1. Introduction

The ISOPHOT data processing and accompanying flux calibration of science observations contain calibration factors and a large number of corrections which have been determined mostly from dedicated in-orbit calibration observations. Since the calibration factors and corrections have a finite accuracy, each will contribute to the total error budget of an observation. The impact of a certain parameter on the photometric accuracy can vary from mode to mode. Due to the many observing modes that can be employed, the determination of the photometric uncertainty of a given observation is not straightforward and may require knowledge of specific details of the observation.

This document provides a detailed description of the different elements that contribute to the overall ISOPHOT error budget up to Derive_SPD. The document follows closely the ISOPHOT pipeline processing scheme which is fully described in the IDUM version 4.

In a later version of this document we will include the error budgets for Derive_AAR. The remainder of this document contains the sections:

• Section 2: definitions are given of the terms releated to ISOPHOT photometric accuracy issues.
• Section 3: the uncertainties introduced in each relevant processing step in Derive_SPD is described. Instrumental corrections and the photometric calibration are performed in Derive_SPD.
• Section 4: summarizes the instrumental effects not corrected for in OLP version 7.
• Section 5: some extra attention is given to the uncertainties in the (point) source flux due to the subtraction of the background emission. ISOPHOT observations provide several methods for separating the source from the background flux.