Introduction

A significant number of ISO proposals contain ISOPHOT observations of very faint sources of 50 mJy or less. Currently these proposals use one of the following observing modes to detect these faint objects:

• chopping between the source and one/two background positions using AOTs PHT03 or PHT22;
• staring observations of the source and a few background positions using PHT03 or PHT22, or the sparse map AOTs PHT17, 18, 19, or PHT37, 38, 39;
• Extended raster maps using PHT03 or PHT22 in raster mode;
• Oversampled maps with PHT32.

In this report we present the status of the ISOPHOT performance for the detection of faint sources at the longest wavelengths from 60 to 200 micron. This includes the following items:

• natural and instrumental limitations of which the observer must be aware and how the various observing modes try to overcome these instrumental difficulties,
• detector sensitivity limits that can be presently achieved, and
• recommendations for the selection and optimisation of the most suitable observing strategy.