Tubes for fluorescent observations have an exit pupil size of 3-4 mm, pipes of the so-called twilight vision is equipped with a pupil, whose size is in the range from 3 to 7 millimeters. No matter how persuaded you, the seller, know that a telescope provides the ability to monitor objects during dusk or in low light conditions. For observations in the night time of the day is a special night vision devices.
Choose those models, the size of the exit pupil which is as close to the size of your pupil in daylight it has a size of 2-3 mm, at night - 6-8 millimeters. To determine the size of the exit pupil, divide the objective lens diameter to increase the pipe. These indicators should be marked on its body. For example, the inscription 8x30 means that the pipe has a magnification of 8 times and the diameter of the lens is equal to 30mm.
Note his reflection in the lens of a telescope: if the production device has been used high quality ar coating, the reflection is not quite clear. The color of the coating does not matter. Check uniformly enlightened the entire surface. To do this, stand with your back to bright light and point it at the lens tube. If you shake it in different directions, you will see the image of the light source in different colors. Among them should not be white.
Think about what magnification you need for monitoring. Buying the device with the increase more than 10-12 times, get an optional tripod. Use a long glass tube with high magnification without special support will be especially difficult at night. Select several products and compare them to each other. If room size allows, watch the surroundings through the pipe. When you use tubes of poor quality, you will see colored edges around objects, blurred and fuzzy image, low contrast between light and dark objects.
Test pipe in the home. To identify defects, invisible in daylight, at night. View the stars through a telescope pipe: they must look like dots with no halos, surrounded by luminous rays. The distortion that occurs when moving pipe from center to edge, should not be large. Listen to your feelings: prolonged use of the optical device you should not encounter the fatigue and inconvenience.