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6.2. Aberrations and object type 2Extended objects Turning to extended objects, there is again the main distinction between faint and bright, on one hand, and contrasty and low-contrast objects on the other, with all combinations in between possible. In general, most faint extended objects (nebulae, galaxies) are also of low contrast, while bright extended objects are either of high, or low contrast. The Sun and the Moon are well known bright contrasty extended objects. The Sun is in the category of its own, with every square arc second on its surface being nearly as bright as -11 magnitude star; even through an D5 filter (w/glass optical density cutting transmission down by a factor of 105), each square arc second of its surface is still brighter than a 2nd magnitude star. The Moon, without filter, comes to little fainter than 3rd magnitude star per each square arc second of its surface. Thus, both of these objects are, diffraction-wise, at the level of bright stellar objects, with one important exception: most of the details on their surfaces are of lower, or much lower contrast than those of bright stars against sky background (with the average sky background being ~24 magnitudes dark, the difference in intensity vs. bright stars is over 20 magnitudes, or over 100 million times). Hence, it is to expect that the minimum angular size required by the eye to start recognizing a detail as extended (vs. point-like) is somewhat larger here. Assuming that it is roughly two times as large as for bright stars, leads to a minimum magnification that will start showing the effect of aberrations/diffraction at roughly double those for bright stars. Another difference is that with further magnification increase, image quality will deteriorate at a faster pace. Main reason for this is that higher magnifications will expose to damage done by aberrations to the smaller detail substructure, making the image as a whole less well defined. Low-contrast extended objects are the category most resistant to the negative effect of aberrations. They are just to faint to allow the eye to detect contrast changes nearly as efficient as with bright details. While nominally contrast does change just as much as in bright details, it is not likely to be seen. For instance, even relatively bright object as Orion nebula (M42), with integrated magnitude of 3.7, has an average brightness per square arc second of nearly 10th magnitude - not a subject to much of a change in appearance due to the effect of aberrations in most amateur telescopes, regardless of magnification. What has more of an effect is the background brightness in a telescope; hence dark skies, good baffling and low surface scatter are more of a factor for this kind of observing than the effect of aberrations - up to a reasonable level. Finally, telescopic images of bright low-contrast objects, most notable being planetary surfaces, are also affected differently by optical aberrations. Their surfaces are not as bright - Jupiter, for instance, shines as if having a 6th magnitude star in each square arc second - which lowers their aberration sensitivity. However, it is more than offset with their other feature: low detail contrast. While it also has a good side, which is larger minimum angular size required for the eye to start recognizing extended shapes and, therefore, higher magnification needed to begin noticing the effect of aberrations than with stellar and bright contrasty extended objects, that is where the benefit ends. Once the low contrast details are visible to the eye, so is the effect of aberrations, and it is greater than for bright contrasty extended details. Main reason for it is that low contrast details also have inferior resolution limit. Consequently, for any given detail size, low contrast detail is closer to being shifted to a challenging resolution level - or no resolution at all - for any given level of aberrations. This can be illustrated with eye CSF (contrast sensitivity function) graph (FIG. 39). It implies that the eye is,
FIGURE 39: Limit to resolution of MTF-like line pattern on the retina is determined by eye contrast sensitivity. The level of sensitivity is highest in photopic mode (approximate average shown in yellow), somewhat lower in mesopic mode (green) and lowest in scotopic mode (pink). Average planetary vision mode is within mesopic, as opposed to lunar vision mode, which is at a higher, scotopic level. At the same time, average planetary detail contrast level is lower than that of average lunar detail. The contrast sensitivity graph also shows an additional factor of resolution limit, related to the angular size of a detail on the retina. In scotopic vision mode, maximum contrast sensitivity is for ~8 cycles per degree (~7.5 arc minutes detail size). In the average mesopic mode it is ~5 cycles per degree (~12 arc minutes detail size), and in average scotopic mode between 2 and 1 cycles per degree (~0.5 to 1 degree detail size). While differences in contrast sensitivity at mid-to-large detail sizes are relatively small within photopic and mesopic mode - when you take a closer look, you see that contrast sensitivity in the either varies up to 3%-4% - they plummet as the detail size goes bellow 20 cycles per degree (3 arc minutes). In the scotopic mode, contrast sensitivity variation is somewhat greater, ~10% in this part of resolving range. Note that contrast sensitivity levels shown are somewhat higher than those practically achievable in field conditions, in particular that for the photopic limit, which is as high as ~0.6% on the graph (it is achieved under laboratory conditions). However, the main proportions remain in place. under optimum conditions, capable of detecting an astoundingly low level of contrast difference. For contrasty, bright lines, it is nearly as low as 0.5%. It would imply that as little as 1% change in contrast level could noticeably - although still very slightly - affect image quality, as perceived by the eye. That would be comparable to the level of aberration indicated by ~0.99 Strehl, 1% reduction in aperture size, or the effect of D/15 linear central obstruction. In practice, this small changes in contrast cannot be detected at the eyepiece. For instance, nominal contrast difference between the two maximas and the minima between two stars at the Dawes' limit is still as high as ~4%. There are several factor that could be causing this discrepancy. One is the ever present seeing error. Even if it is as small as 1/30 wave RMS, on the average - and only the smallest apertures in a very good seeing are privileged to it - it already lowers average contrast by ~4%, with constant fluctuations around the mean value. The effect of as small contrast difference as 0.5%, already at the very limit of detection, is likely to be lost in such conditions. Also, contrast difference resulting in loss of threshold resolution is may not, and probably doesn't, be apparent to the eye as a contrast level change per se. In addition, there are very few details that would be affected, or lost - even if this level of change in contrast could be detected. Lastly, it is likely that the eye strain resulting from peering into the eyepiece has relatively significant effect on lowering eye sensitivity thresholds in general. It is generally accepted that loss of contrast smaller than ~5% (~0.95 Strehl) is either insignificant, or undetectable in field conditions. Small apertures are probably somewhat more sensitive than large apertures, in general, so this figure should be sort of informal average. Follows more details on
how various wavefront aberrations affect diffraction pattern.
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6.2. Aberrations and
object type
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6.3. Diffraction
pattern and aberrations ► |
