Question

Which mirror is used for reflecting type of telescope why

Answer 1

instead of a convex objective lens, a concave mirror is used to collect parallel rays from the object and form an image at the focal point. Then the convex eyepiece lens is used to magnify this image for the viewer.
Concave mirrors focus light at a focal point using reflection, not refraction.
Mirrors are used instead of lenses in large, professional telescopes. This is because an objective lens can have a maximum diameter of approximately 1 metre before it begins to sag, whereas a mirror of several metres in diameter can be used.
Mirrors (rather than lenses) have other advantages:
They can be made very smooth - resulting in undistorted images.
Suitable reflectors can be used to focus all types of electromagnetic radiation.
Mirrors reflect rays of all colours in the same way..