a 
b
Mountings
Alt/Az
There are a great many ways of mounting a telescope. Mountings fall into two major categories. Alt/Az and Equatorial, Figure 1.
a
b
Figure 1. The two basic types of mountings, the alt/az (a) and the equatorial (b). Each mount is similar except that the equatorial mount the azimuth axis is tipped so that it is parallel to the Earth's axis of rotation.
The Alt/Az mounting is the most familiar, Figure
1a. The alt/az telescope consists of
two perpendicular axis with one pointed straight up, the Azimuth axis
and
one at a right angle to the Azmuth axis, the Altitude axis. The
azimuth, (az) axis moves
the
telescope in azimuth, right and left, and the altitude, (alt), axis
moves the telescope
in altitude, up and down. These two motions provide full access to the
sky. Camera tripods and most cheap amateur
telescopes are mounted on
Alt/Az
mounts. The famous Dobson telescope is a Newtonian telescope on an
Alt/Az
mount and not a unique telescope type. The attraction of the Dobson
mount is ease of construction.
In general the Alt/Az mount can be made stronger
with less
material than an equatorial which makes the mounting cheaper than an
equatorial for large telescopes.
For this reason large professional telescopes have adopted the alt/az
mount
for telescopes above 5 m. For small
telescopes there is no real advantage in simplicity in construction
between the two types of mounts. Tracking the motion of the stars is
problematic fot Alt/Az mounts since both axis must be driven together.
The advent of inexpensive computers for driving the Alt/Az mount has
made Alt/Az mountings
popular for amateur GOTO telescopes.
A major problem with the Alt/Az mounting is that it must be driven in 3 axis for astrophotography. The additional driven axis is needed to account for field rotation of the image plan cause by the azimuth axis not being parallel with the Earth's rotational axis.
Equatorial
The equatorial mounting is similar to the alt/az in that it also consists of two perpendicular axis. In the case of the equatorial mount the azimuth axis is tipped so that it is pointed parallel with the earth's axis of rotation, Figure 1b. The axis that is parallel to the Earth's axis is called the polar axis and moves the telescope in Right Ascension, (RA), and the other axis is called the Dec axis and moves the telescope in Declination, (Dec). Making the polar axis parallel with the Earth's a means of providing tracking of the stars by turning only the polar axis, Figure 2.
Figure 2. The equatorial mounting polar axis is aligned with the polar axis. This arrangement allows the telescope to track the stars by only rotating the polar axis.
The ease of tracking of the equatorial mount was a great advantage until recently. The advent of inexpensive computer control for alt/az mounts has lessened this advantage. However, the need to rotate the image plane for long exposure Astrophotography is still a difficulty for the Alt/Az. An Alt/Az mounting was not considered for the 22" telescope. The difficulties in making the computer control system when the mounting was concieved prevented consideration of the Alt/Az mount. The Equatorial was the choice for the 22" telescope.
Equatorial Mounts Types
There are a variety of equatorial mounts that were considered for the 22" telescope,
German Equatorial
The German Equatorial mount is very popular for small to medium size telescopes although they have been used for mounting fairly large telescopes. The German Equatorial mount consists of two axis that are attached at right angles, Figure 3.
Figure 3. The German Equatorial mounting is a simple design. The two axis are attached at right angles with one parallel to the Earth's axis. The optical tube assembly is located off of the polar axis, which requires a counter weight.
The Dec axis is attached to one end of the polar axis. The telescope is attached to the end of the Dec axis. A large counter weight is required on the other end Dec axis to offset the weight of the optical tube assembly. The center Polar axis is supported on a pier. This type of mounting is easy to construct and is fairly inexpensive. The size of the German mount can become unwieldy with telescopes, above 12". The German mount also suffers from the need for meridian reversal. Meridian reversal is the need to swap the telescope from one side of the pier to the other side to prevent the tube from crashing into the pier. In practice this is not a major problem since it can be accounted for. The major stability problem of the German equatorial is that the tube is located at the end of the polar axis. This causes high loads on the north polar axis bearing.
English Cross Axis
The English Cross Axis can be considered a modification of the German mount. In this case the polar axis is extended beyond the declination axis, Figure 4.
Figure 4. The English Cross Axis is a very ridged mounting. Because of the need for a very long polar axis the north polar bearing is very high off of the ground. This type of mounting is best suited for permanent installations.
The English Cross Axis has the advantage over the German Equatorial mount in that the polar axis can be made larger and is supported on two ends. This reduces the bending moment at the union of the Declination axis. Because of the length of the polar axis and the off center mounting of the optical tube assembly the polar axis must be very large. The English Cross Axis also requires a counter weight on the declination axis. Several large telescopes are built using this type. Given the need for a tall north bearing pier and the over all size of the mounting this design was not selected.
English Yoke
The English Yoke is a modification of the English Cross Axis where the asymmetrical mounting of the telescope on polar axis is replaced with a yoke, Figure 5. The advantages in this design are the removal of a counter weight, reduced stress on the polar axis, a lighter polar axis, (the yoke), and the Declination axis and telescope adds stiffness to the yoke.
Figure 5. The English Yoke telescope is a modification of the Cross Axis where the telescope is placed inside of the polar axis and becomes the Yoke. The Yoke mount is more ridged than Cross Axis because the telescope doesn't apply a bending moment on the polar axis. Because of the need for a very long polar axis the north polar bearing is very high off of the ground. This type of mounting is best suited for permanent installations.
The major disadvantage of this type of mount is the fact that the telescope can no longer see polar stars. The amount of the polar sky that is lost is proportional to the tube diameter. The Hooker Telescope on Mount Wilson is an example of this type of mount. Because of the loss in polar sky access this type of mounting is not widely used. For this and the reasons stated for the English Cross Axis this design was not chosen.
Horseshoe
The horseshoe mount is a modification of the English Yoke where the north polar bearing is replaced by a horseshoe. This modification allows access to the polar regions and retains the stability of the English Yoke, Figure 6.
Figure 6. The Polar Disk mounting
replaces
the north polar axis bearing of the Yoke with Horseshoe which allows
access
to the polar sky. The mount needs a for a very long polar axis and the
north polar bearing is very high off of the ground. This type of
mounting
is best suited for permanent installations.
The most famous example of this type of mounting
is the 200" telescope at Mount Palomar. The size problems of this
mounting
are similar to the previous three examples. These mounting types are
best
suited to permanent observatories.
English Fork
If the English Yoke is truncated at the declination and mounted on the polar axis of the German mount the result is an English Fork mount. Doing this compromises some of the stability of the Yoke mount. However , the result is a much more compact design and does not require a counter weight, Figure 7.
Figure 7. The English Fork is a variation of the Yoke and the German mounting. The polar axis of the German mount is mated to one half of a Yoke mount. The resulting mount is less ridged and is more compact than the yoke. Unlike the German mount it no longer requires a counter weight.
The English fork is found in many modern
telescopes.
Many commercial Schmidt cassegrains and several large professional
telescopes
are fork mounted. The 60" at Mount Wilson and the 120" telescope at
Lick
Observatory are examples of fork mounts. The fork mount is a good
candidate
for the 22" telescope mount.
Split Ring
The split ring is a hybrid between the horseshoe and the fork mount in which the declination axis is brought into the polar axis, Figure 8.
Figure 8. The Split ring mounting is a hybrid between the Fork and the Horseshoe.
The transition from a fork mount to a Split Ring is illustrated in figure 9.
Figure 9. The transition of the fork mount into a split ring. As the declination axis is brought into the polar axis both the fork and the polar axis become rings.
As the declination axis is brought closer to the north polar bearing the load indicated by the arrow is brought closer to the bearing center. The degeneration of the fork into the north polar bearing improves the load bearing capabilities of the mount and increases stiffness. The only disadvantage of this type of mount is that the optical tube assembly becomes difficult to access. Several modern telescopes are of the type including the 4m Mayall and the 4 m Anglo Australian Telescope are of this type. The split ring was a candidate for a mounting for the 22"
Mounting Mirrors
Mounting the telescope mirror in a cell is necessary for mounting the mirror in the tube assembly. Most modern large mirrors are thin relative to their size. Mirrors with a thickness ratio of less than 6:1 cannot be mounted in a simple three point static cell without the risk of flexure of the mirror, which would distort the figure of the surface, figure 10a. For example the 22" telescope would need to be 3.5" thick to be mounted on 3 points. The mirror is only 2.75 thick so the mirror cannot be mounted on a three point static cell. There are two designs for smaller mirrors that do not require complex active support systems. That is a nine point flotation cell and an 18 point flotation cell, Figure 10a and 10b.
a 
b
c
Figure 10. Three different arrangements for small mirror cells. A three point static cell and is suitable for mirrors of 6:1 thickness ratio 10a. 10b and 10c are mechanical flotation cells that are suitable for thinner mirrors. 10b is a 9 point flotation system and 10c is an 18 point flotation system.
The nine point cell was chosen for the 22"
telescope.
Large thin mirrors require more sophisticated mounting arrangements .
The
mirror cell for a large, 1 m and above, may require extensive passive
mechanical
support or even active support systems.