2. Observations and Data Reduction
The ISOPHOT observations were performed as part of the observing mode commissioning and of the scientific validation of the off-line processing pipeline (OLP). Therefore, different observing modes (=AOTs, Klaas et al. 1994) were applied and a unique coverage by all 25 ISOPHOT filters was achieved for NGC 6240 and Arp 220.
For Arp 244 the sparse map AOTs PHT17/19 with the largest beam and PHT37/39 were used in order to perform integral photometry of the extended object, whose main body without the tails has a diameter of 3 arcmin in the optical. Exposure times per pointing were 64 s. NGC 6240 and Arp 220 are much more compact, therefore smaller aperture sizes were selected. Typical exposure times for the AOTs PHT03/PHT22 ranged from 32 s in most filters to 128 s in some of the narrow band filters. The on- and off- pointings are listed in Table 1.
Table 1. Pointings of the observations. The on-positions are centred on the objects, the off-positions served for the background subtraction.
The data were reduced using the OLP V6.0 and in parallel the PHT Interactive Analysis (PIA 1) V6.1 in standard processing mode, together with the calibration data set V3.0 (ISOPHOT Data User Manual V3.0, Laureijs et al. 1997). The results of both software systems were in excellent agreement. The fluxes were corrected for beam size effects of the single pointings. To account for the overall uncertainty in signal derivation as well as relative and absolute photometric calibration we have adopted generally 30 for the errors.
© European Southern Observatory (ESO) 1997
Online publication: April 28, 1998