Significance of Space Telescope observations for knowledge of carbon abundance in planetary nebulae

The knowledge of carbon abundance in planetary nebulae and their control stars is very important for studies of stellar evolution and of chemical evolution in galaxies. This knowledge can be achieved by observation of the C III 1907, 1909 A and the C IV 1548, 1550 lines complemented by UV observations of nebulae is coming from the IUE satellite. However the IUE satellite apparently is not able to observe a number of objects which are very important to the subject, such as the planetaries of very low surface brightness. The predictions concerning carbon abundance in planetary nebulae as derived from stellar evolution theory are briefly presented. The value of observations of these objects with the Large Space Telescope is discussed.