Detectors and Imaging
The front line detectors for almost all astronomers are their own eyes. For many, especially when using smaller telescopes, these are also the only detectors. The eye, or more particularly vision, which is the result of the eye and brain acting in concert
- PDF / 378,039 Bytes
- 16 Pages / 439.37 x 666.142 pts Page_size
- 55 Downloads / 180 Views
Detectors and Imaging
The Eye The front line detectors for almost all astronomers are their own eyes. For many, especially when using smaller telescopes, these are also the only detectors. The eye, or more particularly vision, which is the result of the eye and brain acting in concert, is, however, a very complex phenomenon, and some knowledge of its peculiarities is essential for the observer. Thus reference has already been made in Chap. 8 to averted vision, the effect of high contrasts (known as irradiation) and the combination of sub-resolution features (Martian canals). The structure of the whole eye (Fig. 9.1) is well known from school, and need not be considered further here. It is the structure of the eye’s detector, the retina, which is of importance. The retina is a network of detecting cells connected by nerve fibers via the optic nerve to the brain. The detecting cells are of four types: rods, and three types of cones. The names of the cells come from their shapes. The three types of cones have sensitivities that peak at about 430, 520, and 580 nm, and they provide us with color vision. The overall sensitivity of the cones is, however, low. The rods are of only one type, and have a response that peaks at 510 nm. There are about 108 rods and 6 106 cones but only 106 nerve fibers; hence many cells are linked to each nerve. Cones are most abundant in the fovea centralis, and many there are singly connected to nerve fibers. The fovea centralis is the point on the retina where the light falls if we look directly at an object; the rods become commoner and the cones fewer as distance increases away from this point. The light-sensitive component of the rods is a molecule called rhodopsin, or visual purple (from its colour). Under high light levels, the rhodopsin is mostly inactivated and the rods have a low sensitivity. The cones then dominate vision and we see things in color. At low levels of illumination, the rhodopsin regenerates over a period of 20–30 min, restoring the rods to their full sensitivity. Vision is
C. R. Kitchin, Telescopes and Techniques, Undergraduate Lecture Notes in Physics, 181 DOI 10.1007/978-1-4614-4891-4_9, # Springer Science+Business Media New York 2013
182
9 Detectors and Imaging
Fig. 9.1 Structure of the human eye (horizontal crosssection)
then almost entirely via the rods. This effect has two consequences for astronomers. The first is the familiar effect of dark adaptation: immediately on leaving a brightly lit area into the dark, very little can be distinguished. However, after a few minutes, vision becomes much clearer, and after half an hour or so one can often see quite easily even on a moonless night. This adaptation is partly a result of the pupil of the eye increasing in size, but much more the effect of the regeneration of the rhodopsin, which increases the retina’s sensitivity by a factor of 100 or more. The rhodopsin is destroyed quickly by strong light, and so the dark adaptation can easily be lost if the observer is careless with a torch or turns on the main li
Data Loading...
