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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?

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    $\begingroup$ Have you given any thought to the role of the secondary mirror? It not only folds the path of the rays, it does something else too. You want to do imaging at the focal point - if you put a mirror there, how do you see your image? $\endgroup$ – Floris Jul 10 '15 at 18:06
  • $\begingroup$ Part of the rationale for the secondary mirror is to lengthen the effective focal length to mitigate optical aberrations. An interesting optics problem is to figure out what kind of mirror to use a secondary mirror. As @Floris mentioned, you do imaging at the focal point. $\endgroup$ – Tetradic Jul 10 '15 at 20:05
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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).

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The telescope is afocal system meaning that they both the objective and the eye piece share same focal point.

If you place the eye piece at the focus of the objective you will pretty much have just objective lens. It is still a magnifier but one lens.

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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.

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  • $\begingroup$ You should mention earlier that this is for Newtonian reflectors, doesn't apply to e.g. Cassegtain designs. $\endgroup$ – Kyle Oman Jul 27 '18 at 6:05
  • $\begingroup$ @KyleOman Imagine a cassegrain. Place the primary’s focal point onto the secondary. A problem is now created. The now expanding light cone will be too large to exit through the primary. $\endgroup$ – Lambda Jul 27 '18 at 16:12

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