Any obstruction placed in the light path of an imaging system prevents
waves from a portion of the wavefront to reach the focal zone. The
consequence is change in wave contribution at every point of
the diffraction pattern. The effect
is similar to that of wavefront aberrations in that it changes
pattern's intensity distribution, the specifics of it depending on
the form and size of obstruction. Those common to amateur telescopes
are: (1) central obstruction, a solid circular obstruction
usually caused by the secondary mirror or its cell, (2) secondary
holder vanes and (3) apodizing mask. The light loss is
significant only with apodizing mask. Real concern is the change in
point-object image intensity distribution caused by the changes in
pupil transmission properties, and its effect on image quality.
7.1. Central obstruction effects
Much has been said about the effect of central obstruction in the amateurs circles, most of it being speculation. The common notion is that it reduces effective linear aperture for low-contrast details by as much as obstruction diameter. Informal attempts have been made to find a theoretical basis for this empirical "rule". Not a few amateurs "tested" it and often concluded that it "works". What tends to be neglected is the pretty obvious fact that in any such comparison there is more than just a single factor - central obstruction - affecting low-contrast performance. Most of these factors - seeing error, overall optical quality, sensitivity to miscollimation and thermal errors, light scatter, baffling - favor smaller unobstructed aperture, usually a high-quality apochromatic refractor, over the larger reflector or catadioptric. Consequently, if the rule "works" empirically, it inadvertently proves it incorrect, as long as it insists on the difference in performance coming from the effect of central obstruction alone.
While the effect on image contrast is the most important one in most cases, it is not the only one. All of the effects of central obstruction are dependent on its size relative to the aperture. Hereafter, the relative central obstruction diameter in units of aperture diameter D is denoted by ο (left). As it will show ahead, the effects of central obstruction are more closely related to its relative area vs. area of clear aperture (or, alternately, to the relative annulus area), than to its relative linear size. While specialized optical instruments can have central obstruction well in excess of 0.5D, in most of amateur telescopes it is in the 0.2D to 0.5D range. Effects of obstructions significantly larger than 0.5D is not addressed, but they are mostly covered by given general relations.
The effects of central obstruction are:
(1) reduction in light transmission by a factor of (1-ο2), resulting from pupil obscuration,
(2) reduction in the relative peak diffraction intensity - and the energy content of the central maxima - by a factor (1-ο2)2, a
(3) reduction in the size of central maxima, and
(4) longitudinal extension of the diffraction pattern by a factor 1+ο2.
Reduction in the relative peak intensity given with (1-ο2)2 includes the effect of obscuration (in other words, keeping the flux in the annulus identical to that in the clear aperture would result in the peak intensity reduction equaling 1-ο2). Since obscuration effect doesn't affect the relative intensity distribution, only the absolute intensity at every point of the pattern, the effective peak intensity reduction in the context of the relative intensity distribution is given by (1-ο2). However, since the first minima shrinks in size, the total loss of energy from the central maxima is correspondingly larger.
The consequence of this is that the central maxima in aperture with CO, unlike, generally, apertures with aberrations, retains the average brightness of aberration-free aperture. As illustration, at left are plotted PSFs for 1/4 wave of primary spherical aberration and for 0.33D CO, both having nearly equal proportion of energy in the central maxima, with the former often used as comparable in effect on image quality. The plots show the difference in energy distribution, and the diffraction images of equally bright stars at 1.75x the diffraction limit of λ/D in radians (multiplied with 206.265 for arc seconds), correponding to MTF spatial frequency of about 0.57, show beter contrast of the obstructed aperture. It can be attributed to the brighter central maxima, and also less energy in the first dark ring.
The total relative loss of energy from the Airy disc is well approximated with double the relative obstruction area in the pupil, or 2ο2. Consequences of the latter in regard to intensity distribution within diffraction pattern differ somewhat for near-perfect wavefront on one side, and aberrated wavefronts on the other. Depending on the type and size of wavefront deformation, presence of central obstruction may improve, worsen, or have no appreciable effect on wavefront quality within the annulus, compared to the quality of the entire wavefront. This effect is small to negligible for the usual range of central obstruction sizes.
On the amplitude level, central obstruction (CO) effect on the intensity distribution I'(r) of the diffraction pattern - or PSF of the obstructed aperture - normalized to 1 for the peak intensity of clear circular aperture, can be generally expressed as I'(r)=(ΣA-ΣAo), with ΣA being the wave amplitude sum at a point of radius r in the image plane for unobstructed aberration-free aperture, and ΣAo the amplitude sum for the obstruction area, assumed to be a separate aperture. With coherent light, where individual wave amplitudes first sum up, and then square up for the intensity, the peak intensity reduction is (1-ο2)2, without the obscuration effect. With the field light, generally incoherent, with individual wave amplitudes squaring up individually for the intensity, and then summing up for the combined intensity, the peak intensity reduction remains proportional to the annulus area, i.e. given as (1-ο2).
Since amplitude contribution from every point in the pupil equalizes as r approaches zero (i.e. at the central point), the amplitude sum from the obstructed pupil area ΣAo for the central point relates to the clear pupil amplitude sum ΣA as ο2 to 1, with the new peak proportional to 1-ο2. However, since relative intensity distribution is measured vs. peak intensity normalized to 1, this new peak intensity becomes unit intensity with respect to which is intensity distribution measured. What in this context causes energu loss from the central maxima is solely it's smaller diameter, which for the usual range of CO sizes shrinks nearly in proportion to 1-ο2 (for linear obstructions larger than ~0.35D, more accurate is 1-ο2+ο4). Since the central maxima volume, representing its share of energy, changes nearly in proportion to its base area (figure of the bell-shaped curve does not stay strictly identical with the change of CO size, but the deviations are near-negligible), the energy loss from the central maxima due to CO can be conditionally expressed as the relative peak diffraction intensity drop I'(0) in an obstructed aberration-free aperture vs. that in a clear aberration-free aperture, I(0), normalized to 1, as:
I'(0) = (1-ο2) 2 (60)
which is, in effect, the relative annulus area (in units of the clear
aperture area) squared (keeping in mind that it is different in what
it represents from the formally identical expression for the drop in
the absolute peak intensity). In this context, it implies that the
size of CO corresponding to a given peak diffraction intensity
I'(0) is ο=(1-√I)1/2.
With the relative intensity distribution - the basis for contrast
transfer - being independent of the actual peak intensity, PSF of an
obstructed aperture is conventionally normalized to unity for its
actual peak intensity, i.e. given as
Changes in the incident flux (point-source intensity) do affect actual pattern brightness, but not the relative intensity distribution within diffraction pattern, which is what determines the efficiency of contrast transfer.
NOMINAL CHANGES IN INTENSITY DISTRIBUTION WITH THE CHANGE OF C.O.
Following table presents
intensity distribution within the fourth minima for unobstructed and
selected obstructed apertures. Pattern radius r is in units of
λF, and intensity I'(r) is normalized to peak diffraction intensity, to reflect more clearly relative changes in intensity distribution; EE' is encircled energy. Analogously to the central intensity, the actual relative encircled energy of the obstructed aperture is smaller by a factor (1-ο2)2, or EE=(1-ο2)2EE'. For ο=0.30, energy encircled within first minima is 0.565, with 0.263 in the rings, but their values relative to the peak intensity - which is what matters for contrast transfer - are as shown in the table. If the photon flux in the annulus would equal that in the unobstructed aperture, both figures would be larger by a factor of 1/(1-ο2)2, or 0.682 and 0.318, respectively, and the central intensity would be identical to that of the clear aperture.
As plots above indicate, most of the energy lost to the central maxima is transferred to the first bright ring with obstructions of 0.4D and smaller, while larger obstructions also brighten the 2nd bright ring. Logarithmic PSF shows much better the effect of obstruction on intensity and pattern of fainter rings (for clarity, given only for the ο values of 0, 0.30 and 0.50).
The increase of energy transferred farther away is moderate up to about 0.3D CO. With 0.4D, energy beyond six Airy disc radii from the pattern center is more than 50% greater than for unobstructed aperture, and for 0.5 CO it is more than doubled. This is not unusual; for instance, primary spherical aberration (0.4 wave P-V) causing near identical relative drop in the peak PSF intensity as 0.5D CO increases the energy beyond that point less than 20% below the increase due to the obstruction. Coma (2/3 wave P-V) more than 40% below, but considering its magnitude in the outer field of fast paraboloids, it will brighten background much more than central obstruction (which is normally significantly smaller than 0.5D). Astigmatism (0.6 wave P-V) actually reduces the energy level beyond that point, and so on.
Nominal change in the normalized peak intensity, given by ΔI = 1-I'(0), closely approximates the relative amount of energy transferred from the disc to the rings area. For instance, 30% CO (ο=0.3) will lower normalized central diffraction intensity produced by a perfect wavefront from 1 to 0.828; at the same time, relative transfer of energy from the Airy disc to the rings area is ~0.17, or 17%. A simple rule of thumb is that the relative loss of energy up to ο ~0.4 is closely approximated - as a ratio number - by 2ο2, and for ο~0.5 and larger by 1.9ο2.
THE CONDITIONAL "RELATIVE PEAK INTENSITY" OF OBSTRUCTED APERTURE VS. STREHL RATIO; MTF
For instance, 33% linear CO (ο=0.33) effectively reduces normalized central intensity to 0.79. Does that make its effect comparable to that of 0.79 Strehl wavefront error equivalent? Yes and no.
There is a distinct difference in the energy distribution change caused by CO compared to, say, primary spherical aberration at the best focus. Looking at the energy in the central maxima as the volume under the bell-shaped curve, the main effect of spherical aberration is reducing the height of this volume, with the reduction of volume base (width) being nearly negligible in comparison (for given energy density under the volume, as illustrated on FIG. 2). Specifically, 1/4 wave P-V wavefront error reduces central intensity by 20%, while shrinking the central maxima less than 3% linearly. The difference in volume (energy) vs. aberration-free volume is transferred to the rings area; it is approximately 20%, closely approximated by the relative reduction in central intensity.
On the other hand, the relative volume height corresponding to the central maxima is reduced only by 11% due to the effect of 33% CO, but its base is also reduced by nearly as much linearly. When normalized to 1 for the new (reduced) peak, the relative energy loss comes from its base radius smaller by a 1-o2 factor; since it is the square of it that determines the volume for given height, the relative energy loss is also around 21%.
Thus, the relative volume reduction - i.e. energy loss to the rings - is nearly identical to that caused by 1/4 wave P-V of spherical aberration. Another similarity is that most of this energy goes to the 1st bright ring in either case. As a result, contrast transfer over the range of MTF frequencies where the rings energy is dominant factor - from 0 to about 0.4 - is also very similar.
However, due to the smaller central maxima, the obstructed pattern rebounds in contrast transfer not only above the aberrated clear aperture level. In the range of MTF frequencies where the dominant factor of contrast transfer becomes the size of central maxima, generally from about 0.4 to 1, contrast transfer for aperture with D/3 CO also rises above that for clear aberration-free aperture.
The similarity extends to the contrast drop-off for the range of spatial frequencies below ~0.5 (approximately, left side of the MTF graph), which is the range of resolvable low-contrast details. In other words, for this range of spatial frequencies, the peak intensity I'(0) resulting from CO is comparable to the Strehl ratio for wavefront aberrations with respect to the effect on contrast and resolution. They both indicate relative amount of energy transferred to the rings area, the main factor determining contrast level at low- to mid-frequencies of the MTF.
Thus, with the RMS wavefront error ω in terms of the Strehl ratio S being given by ω=0.24√-logS, direct relation can be established between the relative linear size ο of CO and similar in effect RMS wavefront error ω (in units of the wavelength) with respect to low-contrast detail effect as:
ω~0.24[-log(1-ο2) 2 ]1/2 (61)
(it can be simplified to an empirical approximation, ω~0.24 ο for ο <0.3 and ώ ~0.25ο for 0.15<ο<0.3).
For ο =0.325, this gives ώ~0.075, practically equal to 1/4 wave P-V of primary spherical aberration level. Comparison with the effect of spherical aberration is most appropriate, due to both CO and spherical aberration causing radially symmetric intensity distribution, with the predominant pattern change being brightening and widening of the first bright ring.
However, the actual MTF graph (FIG. 104) indicates that the above formula is somewhat pessimistic in regard to the effect of CO. Obstructed aperture has significantly better contrast transfer - even better than that of a perfect aperture - in the right half of MTF frequency range (i.e. for details smaller than about 2 λF linear, or 2 λ/D in radians. Even in the left half of the graph (range of resolvable low contrast details), obstructed aperture has an edge.
The reason is the effect unique to CO (at least in its extent), namely, the reduction in size of central maxima caused by it. The linear reduction is closely approximated by a factor (1-ο2 ) for obstructions of ~D/3 and smaller, and by a factor (1-ο2 +ο4 ) for larger obstructions, up to ~0.7D. Good approximation for the 1st minima reduction ratio for any obstruction size is 1-οn , with n=2+[ο2 /(1-ο)], or n=a+(1/a), with a=1-ο. The smaller central maxima, combined with more of the outer energy contained in the 1st bright ring, gives to the obstructed aperture an edge in contrast transfer efficiency with respect to the spherical aberration error of near identical nominal energy loss from the Airy disc. In effect, point-object-resolution-wise, central obstruction makes the aperture act as larger by a 1/(1-ο2 ) factor with added equivalent of (1-ο2 )2 Strehl of primary spherical aberration.
* There is a small difference between polychromatic (photopic) and monochromatic MTF plot even for non-refractive optics, showing as lower contrast transfer in the ~0.4-0.6 frequency range of the polychromatic MTF, due to the longer wavelengths forming slightly larger Airy disc. However, it doesn't change appreciably plot appearance, due to perfect aperture being affected in the same manner.
This effect is present at all obstruction sizes. As a result, size of CO causing similar contrast drop to that caused by the amount of low spherical aberration indicated by Eq. 60 is, for the range of resolvable low contrast details (left half of the MTF graph), nearly 10% larger, linearly. Thus better approximation for the RMS error of spherical aberration similar in effect to that of the central obstruction ο for this frequency range is given by:
ω~0.22[-log(1-ο 2 )2 ]1/2 (61.1)
(it can also be simplified to an empirical approximation, ω~0.21 ο; it is within a couple of percentage points from the true value for ο ~0.4 and smaller) .
Note that the MTF output varies somewhat from one program to another. For instance, Aberrator and ATMOS show more of a contrast drop resulting from CO. It still has an edge over spherical aberration, but the numerical constant in the above approximation is between 0.22 and 0.23.
Analogous to the contrast transfer of a perfect aperture (FIG. 45, top right), that of an obstructed aperture is given by the overlapping area of two unit-diameter circles, but this time for the two annuli, and relative to the annulus area.
As mentioned on the MTF page, the standard MTF plot, as the one above, shows contrast transfer for sinusoidal intensity distribution. As the pattern of intensity distribution and/or detail shape, changes, so does its contrast transfer. This can be illustrated comparing MTF for standard sinusoidal pattern with the contrast transfer function (CTF) for square-wave distribution (i.e. clear dark and bright lines), as shown on FIG. 105.
The overall effect on contrast transfer here is similar, with probably the most significant difference being that the limiting low-contrast resolution (the pattern in top right corners are high-contrast patterns, for which cutoff frequency lies at the horizontal scale) is somewhat reduced for the standard sinusoidal MTF pattern, but not for the square-wave pattern.
Next page addresses apparent inconsistencies and the question of applicable concept in the standard theoretical treatment of the effect of central obstruction.