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7.2. Spider vanes diffractionPAGE HIGHLIGHTS 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, change in the
normalized central diffraction intensity 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 the degradation factor caused by central obstruction (Eq. 60). Average spider area is somewhere between 1% and 2% of the clear aperture area. That puts the average spider vane central intensity degradation factor between 0.98 and 0.96. As plots at left show, it decreases linearly with the increase in obstruction (quadratic plots are near flat sections of very wide parabolas). Analogously to the effect of central obstruction, vane obstruction reduces central intensity of the main pattern by a (1a)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 this degradation factor can be written as I(0)'~(12a). As with the CO effect, this diffraction peak degradation includes the obscuration effect, which does not affect the relative energy distribution within diffraction pattern. Factoring that in, the relative degradation is given simply by (1a). Another similarity with the CO effect is that the 4vane spider also makes the central diffraction maxima smaller. However, while in the case of CO it remains circular in shape, the 4vane spider makes it slightly rectangular, by slightly flattening the central maxima on four sides where the spikes come out. The reduction is roughly commensurate to the area of obscuration, thus much smaller than with the standard CO sizes. Simulation at left shows it for a 3inch aperture and 4vane spider with 5mm thick vanes (no central obstruction), thus creating nearly 17% obscuration  much more than the vanes usually do. The MTF is also, for clarity, shown for this exaggerated scenario. The contrast transfer improvement over the perfect clear aperture is for the orientation shown; for the pattern rotated by 45 degrees it would be smaller. Compared to the CO effect, this contrast improvement in this orientation drops off quickly after the initial gain at mid frequencies, probably due to the vanes bringing energy into the first dark ring. With the standard thickness vanes, these effects are negligible, more so considering that the actual vane area is reduced in the presence of CO. The relative intensity distribution is, again as with the CO effect, based on the peak intensity w/o obscuration effect, normalized to 1. Energy distribution between the central maxima and the rings depends, in addition, on the volume corresponding to the central maxima, relative to the volume of the rings area. In the absence of exact calculations, best reference point is the effect of CO with comparable relative obscuration. Taking the same 3inch aperture, 17% obscuration area corresponds to 41% linear CO. Graph below summarizes the effects of crosslike and circular obscuration alone, for 17% areal obscuration. The corresponding PSFs show that the 1st bright ring is significantly less bright with the crosslike obscuration than with with CO, with uneven intensity (given for three orientation, zero degrees being along the horizontal, or vertical central line of the PSF pattern), but with generally more energy in the outer area. Encircled energy graphs top right show that this energy beyond the 1st bright ring is significantly higher, despite the crosslike obscuration encircling more energy in the central maxima (approximately 80% of the energy content of Airy disc, vs. nearly 70% for the CO). The more energy beyond the 1st bright ring for the crosslike obscuration is a consequence of roughly three times more energy in the 1st bright ring with the CO. Taking that the relative encircled energy in the central maxima is, as a quality indicator, comparable to the Strehl, and for relatively small disturbance levels nominally nearly identical, leads to the conclusion that a 4vane spider Strehllike figure is somewhat higher than for a CO of the same relative obscuration. In other words, a standard 12% obscuration 4vane spider would degrade the effective Strehl by less than the corresponding 12% obscuration CO, i.e. less than 1014% linear CO. Factor that complicates such comparison are the differences in the size of central maxima. As the magnified inset top left shows, average central maxima reduction due to the crosslike obscuration is less than half of that due to the CO. So are the related effects of increased contrast on the left side of MTF graph and higher cutoff frequency. For small relative obscurations, such are those by the typical 4vane spiders, these highfrequency gains can be safely neglected, more so considering that the spider effect considered here includes its nonexistent central portion behind the CO. The effect is limited to the contrast drop in mid and low frequencies, as shown on the bottom right. With 17% relative obscuration, contrast drop at the 0.2 frequency is 12%; a thinvane spider obscuring 1% of the area would have little over 0.5% contrast drop there, as little as 0.995 Strehl of aberration averaged evenly over all frequencies. 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 4vane 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.
VANE AS AN INVERSE NARROW SLIT Spider diffraction effect is often illustrated by the effect of a narrow slit. Makes it easy to understand why is spike orientation perpendicular to that of the vane. 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 edgetoedge 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/π 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 the 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 widthtoheight 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 β W =Wx π /ƒ and β H =H yπ /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= βW λ/π W (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=π ΦW/ƒλ 2 , 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=π Φ/(2λ F)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).
FIGURE 109: TOP LEFT:
Spider vane diffraction effect is one of a narrow, elongated
rectangular aperture. Its diffraction pattern extends
appropriately less in the direction of elongation, than in the
direction orthogonal to it. The ratio between these two pattern
extensions is 1/ς, with ς being the
widthtolength ratio of the vane. Consequently, it makes the
central spider vane maxima ~1/ς times wider than the Airy
disc of a telescope. Peak PSF intensities of the vane vs. Airy
disc are proportional to the ratio of their respective areas.
CENTER: Intensity
distribution* within diffraction pattern of 2x200mm opening
centered over 200mm ƒ
/10 mirror (so ς=0.01),
calculated by OSLO. Its central diffraction maxima is nearly 1mm
long (purple), and nearly 0.01mm wide
(green, magnified below). At 0.0134mm
and 550nm, the Airy disc is only slightly wider than the
spike width, and nearly 80 times higher (blue).
The central vane maxima is ~70 Airy disc diameters long, but how
much of it is visible  if any  depends on star's telescopic
brightness and magnification. Practically all visual effect of the
vane results from its central maxima. With ~80 times (the
reciprocal of the area ratio) lower intensity than the
telescope's PSF central maxima (showing intensity distribution
over the Airy disc), it is fainter here than the first bright ring.
Its own second maxima is another 21 times fainter, nearly as faint
as the 5th bright ring of a perfect aperture. However, similarly
to the first bright ring, the central spike appears nearly half as
bright as the disc at high telescopic brightness levels, due to
the logarithmic intensity response of the eye (since its actual
intensity is so much lower, it fades away much sooner with the
drop in brightness, as its intensity approaches visual threshold
of detection). Above, to the right, the highmagnification
simulation of the effect of a 4vane spider on the diffraction
pattern of a bright star, with the spikes clearly visible,
extending far out from the central disc. Note that, all else
equal, the spike width is greater the greater central obstruction,
due to the inverse slit aperture formed by the vane having its
length reduced by a factor (1o)/2, o being
the relative linear obstruction size (for instance, spike maxima
in the presence of D/3 central obstruction will be three times
wider than that of a vane spanning the entire aperture; with
the combined intensity of its two sections lower by 1/3).
BOTTOM LEFT: PSF
of a double slit/vane has double the central intensity of a single
slit/vane of the same width, with the central maxima of a single
vane effectively fractionalized into 2s1 maximas
(including central maxima), s being the slit/vane
separation in units of their (equal) width W.
BOTTOM RIGHT: PSF
of a multiple slit/vane has the central intensity higher by a
factor of N than a single slit, N being the slit/vane
number. Within the width of diffraction envelope of a single
slit/vane central maxima, it forms central maxima narrower
approximately by a factor 1/5(N1)s, with (N2) subsidiary
maximas between subsequent principal maximas (sth
principal maxima is suppressed by a diffraction envelope null). Since at these small angles there is practically no difference between the angle and its tangent, linear length of the spike maxima is approximately 2λ ƒ/w, with ƒ being the mirror focal length; substituting the vane width w in terms of the aperture diameter as w=ς D gives the linear spike length as ~2λ F/ς, F being the telescope focal ratio. 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 nearly 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 on the bright stars and planets 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 peak diffraction intensity 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.
Using the same concept of inverse aperture, the actual energy distribution of a curved vane can be simulated. It is shown, for comparison, with those for the straight vane, and for a significantly wider vane with one straight and one curved side (note that the patterns are in their actual orientation vs. aperuresvanes). These energy distribution patterns are deducted from the diffraction distribution of the clear aperture, changing its appearance. The full arc of the curved vaneaperture is 90 degrees, but the angle of energy spread is only about 60 degrees, or little more. Three vanes of this shape, properly positioned, would spread their energu in a nearly continuous radial pattern, eliminating spikes (even a single curved vane would probably make it invisible). This vane still produces an elongated central maxima, but some 15 times shorter than a straight vane of the same thickness, thus much less pronounced.
The middle vane shape illustrates that curving and "styling" the vane shape has little influence on their diffraction effect; in general, the inverse energy they are throwing onto the main diffraction pattern will be determined by their relative size in its quantity, and by their shape  approximately  in its shape. A vane like this (it would make little difference if both sides were curved) would produce significantly shorter spike than a thin straight vane, similar to the straight vane of similar thickness. It is still well over twice longer than the curved vane spike, and with much more energy  proportional to its area  transferred out of the Airy disc. There is no shape that will magically take vane diffraction away. Following the same concept of inverse aperture, the pattern of energy taken out by curved vanes can be also illustrated. For a single circular vane, which seems to be the shape that nearly everything goes for, simulation below shows no spike structure down to 0.002 intensity normalization, i.e. where only portions with intensities below 1/500 the central intensity show as shades of gray (left). Simulation next to it shows its pattern as
inverse aperture: most of the energy is concentrated in the small central maxima, with the rest of it spreading out evenly, in concentric circles. On the other hand, the 3vane curved spider with 60degree arcs creates uneven energy spread, with a 6fold spiky structure (not surprising looking at the patern produced by a single 60degree section). Those should be undetectable most of the time, but could become visible on very bright objects. More important, however, is that the total of energy transfered out of the Airy disc is proportional to the vanes area, and the combined vane length of the 3vane spider is half the circumference of the circular shape they belong to. Despite the vane apertures having proper orientation to cover 360° spread, this edge arrangement is not fully representative of the actual vane arrangement, because the vane apertures are more widely separated. Looking at a single 60° vane aperture at its actual position, we see that the diffraction pattern is quite similar to that of the alike edge aperture (below, top).
The spread widens as the vane gets thinner, with the central maxima remaining unchanged. If the top aperture from the triple 60° arrangement is removed (it is shown on the picture), the pattern shows spiky structure, with only four spikes (top right). A single curved vane belonging to a circle of the same diameter as the aperture forms a 120° arc, hence it spread falls short of 360° (bottom, left). For 360° it needs to be having a 180° arc, i.e. to be belonging to a circle of 30% smaller diameter (bottom middle). Finally, diffraction pattern of an actual 3vane curved spider does show the presence of a 6spike structure, but significantly less pronounced (bottom right). Unlike the whole aperture, where the Airy pattern is much more intense than the vane energy, hence requiring very low normalization in order to show its effects, vane aperture patterns have much lower central intensity and show energy spread with even when normalized to their peak intensity. As a reminder, the inverse apertures of central obstruction and spider vanes take energy out of the principal diffraction pattern, changing its relative intensity distribution; this means that the bright areas are where most of the energy is taken out. Depending on whether the matching areas of the principal pattern are bright, or dark, the combined intensity will be lessened, or reinforced. PSF of some other spider shapes are shown here.
