Sub-beam prism used at the Bosscha Schmidt telescope

A Schmidt telescope sub-beam prism was tested using five blue and three visual plates. In the B and V-bands the extent of the diffraction pattern amounts to 1.56 and 2.03 arcsec respectively. These values are in the order of the average seeing at the observatory. The contribution from the chromatic aberration effect is negligibly small, 0.48 and 0.37 arcsec in B and V respectively. The slope of the regression line in the 7 magnitude range is small. Therefore the magnitude difference between primary and secondary images is independent of magnitude. Averaging the magnitude differences gives a value of 4.2. In view of the ratio of the prism to corrector diameters, the result is encouraging.