Since the phase surface is not axially symmetric, its MTF depends on which way the stripes of the MTF target are oriented. If we calculate the MTF at an average orientation, we can see how the behavior improves with better collimation. Five curves are depicted in Fig. 6-3. The first is the perfect circular aperture. The next is a 30% obstructed but otherwise perfect aperture. Third is the filtering caused by a shift of the focal plane sideways by the generous 3 arcminute tolerance. If this shift is doubled, the contrast sags considerably in the fourth curve. The last and bottom curve is the transfer expected from a severely misaligned mirror. Any alignment effort whatsoever results in better collimation than the last curve, but some Newtonian owners are so frightened of producing worse performance that they refuse to touch the adjustment screws.
The worst MTF curve dips quickly. The interesting behavior is found low on the spatial frequency axis. The initial drop indicates that the telescope is performing as well as a perfect aperture of about 1/4 the full diameter. Severe misalignment reduces the low spatial frequency contrast to that of a 2.5-inch telescope!
Even correcting the misalignment until the axis is tilted by twice the tolerance only improves the contrast so this mirror acts like one of half its aperture. This MTF curve is about the same as would occur for a 50% or
6.4. Filtration of a Misaligned Newtonian 107
Fraction of maximum spatial frequency 10-inch (250 mm) f/4.5
Fig. 6-3. Modulation transfer function of steadily worsening Newtonian alignment (45° orientation of MTF target bars with respect to the direction of misalignment).
60% obstructed aperture. No one would stand for such huge obstructions, but many telescope owners casually accept misalignments of this magnitude.
The filtration of even a slightly misaligned telescope is enough to severely degrade the images. Be fearless in attempting to collimate your telescope. You can hardly do worse than an unaligned instrument, and the potential improvements obtained with only a small effort can be profound.
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