4 Calibration

The extended emission calibration has changed by a factor of about 2 between PIA version 6 and PIA version 7. A large part of this factor comes from the different footprint solid angle values used in the different PIA versions.

  
4.1 Footprint observations around Saturn

During the revolution 409, ISOPHOT made several tens of pointing around Saturn at distances between 4.2 and 45.4 arcmin in order to map the extended wings of the PHOT footprint at 170 $\,{\rm\mu m}$during about 5 hours. Observations were made in the Y and Z satellite axis directions. Figure 9 shows the observed pattern on an IRAS 100 $\mu$m map. All directions were observed twice, back and forth. Observations were performed with the following AOTs: PHT 37-38-39 (sparse maps; PHT37: FCS; PHT38: sky; PHT39: FCS) for distances between 4.2 and 27.6 arcmin, and PHT25 (absolute photometry) at the largest distance. Integration times are around 150 s for PHT38, 200 s for PHT37-39 and 400 s for PHT25.

We use PIA V7.2.2 for the data reduction and calibration. For each detector and each position, we keep only the second half of the data corresponding to the stabilised signal. These observations have no transients induced by cosmic rays and we do not apply any flat-field correction. Sixty seven files were used, the others being either unreadable (3 files) or saturated (3 files). Results are presented in Figs. 10 and 11 for all data in the Y and Z direction respectively. Each pixel is plotted and is used as an independent measurement.

4.2 Comparison of the footprint model with Saturn observations

A model for the ISOPHOT footprint has been developed at Heidelberg (Klaas et al., private communication) based on the ISOCAM footprint routines. This model includes the optical characteristics of the telescope, the primary and secondary mirrors, and the filters and detectors. We have used this program to compute the ISOPHOT footprint up to 20 arcmin with the 170 $\mu$m bandpass filter. Results of the model are shown in Fig. 12 together with the Saturn measurements for the Z axis. To make this comparison, we have assumed for Saturn a flux of 32000 Jy and removed the background using the PHT25 measurement.

We see a very good agreement between the model and the measurements. However, around 4.5 arcmin from Saturn, some data, coming from one Y direction scan (only one position), have a significantly higher flux than the model prediction. Data at similar distances from Saturn in other scans cannot be used due to saturation problems. Therefore, we interpret this discrepancy as due to detector saturation problems (one can note, however, that the contribution of these data points to the solid angle is lower than a few percent).

Figure 8: Map of the FIRBACK/ELAIS North 2 (FN2) field.

Figure 9: ISOPHOT pointing positions (circles) around Saturn on the IRAS 100 $\mu$m map. Saturn positionnal changes during the observations have been computed using the ephemerids of the Bureau des Longitudes (Berthier 1998) and are represented by a small segment at the middle of the cross.

Figure 10: Y direction measurements around Saturn. Squares are forward data, crosses, backward data. Dispersion bewteen the 4 pixels at each position comes mainly from the non correction of the flat-field.

Figure 11: Z direction measurements around Saturn. Triangles are forward data, diamonds, backward data.

Figure 12: ISOPHOT footprint model (continuous line) compared to ISOPHOT measurements around Saturn for the Z axis.

In conclusion, the ISOPHOT footprint measurements and model at 170 $\mu$m are in very good agreement. We therefore use, in the following, the model (from Klaas et al., private communication) as the definitive footprint at 170  $\,{\rm\mu m}$.

4.3 The extended brightness photometric correction factor

In PIA V6.5 the solid angle used to convert the flux in brightness was that of the pixel ( $1.88\times10^{-7}$ sr); in PIA V7.2.2, it is the footprint model's one but truncated at 4.1 arcmin ( $2.67\times 10^{-7}$ sr). The full solid angle of the footprint is equal to $3.0\times 10^{-7}$ sr. Therefore, there is a photometric correction factor to be applied to the extended emission:

$$\begin{eqnarray*}B_{\nu} &=& B_{\nu}({\rm piaV72}) \times \frac{\Omega_{\rm piaV... ...rm footprint}}\\ B_{\nu} &=& B_{\nu}({\rm piaV72}) \times 0.89. \end{eqnarray*}$$

After correction of the absolute calibration of the extended emission, point source fluxes (given in Dole et al. 2001) are computed using the footprint model (Klaas et al., private communication).

4.4 The rejection rate

Unique measurements have been done by the ISOPHOT team during the eclipse of the sun by the earth (Kranz et al. 1998; Klaas et al. 1998a; Lemke et al. 1998) by pointing at a sky region at 60$^\circ$ from the sun before, during, and after the eclipse. These measures reveal no signal variation, leading to an upper limit of 10^-13 at 60$^\circ$ for the straylight rejection rate. This exceptional measurement clearly shows that there is no contribution to the flux from the far-side lobes. This demonstrates that ISO is able to make absolute measurements of the extended emission and gives a high degree of confidence to our absolute photometric calibration.

4.5 Comparison with DIRBE extended brightness

We can now compare the extended ISOPHOT FIRBACK brightness with the predicted one using DIRBE and HI measurements. Because of the size of the DIRBE beam, this comparison is only feasible due to the flatness of the FIRBACK fields; this flatness is observed with DIRBE on several degrees around each FIRBACK field.

The sky measurement is the sum of the zodiacal light, Cosmic Infrared Background (CIB) and dust interstellar emission. For each field the zodical light, at the time of the observation, has been computed using the Reach et al. (1995) DIRBE model. The CIB is extrapolated at 170 $\mu$m using the DIRBE measurements of Lagache et al. (2000). For the dust emission, we compute its contribution using (1) the HI column density from the Leiden-Dwingeloo survey, Hartmann & Burton 1997 (we prefer to use the HI column density rather than the DIRBE brightness since DIRBE data are very noisy in FIRBACK fields) and (2) the emissivity from Lagache et al. (2000). The final predicted emission at 170 $\mu$m for the three fields is shown in Table 2. It is in remarkable agreement with the measured ISOPHOT brightness.

 

 

Table 2: Cosmic Infrared Background (CIB) from Lagache et al. (2000), zodiacal (from Reach et al. 1995) and dust emission (from Lagache et al. 2000) at 170 $\mu$m for the three FIRBACK fields (in MJy/sr). The total emission is the sum of the three contributions for each field. There is a remarkable agreement between the predicted brightness and the measured ISOPHOT one.

	FN1	FN2	FSM
CIB	$1.10\pm0.2$	$1.10\pm0.2$	$1.10\pm0.2$
Zodiacal	$0.71\pm0.1$	$0.80\pm0.1$	$0.75\pm0.1$
$N_{\rm HI}$ (cm-2)	$8.2\times 10^{19}$	$7.7 \times10^{19}$	$1.0 \times10^{20}$
Dust	$1.17\pm0.35$	$1.09\pm0.33$	$1.42\pm0.43$
Total predicted	$2.98\pm0.41$	$2.99\pm0.40$	$3.27\pm0.48$
PHOT measured	$3.01\pm0.14$	$2.97\pm0.17$	$3.39\pm0.12$

Figure 13: Comparison of the ISOPHOT 170 $\mu$m measured flux with the IRAS extrapolated ones. Data are compatible with a slope of unity.

4.6 Comparison with IRAS point sources measurements

The absolute flux calibration of ISOPHOT is derived using calibration standards such as planets, asteroids or stars (Klaas et al. 1998b; Schulz et al. 1999; ISO Consortium 2000a, 2000b). One can however check, using IRAS detected sources in FIRBACK fields, the consistency between IRAS 60 and 100 $\mu$m flux and the ISOPHOT 170 $\mu$m one. However, we have to keep in mind that it is very difficult to extrapolate the IRAS fluxes at 170 $\mu$m (one can have a factor 2 to 3 in the extrapolation using different models or black-body temperatures).

Twelve FIRBACK sources, well identified as very nearby non interacting galaxies, have IRAS 60 and 100 $\mu$m flux counterparts, measured with SCNAPI. We extrapolate the 60 and 100 $\mu$m IRAS flux using the template spectra of Dale et al. (2000). These spectra are based on the IRAS 60/100 color ratio and are thus well adapted. The comparison is shown in Fig. 13. Uncertainties on the extrapolation, that take into account only the errors on the 100 and 60 $\mu$m fluxes (and not the model uncertainty), are very large. Data are compatible with a slope of unity; but this comparison is only illustrative.