Seeing In the Dark Part 1

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Seeing in the Dark - Part 1

Night Vision

To make the most of astronomy, you need to learn a set of new skills; skills that will help you to identify and see faint objects more clearly.
The first thing to learn is that you need to find a nice dark place and give your eyes time to adjust to the dark. Avoid areas that are lit by bright street lights, car headlights or bright lights of any kind.
To set up your telescope and read star charts you will need a colored flashlight. That is a flashlight that gives off a red colored light instead of a white one.
The reason for that is that it takes longer for you eyes to readjust to the dark when you use a white light than it does when you use a red one.

Averted Vision

Once your eyes have become used to the dark you can now see faint objects. One method used by astronomers to see very faint objects is to use averted vision. Averted vision is where you look at things indirectly or out of the corner of your eye. This is useful for looking at the fainter objects, such as nebulae. This can be a little frustrating at first; it is a natural reflex to look directly at an object instead of glancing at it with adverted vision. During the daytime when there is a lot of light available you can look at things directly. Direct vision causes the image to focus at the center of your retinas. However the center of a retina has fewer cone cells. It is the cone cells that are extra sensitive to low light. Averted vision is especially helpful for viewing galaxies, nebulae and comets.

Seeing

You might be surprised to learn that ‘seeing’ is a technical term in astronomy. It does not refer to how good you can see but how clear and undistorted the atmosphere is. If you can see the stars and they are twinkling rapidly, then that indicates that it is a ‘poor’ seeing night. Things that will affect seeing include light pollution, dust, clouds, wind, thermals, smog and moisture (you can notice moisture clearly when there is a ‘ring’ around the moon. If you know anything about gliding then you will most likely be familiar with thermals. Thermals occur more frequently on hot days, especially where the ground is heated unevenly so that air is rising in one area. When the air is pressurized unevenly; light travels through it unevenly. That is why they like to build observatories on top of mountains and why the Hubble Space Telescope was able to provide us with such great images even though it’s aperture was smaller than the larger ground based telescopes. So the best seeing is done far away from the city smog and lights, up on a high mountain and during cold (but not cloudy) winter nights. You might not have to go up a high mountain to make the most of an amateur telescope but the better the conditions that you can find the clearer your viewing will be.

The Moon

Ok so you’ve packed the telescope into the car, driven to a nice dark place, and you are quite relieved to see that the clouds have vanished. You look up and there are countless stars in the sky, you hurry to set up your telescope but then you look over to the East and see that the sky is starting to glow in that direction, in fact you look around and notice that the whole sky is suddenly brighter. Here comes the Moon and with it your chance to view the faint deep sky objects. So if you are planning a trip out into the country to see faint objects then it pays to plan your trip around the phases of the Moon. If the Moon is new it will appear as a sliver in the West and set soon after dark, that’s a reasonably good time to see faint objects. Each night after a new Moon, the Moon will start to get brighter and rise roughly fifty minutes later each day until two weeks later it is a full moon that rises close to sunset and dominates the sky all night. While the new moon, up to the first quarter, is excellent for viewing craters on the Moon, (since the shadows appear longer from the Earth), the Moon becomes increasingly brighter until only the brighter stars are visible. When the Moon is full it is hard to see much detail on the Moon through a telescope that’s because the shadows are practically invisible at that time. The Moon starts to rise after sunset as it approaches its thirds quarter stage. The third quarter Moon will rise around 12pm and start to lighten the sky a while before then, so once the third quarter stage has passed and the Moon start to approach the New Moon stage you should be able to get some very dark nights in the country away from the city lights. However as I will point out in the next part of this series, it is actually easier to get to know the constellations when it is not too dark. So a bright Moon is great for getting to know the brighter constellations.

Getting to Know the Sky

It is one thing to have a telescope and to be able to see in the dark but to make it interesting, it helps if you know what you are looking at so ‘Part Two’ is about learning the constellations.

Next (Part 2)

I am a Creationist and for anyone who might be interested I have posted a page with links to resources related to Creation.

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