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◄ 7.1.1. Telescope central obstruction: size criteria   ▐    7.3. Apodizing mask ►
 

7.2. Spider diffraction

More often than not, central obstruction by a smaller secondary mirror is accompanied with aperture obstruction caused by its support structure - so called spider vanes. Their effect is generally small, but it can be significant. That makes them worth of a closer look.

Unless the secondary mirror cell is supported by an optical window, the supporting vanes - so called spider vanes - are in the optical path, altering emitting area of the wavefront and, thus, creating diffraction effect. As long as the pupil area obstructed by the vanes remains relatively small, spider diffraction is more of a cosmetic damage than seriously affecting contrast level (FIG. 108). Analogously to the central obstruction effect, what  

can be thought  of as a Strehl ratio degradation factor caused by spider diffraction is, in effect, the ratio of the clear (annular) pupil area with and without the vanes, squared, or                        

with N being the vane count, τ the relative vane thickness and ο the relative size of central obstruction, both in units of the aperture diameter. The negative factor equals the relative spider area in units of the clear aperture (i.e. annulus) area; this is consistent with degradation factor caused by central obstruction (Eq. 60). Average spider area is somewhere between 1% and 2% of the clear aperture area. That puts an average spider vane contrast degradation factor between 0.98 and 0.96 - below the level of 1/30 wave RMS wavefront error. As plots above show, it decreases somewhat with larger obstructions, but that is, of course, paid for with significantly greater combined Strehl degradation factor due to the obstruction.

Analogously to the effect of central obstruction, vane obstruction reduces central intensity of the main pattern by a (1-a)2 factor, a being the relative vane area in units of the clear aperture area (for spider vanes, it is the area of annulus), by lowering constructive interference within central maxima and intensifying it in the outer potion of the pattern. For small values of a, typical for spider vanes, the Strehl degradation factor can be written as S'~(1-2a).

It also closely approximates the combined Strehl degradation factor of the spider and c. obstruction in the left side of MTF graph (extended low-contrast detail resolution) if a is their combined relative area in the aperture (this approximation is also good for the PSF maxima degradation factor for central obstructions smaller than ~0.35D).

This is not quite in agreement with the popular notion that the contrast effect of spider vanes is directly proportional to their area, relative to the area of aperture. The misconception probably comes from misunderstood sequence in Suiter's "Star Testing Astronomical Telescopes", where he states that the initial quick contrast drop is in proportion to the vanes area. However, looking at the MTF graph, it is easy to see that this initial drop in contrast remains nearly unchanged linearly for nearly 2/3 of the MTF range. In other words, the actual contrast loss keeps increasing as the relative contrast value decreases for smaller spatial frequencies (detail size). The average contrast loss caused by vanes is, therefore, considerably higher, as given by Eq. 65.

However, it should be emphasized that this theoretical approach is strictly valid only in the context of near monochromatic point source - i.e. coherent light. In the real world, light processed by the telescopes is typically polychromatic, i.e. partly incoherent, in which case the effect of light obstructions on intensity distribution within diffraction image is significantly smaller. As in the section about the effect of central obstruction, the text continues with the standard coherent light context, but keeping in mind that it is not directly applicable to the field conditions.

Depending on the object of observation, the actual spider effect can be much smaller, due to the energy being thrown so far from the Airy disc. For instance, a spider wane D/100 thick will have its principal spike length superimposed over diffraction pattern nearly 100 Airy disc diameters long (only a portion of it visible at best, depending on its telescopic brightness). For a 10" aperture, that is nearly 1 arc minute from the disc center. That would place most of the spike energy out of a relatively small object, not influencing its contrast. For Jupiter, roughly 2/3 of the principal spike fall outside the planet's disc, with 1/3, or so, of the spikes' energy left in, lowering the contrast. Assuming 4-vane spider and 25% obstruction, it would cause little over 1% actual average contrast loss (nearly 0.99 Strehl equivalent), not 4% as indicated by Eq. 65. On the other hand, on large objects like the Moon, nearly entire spikes' energy remains within the image, and the effective contrast degradation factor is ~0.96.

There are various vane configurations possible, but the only result is a different form of energy distribution - the amount of energy transferred out of the Airy disc remains unchanged for any given vanes area. Given size of central obstruction, the vane area is directly proportional to its width - the wider vanes, the more energy spread out, the higher its peak intensity, but the shorter spike length.

Spider diffraction effect is often illustrated by the effect of a narrow slit. There doesn't seem to be clearly defined width above which diffraction effect becomes that of an aperture. Hecht uses narrow slit relation for a 0.5mm by 30mm slit, which is very similar to the vane configuration, even if it is, evidently very wide with respect to the wavelength. Specifically, intensity distribution within diffraction pattern created by a slit aperture placed in front of an objective with focal length ƒ is described  by:

with I(0) being the intensity as a function of point radius, for the central intensity normalized to 1 (actual intensity depends on the slit area), β=Sπsinθ in units of the wavelength λ, S being the edge-to-edge separation (i.e. either the slit width, or length) and θ=r/ƒ being the point angle in the image plane, in radians, with r the linear point height (linear radius r=β/πi in units of λƒ/S). The numerator angle is in degrees, denominator angle in radians. The minimas occur for β=aπ, with a=1,2,3,4... First maxima is for β=θ=0, and every subsequent maxima at β=tanβ (with β at left in radians), or for β=bπ, with b=1.43, 2.46. 3.47... This gives the second maxima intensity (for β=1.43π) as 0.047 of the central intensity, the third maxima as 0.016, the fourth 0.008, and so forth.

    With the slit height much larger than its width, diffracted energy drops to first minima much quicker in the plane perpendicular to the slit height, negligible in comparison to the energy spread in the plane perpendicular to the slit's width (for instance, with the width-to-height ratio of 1:100, sinθ in β=Sπsinθ has to be 100 times smaller in order for β to have any given value, including the first minima at β=π.

    What seems to be more appropriate reference shape for the vane in the visual wavelengths is rectangular aperture. Its width is still much smaller than its height, but it is quite large relative to the wavelength. In the case of rectangular aperture, intensity distribution within diffraction pattern is described with a double squared sinc function:

where subscripts W and H stand for aperture's width and height, respectively, along its sides, with βW=Wxπ/ƒ and βH=Hyπ/ƒ, also in units of λ, where x and y are the linear point coordinates in image plane  in the horizontal and vertical direction, respectively. Evidently, this relation gives identical distribution of minimas and maximas in the two perpendicular planes as Eq. 66 (one perpendicular to the vane width, the other to vane's height), with either x or y being the equivalent of r under Eq. 66, and either being zero along one of the two perpendicular axes in image plane. Still, this relation is more complete since directly determining intensity distribution along both perpendicular axes, and in any chosen direction in the image plane.

With the first intensity minima falling at a constant nominal value of β, its angular radius, given by θ=x/ƒ=βWλ/πW (for small angles sinθ=θ in radians) is inversely proportional to the width W. Thus, the longer the vane, the more narrow its spike; the wider vane, the shorter its spike. A 200x1mm vane - so with W=1 and H=200 (neglecting central obstruction), for λ=0.00055mm will produce first maxima nearly 4 arc minutes long (for βW=π) and about 1.1 arc seconds wide (βH=π); a vane twice as thick will produce maxima half as long, with its width unchanged. Thicker vanes may appear to be producing less intrusive, shorter spikes, but they drain more energy from the Airy disc, causing greater negative effect on the contrast level.

The actual spike peak diffraction intensity, similarly to circular aperture, is proportional to its area, and can be written as I=πΦW/ƒλ2, where Φ, W, ƒ are the flux (blocked by the vane), vane width and focal length. The flux as a product of vane area and flux per unit area, and can be written as Φ=WHu, u being the flux per unit area. The W/ƒ factor reflects the effect of spike size (i.e. length), with the intensity being proportional to the width for given flux, and proportional to the square of it considering that the flux also changes in proportion to the width. Obviously, if we scale a vane to a twice larger aperture, its area - and flux blocked out - increases fourfold, and with it its spike intensity. But the same happens with the aperture's diffraction pattern, whose intensity, I=πΦ/(2λF)2, also changes with the flux, and their brightness relative to each other does not change.

If, however, it is the focal length ƒ that doubles, spike intensity relative to the star diffraction pattern also doubles, since the latter is spread onto four times larger, and the former only two times larger area.

The form of pattern change is determined by the vane profile in the pupil, which in turn determines intensity distribution of the vane as an aperture. Straight vane projects a spike that is centered over diffraction pattern, as illustrated on FIG. 109. Since "dark aperture" created by the vane becomes a part of the wavefront, it projects a spike centered at the chief ray (i.e. center of the diffraction pattern), extending orthogonally to the vane orientation, regardless of its orientation in the pupil, or length (shorter section will produce wider, fainter spike).

As the two insets on the bottom of FIG. 109 show, it is possible to reduce diffraction effect of a spider vane by replacing a single vane with two or more parallel vanes. Multiple vane replaces a single central maxima of a single vane with multiple subsiding maximas covering bearly identical width angularly. Intensity of the central maxima is proportional to the combined vane area, thus for the reduction in energy transferred from the Airy disc, such multiple vane would need to have unit vanes of lesser width than a single vane it would replace.

Curved vane spider

The intense spike produced by a straight vane can be visually eliminated by curving the vanes. The result is a curved vane spider. Diffraction effect of a curved vane can be illustrated by breaking it into a number of smaller, practically straight sections, with varying orientations (FIG. 110a). While the total amount of energy produced by a curved vane is identical to that of a straight vane of equal length and thickness, it is spread out wide, making it practically invisible (it still lowers the contrast the same, on average).

The Strehl degradation factor is somewhat different from that for the straight vanes (Eq. 65):

with α=180/N being the vane arch angle in degrees. However, the result is only slightly lower for given count (N) and relative thickness (τ) of the vanes, reflecting the slightly greater curved vane length.