Steady, & guys, sorry, but 20x is woefully underpowered for those kinds of distances. Especially for literally reading something at that distance.
Regarding telescopes, I know of several which get great reviews
(none under $1k by themselves). But one big issue which you're going to run into is that to get the magnification needed to see sail numbers at that distance, is that Any, repeat, Any; haze, moisture, mirage, dust, etc. in the air, is going to make using that much magnification impossible. And such is all but ever present around the sea.
The stuff in the air (denoted above) essentially, well, literally, scatters the light reflected by the target, which you're trying to see. Thus distorting it to the degree that resolving said images
is probably just not possible.
Plus, you're trying to resolve a moving target, & one which, at that, doesn't move in a manner which the eye's not used to tracking. Meaning it's moving in a quasi random manner. Not lineally or in an arc
. Like your eye's used to. Making it that much tougher to focus on & resolve.
Especially as boats are continually jerked around moving through waves, & sails
flap & flutter with the breeze. Plus they're attached to the mast
, which is a magnifier of any motion that the boat goes through.
It might be possible using a computer hooked up to a telescope, which has software
with algorithms that'll render "it's best guess" as to what it "sees". And then it'll put up (a composite of) said image on your screen
Oh, & you'll likely also need a motorized drive on the optic to track the target. And track it in a way which the computer's software
moving the optic, can predict.
That's how big time celestial is done, & has been for decades. High end amateur, & Professional. Ditto on look down satellites.
Somewhere over land, on a perfectly clear day, with zero mirage it might be possible to get & use that kind of magnification/resolution, but otherwise, you can likely forget it.
I mean you probably can't read sail numbers with the naked eye @ 100yds. And 4 miles = 8,000yds, so....
Though I may be wrong. Then again, this ain't an optics forum. And unless you know guys who build things like the Hubble...
Do some reading on the usual high end models. One other option might be a surveyor's scope
, the big ones. Plus astronomical telescopes, as mentioned.
But the thing is, getting into that kind of glass, plus a good tripod, as well as a good tripod head
, will likely set you back a good bit more than $1k (several times that is likely).
And you need all 3, as without a steady base, & a smooth way to pan your optic, you're sunk at the starting gate. Especially as with the kind of magnification you're talking about, your field of view at such powers, will be tiny, so following motion via hand operated gear will be a "challenge" to say the least.
Plus, without the premium vibration dampening built into what's holding up & panning the optic, you're sunk again. As the motion of everything which touches the optic is magnified by 80x, or whatever. Such as one's heartbeat & breathing. I mean that's discernable with plain old 7x binoculars.
Maybe using a digi-scoping kit would work. Which is where you have a digital camera
bolted onto the same plate as the telescope. Mounted on top of the head
on top of the tripod.
Then, as you snapped off pics (using remote
shutter activation so as not to disturb your optics), & pulled them out of the camera
via your computer, you could magnify them sufficiently (& or with the right software) so that you could read sail numbers. Maybe. Though again, such is a several $K setup.
Ditto on possibly using a high end camera, with image stabilization, & lens the size of a bazooka, on a tripod. Again with a remote
shutter, & image downloading to a computer.
A place to start might be www.OpticsPlanet.com
But my friend's the astronomer, not me. Neat stuff those astronomers have though. Albeit Really pricey.
I hate to sound defeatist, however, based on what I know, it is what it is.
Though I would be curious to find out if you come up with something which works. So good luck.