2. Observing Hacks
Section 2.1. Hacks 1132
Hack 11. See in the Dark
Hack 12. Protect Your Night Vision from Local Lights
Hack 13. Describe the Brightness of an Object
Hack 14. Identify Stars by Name
Hack 15. Identify Stars by Catalog Designations
Hack 16. Know Your Constellations
Hack 17. Understand Celestial Coordinate Systems
Hack 18. Print Custom Charts
Hack 19. Keep Your Charts at the Eyepiece
Hack 20. Locate Objects Geometrically
Hack 21. Learn to Star Hop
Hack 22. Learn to See DSOs
Hack 23. Observe Shallow-Space Objects
Hack 24. Slow Down, You Move Too Fast, You've Got to Make the Evening Last
Hack 25. Learn Urban Observing Skills
Hack 26. Sweep Constellations
Hack 27. Maintain an Observing Notebook
Hack 28. Develop an Organized Logging System
Hack 29. Plan and Prepare for a Messier Marathon
Hack 30. Run a Messier Marathon
Hack 31. Photograph the Stars with Basic Equipment
Hack 32. Discover and Name a New Planet
2.1. Hacks 1132
Locating and observing astronomical objects requires developing a special set of skills and practices, most of which are not intuitive. It requires a detailed knowledge of the night sky and of specialized astronomical terminology and conventions. There are things you must know and be able to do if you are to be successful.
Just finding the object you want to view can be difficult. The night sky is huge, and many astronomical objects are tiny, dim things. Even after you have found the object and verified its identity, teasing out the maximum possible amount of visible detail is very challenging.
We've watched many beginning observers encounter the same frustrating problemswhat we call the "newbie blues"and we've helped more than a few of them over the hump. All of them, particularly those who have go-to scopes, hope there are shortcuts to learning to observe. There are no shortcuts. A go-to scope is no better substitute for learning the night sky than an automatic transmission is for learning how to drive. Learning to observe is a hard-won skill, but one you can be proud of achieving.
In this chapter, we tell you what you need to learn, know, and do to locate, describe, and observe astronomical objects.
Hack 11. See in the Dark
Have you ever wondered why all cats are gray in the dark?
Our eyes function in two entirely different modes, depending on how much light is available. In daylight or bright artificial light, our eyes function in day vision mode. After dark, our eyes shift to night vision mode. The physiological changes that occur in our eyes during the shift from day vision to night vision are called dark adaptation. Dark adaptation occurs slowly, typically requiring 25 minutes for 80% adaptation and 60 minutes for 100% adaptation. That's why astronomers get upset when someone shows a bright light.
There are many misconceptions about night vision and dark adaptation, even among astronomers. To understand the process of dark adaptation, you need to understand something about the physiology of the human eye. Our eyes have two types of light sensors, called rods and cones. Rods provide monochromatic vision, but are very sensitive to light. Cones provide full color vision, but are relatively insensitive to light.
Cones and rods are unevenly distributed over the surface of the retina. Cones predominate in the fovea, the center of the retina, where they are densely packed. The fovea contains about 200,000 cones in an area of about one square millimeter, and thus provides acute resolution of fine detail. The entire retina contains about only 7,000,000 cones. That means cones are very sparsely scattered outside the fovea, just enough to show brightness and color with little detail in your peripheral vision. Rods predominate outside the fovea. The entire retina contains about 130,000,000 rods. They are less densely packedat about 90,000 per mm2than cones in the fovea, but much more densely packed than cones outside the fovea. Accordingly, rods provide poor resolution of fine detail relative to the cones in the fovea, but much higher resolution than the sparsely scattered cones outside the fovea.
Rods and cones detect light by using dyes to absorb it. As light is absorbed, the dyes bleach and a signal occurs to indicate that light has been sensed.
There is only one type of rod, which is why rods provide monochrome vision. There are three types of cones, one for each of the primary colors of light: red, green, and blue.
2.2.1. Vision Modes
Broadly speaking, there are three modes of vision:
Technically, mesopic mode isn't a separate mode, but a combination of photopic mode and scotopic mode. Mesopic mode occurs when part of your eye functions in photopic mode and part in scotopic mode. Here are two examples of mesopic mode as it applies to astronomy:
2.2.2. Night Vision Fallacies
Here are some common fallacies about night vision and dark adaptation:
220.127.116.11 Dark adaptation is all-or-nothing
Wrong. Many astronomers believe that any exposure to light damages overall dark adaptation. In fact, not only does each eye dark adapt separately, but each cone or rod also adapts individually and cones do so separately from rods. That means you can keep one eye fully dark adapted even if the other eye is exposed to bright white light. Also, because cones adapt separately from rods, you can use photopic vision for viewing charts, recording observations, etc., without harming the scotopic dark adaptation of your rods.
18.104.22.168 Pupil diameter is critical to dark adaptation
Not true. The human pupil varies from as small as 2mm in diameter under bright lighting to as large as 8mm under dark conditions, a range of only 4:1 linearly and 16:1 areally. In fact, the usual range is less. It's very rare for a person more than 20 years old to be able to dilate to 8mm; 7mm is the more usual maximum in young adults, 6mm at age 35 to 45, and 5mm is common in people older than 55 or 60. At most, then, the range of brightnesses controlled by pupil diameter is 16:1, and a range of 12:1 or even 6:1 is more usual. In fact, directional sensitivity reduces that factor still more, into the range of 10:1 to 4:1. The range of brightnesses detectible by the human eye is about 10,000,000,000:1, so pupil diameter plays only a miniscule role compared to the sensitivity level of the rods. Also, the pupil constricts and dilates very quickly compared to the time needed for rods to recover their dark adaptation.
22.214.171.124 A dim green light is the best choice for preserving night vision
Nope. This myth probably persists because the military uses dim green lighting in some tactical situations and because military night-vision scopes produce a dim green image. In fact, the military uses dim green light because the photopic (cone) vision needed to provide high visual acuity is most sensitive at green wavelengths. But any green light bright enough to trigger your cones is much more than bright enough to destroy the dark adaptation of your rods, eliminating your night vision.
126.96.36.199 A bright red light destroys night vision
Red light preserves dark adaptation for a simple reason. The rhodopsin pigment in rods is completely insensitive to light at wavelengths longer than about 620 nm, which is to say deep red. Although the erythrolabe dye present in your L cones has peak sensitivity near 564 nm, its sensitivity extends far into the red part of the spectrum. That means you can use a very bright red light without damaging your scotopic vision at all.
This is an excellent reason to use a red LED flashlight rather than a standard flashlight with a red filter. LED flashlights emit light at one specific wavelength, and red LED flashlights emit at a wavelength to which rhodopsin is insensitive. Red filters, on the other hand, also transmit a fair amount at light at shorter wavelengths, so a bright red-filtered flashlight can impair your night vision.