You've probably heard of "Digiscoping", which is using a digital camera with a telescope rather than using a telephoto lens. Image quality is lower, but scopes are often cheaper and if you have one with you, why not use it. It's popular with birders who tend to carry spotting scopes for their birding, and with a small digital camera pointing through the scope they can get some OK images.
However, that's not what I'm describing here. In this case I'm taking a high quality telephoto lens and turing it into an equally high quality spotting scope.
In principle all you need to do to turn a lens into a telescope is add an eyepiece. Once upon a time (and in a land far, far away...) you could actually buy an optical assembly which you could mount on the back of your lens and which would turn it into a telescope, but I haven't seen one in many years. See Update
The problem with this simple approach is that it will give you an inverted image. That's fine for astronomy, but not so good for bird watching. While you can (or at least could) find an "erecting eyepiece", they tend to be of lower quality (and maybe higher price). There are potential solutions to this problem which involve the use of either a 90 degree or 45 degree adapter. These are sometimes called "star diagonals" because they are often used on astronomical telescopes where it's much easier to view objects in the sky if you are looking down into a 90 degree adapter than looking straight up with a direct eyepiece.
The 90 degree adapter gives you an image that's the right way up, but left and right are reversed. Such adapters either use a mirror or a right angle prism. The prism based units tend to be higher quality since are more efficient (less light loss) and cost more. If you wanted to make a cheap adapter, you'd use a mirror. Mirror based diagonals can be excellent, it's just that smaller ones rarely are. The 45 degree adapter gives you an image that's both the right way up and has left and right where they should be. The downside is that the optics of a 45 degree adapter (an Amici prism) tend to be more expensive and optically complex.
The problem with both 45 and 90 degree adapters is that they add to the length of the optical path. The focus point of a camera lens is quite close to the rear flange. For example Canon EF series lenses designed for use on EOS cameras bring the light to a focus 43mm past the rear lens flange. While you can certainly get a telescope eyepiece to that position, once you add a 45 or 90 degree prism to the back of the lens, you can't then get the eyepiece close enough to allow infinity focus.
So what to do? Well, it turns out that if you don't mind doing a little mechanical work you can actually get an adapted 90 degree diagonal close enough to the lens so that with a suitable eyepiece in it you can focus to infinity. While I'm not going to provide exact plans on how to do this, I will describe what I did myself.
First I found a 90 degree "star diagonal" from an old Meade refracting telescope I picked up a flea market. This was a 0.965" unit. They also come in 1.25" and 2" sizes, but larger sizes are more expensive and add more distance to the optical path, so for this application they are less desirable (in fact I'm not sure you could get them to work). A typical adapter of this type has a 0.965" diameter tube screwed into one face, and this normally slips into the 0.956" internal diameter focusing tube of the telescope. Screwed into the other face of the unit is a 0.956" internal diameter eyepiece holder, into which slips the 0.965" diameter eyepiece. This is shown in the diagram below:
The trick with adapting such a 90 degree star diagonal for use with an EOS lens is to minimize the distance from the lens to the prism and from the prism to the eyepiece. Depending on the nature of the 90 degree start diagonal you have, this may mean some cutting, sawing, filing and drilling.
Once I'd modified the star diagonal I had to find a way to attach it to the lens. The easiest way to do this is to mount it on an EF mount rear lens cap. However the standard Canon cap is a little too deep and moves the optics too far away from the lens flange. In my collection of camera parts I found a 3rd party rear lens cap which was thinner. I also noticed that the Tamron caps were thinner then the Canon caps, so maybe you could use a Tamron cap. I drilled a 3/4" hole in the center of the cap and then attached the prism housing of the star diagonal to it. I drilled and tapped holes into the prism housing, but you could equally well just glue the two pieces together. The adapter is shown below and you can see the hole in the cap plus the screws I used to attach it.
So that's about it. I'm not giving exact instructions because I don't know what right angle adapter you will find or what lens cap etc. you will use. What I am showing is a "proof of principle". You can make an excellent eyepiece adapter for Canon EF lenses (and probably most other brands of lenses) using a 0.965" star diagonal and keeping the optical path as short as possible. With the arrangement shown it is possible to focus to infinity. Actually the distance scale on the lens is pretty accurate for focus, though that's more by luck than design!
If you build something like this and find you can only focus on objects that are fairly close, the optical path between the lens and the eyepiece is too long. You have to do whatever you can to minimize the optical path between the eyepiece and the lens until you get infinity focus. That may require some surgery on the 90 degree adapter or eyepiece.
If you like to both watch and photograph wildlife, adapting your lens to become a telescope means that you have one less piece of equipment to carry. With quality optics (eyepiece and diagonal) it makes a very, very high quality spotting scope.
I'm using a 22mm eyepiece because that's what I had on hand. Shorter focal length will give higher magnification, but might require an even shorter optical path, and that's going to be tricky. Magnification is given by dividing the focal length of the lens by the focal length of the eyepiece. In this case a 300mm lens with a 22mm eyepiece gives 13.6x. With a 1.4x TC on the lens it becomes 19x and with a 2x TC on the lens it becomes 27x. With a 500mm lens the numbers are 23x, 32x and 46x. There's no reason why you couldn't use this adapter on a telephoto zoom, though it will work best with fast telephoto primes. I've also stacked a Canon 2xTC and a Tamron 1.4x TC and the visual image quality remained excellent with high contrast and high sharpness. That combination produced 38x with the 300mm lens and 64x with the 500mm lens.
Garage sales and flea markets are a good source for old telescopes and telescope parts. I've found three usable scopes in the past couple of years for $5-$10 each. Failing that, try which is always a decent place to find stuff like this. You can also find telescope components sold by Amazon, Adorama and B&H Photo (see links below) if you want to buy new.
Since this article was first written, Kenko have come out with a line of commercial lens to eyepiece adapters for Canon, Nikon, Sony and Pentax lenses. They are available in both "straight through" and "45 degree angle" models and cost $179. The fixed eyepiece has a focal length of 10mm, giving a magnification of 10x for every 100mm of lens focal length, so a 300mm lens becomes a 30x spotting scope and a 500mm lens becomes a 50x spotting scope. The adapter has a tripod mount built in, so it can be supported when using smaller lenses without tripod mounts (e.g. a 100mm lens to give a 10x scope). The tripod mount in the adapter is good for lenses up to 800g (about 1lb 12oz).
These adapters use a roof prism and 5 optical elements in 3 groups to extend the back focus distance and produce an image that's the right way up with left and right in their correct directions. This makes them ideal for terrestrial viewing. For astronomy an adapter with minimal optics (but inverted or left/right reversed) may give slightly higher optical quality and can be built to accept interchangeable eyepieces for different magnifications - but you have to build that yourself! $179 for a commercial adapter seems like a pretty reasonable price and 10x per 100mm is a good choice. Focusing is done with the lens, so the lens they are used with must have manual focus capability (without needing any power).
Here are links to the available adapters: