The observations of nuclei of three planetary nebulae, NGC 6826, NGC 7662 and IC 4593, have been made as part of a more extensive program with the aim of obtaining their absolute spectrophotometric gradients (ϕ) and photographic magnitudes (from the spectrogram). The spectra were obtained with an 8–12′ Schmidt telescope combined with an objective prism (the dispersion is 420 Å/mm at Hγ). The preliminary data which were obtained at different times, particularly the spectrophotometric gradients of the nuclei of these nebulae, did not agree with each other.