by Chris Peterson » Tue Jul 26, 2016 1:04 pm
Knight of Clear Skies wrote:Chris Peterson wrote:Nevertheless, there is no real distinction that needs to be drawn in an astronomical image made with an objective designed to attach to a camera and an objective designed to have an eyepiece. All that is really relevant is the aperture and the focal length- the key parameters for any imaging optical system. The rest is details. Nice to know, but not essential.
A short telephoto lens is very similar to a small telescope, I agree.
The focal length is entirely irrelevant. In terms of optical function, there is no difference between a camera lens and the instrument we call an astronomical telescope.
However, not all camera lenses can be usefully described as telescopes (an optical device designed to magnify distant objects).
If you use that definition of telescope, than no camera lens qualifies. Neither does any astronomical telescope used as a focal imager. Optically, telescopes are afocal. They require both an objective and and ocular, and when we image we lack the latter. Imaging systems are focal. They do not magnify.
To take an extreme example when using a fisheye lens with a field of view of 180 degrees the photographer has to be careful to keep their feet out of the shot.
Which is as much an imaging telescope as the Keck or the HST.
50mm focal length is about the point where describing a camera lens as a telescope becomes misleading I'm afraid.
Not at all. In an imaging system, it is trivial to construct a camera with a 50mm lens that provides a higher resolution image of a target than a different camera with a 1000mm lens.
[quote="Knight of Clear Skies"][quote="Chris Peterson"]Nevertheless, there is no real distinction that needs to be drawn in an astronomical image made with an objective designed to attach to a camera and an objective designed to have an eyepiece. All that is really relevant is the aperture and the focal length- the key parameters for any imaging optical system. The rest is details. Nice to know, but not essential.[/quote]
A short telephoto lens is very similar to a small telescope, I agree.[/quote]
The focal length is entirely irrelevant. In terms of optical function, there is no difference between a camera lens and the instrument we call an astronomical telescope.
[quote]However, not all camera lenses can be usefully described as telescopes (an optical device designed to magnify distant objects).[/quote]
If you use that definition of telescope, than no camera lens qualifies. Neither does any astronomical telescope used as a focal imager. Optically, telescopes are afocal. They require both an objective and and ocular, and when we image we lack the latter. Imaging systems are focal. They do not magnify.
[quote]To take an extreme example when using a fisheye lens with a field of view of 180 degrees the photographer has to be careful to keep their feet out of the shot.[/quote]
Which is as much an imaging telescope as the Keck or the HST.
[quote]50mm focal length is about the point where describing a camera lens as a telescope becomes misleading I'm afraid.[/quote]
Not at all. In an imaging system, it is trivial to construct a camera with a 50mm lens that provides a higher resolution image of a target than a different camera with a 1000mm lens.