I cannot see anything through the telescope
What has happened? The telescope should be lined up on an object and the lens cap is off, but I cannot see anything through the eyepiece.
The sky has gone cloudy!
Solution: Cloudy skies often thwart the amateur astronomer, but it is not unusual to be midway through an observing session only to find unexpected clouds appear seemingly from nowhere. I speak from experience that, on occasion, unexpected clouds can cause objects to miraculously vanish from view while peering down the eyepiece. It is worth a glance upwards to make sure a clump of cloud has not appeared.
The eyepiece is too powerful
Solution: Changing the eyepieces in the telescope will change the magnification, but this comes with a drawback. Increasing the magnification also increases the darkness of the image and it is possible to increase the magnification so much that the image becomes too dark to be useful. There is a really handy rule of thumb to work out the maximum magnification of a telescope… Take the aperture in millimetres and double it. This gives you the maximum useful magnification before the image starts to get too dark. To work out the magnification of the eyepiece and telescope combination, divide the focal length of the telescope by the focal length of the eyepiece. This gives the magnification. If it is greater than the maximum useful magnification, then try another lower power eyepiece, which will be denoted by a longer focal length, usually measured in millimetres.
Your eyes are not dark-adapted
Solution: The human eye is designed to be able to see in the dark, but this dark adaption is not instant. It takes time for your eyes to adjust to seeing in the dark. In fact, it can take up to 40 minutes for the process to complete. There are two changes that take place when you are immersed in a dark environment. The iris opens up in just a matter of seconds; this is to allow more light to enter the eye. There is also a change to the chemicals in your eyes to make them better at seeing in the dark, but this can take quite some time, as long as 40 minutes. If you have just popped out from a normally illuminated room, it is quite possible that your eyes are not yet adjusted to the darkened environment. Give your eyes time to adjust and you will be amazed at what you can see.