Mounting altazimutale
The mounting altazimutale is a Monture comprising a vertical axis, or axis of azimuth, and a horizontal axis, also called axis height. This provision makes it possible to direct the instrument of aiming easily. Its mechanical realization is much simpler than the majority of the equatorial mountings.
Mountings altazimutales can be motorized in order to compensate for the rotation of the vault of heaven. This compensation as well as requires a control on the axis height on the axis of azimuth, and is in general ordered by a data-processing device.
This follow-up on the two axes does not return therefore mounting directly suited to the photography of the sky. Indeed, when the movements are compensated in height and azimuth, the field observed turns slowly and achieves a complete rotation in a sidereal Jour. To include/understand this phenomenon, it is enough to imagine that the instrument is pointed towards the celestial Pôle: no movement is then necessary in height nor in azimuth. On the other hand, it is clear that the field concerned turns apparently around the celestial pole. The turntable supporting the instrumentation can be put in rotation in order to compensate for this effect. This accessory bears sometimes the name of “derotator of field”.
In its not motorized version, this type of mounting equips the Lunette S and the Téléscope S amateurs used mainly for the visual observation. The mounting of the Dobson telescopes is a mechanical example of realization simplified in this field.
The large scientific telescopes built since the end of the 20th century also use a mounting of this type, in the place of the old equatorial, heavier, more expensive mounting mechanically and requiring a shelter of bigger size.