Calibration

As you see from the discussion given in Chapter 2 and Chapter 3, the C05 observations take much longer than previously assumed. It is of interest here that for C05 observations only the same fraction of calibration time is available as for all other AOTs. As a consequence, the observatory together with the ISOCAM consortium has reached, the conclusion that not all optical configuration can be fully calibrated for polarization. We will be able to fully calibrate no more than three filters of the LW detector in-flight. The filters planned are (see ISOCAM Observer's Manual, p. 22) :

• LW1: = 4.5 ,
  
• LW8: = 11.4 ,
  
• LW9: = 15.0 ,
  

These three filters have been selected as a compromise, from the total requested sample of filters in the present mission data base including all C05 observations. For these three filters we plan to provide a PSF for each of the three polarizers and to establish the instrumental polarization, which seems to be wavelength dependent.

In addition we plan to derive the instrumental polarization by mean of raster observation on the zodical background in the two broad band filters:

• LW2: = 6.75 ,
  
• LW10: = 11.5 ,
  

Please note that for both of those filters we will not be able to provide a PSF in each of the three polarizers.

From the ground based calibration measurements one has a measure for the LW7 filter. The instrumental polarization for LW7 as measured in the lab reads:

• fractional polarization: p = 2% and
  
• polarization angle:
  

Here, is measured along the spacecraft Y-axis, so in the direction of polarizer 1 and not anti-clockwise with respect to North.

However, the lab measurements are affected by Fabry-Perot effects between the ISOCAM polarizers and the calibration unit, so one cannot be absolute sure how much one can trust those figures.

In case you are not using one of the filters LW1, LW8 or LW9, you can still perform your own calibration observations. For those calibrations we recommend to use one of our non-polarized photometric standard stars and use the same observing strategy, but double the value of , as given by Equation 3.2. For this extra calibration, the additional observing time is from your project. It will be of order 12000s (twelve-thousand seconds).