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======================================================================== ==> In this proposal, more time is being requested for JGURTLER.STARDUST ======================================================================== We propose to map the circumstellar dust envelopes of post-AGB stars at 50 microns using the PHT-C camera. This study is an extension of our ISO open time project "Analysis of dust envelopes around evolved stars". Until recently, the extent of these dust shells was thought to be too small to be mapped by ISO. But new evidence has appeared indicating these envelopes to be much larger than originally believed. There are two pieces of evidence: (1) Current radiative transfer models seriously underestimate the extent of the envelopes in the N-band (10.6 microns); this effect may be even more important at longer infrared wavelengths. (2) The diameter of the post-AGB object OH 231.8+4.2 determined from 12CO interferometer data of 40 arcsec is much larger than expected from standard radiative transfer calculations. The extent of this object in our N-band image is about 4 arcsec. Taking the case of OH 231.8+4.2 as a clue, we have selected those sources from our N-band survey of post-AGB objects for the project which are clearly resolved (see Fig. 1 and 2). Note that the PHT-C maps of post-AGB dust envelopes would be the first at wavelengths longer than 10 microns. The discovery of extented emission at 50 microns around post-AGB stars would be important news just by itself. We will also analyze the PHT-C data together with our N-band images. In addition, the results will be compared with our ISO 50 micron maps of dust shells around semi-regulars; first data have been successfully reduced. The ISO data will have an important impact on our radiative transfer model and thus on the understanding of the structure and evolution post-AGB dust shells.