Tricks of Trade

Passed down from generations previous to generations next, here are some tricks of trade, or so to speak, that will help you during your next observation session or star party.

Dark Adaptation

The eye has an “iris” (an aperture regulator) that controls how much light falls into the eye through the “pupil” (the aperture). The moment you see a bright light, the iris immediately contracts the pupil to allow less light. For observing faint celestial objects, the iris must open the pupil as wide as possible. Thus, one needs to be in dark surroundings for a while for this to happen. Moreover, the absorptive pigments (“Rhodopsin”) in the eye are also designed to avoid excess exposure. The moment we look at a bright light, this pigment bleaches (i.e. becomes reflective rather than absorptive) and reduces the sensitivity to light. However, it takes about 30 minutes for the pigment to regenerate. Thus, one must ideally observe from a dark location with no surrounding lights, and allow the eye to adapt to the darkness for about half an hour to become more sensitive to dim light.

Averted Vision

Light-sensitive cells in the eye are not distributed uniformly throughout the retina (which is the light-sensitive “screen” of the eye). The eye is constructed such that looking straight at an object makes light fall onto a region called the “fovea centralis”, which has a large number of cone cells. The large density of cone cells helps the eye see more detail while looking straight at an object. Now, cone cells are sensitive to color, but not to dim light. Thus, the eye is “optimized” for viewing details in bright objects. But for faint objects, the cone cells are not responsive. It is the rod cells that are sensitive to dim light. The region in the retina which has a lot of rod cells, is not centered on the visual axis, but is off center. Thus, one can sense dim light better by not looking straight at the object, but looking away from it!

The technique of averted vision is based on this concept. In summary, it is “stare away from the object to see it better”. People who predominantly use the right-eye, must look towards the right of the actual position of the object, but be aware of the actual position of the object. People who use the left-eye, must look towards the left of the actual position of the object, but be aware of the actual position of the object. This will greatly enhance what you can see. Practice using averted vision is what differentiates a first-time observer from a seasoned observer. Mastery over averted vision is important to be able to pick out faint detail in objects.

In the eyepiece of a telescope, averted vision is easy to use. If you use your right eye, stare to the right edge of the eyepiece (assuming the object is in the center of the field) and if you use your left eye, stare to the left edge. But be aware of the center of the field of view.

Averted vision can greatly improve views of objects. It can make globular clusters easier to resolve, show spiral structure in spiral galaxies etc.

Star Hopping

Star hopping is a technique to locate faint objects which are not visible to the naked eye. It uses bright stars as a guide to locate fainter objects. A known star is first tracked in the finder scope, and then the scope is moved so as to follow a known pattern of stars in the sky till the object is reached. Once the target object is reached, higher magnifications can be used for observation.