Since it is expected that the dispersion of 40A/mm (1200 L/mm grating) will be used the most, this dispersion is taken as the standard. It was found that to a good approximation exposure times are inversely proportional to dispersion. The exposure times for the comparison arc are independent of dispersion.
Exposure times are quoted which produce a density of 0.8 above fog in the middle of the spectral range (unless otherwise specified). Seeing conditions of 1 to 2 arc seconds are assumed using the 1 arcsec slit. Needs to be updated Image trailing uses the full 2,mm of slit exposed by the star aperture in the dekker. It is estimated that at the time of commissioning that there was 0.4 magnitudes of extinction at the zenith by dust.
The exposure times are quoted for stars of zeroth magnitude.