Z e n t r a l i n s t i t u t ftir Astrophysik d e r Akademie d e r Wissenschaften der DDR Mit 15 Abbildungen. (Eingegangen 1975 Juli 11)Twelve Ap stars with known periods and fairly large amplitudes were photoelectrically investigated in a ten colour system (A 3400 A to A7600 A). The amplitude-wavelength relations following from the light curves cannot be explained by the influence of the backwarming effect. These relations point to complicated structures in the atmospheres of the peculiar regions of the magnetic stars. Zw6lf Ap-Sterne mit bekannter Periode und grbker Amplitude wurden in &em Zehnfarbensystem

The University Observatories of München and Göttingen and the Landessternwarte Heidelberg are building in cooperation with ESO two almost identical FOcal Reducer /low-dispersion Spectrographs (FORS) for the ESO Very Large Telescopes. FORS allows low-dispersion multiobject spectroscopy (19 slits) and longslit spectroscopy in the wavelength range of 330 to 1100 nm. A set of standard grisms with reciprocal dispersions of 45 ...230 Å/mm working in the first order are foreseen. With a slitwidth of 1 arcsec the resulting spectral resolutions range from 180 to 1800.For further FORS details see Appenzeller and Rupprecht (1992) and Seifert et al. (1994).The standard grisms are located in a grism wheel in the parallel beam between the collimator and the camera. Seven of eight positions are available for grisms. The free diameter of the grisms is 135 mm to cover the whole field of view of FORS. To avoid reflection ghosts the entrance surfaces are all tilted by .

Z e n t r a l i n s t i t u t f a r Astrophysik d e r AdW d e r D D R Mit z Abbildungen. (Eingegangen 1975 O k b k 3) Der Aufbau eines lichtelektrischen Fotometers mit spektraler Zerlegung durch ein Gitter wird bescbrieben.The construction of a photoelectrical photometer using a grating is described.

Focal reducers are nowadays widely used on large telescopes as multi-mode instruments. Some general aspects of the design of focal reducers are discussed. FORS, the focal reducer/low-dispersion spectrograph for the ESO VLT is presented as an example for such a multi-mode instrument on a large telescope.

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