I assume there are limits to how far a powerful telescope will let you view items on earth landscapes due to limitations imposed by the atmosphere. What are the practical size limits for how large can you can (or should) make a telescope used for viewing objects on the earth.
Maybe I should be more clear re what I’m talking about.
Assuming I want to have a telescope perched on a cliff overlooking the ocean to look across the ocean and let me know about approaching fleets appearing on the horizon. Is there a point where bigger is not better with respect to how large a telescope you could or should use for that purpose?
Pawn Stars recently had a 70 lb. pair of World War II-era Nikon Coastwatcher binoculars taken from Guam in 1944.
They were huge. At least three or four feet long. I can’t find a picture on Google. I’m pretty sure thats about the limit for useful earth use. I’m pretty sure if anything bigger was possible they would have been used on Guam. They were looking for an invasion fleet.
Canon make a 5200mm telephoto lens, which is a ridiculous size and can take images of objects 18-32 miles away. It’s 1.8m long.
I wonder at what point the curvature of the Earth plays a role?
It’s less than you think, unless you are high up:
So that 5200mm lens will “reach” the horizon for heights about upto 180m above sea level? Blimey!
That’s pretty meaningless unless you define the size of the object. The crappiest point-and-shoot camera can snap an object 93 million miles away with no problem if you point it at the sunset.
Are these binoculars similar to the ones they showed? Most of these are 20x120 (20x magnification, 120mm aperture). Nikon still makes them, by the way.
I think that size is a compromise between field of view and magnification. If you have much higher magnification than 20x, it would take forever to scan the horizon, and there’s a big chance you’d miss something. And at 20x magnification, it’s meaningless to have more than 120mm aperture for visual use, because the exit beam will become larger than the pupil of a human eye.
But if the goal is to get the most detailed view of a specific target, and if you are on solid ground, you can use higher magnification. I don’t know what the practical limit is; obviously it would depend on how still the air is. When looking directly up at a planet, ~600x is usually the limit even under exceptionally good seeing. My WAG is that there’s usually no benefit to >100x for terrestrial viewing, unless you’re on top of a very high mountain.
Keep in mind too that if a distant object has any height to it, you will see the top of it even when its base is beyond the horizon. So if you’re on top of the Burj Khalifa, and the horizon is 69 miles away, you would have an unimpeded view of the top of another Burj Khalifa 138 miles away from you (even though the rest of that other BK would be obscured by the earth).
That matters if you’re scanning the horizon for an invasion fleet whose ships extend 50-100 feet above seal level.
This was supposedly one of the earliest arguments in favor of a round earth: if the earth was flat, you would expect receding ships to just become smaller and smaller, but instead it was observed that they disappeared below the horizon well before becoming unviewably small.
I’ve looked through a twelve inch telescope at a distant hill and that worked fine, but there was quite a bit of heat haze. Doubling the size of the aperture might not have yielded any benefits, unless seeing conditions were very good.
I’d add that by restricting your field of view, high magnification also amplifies any vibrations, potentially jiggling your image along with dimming it somewhat because the available light is spread out more. (In terrestrial viewing, the latter is of course less important than in astronomical viewing.)
When talking about telescope size, the usual first metric is the diameter of the objective or mirror. Focal length is somewhat secondary. If there were no atmosphere the diameter of the objective provides the fundamental limit on resolution - the larger the diameter the better the resolution - this is the Rayleigh limit. However when looking though the atmosphere the resolution is typically limited by thermal gradients in the air that lead to apparent changing light paths. You get heat haze looking across the ground, or twinkling looking at stars. Worse, the greater the diameter of the telescope the more bubbles of turbulent air you include, and the resolution can actually drop. This puts a fundamental limit on the resolution you can achieve. There is simply no point making a terrestrial telescope of larger diameter than is required to meet the other usability needs - as you can’t get better resolution. This angular resolution naturally translates into the magnification of the telescope. You want to match the angular resolution the telescope achieves to the eye’s resolution when viewing through the telescope. There is no point making the telescope provide a greater magnification than this - you are just magnifying blur - and you get what is sometimes termed empty magnification. The next trick is to match the exit pupil diameter of the telescope to the pupil diameter. There is no point having the exit pupil of the telescope (the effective diameter of the beam of light exiting the eyepiece) larger than the pupil of the observer. If the telescope is to be used mostly in daytime, the observer’s pupil won’t be all that large - say 2mm. At night a dark adapted pupil might be 6mm. Again, this puts a limit on the useful diameter of the objective - there is simply no point making the objective bigger than the required exit pupil diameter times the highest desired magnification. And since the maximum useful magnification is limited by atmospheric turbulence you end up with another effective limit on the size of any terrestrial telescope.
In general you just don’t see diameters much more than 100 to 150mm. The most extreme would be something like the Fujinon 150mm binoculars. One of the more famous telescope would be the Questar, which is 3.5 inches. Both of these cost remarkable amounts of money.
Interestingly, if you are looking up (at the stars) you are often only looking though one layer of turbulent cells, and a telescope of 200mm or so is no bigger than the average size of a cell, so it actually sees a very high quality image - it is just that the image waves about all over the place. This leads to some very neat tricks to achieve very high resolution pictures. You can use an active mirror to stabilise a planet in the field of view. This is like a poor man’s version of the active image stabilisation professional big telescopes use that use a laser generated guide star. You can only stabilise a tiny area with one mirror, but it is enough to get a planet. You can also take high a speed series of images and automatically select the stable ones, and automatically translate and stack them to build up a stable long term exposure to get very high resolution images over a wide area of sky. It becomes possible to get Hubble beating resolution from the ground this way. But the terrestrial viewing you are going sideways though the heat, and intersecting many turbulent bubbles, so there is little chance of doing much good, at least for now.
The other replies have covered most of the salient points. I wanted to mention something, though, about telescope light-gathering.
Larger telescopes are only made larger so that they can gather more light under very dim conditions (e.g., the night sky). If there is plenty of light around, as in daytime, you don’t need a large (diameter) telescope, because you don’t need to gather extra light. There’s plenty of light already.
So a fairly small diameter telescope (that corresponds to all of the descriptions and limitations above) would suffice. The limitation on this telescope, as mentioned above, is that you are magnifying the blur. Once you’ve increased the magnification to the point where the image is blurry, extra light gathering won’t help, and will probably hurt.
J.