Method of Bigourdan

The method of Bigourdan is a method of put in station of a mounting of Télescope invented by the astronomer Guillaume Bigourdan. Its advantage is to allow an erection even when one cannot locate the celestial Pôle and that one does not know the Latitude place. It also makes it possible to obtain a better precision than with a direct pointing of α Ursae Minoris (the pole star). However, it requires much time to obtain a precision sufficient for the Astrophotographie.

Description of the method

This method requires the use of a Oculaire réticulé, and of focal as short as possible. The use of a Boussole can also facilitate the operation. The mounting must be placed on a quite horizontal support.

Foot-note: the image provided by a Telescope is reversed (haut-bas), while the telescopes derived from the Cassegrain restore a right image. On the other hand, the telescopes of the type Newton return an image of axial Symétrie, therefore reversed (haut-bas) (the optical axis of the eyepiece being normally horizontal). In addition, the use of an elbow to return the image to 90° conversely generates one (right-hand side-left) of the image of axial symmetry.

Adjustment of the axis of longitude

To make swivel the tube of the telescope so that it is parallel to the time axis of mounting
Bloquer the brakes of the time axes and variation, and slightly unscrew the thumb screws of the axes of latitude and longitude so that they are mobile but stable
Pointer the tube of the telescope towards a star easy to recognize and located more close possible of the celestial equator and the meridian line Southern
Tourner the réticulé eyepiece of the telescope so that the pilot wheels are horizontal and vertical, then to bring star located to the center of the reticle
Attendre a few minutes, the duration of waiting depends on the enlargement eyepiece and must highlight the rotation of the celestial Sphère
If the star derived towards the horizon ( i.e. under the horizontal wire for a not reversed image haut-bas ), to slightly swivel the axis of longitude towards the Western, i.e. towards the line since the south is looked at, more especially as the vertical drift is important; and conversely, if the star derived towards the zenith ( above the horizontal wire ), to swivel the axis of longitude towards the left
the operation must be repeated because the adjustment is done by groping, it can be necessary to change star locates because it must always be nearest possible southern meridian line; the adjustment of the axis of longitude is satisfactory when the vertical drift is negligible
to tighten the screw of the axis of longitude

Adjustment of the axis of latitude

the screw of the axis of longitude is tightened and the brake of the axis of variation is also blocked, the slope of the reticle of the eyepiece should not be changed
Débloquer the brake of the axis of right ascension and to turn the tube of 90° towards the Is, i.e. towards the left since one always looks at the south
Pointer an easily recognizable star nearest possible to the meridian line is and the celestial equator, and to bring this star to the center of the reticle of the eyepiece
Resserrer the brake of the axis of right ascension
Attendre again a few minutes
If the star derived towards the horizon ( i.e. under the horizontal wire for a not reversed image haut-bas ), to raise the time axis around its axis of latitude until bringing back star on the wire; and conversely if the star derived towards the zenith
the operation can be repeated, when the adjustment is satisfactory, to tighten the screw of latitude

The brakes of the axes of right ascension and variation can be freed, the time axis is now parallel to the axis of rotation of the Earth and point towards the celestial pole of the hemisphere observed. The precision reached with the method of Bigourdan depends on the enlargement on the eyepiece and the number of times that the operation is repeated. For the erection of a mounting on mobile support, which is the most frequent case, it is generally not possible to obtain a precision higher than 0,5°.

See too

Other methods of erection:

Put in station of an equatorial mounting
Method of King

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