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▪ CONTENTS ◄ 5.1.2. Limitations to the standard model ▐ 5.1.4. Seeing error: Strehl, resolution, OTF ► 5.1.3. Seeing and telescope aperture
PAGE HIGHLIGHTS Since the atmospherieinduced wavefront error  so called seeing error  changes with (D/r0)5/6, it will vary, for given atmospheric coherence length (Fried parameter) r0, with the aperture size D.
Eq. 52.1 and
52.2 give following averaged RMS errors in the average 2 arc
seconds seeing (r0~70mm
for 550nm), over the span of amateurs' apertures, with the
corresponding Strehl values calculated from the seeing Strehl
approximations:
While these figures appear rather pessimistic, one should keep in mind that they are only averaged values. Statistically, one of many possible breakdown's for the dismal 0.69 wave RMS longexposure error at (D/r0)=5.7 (for 400mm aperture in 2" seeing) is 1.38 and 0.34 wave, half the time each. Likewise, its short exposure error splits to 0.5 and 0.12 wave RMS. Placing the visual error level roughly in between the two would give 0.94 and 0.23 wave RMS. This means that half the time the image would be a real mess, but the other half it would be significantly better, with the visual Strehl of around 0.35, comparable to little over 1/2 wave PV of spherical aberration. Also, the tilt error effect is not detrimental at very small apertures (smaller than ~4") where, in average seeing, it causes image to move about randomly as a whole, relatively slowly, without significantly affecting its visual quality. Thus, in this aperture range, the size of seeing error in visual observing is mainly determined by the wavefront roughness component (Eq. 52.2). And, since these figures are based on the standard Kolmogorov model, which assumes an infinite outer scale of turbulence, the actual error is somewhat smaller for the longexposure modality due to the smaller tiptilt error component with a finite actual scale of turbulence. As the aperture increases  or the seeing worsens  the eye has more difficulty to keep up with the frequency of image motion, as the diffraction pattern gradually breaks into speckle structure, with an increasing number of pattern components moving randomly within the speckle envelope. The result is progressive blurring and bloating of the star image. The roughness error component too increases with (D/r0)5/6 until, at a sufficiently large aperture  or in sufficiently bad seeing  diffraction pattern disintegrates into a random speckle structure (FIG. 77). The illustrations show pattern structure as a shortexposure snapshot; physical size of the Airy discs is identical (i.e. focal ratio is the same for all apertures) to make the atmospheric effect directly scalable.
FIGURE 77: Illustration of a point source (stellar) image degradation caused by atmospheric turbulence (linear pattern size, identical fratio). Left column shows best possible average seeing error in 2 arc seconds seeing (r0~70mm @ 550nm) for four aperture sizes. The errors are generated from Eq. 5354, with 2" aperture having only the roughness component (Eq. 54), and larger apertures having tilt component added at a rate of 20% for every next level of the aperture size, as a rough approximation of its increasing contribution to the total visual error (the way it is handled by the human eye is pretty much uncharted territory). Columns to the right show the possible range of error fluctuation, between half and double the average error. Best possible average RMS error is approximately 0.05, 0.1, 0.2 and 0.4 wave, from top to bottom (the effect would be identical if the aperture was kept constant, and r0 reduced). The 2" aperture is little affected most of the time; 4" is already mainly below "diffractionlimited", while 8" has very little chance of ever reaching it, even for briefly. The 16" is, evidently, affected the most; D/ro ratio for its x2 error level is over 10, resulting in clearly developed speckle structure (magnification shown is over 1000x per inch of aperture, or roughly 10 to 50 times over the practical limits for 2"16" aperture range, respectively. Also, since the angular blur size is inverse to the aperture size, the x2 blur in the 16" and 2" aperture are roughly of similar size angularly). The speckles, being formed by the energy coming from the fragments of the broken wavefront over the pupil that meet in phase at the points randomly scattered around Gaussian image point, approximate in size diffraction disc (this apply to the brightest, so called firstorder speckles; fainter speckles are smaller in size). The diameter of the 1storder speckles envelope, the bright central core of the entire star image, roughly analogous to the FWHM (FullWidthatHalfMaximum) of the PSF of aberrationfree diffraction image of an aperture of diameter equal to r0, is approximated by λ/r0 in radians. This means that it is r0, not the aperture, that sets the resolution limit. The speckle phenomenon is not only characteristic of large professional telescopes. Even mediumsize amateur telescopes in bad seeing begin to develop speckle structure. Probably the simplest model to use for explaining the mechanism of speckle formation is to assume that the aperture of a telescope is, as a result of atmospheric turbulence, effectively broken into subapertures, each approximately of the diameter equal to r0, with a different, varying amounts of tilt. The resulting diffraction pattern is a product of superimposed subaperture patterns. Similarly to a multiaperture telescope, the combined pattern shows tendency to segmentation  mostly in the rings area  with the number of firstorder radial segments (spikes, lobes, etc.) approximately equaling that of subapertures. Unlike multipleaperture telescope, the seeingformed subapertures are in an optical disarray, poorly defined and in a constant motion. Thus the increase in number of "subapertures" quickly causes disintegration of the bright central disc and formation of the random speckle structure which is stationary only within time intervals smaller than ~20 milliseconds (shortexposure image). Since speckles originate from wavefront roughness, which is proportional to (D/r0)5/6, their number will be larger for larger (D/r0) values, while their average angular size will be smaller in inverse proportion to the aperture size (thus, fully developed speckle pattern remains of similar angular size, regardless of aperture). The number of 1storder speckles N can be approximated as a ratio of the area of the speckle envelope's FWHM (approximated, angularly, by λ/r0) vs. area of a single speckle (taking 2λ/D for the average angular separation of speckle centroids), or N~[(λ/r0)/(2λ/D)]2=(D/2r0)2; hence the spackle structure begins to form, at least statistically, at (D/r0)~3, and should become apparent at (D/r0)~4 and larger. Note this approximation may somewhat overestimate the number of speckles, since based on r0, which is the seeing FWHM that includes the tilt error, i.e. random motion of the speckle structure around its central point. Formation of the dynamic speckle structure changes the realm of a telescope; it voids its resolution limit, making it dependant on the atmosphere, not aperture size. Energy spread caused by it also significantly lowers telescope's contrast level. Since the error increases with aperture, given large enough difference in aperture size, there comes the level of seeing error which may take away the resolution/contrast advantage from the larger aperture and award it to the smaller one (FIG. 78).
FIGURE 78: Effect of seeing on the visual timeaveraged seeing PSF for three relative aperture sizes, referred to, from the smallest to the largest as D, 2D and 5D, respectively. The sizes of seeing disc are only approximate, since the effect of tilt error in visual observing varies from small/negligible at (D/r0)<2 to significant/dominant at larger D/r0 values. Top image (A) shows the approximate visual size of the central diffraction maxima in the three apertures for zero seeing error, and for D/r0=1 in the smallest aperture., D At this level of seeing error, an aperture is visually somewhat above diffractionlimited level (0.80 Strehl), with its central maxima remaining intact and unchanged in size. In 2D aperture, (D/r0)=2, with significantly more energy transferred to the rings area, but with the central maxima also remaining mainly intact and with no significant change in size (it is shown slightly enlarged). The 2D aperture, therefore, retains most of its resolution advantage, and some of its contrast advantage as well. In the 5D aperture, (D/r0)=5, at which level the speckle structure clearly shows. Consequently, averaged FWHM radius is approximated by D/2r0 in units of λF. That gives 2.5λF, or five times the aberrationfree FWHM for this aperture. In other words, averaged FWHM of the 5D aperture is now nearly identical to that of five times smaller aperture, with similar resolution limit and contrast level (the latter depends also on the specifics of intensity distribution, but for simplicity we'll assume it indicated by the FWHM angular size). However, as seeing fluctuates, it is still possible that (D/r0) falls below ~3, regaining nearintact central maxima and good part of its contrast/resolution advantage. Bottom image (B) shows these three apertures as seeing worsens by a factor of 2. Now (D/r0)=2 for the smallest aperture, which means that it still has its central maxima near intact most of the time. For the twice larger aperture, (D/r0)=4, with its averaged FWHM grown nearly to that in twice smaller aperture. Since it is still likely to drop bellow (D/r0)=3 with moments of better seeing, it does retain its central maxima near intact some of the time, allowing it do realize its aperture advantage during those moments. However, (D/r0)=10 in the 5D aperture means that it has fully developed speckle structure, with very little chance to fall bellow (D/r0)=3 (it would happen only in seeing more than three times better than the average). With fully developed speckle structure, its resolution limit is approximated by λ/r0; since r0 is 1/10 of its aperture, its resolution now is also 1/10 of its maximum theoretical resolution λ/D, or only about half of the resolution power of five times smaller aperture. Further doubling the seeing error brings the smallest aperture to (D/r0)=4, and twice larger aperture to (D/r0)=8. The latter is now at a similar level as the 5D aperture in twice better seeing, while the smallest aperture still have realistic chance of falling bellow (D/r0)=3, in which case it outresolves, and outcontrasts both larger apertures. Yet another doubling of the seeing error takes this advantage of the smallest aperture away  but now an aperture twice smaller than this one will outresolve and outcontrast the three larger apertures. This general scenario would play out with, say, 150mm, 300mm and 750mm apertures in 1, 2 and 4 arc seconds seeing (i.e. r0 of 144, 72 and 36mm, respectively). Image below illustrates how angular size of diffraction pattern and its appearance change with the decrease in atmospheric coherence length r0 (top) and increase in aperture D (bottom).
Decrease in r0 for given aperture results in the progressive break up and enlargement of the diffraction pattern. For given r0, angular pattern size for (D/r0)>3 will be nearly constant, approximated by λ/r0. As D/r0 decreases below 3, the central maxima gradually regains its integrity, and limiting resolution shifts toward λ/D. Despite the central maxima becoming larger than speckles in larger apertures, its diameter becomes smaller than that of their primary speckle structure. Consequently, for 1.5<(D/r0)<2.5, the smaller aperture has better resolution than two or more times larger apertures. At (D/r0)~1, resolution level in the smaller and much larger (five or more times) apertures are similar; further decrease in aperture results in decreased resolution in smaller vs. larger apertures. If we take for the smallest aperture D=50mm, and r0~100mm (~1.5 arc second seeing), then the bottom patterns, from left to right, correspond to 50, 100, 175, 300, 600mm and 1.2m apertures. Keep in mind that these represent averaged, statistical errors. In actual field conditions, as seeing fluctuates, 175mm aperture will have its D/r0 in the 1.52.5 range during periods of better seeing, outresolving both smaller and larger apertures. Even 600mm aperture is likely to have moments when D/r0 drops to ~2.5, or somewhat bellow; that is, however, very unlikely for 1.2m aperture, which remains inferior to 150300mm apertures (approximately) resolutionwise. EFFECT ON CONTRAST The degree of pattern integrity directly determines the maximum usable pointsource magnification, defined as one needed to achieve limiting visual resolution allowed by seeing. Limiting magnification for extended object is more complex, varying with detail's inherent contrast, brightness and shape. In general, contrast of extended objects is affected more than pointsource resolution  and particularly those with low inherent contrast  thus seeinglimited usable magnification for this type of objects should be somewhat lower. An indication of the effect of seeing on contrast and resolution in smaller vs. larger aperture is best presented with MTF graph (FIG. 80).
FIGURE 80: TOP: Taking the near optimum D/r0 stellarresolutionwise of ~2 and assuming that at this error level eye still filters out most of the tilt error component, would place the corresponding visual MTF closer to that for the shortexposure MTF. Contrast over the range of resolvable bright lowcontrast detail would be at the level of a 40% smaller aperture, (dashed gray extending to the horizontal scale) while its threshold for these details would be reduced by about 1/3. Its stellar resolution would be near its theoretical limit.
On the other hand, a 2.5 times larger aperture at
D/r0~5,
would have its visual MTF closer to the longexposure MTF for that
D/r0
level. Its lowcontrast detail transfer would have been at the level
of five times smaller perfect aperture, with its bright lowcontrast
detail threshold reduced by about 2/3. Its stellar resolution would
be about 1/3 of its theoretical, which means that it could be
inferior to 2.5 times smaller aperture, which has 40% of its
limiting resolution. The approximate midfrequency contrast levels,
at the level of 40% and 80% smaller aperture, indicate that after
applying the 2.5 aperture differential factor the larger aperture
would still have an edge in bright low contrast detail resolution.
It is confirmed by reducing the small aperture visual MTF plot's
width 2.5 times (dotted orange), making it directly comparable to
the larger aperture's visual MTF. It shows that the smaller aperture
has better contrast transfer over all frequencies with nonzero
contrast in the larger aperture, becoming significant for
frequencies larger than ~0.4. It also has better limiting resolution
for highcontrast details (MTF cutoff frequency), but somewhat
inferior bright lowcontrast detail resolution threshold. That seems
illogical at first, but it is a consequence of the threshold line 
which represents the minimum contrast required by eye at given
normalized frequency  being steeper in the smaller aperture when
scaled relative to the larger aperture, since any normalized
frequency
ν
in the latter corresponds to 2.5 times higher frequency in the
former, and a higher spatial frequency requires higher minimum
contrast. This crude approximation using MTF model indicates that the smaller aperture would probably have relatively small advantage in both, planetary and stellar resolution, when at the seeing error level of D/r0~2. The advantage is small enough that could not materialize if the assumptions are somewhat biased toward smaller aperture. However, considering that most other factors  inherent optical quality, thermal equilibrium, absence of central obstruction, better collimation  generally favor smaller aperture, it makes likely the scenario suggested by the size of seeingaffected central maxima, i.e. that an aperture at D/r0 seeing level would outperform both, significantly smaller and significantly larger apertures (plots for twice smaller aperture would fit between D/r0=0 and D/r0=2, thus it is obvious that it would be outperformed by both larger apertures). Main difficulty in accessing more accurately effect of seeing on visual performance in apertures of different sizes is caused by the gradual, unspecified transition from seeing error effectively including only the roughness component (shortexposure error level) at small aperture sizes (i.e. at low seeing error levels, approximately D/r0<2) to also fully including the tilt error component at large seeing error level (approximately D/r0>5, long exposure error level). The dynamics of this transition can be only hinted at from the seeing MTF for both error levels over a wide range of seeinginduced error (FIG. 81).
The very nature of seeing  its constant fluctuation around the average value and, in an extended time frame, fluctuation of the average itself, benefit larger aperture more when swinging above the average. For given seeing level, D/r0 value most of the time remains within ±50%, and ~25% deviations can represent appreciable portion of the time. With the seeing average following similar pattern  it can be more, or less at any given night, but this should be reasonable assumption, within the range of a rough average  r0 could reach more than two times larger size, and could be nearly twice larger over relatively significant portion of the time (on the other hand, it would be also more than twice smaller, and nearly twice smaller over nearly as much of the time).
FIGURE
82: Simplistic scheme of the variation in r0
around its averaged value. D/r0
changes in inverse proportion to it, and seeing error in proportion
to (D/r0)5/6.
For the maximum amplitude of deviation from the average A, the
portion of time within which r0
will differ from A by ±a,
or more, is at
least roughly approximated with t~0.5(a/2A),
CONCLUSION What all this implies is that it is possible for a significantly smaller aperture to outperform the larger one, but it requires them to be, and remain at a specific seeing error level, generally involving compromised seeing conditions. It is only possible when D/r0 in the larger aperture is ~4, or larger, in which case a smaller aperture with D/r0~2 will have  all else equal  better resolution and contrast transfer. In other words, smaller aperture could perform better while the seeing error is large enough, but if it lessens sufficiently due to seeing fluctuations, larger aperture would rebound and perform better. Typically, seeing fluctuations are wide enough for that to happen. This is, of course, very simplified concept, omitting the role of other induced errors (thermals, collimation), inherent optics errors, as well as observer's individual limitations and experience, but it is necessary to assess the effect of seeing alone. Other factors can also come into play, such as luminosity of the telescopic image while observing lowcontrast details. Apertures smaller than about 6 inches in diameter will provide less than optimum image brightness at high magnifications, which negatively affects eye's lowcontrast detail resolution.
Following page addresses other important aspects of the seeing error:
specifics of the seeing PSF, Strehl, and OTF.
