The modern use of a fast primary mirror allows the optical tube assembly to be shorter. For any given optical and structural tube configuration, the location of the rotational axis is determined independently from the mounting configuration. An alt-az and an equatorial fork mount will have the same swing radius so will require the same size dome. The equatorial mount may require additional floor space to the south (in the northern hemisphere) depending upon the latitude of the site.
Alt-Az Myth #2: The alt-az configuration simplifies the radial support of the primary mirror, which may have some cost savings and optical blank considerations. Also, the alt-az mount may simplify some structure and bearing considerations for static loads, but the alt-az and the equatorial configurations share a very similar optical tube assembly so have the same dynamic (i.e. wind and acceleration) loads and require the same dynamic stiffness characteristics. The dynamic loads dominate the stiffness requirements for modern high performance telescopes of moderate size, so there tends to be no structural cost savings. The alt-az telescope also has field rotation, which requires (for imaging) a precision computer controlled field de-rotator and complicates guiding. This additional hardware costs more and uses up back focus causing the secondary obscuration to be larger resulting in more light loss and increased diffraction (i.e., less light focused into the Airy disc and more light distributed into its surrounding diffraction rings). The alt-az telescope also has a singularity at the zenith preventing the telescope from accurately tracking an object through the zenith. The size of the non-observable cone can be quite small depending upon the dynamic response of the telescope and it's servo system. The dynamic loads applied by the control system to minimize the non-observable cone can be substantial. The greater load and dynamic response requirements increase the cost of the servo system. Good pointing and tracking of an alt-az telescope is more difficult to achieve partly because of time keeping and partly because of the dynamic range of the azimuth velocity. The excellent pointing reported for the large alt-az telescopes is more a result of human effort than anything inherent with the configuration. Service access to Cassegrain instruments is limited by the azimuth structure in small and moderate sized alt-az telescopes. Mirror and mirror cell handling is more complicated requiring more expensive handling equipment. The equatorial fork mount has clear access to the instrumentation. In summary, the optical support requirements or the requirement for a Nasmyth platform may require an alt-az configuration, otherwise, a properly designed equatorial fork mount telescope has cost and operational advantages. One of the myths about equatorial telescopes is that they require a larger dome. Older telescopes were made with very slow (by today's standards) primary mirrors. These old telescopes have optical tube assemblies that are much longer than a modern telescope and do require a large dome. Because the optical tube structure can be identical to that of an Alt-Az telescope, the equatorial fork mount telescope has the same swing radius and can fit into the same size dome. The polar axis does extend to the south requiring more space, but this space can be provided inexpensively in the non rotating part of the building. Another advantage of the equatorial telescope is not readily apparent. Because the space directly below the primary mirror cell is clear of structure (unlike an Alt-Az) the primary mirror and cell may be lowered from the telescope to a handling cart. The Alt-Az fork (yoke) structure would prevent this action complicating the handling of the mirror and cell. Many Alt-Az telescopes require removing the secondary assembly and the trusses to lift out the primary mirror. This requires considerable effort and optical collimation after a simple washing of the mirror, for example. Also a dome crane is needed increasing the size and cost of the dome. Access to install or remove instruments is also much better for the equatorial compared to the Alt-Az. A larger dome may be needed to handle the instruments for the Alt-Az telescope. A hydraulic operated scissors lift table may be built into the floor directly below the telescope primary mirror cell. The lift table can be used to install/remove the primary mirror and cell and to handle the Cassegrain instruments. With this system two people can safely remove or install the primary mirror and cell. The cost of the system is very low compared to the cost of a dome crane and the additional dome structure, slip rings, etc. With no field rotation, auto guiding becomes far less complicated and there is no need for field derotators at the Cassegrain, bent Cassegrain focal plane locations, and for moderate integration times at the Nasmyth focal plane location. The cost of the precision field derotators is high and the additional complexity increases the cost to maintain the system. Alt-Az mounts require accurate time in order to calculate the proper drive rates. For an equatorial telescope, time errors only introduce an offset error which is zeroed when the telescope is initialized. There is only a small error introduced into the refraction correction. EQUATORIAL MOUNT: The equatorial mount will consist of the following subassemblies: (a) Declination bearings and housings These subassemblies are an exact analog of the Alt-Az subassemblies with different names. Many of the parts are identical to the Alt-Az mount. © 2001 Frank Melsheimer
(b) Declination drive assembly
(c) Fork (yoke) with Nasmyth platform on one fork tine
(d) North polar axle hydrostatic bearing assembly
(e) Right Ascension drive assembly
(f) Pedestal assembly