Dark Adaptation and Stargazing
To fully appreciate the night sky, astronomers need to understand the function of the eye and specifically the retina.
The retina is the light-gathering neural tissue on the inside surface of the eye that transforms light into electrical energy. This electrical energy is sent to the brain and then is interpreted into the sense that we call vision. Vision is the end result of a photo-chemicalelectrical reaction started by a photon of light striking a single cell in the retina called a rod (or a cone). Increasing the sensitivity of these rods is the key to seeing more stars.
I mention rods above (and throughout this article) because it is rods that will be doing most of the low light ‘seeing’ during an evening with your telescope. Photopic and mesopic vision, which refer to daylight and dusk light levels are mostly cone type vision. Cones also provide the perception of color to our world. The switch to mesopic vision is easily noted by a decrease in color perception and the switch to black and white vision. The switch to fully rod function is called scotopic vision. In the following discussion, I am referring to scotopic vision.
Cones generally require way more light to fire, and use a slightly different type of photochemical than rods. Our most acute (or sharpest) vision comes from an area in the retina called the macula. The macula is virtually all cones and works poorly in low light situations. The fact that we have all seen a star out of the “corner” of our eyes only to have it disappear when we “look at it” (with the macula) attests to the poor function of cones in stargazing. This ability to look “off center” and use the mid-peripheral part of the retina that contains lots of rods is a time-honored way to sight stars that would be impossible to see with the macula (where the cones are).
Increasing the light sensitivity of rods in the retina is as simple as waiting the required time to increase available rhodopsin (also called ‘visual purple’) within the rod itself. Rhodopsin is the photo-chemical that starts the cascade of events that lead to a light stimulus being registered in the brain ( that is, ‘saw it’ or ‘not’). In simplistic terms, the more rhodpsin the higher the sensitivity. The key then is how to increase the available amount.
The cascade of events that begins when a photon of light being absorbed by a molecule of rhodopsin in the rod changes the molecular shape of that molecule, rendering it unable to start the cascade again until ‘reset’. Time is what resets the rhodopsin after activation. Literature suggests that most rods are reset (or recharged) up to 80% to 90% of full capacity at 10 minutes of no activity and that they are fully reset at closer to 30 minutes of no light. Now, keep in mind, we are talking about no light going into the eyes for 30 minutes before they are fully dark-adapted.
What about red lenses? Red tints over flashlights do help keep the rhodopsin counts high in rods. The rhodopsin in human rods is moderately insensitive to the longer wavelengths of light (say from 650 nanometers and longer) so in theory the rods will not ‘fire’ and therefore not decrease the store of rhodpsin. There are some references that state that any light hitting a rod will decrease its sensitivity, so the red light theory may not be 100% true. Anecdotes abound with the merits of red lenses, so use what works for you.
The use of hoods and/or patches seems to be a time honored way of getting and staying dark-adapted. Keeping one eye patched (and dark adapted) while the other eye is used for referencing charts or taking notes (and obviously not fully dark adapted) will not affect the dark-adapted eye. A hood placed over the head of the observer and his or her telescope eyepiece will also help keep most any stray light out of the eye of the observer. Any light also includes stray atmospheric light as well as the wrongly pointed flashlight. Patching the observing eye when out of “the hood” would also help keep it adapted. Remember that once the rods have been exposed to light the whole dark adaptation process begins anew.
Not mentioned in the literature is the fact the ‘observing eye,’if looking at a bright enough object through a telescope, would lose its adaptation. A big scope looking at a bright (and now highly magnified) planet for instance would surely push the rhodopsin levels low enough in the rods to cancel all the previous 30 minutes (or more) of trouble to fully dark-adapt. I would assume that neutral density filters could be used to decrease intensity. Also, the rods of the eye are best suited to pick up motion, especially in the mid-peripheral region of the retina directly surrounding the macula. Because of this fact, there are citations that mention the “tapping” or “bumping” of the telescope being viewed through so as to cause “motion” and possibly allow the viewer to spot a previously unseen star.
Keep in mind as well that vision is contrast. Recall the uncomfortable feeling of driving too fast in a heavy fog. There is plenty of available light, but what we “see” is limited by a severe lack of contrast. Our eyes’ ability to detect very faint light sources is truly remarkable but only if that light source is sufficiently brighter than its background. Sparkling clean optics is the key here. All pieces close to the focal plane are suspect, including the eyepiece, diagonals and any secondary mirrors, and should be kept spotless.
As in many things, trial and error seems to be the rule. I’ll hope that I have provided a little more information so that you might try and not error. Understanding is the key. Now, go have some fun in the dark.
Dr. David Kirscher is an Optometric Physician who has been practicing on Bainbridge Island for the last 18 years. Trying to get patients to see their best has been his goal—regardless of the circumstances...