In a reflecting telescopes, light is reflected from a primary concave mirror to a secondary mirror. But the secondary mirror is never placed at the focal point of the primary mirror. Why is that? What would happened if it is? Is it because the image would be flipped upside down?
In a reflecting telescope... the secondary mirror is never placed at the focal point of the primary mirror. Why is that?
In a telescope (or microscope... any visual optical system), the image will be seen clearly and in focus. So, if that image is at or near a mirror surface, you'll see, clearly and in focus, every speck of dust that is on the mirror.
Similarly, the bulk of glass lenses (glass might contain small flaws, bubbles, inclusions) is commonly not the site of any image.
The exceptions are a reticle (a scale or pattern intended for size/angle/shape comparison), or an image-boundary defining 'stop' (usually a hole in a blackened metal disk).
The secondary mirror is used to get the light out of the tube and into the focuser. The light cone produced by the primary mirror continues to expand after it converges at the focal point. If the secondary mirror is placed there the light cone will have expanded too wide by the time it exits the tube and enters the focuser tube. The mirror’s location is chosen by determining the optimal diameter of the light cone as it exits the tube. You want the focal point to be outside of the tube just shy of an adjustable eyepiece. Of course there are exceptions but as a general rule this is how it works on most Newtonian reflectors.