Light losses ¹

There are two main reasons that can produces light losses with FOCES spectrograph. The size of the diaphragm at the Cassegrain focus compared with actual seeing value and the slit width at the entrance of the spectrograph.

Assuming a Gaussian seeing of fwhm , an entrance diaphragm of diameter will pass a fraction of the light of a pointsource according to

with light losses as follows :

/ 0.50 0.75 1.00 1.25 1.50 1.75 2.00
F()/F( ) 0.159 0.323 0.500 0.661 0.790 0.880 0.938
mag -2.00 -1.23 -0.75 -0.45 -0.26 -0.14 -0.07

The spectrograph slit instead cuts out a rectangular cross-section of width of the entrance aperture diameter , where now

and = / . Note that there is not light loss when > .
Since determines the spectral resolution , the transfer of photons through the fibre system is controlled by the product of the two formulae above. Thus, for a given spectral resolution there are three ranges of seeing that require a particular choice of an optimal entrance aperture as shown in next figure.

Note that the use of the diaphragm with 130 µ diameter is only suggested for a seeing below 1 arcsec. The larger diaphragms (200 or 300 µ ) are considerably better for medium or bad seeing.

1.- From M. Pfeiffer et al. Astronomy and Astrophysics 1998, 130-381.

Santos Pedraz Marcos
Last modified: Tue Aug 29 16:52:13 WET DST 2000