6.3.2. Telescope aberrations and extended 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 intermediate scenarios 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 D5 filter (w/glass optical density cutting transmission down by a factor of 1/105), 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 arbitrarily 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 that 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 greater 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 certain 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 of its surface - 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 resolution threshold - or below it - for any given level of aberrations.

This can be illustrated with eye CSF (contrast sensitivity function) graph (FIG. 56). It implies that the eye is,

FIGURE 56: 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). In the average mesopic mode it is ~5 cycles per degree (~12 arc minutes), and in the average scotopic mode between 2 and 1 cycles per degree (~0.5 to 1 degree). Differences in contrast sensitivity for mid-to-large detail sizes are relatively small within photopic and mesopic mode - contrast sensitivity, up to 3%-4% (they plummet as detail size goes below 20 cycles per degree, i.e. 3 arc minutes), and larger in scotopic mode, ~10%. Note that contrast sensitivity levels shown, achieved under laboratory conditions, are higher than those practically achievable in field 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 have that privilege - 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 this threshold resolution may not be, and probably is not, 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.

◄ 6.3. Aberrations and object type ▐ 6.4. Diffraction pattern and aberrations ►

Home | Comments