THE MEAN ERROR OF PHOTOGRAPHIC PLATES 
                       by P. AHNERT, Sonneberg


    For a critical examination of small fluctuations of light, especially of 
irregular stars, it is desirable to know the uncertanties of the photographic 
layer.
    A priori, we can suppose that these errors might be smaller at mirrors 
and Schmidt-cameras than at lenses because the images of the stars will be 
unfavourably influenced by the chromatic deviations. Some observers assume 
that the mean error of a plate taken with the Schmidt-camera is about +-0.02m 
or at most +-0.03m. For the plates taken with our Tessar lenses, with a diameter
of 47 and 140 mm, I found a considerably higher value.
    I have estimated the two Cepheids TX and VX Cygni on 45 plates of 
the 140 mm-Tessar seven times on different days. The magnitudes of the 
comparison stars have been derived from the S. A. 40 which was photographed 
on the same plates. Then the mean light curves were drawn by aid of the 
means of the seven estimations. If I suppose that the means of the estimations 
give the true brightness on the plate, further that the mean light curve gives 
the true brightness of the star at each phase, then the deviations of the single
estimations from their means must give the mean error of the observations, 
and the deviations of the single estimations from the mean light curve must 
give the total mean error caused by plate and observer. The square root of 
the difference of squares of both mean errors will then nearly give the mean 
error, caused only by the condition of the plate.
    The mean error of the estimations has been found to be +-0.068m for 
TX and +-0.062m for VX Cygni. The mean error of all estimations in relation 
to the mean light curve was +-0.094 and +-0.107. The mean error of the plates 
alone then becomes +-0.065m and +-0.087m respectively.
    In the same manner I investigated two series of photographic observations 
of R Sge, got with the 47 mm and 140 mm Tessar cameras. Only four single 
estimations have been made. The results were as follows:

Mean error of one estimation                 little camera    big camera
  in relation to the mean value............     +-0.074m     +-0.037m
Mean error of all estimations in
  relation to the mean light curve.........     +-0.101m     +-0.075m
Mean photographic error of one plate.......     +-0.069m     +-0.065m

    If I give half the weight to the observations of R Sge, the mean error 
of a photographic image on plates, taken with our Tessar cameras, becomes 
+-0.073m.
    These results have been confirmed by photoelectric measurements of 
TX and VX Cygni on the same plates. The mean error of one measurement 
in relation to the mean light curve became +-0.065m and +-0.095m respectively, 
in a near agreement with the values found above. The larger mean error of 
the VX Cygni observations possibly has been caused by the faintness of the 
images on some plates near the minimum.