“You must not expect to see at sight… Seeing is in some respects an art which must be learned. Many a night have I been practising to see, and it would be strange if one did not acquire a certain dexterity by such constant practice.” – William Hershel

Observing is a skill : a seasoned observer can see more details and dimmer objects than a novice. noted that

The fact that observing is a skill obtained by practice often leads to confusion when a newcomer cannot see what is obvious to others with more experience. The newcomer may conclude she has bad eyes. Usually, a few steps will remedy most problems. This guide should be useful in addressing issues that newcomers face and should provide some insight into human visual perception.

Averted Vision

The technique of averted vision has been heavily discussed among amateur astronomers and is really of fundamental importance for observing deep sky objects. Averted vision involves not looking directly at an object, but looking a little off to the side, while continuing to concentrate on the object. The technique is based on properties of the structure of the eye (discussed below). By developing the technique, some observers report a gain of up to three or four magnitudes (15:1 to 40:1). There is some evidence that the technique has been known since ancient times, as it seems to have been reported by Aristotle while observing the star cluster now known as M41.

It does matter whether you avert right or left. The most effective direction is that which places the object on the nasal side of the vision. This avoids the possibility the object will be imaged on the blind spot caused where the optic nerve comes up onto the front of the eye.

A similar technique is called scope rocking, and is done by simply moving the telescope back and forth slightly to move the object around in the field of view. It is based on the fact that the eye has evolved to be more sensitive to motion.

#Structure of the Eye

Averted vision can be understood by studying the structure of the eye. The human eye contains two types of detectors: rods and cones. The cones are responsible for color vision, but are much less sensitive to low light than the rods. They are located in a small patch at the center of the iris called the fovea. The rest of the iris is composed of rods, which are black and white detectors. They are largely responsible for peripheral vision and are more sensitive to dim light and motion. Due to the way the rods are wired, (and to some extent lower density) rods have a lower resolution than cones. The nerve cells which connect to rods and cones are called retinal ganglion cells. In the fovea, each ganglion cell may connect to as few as 5-10 cones. The number of connections increases moving towards the edge of the eye. At the outer edge of the eye, each ganglion cell may connect to as many as 1000 or more rods. As a result, among rods more photons are accumulated per ganglion cell, but resolution goes down. The density of the rod cells usually reaches a maximum around 15-20 degrees off the center of vision, and this is the most sensitive. The resolution of the eye falls off rapidly beyond 0.6 degrees (the small region of the fovea). It is four times poorer at 10 degrees radius as it is within the 0.6 degree radius from the line of sight.

Blind Spot

It is an interesting fluke of nature that in humans (and all other vertebrates) the ganglion cells lie on top of the rods and cones.[1] To get to the brain the optic nerve fibers come together and pass through a hole in the retina, creating a blind spot. The blind spot is on the side of the eye opposite to the nose. The reason we do not notice a large gaping hole in our vision is that the visual cortex fills in the blind spot using information from the other eye. If only one eye is being used, the blind spot is filled in with a sort of interpolation from the surrounding area. This interpolation (or extrapolation, if you prefer) is quite good and is capable of interpolating color and shape. It is quite easy see your blind spot though, using this excellent webpage. You can also map your blind spot using this applet to see just how large it is and what shape it is.

[Incidentally, there are also small blood vessels which pass over the retina and go into the vitreous humour. They cast shadows on the retina (mini blind spots) which are normally not seen due to neural adaptation. It is possible to see the blood vessels in your eye if someone shines a bright light into your eye, thus changing the shadows. (you may have noticed this at the optometrist)]

Night Vision

For the eye to become fully dark adjusted takes about 30 minutes. The chemical that is responsible for detecting light in the rods is rhodopsin, also called “visual purple”. Visual purple is chemically altered when light hits it, and must constantly be regenerated. It has also been showed that nervous system accumulates photons differently under low light conditions. A strong signal coming from a collection of 100 rods in bright light will be equivalent to a weak signal coming from a collection of 1000 rods in dim light. The nervous system thus allows for greater convergence of signals from rods in dim light. Dark adaptation allows the human eye to see a remarkable range of brightness levels, spanning 9 orders of magnitude – a factor of one billion.

Rhodopsin is not sensitive to light below 620nm (red), so this is why red lights are used in astronomy. Red LED lights are better than white lights with filters, because LEDs are guaranteed to emit only specific wavelengths in the red. Filters, on the other hand, are never perfect and always allow small amounts of other wavelengths to pass through.

Night vision develops independently in each eye. So, it is possible to cover one eye to allow it to remain dark adapted while the other looses its adaptation. Indeed, dark adaption occurs at the level of individual rod and ganglion cells, so a bright light (such a car’s headlights in the distance) will destroy dark adaptation in part of the eye but will not necessarily ruin overall dark adaptation. To maintain dark adaptation in their eye, some hard-core observers use an eyepatch when not looking through the telescope. You can also use a towel to shield out stray light. (see the book Astronomy Hacks pg 84)

In summary, there are three “modes” of vision discussed in literature on the subject:

Photopic mode – “day vision” – high resolution color imaging. Peak sensitivity in green at 555nm.

Scoptic mode – “night vision” – rods only – low resolution black and white imaging. Peak sensitivity in blue green at 505 nm.

Mesoptic mode – a mixture of photoptic and scoptic vision which occurs during moonlight and similar light levels.

In mesoptic mode, when cones are reaching the limits of their detection ability, perception of color changes. Cones are least sensitive to red, so red is the first color to disappear. This is then followed by the disappearance of other colors as light levels get dimmer. Often, observers report nebula to be blue-green, because that is the area of the spectrum where the cones are most sensitive. This change of colors in low light conditions is called the “Purkinje Effect”.

Eye Placement:

Vertical displacement and eye relief

All telescope eyepieces have an optimum viewing distance, called the “eye relief”. It is often specified on the eyepiece packaging and is measured in millimeters. Too little eye relief can be vexing because one is in danger of bumping the telescope. On the other hand, too much eye relief can be problematic because it becomes difficult to figure out where to place one’s eye. Often, newcomers try to get as close as possible to an eyepiece. While this may work ok for many eyepieces with wide apparent fields, with others the image will be lost around the edges. Additionally, over time eye lashes can smudge the optics and damage coatings.

Centering and exit pupil

In addition to being the correct distance from the eyepiece, it is important to have the eye properly centered. The light passing through an eyepiece is focused down to a small cylinder of light. The smallest diameter of this beam is called the “exit pupil”. Exit pupil can be determined from the following formula:

exit pupil = eyepiece focal length ÷ telescope f/# (ex.: 32mm ÷ f/10 = 3.2mm)

Many visitors to the Hirsch Observatory report not seeing anything, simply because their eye is not centered over the exit pupil. When the exit pupil is bigger than the pupil of the observer’s eye, not all the light that comes out of the telescope can make it to the observer’s retina: some will be blocked by the pupil of the eye. So, although lower power eyepieces will make an object brighter, when exit pupil exceeds the size of the eye’s pupil, no further brightening will be noticed. The diameter of the pupil of the dark-adapted human eye varies from person to person, but tends to decline with age. As a rule of thumb, persons younger than about 40 will likely have dark-adapted pupils of 7 mm diameter; older people will have smaller ones. [The psychology of pupil dilation is a fascinating subject, but is not relevant to astronomy.]


A common question is whether eyeglasses should be worn. In most cases, if a telescope is being shared among several people, it is easier to keep glasses on, rather then having to refocus the telescope. And, in general, it is better to keep glasses on, because eyeglasses correct for astigmatism, which can not be corrected by focusing the telescope. (However, focusing can correct for near-sightedness and far-sightedness). Astigmatism essentially occurs when a lens focuses differently along horizontal and vertical planes. It is usually checked for by optometrists and corrected with eyeglasses which have different radiuses of curvature in different directions. Studies show astigmatism is quite common, and occurs in about one in three people. You can test for astigmatism by removing your glasses and rotating your head while looking in the eyepiece. Astigmatism will be more apparent at low powers, with a larger exit pupil and less apparent at high powers. For those with little or negligible astigmatism, some benefits may be obtained by not wearing glasses. At the very least, it eliminates a small amount of light loss from scattering. It may also be of practical necessity, if the eyepiece has very little eye relief, and there is not enough room to get close enough to the eyepiece. Finally, all eyeglasses have slight achromatism, which increases towards the edges. This can be observed by looking at an object with high contrast, such as the edge of a backlit object. Either blue or red will be observed depending on the geometry of the situation.

Perceptual Blindness

“Perceptual blindness” is a term used by cognitive psychologists. It is essentially the principle that people have a much easier time seeing things when they know what they are looking for, and will not notice things that they are not familiar with. Vision is much more than just the collection of photons in the eye and corresponding electrical impulses. Images must be interpreted and objects discerned, though a complex neurological system which is not fully understood. For instance, someone who is looking for a galaxy may expect something with spiral arms or a small bright object. They may not realize that they should look for a fuzzy, dim elliptical patch. The stimulus is present, but it is being ignored by the brain as “uninteresting”. Such ignored stimuli may have effects on the brain but will not reach consciousness. In addition, trying to concentrate on finding a particular object may lead one to ignore other features in the surrounding environment.

To give an often quoted example of “perceptual blindness”, there is legend which says that that when Christopher Columbus arrived, the natives where not able to see the ships approaching until they were very close. Perhaps one “wise sage” was able to see them and ran back to the village to alert the others. They arrived but were not able to see the ships even though they were in clear sight. It is easy to picture how an untrained eye would have difficulty spotting the ships on the horizon, but many popular accounts claim they could not see them even up close. The pseudoscientific movie “What the Bleep do we Know” went so far as to claim as historical fact that the Indians could see the waves caused by the ships, but not the ships themselves, which is wrong and misleading. However, a slightly different type of “perceptional blindness”, perhaps better called “conceptual blindness” can occur when objects are in clear (conscious) sight. For instance, an Indian told to look for a “boat” may look very hard for a bamboo raft while not interpreting a sailing ship as a legitimate boat. Upon seeing the sailing ship, he will have difficulty figuring out what it is. For instance, the Aztecs described the sailing ships as “floating islands” because they had no other concepts to associate the ships with (incidentally, they also referred to horses, another thing they had never seen before, as “large dogs”).  Here is another anecdotal example: a surprising number of students are not aware that there is an observatory on top of the Science Center. My personal theory is that students “see” the observatory on a daily basis, but it does not register to them consciously. If it is consciously seen, then it may not be associated with prior concepts of “observatory”. Instead, it may be taken as just “another thing on the roof”, among fume hoods and AC units. The same type of thing happens all the time in astronomy.

Relax: (“Zen of Observing”)

A general piece of advice for observing is to relax. Keeping both eyes open is usually recommended for maximum relaxation. If the field of view of the other eye is distracting, it can be covered with one hand. An observing chair is usually necessary to achieve proper relaxation. A serious observer will want to spend at least a few minutes at the eyepiece observing an object, which can be stressful if one is bending over or in an otherwise awkward position. According to one website, “Fatigue and muscle strain increase random eye movement.” Certainly, being under stress makes it harder to concentrate.

Exposure time

** There appears to be at least one website saying that the human eye can “accumulate photons” like a CCD chip, and can have an effective exposure time of several seconds. This comes from the testimony of Roger N Clarke, a well known observer. There is no evidence for this as far as I can tell. One website give the exposure time of the eye as being around 1/5 of a second.

The factor of time and patience

** Patience is a particularly important virtue in astronomy. Through patience, one can see many amazing things, such as spectacular shooting stars, satellites passing through the eyepiece and minor structural details in nebula and galaxies. The reason patience pays off threefold: first, takes a while to discern details in objects at low light because finding the optimum averted vision position takes a while. Secondly, atmospheric turbulence varies seeing conditions dramatically. Planetary observers used to sit at telescopes for many hours waiting for brief seconds of perfect seeing. (nowadays, CCD chips are used) Finally, being patient allows one to see random transient phenomena, as already noted.

Finally, the most patient of all observers – comet hunters –  are rewarded when they finally discover a comet. The average time to find a comet, including accidental discoveries, is around 400 hours (not including accidental discoveries, which are rare). Some claim that the average for subsequent discoveries is 200 hours. The famous comet hunter David Levy logged 916 hours before discovering his first comet. The Japanese, by far hold the record for amateur comet discoveries. Japanese observers also hold unofficial records for most time spent before first discovery: “In 1987 Noboru Nishikawa took 3024 hours in 2389 sessions to find his first comet (1987a). In 1990 Yuji Nakamura discovered his first comet after searching 2236.5 hours in 1558 sessions.”[2]

From The Amateur Astronomer’s Handbook by James Muirden:

No opportunity should be lost to train the eye to work with the telescope; to observe the same object with different powers so as to see the effect of magnification; to try to see faint stars; and to draw planetary markings. In the beginning, to be sure, this may all seem to be wasted effort; the observing book will fill up with valueless sketches and brief notes of failure. But this apparently empty labour is absolutely essential; for, as the weeks pass, a steady change will be taking place. Objects considered difficult or impossible to see will now be discerned at first glance, and fainter specters will have taken their place. Indeed, these former features will now be so glaringly obvious that the observer may suppose that some radical improvement has occurred in the observing conditions. But the credit belongs entirely to the eye…”

Saccades and Microsaccades

Saccades are rapid motions of the eye. The eyes resolution drops off almost exponentially from a small area of high resolution in the fovea. Because of this, when we look a at a scene, the eye jumps from place to place. In the tiny time during these jumps (measured in ms), the optic nerve shuts off.

Microsaccades are tiny jerky movements of the eye which occur normally when the eye is relaxed. They correspond to a resonance of the eye around  30-70 hz. Microsaccades are currently a rather poorly understood phenomena, and research is being done to understand their purpose. It appears that they allow for a greater area of high resolution.

Microsaccades may also explain why it is difficult to concentrate on a point for a long period of time. As noted before, concentration on a specific object is very unnatural for the eye. An interesting experiment to perform is to concentrate on a star for a few seconds.  Eventually, you will see stars in the surrounding field of view fade away! In fact, its likely that the star you are looking at will fade or even disappear completely. This phenomena is called Troxler’s fading, and at least some research suggests the purpose of microsaccades is to prevent this fading.[3]

The cause of Troxler’s fading is the adaptation of neurons in the visual cortex. This follows the general principle in sensory systems that an unvarying stimulus eventually disappears from awareness. For example, if a small piece of paper is dropped on top of one’s arm, it is felt for a few seconds, then the sensation is no longer present, (at least, not in direct consciousness) because the tactile neurons have adapted. But if one jiggles one’s arm up and down, giving varying stimulation, one continues to feel the paper until it falls off one’s arm. This idea is supported by experiments where an image is projected so that it remains fixed on the retina (known as a “stabilized retinal image”).  This can be achieved by monitoring the motions of the eye and quickly counteracting the motions. The image is observed to fade away within a few seconds. This also explains why afterimages, which are caused by a “bleaching” on the retina fade away quickly.

Eyepiece and filter selection:

Eyepiece and filter selection is an entirely separate subject, which is covered on many articles online. For instance: Astronomical Telescope Eyepieces: A Discussion for the Beginner


[1] Wikipedia: Evolution of the Eye (c.f. Feynman Lectures on Physics, Vol. 1).

[2] Comet Comments (there are many websites on comet hunting with similar statistics)

[3] Martinez-Conde, S., Macknik, S., Troncoso, X., & Dyar, T. (2005). Microsaccades counteract visual fading during fixation  Journal of Vision, 5(12):72, 72a,

[4] Purkinje Effect on Wikipedia

Wikipedia: Saccades**

Wikipedia: Microsaccades

Wikipedia: Troxler’s Fading

Wikipedia: Stabilized retinal images

The Caldwell Objects and how to observe them

Further Reading:

Is it possible to see a single photon?