Spotting Scopes
Many spotting scopes are capable of being used successfully for digiscoping.
The most important feature of a spotting scope that will be used for digiscoping is the size of it's objective lens.
Light entering the camera lens will be severely reduced at the high magnifications achieved by a spotting scope......this restricts the shutter-speeds available to the camera and motion blur and camera-shake will become more of a problem. Spotting scopes with an objective lens of 77mm, 80mm, 82mm or greater are advisable. Smaller scopes may give acceptable results in bright conditions, but the smaller the objective lens, the less detail the scope can resolve.
Another key element of spotting scopes is the quality of the glass used. Most manufacturers produce two versions of each model, one with standard coated glass and another with high quality glass. These high quality glassed scopes offer far better image quality for digiscoping purposes, though the difference is less apparent when viewing through the scope with the naked-eye.
Various names are given to theses 'special' glasses: APO, Fluorite, ED and HD......They all offer slightly different properties, but the end result is a superior image in every respect.
In addition to each model being available in standard or high quality glass, every model is available with a straight eyepiece mount or an angled eyepiece mount. I don't think this is particularly relevant as far as digiscoping is concerned, and is a matter of personal prefference.
Here's a brief look at the main spotting scopes available:

Kowa TSN 1,2,3,4. This range has 77mm objective lenses. TSN 3,4 use Fluorite glass. No longer in production, but excellent value if purchased secondhand (many of my pics were taken with the TSN4).

Kowa TSN 821,822,823,824. This range has 82mm objective lenses. TSN 823,824 use Fluorite glass. Had a recent face-lift and called the 'M' series. The Zoom eyepiece is one of the best around. Many birders would put the 823/4 and zoom eyepiece ahead of the Leica in terms of optical performance.

Leica Televid 77, APO77. These scopes have an excellent reputation for colour accuracy and produce great results. The zoom eyepiece isn't the greatest around, though it's getting better with each new version. Leica have now released a astro e.p. adapter for th Televid 77.

UPDATED: Nikon produce a number of excellent spotting scopes. The new Nikon eyepieces may not be
Digiscoping-friendly with a number of commercial adapters. Very few digiscopers/birders seem to use Nikon scopes in the U.K?, so there's a lack of feedback on how they perform. I've seen pics taken with the ED78III and these were very good. Nikon now produce their own adapters for use with the new MC series e.p's .....seem to work perfectly, see photo at bottom of page.

Swarovski 80, 80HD. These scopes use 80mm objective lenses. Probably the most popular spotting scope for digiscoping, due to the fabulous 20-60x zoom eyepiece. I now use this scope for all my digiscoping. An Astro e.p. adapter is also available, and talk of a Swarovski digiscoping adapter (talk from Swarovski) New series of 80mm scopes now available, cosmetically similar to the 65mm.... though subtle internal changes may bring about higher quality images?

Swarovski 65HD. This new scope gives an amazing amount of light considering it's objective lens size. I've heard very good reports on it's performance for digiscoping. The size and weight are certainly a bonus....though the price is very high. In the U.K. it's price with a zoom e.p. is almost the same as the 80HD.

Zeiss 85T*FL. It looks very similar to the Leica 77, and includes two focus for coarse tuning, the other for fine focus adjustment. Three eyepieces are available, a 30x, 40x and a 20-60x zoom.  The design looks quite 'stubby', but that may be due to the fact that it has a larger objective lens than most scopes. Weight is slightly lighter than a Swarovski AT80HD at 1450g, and considerably lighter than the Leica. Obviously the scopes are Nitrogen filled, waterproof models. Have seen a few digiscoped photos from this scope, look o.k. but obviously further proof needed. Zeiss UK can contact me via the homepage e-mail address :-) I've seen rather mixed reviews of the 85FL, some concern over the zoom e.p performance....though some suggest this is down to the actual scope body rather than the e.p.

Pentax 80. This much talked about scope is interesting in that it uses large (monstrous) astro type eyepieces. There's little doubt about the quality of these eyepieces, but some questions raised about why some reviewers love the optical performance.....and others think it's rubbish. It has been said that quality control may be at fault, leading to rogues examples (are the rogue scopes the good ones or the bad ones?) To be released in the U.K. in Autumn 2002. Also the Pentax 100 available soon. I'm unaware of any adapters large enough to be used with the zoom eyepiece.
Eyepieces are available in many magnifications for each model of spotting scope. Results from modern zoom eyepieces are no worse than fixed magnification wide-angle ( see Equipment Reviews ), long eye-relief versions. Obviously, zoom eyepieces also allow greater flexibility (the ability to go to portrait format at 35x in a second), though the 4x zoom on the Nikon 995 makes this less of an issue than the 3x zoom of the 990. A greater field of view is available from fixed eyepieces, but doesn't have any benefits when the camera is up against the eyepiece, though you can locate the subject slightly easier when conventional viewing by eye. It's probably best to stick to the eyepiece you use for normal birding, you'll only have the camera against the scope for a small percentage of the time...birding first, digiscoping second.
Latest Eagle-eye Opticzooms Have just announced that they will be producing an eyepiece that will fit on most spotting scopes (via a small adapter). This eyepiece is 12x magnification (though that may change on different scopes) and aimed at the videocam user, but it will allow digital still cameras with large diameter lenses to be used with far less vignetting than with conventional eyepieces. Carlo has said that results with a Casio 3000 camera were impressive, with no vignetting apparent at the top-end of the cameras zoom.
                              Digital Cameras For The Task
Almost every digiscoper uses a camera from the Nikon Coolpix range, either the 880,950,990,995 and the latest model the 4500. One of the main reasons why this is the case is that the zoom lens mechanism is situated inside the camera, so there is no external movement when zooming-in or when focussing. Therefore the camera can be placed up against the scope's eyepiece without the risk of a collision between camera lens and eyepiece.
Another vitally important feature of the Coolpix range is the small diameter of their lenses (we suffer from slow f 2.5 lenses though). 
It really is a method for Nikon Coolpix cameras, though a handful of other cameras can produce good results. It is possible to connect any digital camera to a scope's eyepiece, but the results will often be poor. Eagle-Eye U.K.
manufacture adapters to suit various cameras, even those without a filter thread on the lens.
The new CP4500 has been released. A 990/995 clone with a 4.1mp ccd, smaller and lighter than the 9** series. Same lens as the 995 with the 4x optical zoom and f2.6 at it's widest. The ugly looking pop-up flash bump has gone and the movie mode has audio. Price in the U.K. has been suggested at about £600 but I have seen it on offer for £529. LCD monitor has shrunk to 1.5 inches (new sunshade needed?), but it's  far clearer and brighter than the old 1.8 inch ones. The control layout has also been changed. I now have a review of this camera
All in all it's a very welcome camera, not because of the quality of pics that it 'may' deliver....but becuse it assures the future of digiscoping for a couple of years. New digiscopers won't have to scour the secondhand market to get a decent digiscoping camera. Hopefully this new camera will help to keep the momentum of digiscoping at the current pace, or even exceed it....the results of this will be noted by manufacturers of birding optics and digital cameras, and maybe getting them to cater for our need with new products.

CP5000: This camera can be used for digiscoping, though there's little reason to choose it over a cp9** style camera. Excellent results have been seen but in my own use it has exhibited vignetting at most zoom levels on the camera, others have had better success (often with wide angle eyepieces). Because of it's limited zoom range, may not get you close enough to the subject with lower power e.p's

LATEST: Leica have their own complete digiscoping set-up involving their Digilux 1 4mp camera together with an adapter to connect to their eyepieces. I had a play with this set-up at this year's Birdfair. Extremely disappointing performance, only recommended for use on their 20x e.p's and then with plenty of vignetting at all camera zoom position, bar full wide angle. Zoom control on the camera was poorly situated for the digiscoper at the front of the camera. See photo at bottom of the page.
Digiscoping is a method of taking extreme telephoto images with a digital camera and spotting scope. Spotting scopes are terrestrial telescopes used by birdwatchers to get a close-up view of birds. Relatively cheap digital cameras can be placed up against the eyepiece of a spotting scope to achieve magnifications that would be unthinkable with conventional camera lenses.
The quality of the images will vary, being dependant on many factors. These include;
1: The optical quality of camera and spotting scope.
2: The prevailing light and atmopsheric conditions.
3: Ensuring that the camera and spotting scope are as stable and vibration-free as possible, whilst taking the picture.
London Camera Exchange Trevor Codlin at the Winchester branch of this large chain of camera shops produces adapters to suit most eyepieces. Very simple and neat design, basically an alloy tube with a thread at one end and made for the specific eyepiece that you use. Trevor has now improved the design of this adapter by making the tube walls thicker, giving a deeper screw thread and allowing far stronger screws to be used ( with nice big heads for an easy grip.) Another major improvement is the option to have a direct 28mm thread to connect to the Nikon lenses without the need for any step-up ring and reducing vignetting to a minimum. The compactness of the adapter ( size varies depending upon your e.p.) allows it to be left attached to the camera and the whole ensemble carried in a coat pocket, to be attached to the scope's eyepiece when the need arises. The beauty of these custom sized adapters, with a snug fit over the eyepiece, is that the camera lens is perfectly aligned with the eyepiece glass as soon as it slips over the eyepiece.....wobble or mis-alignment isn't a concern and you're ready to start taking photographs far quicker than with any other adapter. Only one securing screw needs to be used in my experience, this can be quickly losened to allow a zoom e.p. to be rotated to a different magnification....without removing the camera/adapter. Phone the shop to get one: (44) 01962 866203

The Eagle-Eye Optic Zooms 'Digimount' adapter. This adapter has been around for some time, but has now been revised with a direct 28mm thread instead of a 37mm thread that required a step-down ring to fit Nikon filter threads. This direct 28mm thread allows the camera lens to get to within 2mm of my Swarovski eyepiece's lens, reducing vignetting at lower camera zoom settings and when. This adapter is well made, incorporating four screws that lock onto the eyepiece. Separate insert collars are available to allow fitting to eyepieces of varying diameter. Care must be taken when using direct thread attachments. If you damage the thread of the Nikon, it is almost impossible to repair. A step-down ring ( left attached to the camera ) acts as protection for the camera's thread, but takes the camera lens further away from the eyepiece.

LE2 PLUS This new and improved version of the old LE adapter is another very well made device with an ingenious springy metal band that clamps to the eyepiece body. The latest version will fit any eyepieces up to a diameter of 2 inches, and will be very handy for those who have more than one eyepiece for their scope. This adapter has three sturdy screws to secure the camera to the eyepiece (pushing the springy metal band onto the eyepiece), and can be attached to the eyepiece in seconds. The large central aperture may allow non-Nikon cameras with larger diameter lenses to be used without significant vignetting....though a low magnification wide-angle eyepiece would need to be used on the scope. Maybe better for N. Americans who would rather purchase from a domestic source.
               Adapters That Connect The Camera To Eyepiece
There are a number of adapters available that will attach the camera to the eyepiece of the scope. I have used these three different versions. Each type has it's merits and all work well, with vignetting kept to minimum.
The Imaging Factory Noise Reduction Plug-in   Two versions of this great Plug-in filter, a pro version ($100) and a basic ($39), extremely good  chroma noise reduction as well.
Mathias Verjerslev's Digital toolbox 'action set'   Very good free ISO/general noise reduction 'Action set' for Nikon 9** cameras
Equipment  That I Currently Use

Nikon Coolpix 4500
Nikon Coolpix 990
Swarovski AT80HD
20-60X Zoom Eyepiece
30X WWA Eyepiece
Nikon SB24 Speedlight & Fresnel screen.
Velbon Sherpa 5300 Carbon Tripod
Velbon Lightweight PH157Q Head
Lexar compact flash
1800mah NiMH Batteries (for cp990)
20 Inch Shutter Release Cable
Cable Release Adapter Bracket - From Jessops U.K. (mod code JESCRAB).

Camera Settings That I Use
Digiscopers around the world use various settings, various combinations of settings give excellent results. These are my usual settings for the Nikon cp990. Most are applicable to the cp995 and cp4500.

M-Rec mode: always used as it allows more controls in the menus to be activated.
Single Focus mode always activated, saves batteries &  focus confusion
Area zone focus usually set on manual.
Focus Confirmation I switch this on occasionally, though it can cause more problems than it cures. Can be deceptive in what is actually in-focus.
Exposure Metering set on area zone/spot, or sometimes centre-weighted.
Focus Mode generally set on Macro, but Infinity and Normal produce good results as well.
Exposure Control Most often used in full manual with aperture fully open and choosing a shutter-speed based upon the linear exposure meter.
ISO: Only in times of real desparation will I go to ISO200, then it is up to a good noise reduction plug-in/action to get a half-decent print.
Photo Size Always at full size and Fine Jpeg setting.
Other Settings Most are left on Auto and are not particularly important. Some discussion on the merits of in-camera sharpening v Photoshop sharpening. I leave my camera control on the medium setting, auto can overdo things every now and again.
Contrast may need turning down on the cp4500, though it may be that I have an errant one.

Software That I Use
Photoshop 7.0 

iNova Noise Reduction 'Action'   A good free ISO noise reduction 'Action' for Nikon 9** cameras
View my Digiscoping pages or my Bird photo galleries below
In addition to these adapters, there are several others from third party manufacturers. Nikon, Leica and Kowa also produce digiscoping adapters for use with their own equipment.
An excellent image sharpening plug-in for images going off for print. Not cheap and not for use on small images.
Another chroma/luma noise reduction plug-in. Seems better than Imaging Factory Pro at luma reduction (light pixels that don't belong), but does cost more.