravitational force tends to
deform larger pieces of glass, especially if they are relatively thin.
The form of deformation depends on the position angle, as well as on the
support points distribution and level. While the error induced is
usually low with proper care taken in mounting the elements, it can
become significant if neglected.
Common characteristic of induced
telescope aberrations is that they do not have pre-determined level.
Unlike the aberrations inherent to the optical set, they vary with the user, telescope and the circumstance. The effect on
image quality is directly related to the RMS wavefront error they cause,
which is often times hard to determine. Partly due to
this elusiveness, they are, in general, less well known of, and taken
less seriously than intrinsic telescope aberrations. However, there is no
difference in the effect of aberration, regardless of its origin.
Aberrations induced to a near-perfect optics can make it perform as a
third-grade system. Thus, knowledge and control of induced
telescope aberrations are unavoidable part of the proper routine of using
a telescope.
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5.1. Seeing error, tube currents
▐
6. EFFECTS OF WAVEFRONT ABERRATIONS
►
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