Final answer:
The normal adjustment in an astronomical telescope is Focus, which involves moving the eyepiece relative to the objective lens to attain a sharp image. Magnification is not the primary consideration and can be adjusted with different eyepieces.
Step-by-step explanation:
The normal adjustment in an astronomical telescope is Focus (A). This involves changing the placement of the eyepiece relative to the objective lens to ensure that the image being viewed is sharp and clear. It is essential for projecting a real image onto a screen and also to account for the viewer's eyesight or for different observational situations. Adjusting the telescope's focus can be achieved while observing astronomical objects to compensate for changes in distance.
Magnification, which may initially seem important, is done through different eyepieces and is therefore adjustable. However, it is not the primary consideration when choosing a telescope. Too much magnification can result in a poor quality image due to atmospheric turbulence. A quality telescope will often come with various eyepieces to maintain a useful range of magnification.