What follows is based on my experience. Yours may vary. I encourage further contributions from readers to improve the content of this page; which discusses the following:
If you have to wear spectacles; then use eyepieces with a large "eye relief" such as Orthoscopic or Plossl designs. These will allow you to see the entire field of view at once. You can use other eyepiece designs; but be warned that your spectacles will prevent you from placing your eye at the best position, thus producing an annoying "keyhole-like" view.
Many good eyepieces now have rubber eye guards fitted. These things leave rings of "gummy" residue on spectacle lenses. If possible, pull off the eye guard and wash it in soapy water to remove the goo. Or use eyeguards that can be folded down over the outside of the eyepiece.
Spectacle wearers will have particular difficulty using eyepieces of very short focal length, and eyepieces with 25mm (0.96 inch) barrels, unless they remove their spectacles.
Contact lens wearers may notice a small grey or white halo around all bright objects when looking through a telescope. This means that your contact lens is either dirty or scratched. This may be totally unnoticeable in daytime use. Soft (hydrophilic) contacts are also vulnerable to ultraviolet light, which makes them slowly go opaque. Australian sunlight ruins mine in about 30 months.
Cataracts not only block out certain viewing directions, they also tend to defocus whatever remains visible. Obviously if you have a cataract-free eye, then use it for telescopes. Some cataracts can be destroyed in situ, but some cases require replacement of the cornea with an artificial lens.
Unlike the cornea, these lenses transmit some ultraviolet radiations so you will be able to see into the near-ultraviolet as well as "visible" light. This can produce some unusual (and interesting) changes to your colour perception. I am told that planets and some emission nebulae look noticeably different after corneal replacement!
One major effect of red-green colour blindness is the perception
of emission nebulae; many of which may be completely invisible to the
affected observer. Hydrogen in nebulae emits mostly red light. However,
if a nebula is described as being visible with an "O-III"
filter (oxygen-3 filter) then it emits a lot of blue-green light (from
oxygen), which a colour-blind observer should see.
Other red objects (eg: Mars, red stars) will appear much fainter to a colour-blind observer. This rules out most variable star observing.
Night blindness is distinctly different from the universal problem of "I have just come outside from a bright room and I can't see anything." This is a lack of dark adaptation, which requires 15-30 minutes to achieve. Night blindness is an inherent lack of sensitivity to dim light; which may be caused by diet deficiency (usually Vitamin A), retinal or corneal damage, or the eye pupil's failure to dilate in dark conditions.
The frequently-quoted figure of 7mm diameter for a fully dilated pupil applies to younger observers. After your 30th birthday, you go downhill ;-) and by the 70th birthday most of us will achieve only 3-4mm dilation. A telescope eyepiece has a calculable exit pupil diameter -- this is the actual diameter of the image passed to your eye. Generally, the exit pupil diameter is roughly proportional to the eyepiece focal length, or inversely proportional to magnification. Which is why older observers see fainter stars with 12x50 binoculars than they do with 7x50s, even though both binoculars collect exactly the same amount of light.
The late Clyde Tombaugh (discoverer of Pluto) mounted his personal telescope onto an old lawnmower chassis and was still moving it by himself in his nineties. Many other observers put wheels on their big Dobsonian's mountings; and some companies sell wheel frames for use with tripod mountings.
There are numerous causes of dexterity problems -- one of which is hypothermia during those long, clear, COLD winter nights! Check that your telescope's controls, clamps, screws, fasteners, etc can be manipulated with frozen or arthritic fingers, or while wearing gloves. The little screws that hold eyepieces in the focuser are a common source of trouble; so if you can't use them then replace them with larger-headed screws.
Similarly, if the focuser is difficult to use then modify it. I saw one
telescope which had bathroom tap handles replacing the normal focuser
knobs -- which were too small for the scope's arthritic owner to use.
Helical-type focusers (which screw in or out) demand strong, steady fingers.
Control buttons on computerised telescopes should be tested before purchase to ensure that you have no trouble operating them. Some telescope controllers can be tiring to hold onto for long periods, and the buttons may be difficult to distinguish in the dark.
Essentially, this is a "tall" Dobsonian mounting, balanced so that the altitude axis coincides with the eyepiece. Note that the weight is NOT directly in front of the telescope (to avoid blocking the view). A large gusset or rib piece is placed on the opposite side from the eyepiece to strengthen the mounting.
The eyepiece will still move, but only in a horizontal plane. If the upper baseplate is enlarged to carry the user+wheelchair and telescope, then you don't have to chase the eyepiece at all. You will need to use ball bearings or small wheels (instead of Teflon blocks) to support the baseplate, which will be carrying a lot of weight. Motorised telescope guidance controls are highly recommended; and for advice on How To Motorise A Dobsonian, see Mel Bartels' home page.
The eyepiece will still move horizontally and vertically. The support arm needs to be solidly built and attached to a strong pier. A bracing of some kind between the arm and the pier (above the arm) will improve vertical rigidity. The arm should be on the south side in the northern hemisphere, or the north side in the southern hemisphere; and be at least one metre long. For greater horizontal rigidity, a pair of arms forming a narrow (horizontal) triangle and meeting under the telescope can be used. You may need two piers.
StarChairs have been used successfully by paraplegic observers; and the inventor has begun experimenting with a modified controller that could be used by quadraplegics.
The Kings Tableland Observatory (55 Hordern Rd, Wentworth Falls, NSW 2782, Australia) has built an "offset pier" mounting which provides an eyepiece at a user-selected height -- with full wheelchair access -- for a commercial Schmidt-Cassegrain telescope. Their design uses a strong, counterweighted, pantograph arm.
The Hyatt Regency Hotel in Hawaii has a large computer-controlled telescope with a stationary eyepiece and full wheelchair access. This was built by Alaska Applied Sciences (Box 020993, Juneau, AK 99802, USA).
At least two other telescope designs provide completely stationary eyepieces. One is the classic Springfield Mounting, the other is the Coude Telescope (and its variations). Unfortunately, neither are usually available off the shelf and both are relatively expensive to make.
Of course you will also need plenty of solid surface around the telescope, and installing the scope in a permanent observatory is highly recommended.
Sorry about the crayon, They won't let me have any sharp objects.