primary mirror

This is a major advantage for solar telescopes, where a field stop (Gregorian stop) can reduce the amount of heat reaching the secondary mirror and subsequent optical components.

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In the Gregorian design, the primary mirror creates a real image before the secondary mirror. This allows for a field stop to be placed at this location, so that the light from outside the field of view does not reach the secondary mirror.

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In a prime focus design no secondary optics are used, the image is accessed at the focal point of the primary mirror.

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The Gregorian telescope consists of two concave mirrors; the primary mirror (a concave paraboloid) collects the light and brings it to a focus before the secondary mirror (a concave ellipsoid) where it is reflected back through a hole in the centre ...

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It is free of coma and spherical aberration at a nearly flat focal plane if the primary and secondary curvature are properly figured.

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The Ritchey–Chrétien telescope ... is a specialized Cassegrain reflector which has two hyperbolic mirrors (instead of a parabolic primary).

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The cassegrain telescope (sometimes called the "Classic Cassegrain") ... has a parabolic primary mirror, and a hyperbolic secondary mirror that reflects the light back down through a hole in the primary.

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The Newtonian telescope ... usually has a paraboloid primary mirror but at focal ratios of f/8 or longer a spherical primary mirror can be sufficient for high visual resolution.

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The Gregorian telescope ... employs a concave secondary mirror that reflects the image back through a hole in the primary mirror. This produces an upright image

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A curved primary mirror is the reflector telescope's basic optical element that creates an image at the focal plane [OF THE PRIMARY IF THERE IS NO SECONDARY]. The distance from the mirror to the focal plane is called the focal length.

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