While the Moon might appear to be serene and peaceful, this was not always the case. The Moon bears the scars of a violent bombardment of the inner solar system by meteors, comets, and other large pieces of rock that ended about 3.8 billion years or so ago, but the good news is that the presence of these scars offers novice star gazers an excellent opportunity to learn how beginners can use an astronomical telescope to best effect. Here is how to get the best views of the Moon.
The image above shows the Moon magnified by ±50 times, which is good for getting the “big picture”, but to see smaller details like individual craters, you need to increase the magnification considerably. However, magnification is probably the single biggest problem novice star gazers have to learn to master, since high magnifications are not always better.
It must be remembered that telescopes do not magnify images. Telescopes only gather light, on the principle that the bigger the mirror or objective lens is, the more light the instrument gathers. Magnification takes place in the eyepiece, with the focal length of the eyepiece determining the degree of magnification, but high magnifications reduce the field of view, which means that at excessive magnifications, an object, or feature on the Moon can break down and become invisible.
Clearly then, there must be a happy middle ground, or magnification at which an object becomes visible, without either getting distorted, or the field of view becomes so small that not all of the object fits inside the field of view. Moreover, as the field of view becomes smaller as magnification rises, all movements of the telescope are magnified as well. For instance, if the telescope’s tripod is not steady and prone to transmitting vibrations to the tube, these vibrations can be magnified to the point where the image becomes so unstable that no detail can be made out.
Nonetheless, the Moon is able to “tolerate” high magnifications better than any other celestial object, which means that magnifications of up to about 200× times can be used to pick out smaller objects and features, such as the central peaks in major craters.
Observe the Moon at the right time
The problem with the Moon is that it can be so bright that an image can be washed out because the telescope gathers too much light. The image above is a perfect example of what happens when the Moon is fully illuminated by the Sun. Because sunlight strikes the Moon straight on, much of the light is reflected straight back at the observer, which does not make for an enjoyable viewing experience.
Therefore, the trick in observing the Moon lies in looking at those parts of the Moon that are not fully illuminated, such as the area around the terminator, which is the area on the Moon where light transitions into darkness. In this area, sunlight strikes the Moon obliquely, and the resulting shadows have the effect of accentuating details that would otherwise be drowned out in the Sun’s glare.
Consider the image above that clearly shows the terminator, along which details of craters are clearly visible at a relatively low magnification. Nothing can be seen to either side of the “grey zone”, but with some practice, the same level of detail can be seen as the instrument is made to track the progression of the terminator.
However, because the Moon “wobbles” in its orbit (that is inclined with respect to the both the ecliptic and the earth’s equator) around the Earth, sunlight strikes the Moon at different angles at different times, meaning that in practice, an object that was visible close to the terminator today might not be visible a few days later.
More tips and tricks
As a general rule of thumb, the best time to view the Moon is during the two “quarters”, which is when only about 25% of the Moon’s disc is illuminated, but even then, there are many factors that can influence an observing session- and not always in a good way. Therefore, to prevent disappointment, here are some tips and tricks to keep in mind to enhance you viewing experience.
Mask off your telescope
If you are using a big telescope, say, a 10-, or 12-inch reflector, limit the instruments’ light gathering ability by masking off some of the aperture, even during times when the Moon is not fully illuminated. Simply cut a hole of about 5-6 inches in diameter in a piece of cardboard, and tape the mask over the telescopes’ open end. However, leave enough room for some light to pass around the secondary mirror- if the hole is too small you won’t see anything, so experiment with masks until you find a configuration that offers the best views.
Orient your view
Can you spot what’s wrong with the image of the Moon as shown above? In this case, the crater Tycho, the bright spot at the top of the globe with the bright ray patter emanating from it, is in the “wrong” place. Some telescopes invert images, so if you have never seen the Moon through a telescope before, your first view might not be what you expected to see: depending on the number of optical elements (mirrors and/or lenses) in the instrument, you might see the Moon upside down, or even mirror-reversed, which can be somewhat disconcerting.
Thus, when you purchase your first telescope, ask the dealer how you will see objects through it, because seeing the Moon upside down might make it vastly more difficult to learn to find your way around the thousands of craters, mountains, rilles, and other objects that make the Moon one of the most fascinating objects in the solar system.