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In this section: dark adaptation, averted vision, troxler phenomenon, magnification, contrast, image motion, observing at the limit, colour vision, binocular vision.
What follows is the collected writing of several experienced observers, giving their opinions about the finer points of visual observing. Extensive use is made of the works of K P Bowen, R N Clark, L Cain, A MacRobert and S J O'Meara.
Bowen (1984) summarizes the route taken by light as it encounters a human eye:
"The light first encounters the cornea, a transparent window of living tissue that is the main image-forming element of the eye. It finally reaches the retina, where two types of light-sensitive elements – rods and cones – detect it. ... The cones distinguish colour, and function best in a bright environment. Rods, on the other hand, cannot discriminate colour, but are better as motion detectors and operate best in low-light conditions. The cones are most sensitive to green light at about 5600 angstroms, while the rods respond best to blue-green light at about 5100 angstroms. The retina can detect and function over an incredible brightness range of 1014 times, a marvel indeed."
In low-light conditions, the eye takes a while to adapt. MacRobert (1985) advises: "Give your eyes at least half a hour to thoroughly adapt. Complete dark adaptation is critical for seeing faint deep sky detail. Most observers never reach proper dark adaptation, because they use torches that are too bright. Studies have shown that even red light when sufficiently bright impairs night vision. Make your own "observing capsule" by draping a dark cloth over your head. I prefer to wear a black T-shirt upside-down, which I can pull over my head as needed. It doubles as a neck-warmer, too. This dark shield cuts off interfering lights and reflections from the telescope tube (which in my case is glossy white aluminium). One observer raises the issue of 'hood humidity' while the warm radiation from your face slows the process of dew formation on the eyepiece, the humidity from your eye and breath speeds it up. "Since the hood traps the moisture from your face, you may need to buy or rig up a little heater for your eyepiece to prevent dew from forming on the lens. A simpler solution is to step away periodically to let the pocket of humid air disperse." An eye-piece heater is probably a better solution.
Another way to maintain dark adaptation is to observe with one eye, while reading starcharts and taking notes with the other. I found the eyepatches supplied by hospitals to be woefully inadequate, being tiny, discrete things. Instead, I cut a large (12cm x 9cm) piece of soft black leather into an oval shape and attached a length of pants-elastic. While at the eyepiece, this flops over my left eye, which can then be kept open and relaxed while taking in no distracting image. In less than a moment, I flop the patch over my right eye and use the left for 'normal' viewing.
The cones are concentrated in the central retina and are the only light-sensitive elements in the fovea centralis, which is an area of our vision about 1.5° in diameter where we have our highest resolving ability.
Outside this area, the rods rapidly gain predominance and reach maximum concentrations in a narrow oval ring located about 19° horizontally and 15° vertically from the fovea. This off-axis concentration of rods is the reason why 'averted vision' (looking slightly away from the point of interest) produces such a dramatic improvement in our ability to see faint objects. The dimmest astronomical sources the eye can detect, whether stars or nebulae, are seen only with averted vision. Practice aiming your eye in one direction while paying attention to something a little off to the side. As MacRobert (1985) points out, "This is what deep sky observing entails almost all the time."
This is a bit tricky at first, because we must bypass our eye's built-in natural instinct to place whatever we're looking at directly on the fovea. Look at a part of the field away from the spot where the desired object is, yet be aware of that location without gazing at it. With practice the technique becomes second nature.
Clark points out that, on the periphery, the eye is about 40 times (4 magnitudes) more sensitive than the fovea. Peripheral vision, Clark notes, is best when the object appears 8 to 16 degrees off-axis in the direction toward the nose. The areas up, down and toward the ear are not quite as sensitive. The blind spot lies 13-18 degrees towards the ear; placing the object here will cause it to vanish altogether.
The eye tends to 'ignore' an image if its position on the retina remains stationary for any length of time. This is known as the Troxler phenomenon, and the eye usually changes its fixation without our realising it about 10 times a second to avoid it. During intense observation, however, a person can fixate on an object long enough to cause the image to fade somewhat. Conscious sweeping of the field or taking a break from the eyepiece for a few seconds will help prevent this fade-out.
A faint extended object (e.g. a galaxy, a bright spot within a galaxy or nebula) should be viewed with enough magnification so it appears several degrees across to the sky. To be detected, it must be surrounded by a darker or lighter background, so the eye can distinguish contrast. Various magnifications should be tried to bring details into the range of best detection. At each magnification, considerable time must be spent examining for detail. Higher magnification should be tried until the object is totally lost from view. The eye should be dark adapted for at least 30 minutes so the photochemical visual purple is at full abundance. Bright stars and extraneous lights will tend to destroy dark adaptation." (Clark 1990:18)
One piece of conventional wisdom is that low magnification should be used for deep sky observing, so that light from a faint object is concentrated on a small area of the eye's retina. Clark argues: "This would be true if the retina worked passively, like photographic film. But it doesn't. The visual system has a great deal of active computing power and combines the signals from many receptors to detect a faint extended object.
"Increasing the magnification spreads the light over more receptors, and the brain's processing power can then bring into view fainter objects having lower contrast. When you switch from a low to a high power, you could gain a magnitude or more in faint stars. This works because high power reduces the surface brightness of the entire field by spreading out the light. Doing this dims the sky background without affecting the total amount of light arriving from small, discrete objects. Stars appear so tiny that their surface brightness hardly looks changed at high power. But even an already dim, diffuse galaxy won't be rendered any less visible when its surface brightness is lowered (at least within limits), because the galaxy's contrast with the sky remains the same. You're actually likely to see it better, because your eye perceives low-contrast objects better when they are large. The neural network in your retina is smart enough to gather and correlate the galaxy's light from a wide area. Deep sky vision is quite different in this regard from the behaviour of 'dumb' photographic film, which responds to surface brightness only."
Clark also notes: "It is normally accepted that the highest power [usefully employed on a telescope] is about 50 to 60 times the objective in inches. This limit is correct only for bright objects ... for fainter objects the eye has less resolution and needs to see things larger, so higher powers are called for. At the limit of the eye's detection ability, the highest useful magnification is on the order of 330 per inch of objective!
"High magnification demands a lot from your telescope. If the mounting is unstable, or the slow-motions not smooth, then each vibration or bump will cause the image to dance about wildly. Further, the field of view becomes narrower as you magnify, making objects a bit more difficult to keep track of. The effects of an unstable atmosphere are exaggerated at high magnification. Clark notes that "magnification also reduces the surface brightness of everything in view. It must not reduce an object's surface brightness below the eye's detection limit, of course, or the object will disappear."
So while higher magnification does decrease surface brightness, MacRobert notes, the total number of photons of light entering the eye remains the same. It doesn't really matter that these photons are spread out over a wider area; the retinal image-processing system will cope with them. At least within certain limits. A trade-off is needed to reach the optimum power for low-light perception: enough angular size but not too drastic a reduction in surface brightness.
Mel Bartels on his "Visual Astronomy" page concludes that there are a number of ways to make an object detectable. Regarding magnification, he says: "Use sufficient magnification to make the background invisible and the object about one degree in apparent size. Most amateurs today use too low of power because their scopes don't track, and because 'that's what everyone else does'. John Dobson was the first large aperture observer to point out the advantages of high magnifications. Al Nagler, and Brian Skiff, among others, have recommended high magnifications. By increasing magnification, you are decreasing sky background brightness, and making the object larger in apparent size, both crucial to detectability. For small extended objects, you may exceed the old double star observers' rule of 50x per inch of aperture."
Nils Olof Carlin summarises: "To detect a faint object, you can increase magnification till the sky is so dark that you have difficulty seeing the field stop, or till the object has an apparent size of one degree, whichever comes first."
The retina has very poor resolution in dim light (which is why you can see a newspaper at night but not read it). Details in a very faint object can be seen only if they are magnified sufficiently. To get the most out of observing, use different magnifications, including very high, and take time to closely examine the object. And don't forget to make a sketch.
Lee Cain (1986) writes that the "most important factor in observing deep sky objects is contrast. The eye is capable of seeing extremely faint objects as long as there is sufficient contrast with the background. ... Cleanliness influences contrast. Dirty optical surfaces are perhaps the biggest enemy of contrast in telescopes. Dirt near the telescope's focal plane is most detrimental, because here the light is concentrated and light-scattering dust is more nearly in focus to the eye. Eyepieces, diagonals, and secondary mirrors should be kept as clean as possible."
"The inside of a telescope should be blackened and well baffled to keep stray light form entering the eyepiece. But the amount of stray light in even a poorly baffled telescope is less than what enters the eye from around the outside of the eyepiece. ... I suggest using a rubber eyecup or draping a black cloth over your head while observing."
Since the low-level light receptors, rods, are good at picking up motion, some observers lightly tap the telescope tube, causing the image to move about. In this way they notice detail in an object that they otherwise wouldn't pick up. How much to rock? With a 40arcmin field of view, I use up to 10 arcmin swings (amplitude). In my experience, the image should not move too rapidly, or it simply becomes a blur - you don't want to generate your own seeing! Also, this techniques seems more effective when used with a telescope than when applied with binoculars.
Schaefer (1989) notes: "Only three parameters have a big effect on the limitng magnitude: aperture, magnification, and the naked-eye limiting magnitude at zenith. A telescope of larger aperture obviously collects more light. Higher magnification spreads the background sky brightness over a larger angle, so a telescopic viewer sees a fainter skyglow and hence can pick out fainter stars. The zenith magnitude is an indicator of sky brightness: the fainter you can see without the telescope, the fainter you can see with it."
An important concept is the probabilistic nature of observing at the limit. Schaefer comments that "the actual limit depends on the observer's confidence. If you're absolutely sure that a star of magnitude 14.0 is always visible, you may still get occasional definite glimpses of a 15.0 star. Faint-star visibility is a probabilistic affair." Clark (1994) expands on this: "Observers looking at extremely faint stars note that the star will blink into view occassionally; it is only seen for brief moments. "By increasing the 'integration time' - the time spent trying to make a detection - a few blinks detected at the same location can build up to the positive identification of stars or faint objects normally considered beyond reach ... The longer you look, the fainter you should see. Laboratory research on the eye's response to faint light shows that detection limits can be likened to a probability curve ... there is no single limiting magnitude for a given telescope aperture."
"The best observers achieve results at the 2 percent level, yet they are actually detecting the target star perhaps only 1 percent of the time. The reason is simple: its difficult to hold your eye steady enough to allow the most sensitive part of your retina to collect photons from a tiny part of the total image. Thus while you might spend 100 seconds trying to detect a faint star, it is challenging to hold your eye steady in the correct position for even 50 seconds. Even the best observers can't do it all the time, and they have practiced the technique long and hard."
Some 5 to 10 minutes are needed to detect faint objects at the 5% probability level. "To push the detection level to the 2 percent threshold requires much longer observation time. You also need to be far away from city lights and have keen averted vision and intense concentration. If you cannot escape all city lights, there are tricks that will help you achieve the faintest possible observation. High on my list is total isolation from extraneous light. Cover your head with a black cloth if you must. Or use higher magnifications, which help reduce the sky background and the interference of bright stars in the field ... Another aid is to tap the telescope tube lightly when trying to confirm a sighting of a faint star, since our eyes are very sensitive to motion. Some observers breathe more deeply or frequently than normal, in the hope that doing so will deliver more oxygen to the eye's receptors."
"But there is more to observing than simply detecting faint stars. The probabilty principles can apply to contrast in extended objects. You may detect a low-contrast feature on a planet by simply observing it long enough for it to 'flash' into view. Planetary observers usually credit seeing changes as being responsible for all fine, low-contrast detail they see. Some of these 'moments of good seeing' might well be the eye and brain combination hitting those low probability levels! ... Again, to reach the lowest levels, you must have excellent skies ... You must be far from cities and work on very clear nights."
Clark (1990) writes that "the human eye is a remarkable detector of colour under bright daytime conditions. But the colour receptors, the cones, do not function at all in the low light levels of night, so no colour is seen."
Shaefer (1993) points out that "the human eye can detect colours from sources brighter than 1500nL. The reason is that photopic vision (i.e. day vision which uses the retinal cones; as opposed to scotopic vision, or night vision, which uses rods) has three types of photopigments, each with a different spectral sensitivity. So the eye simultaneously measures the brightnesses of an image over three different wavebands, much like taking CCD images through three different filters or taking a colour photograph with three different chemical dyes. The situation is also analogous to photometry in the Johnson UBV system, where three intensities are measured."
To detect colour in deep sky objects, do not use averted vision – look directly at the object.
Bowen (1985) reports:
"When low-contrast targets are viewed, there is a 40% improvement in resolution with binocular as compared to monocular vision. Binocular vision also gives an improvement in contrast sensitivity on the order of 40%. Also, there is a lower light threshold; there is a 25–40% gain in the ability to see faint objects."
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