Final answer:
Lag is an undesirable yet useful characteristic of the vidicon camera tube as it can be used for signal averaging, reducing noise and enhancing the quality of images, particularly in telescopic observations where an eyepiece is used for magnification.
Step-by-step explanation:
The undesirable yet useful characteristic of the vidicon camera tube is lag. The vidicon is a type of camera tube detector that is sensitive to electromagnetic radiation and is used to make a record of astronomical observations. While it can cause distortion due to chromatic aberration, where an image appears fuzzy because each wavelength focuses at different spots, the characteristic of lag can be used advantageously.
Lag in vidicon tubes refers to the persistence of the image, which can be utilized for signal averaging, effectively reducing noise in the final image and improving the quality of the observation. This is beneficial in telescopic systems where the eyepiece magnifies the image produced by the objective lens or primary mirror, compounding any noise present in the original signal.
An undesirable yet useful characteristic of the vidicon camera tube is lag.
Lag refers to the time delay between the input signal and the video output, causing a slight delay in the image being recorded or displayed. While lag is considered undesirable as it can result in a loss of real-time video capture, it can also be useful in certain applications, such as when capturing slow-motion footage or reducing the impact of sudden changes in lighting conditions.
Therefore, lag is the correct answer to the question.