### ECH_FORMAT_CENTRE

Determine the appropriate spectrograph settings

##### Description

Firstly, calculate the prism system rotation and position. The required echelle gamma angle is not quite the same as the prism rotation angle. This is because of the 12 degree separation between the incoming ray to to the echelle grating and the outgoing ray to the camera - the prism rotation angle has to be multiplied by an empirical correction factor of 1.011 (I am not sure of the complete justification for this).

Having determined the appropriate echelle gamma angle, the cross dispersion is now correct and it is necessary to determine the appropriate echelle theta angle to give the correct wavelength in the centre of the field. If the order number was not explicitly specified, assume the order within which the given wavelength lies with the free spectral range. Theta is determined simply by stepping along the order and, once having stepped past the required wavelength, binary chopping until convergence.

Finally, determine the appropriate collimator to use and the necessary slit assembly rotation in order to give vertical slit images on the detector.

ECH_INIT must be called before calling this routine.

##### Invocation

CALL ECH_FORMAT_CENTRE (WC, MCEN, THETA, GAMMA, COL_COLOUR, PRISM_POS, SLIT_ANGLE, STATUS)

##### Arguments
 READ WC REAL Desired central wavelength (Angstroms) READ MCEN INTEGER Desired central order. If <=0 the order in which the above wavelength lies within the free spectral range is used WRITE THETA REAL Echelle theta (radians) WRITE GAMMA REAL Echelle gamma (radians) WRITE COL_COLOUR CHARACTER Collimator (UV or WIDE) WRITE PRISM_POS REAL Prism position (mm) WRITE SLIT_ANGLE REAL Slit angle (radians) READ, WRITE STATUS INTEGER Global status value

None known.

##### Authors

F. Diego UCL (ZUVAD::FD)