ISO Explanatory Library : PHT

ISO Explanatory Library: PHT

ISOPHOT Polarisation AOTs PHT50 and PHT51

Authors: Rene Laureijs and Ulrich Klaas

After careful study of the ISOPHOT detector properties and assessment of the necessary calibration programme it has been decided to release the polarisation modes PHT50 and PHT51 with strongly reduced options. The following ISOPHOT polarisation modes have now become available:

      PHT50: P2 detector, 25 micron filter (P_25) with 79" aperture
      PHT51: C200 detector system, 170 micron filter (C_160)

In both cases the minimum integration time is 256 sec per polariser. The maximum S/N one can achieve in C_160 is about 250, but a S/N as low as 80 can also occur in the same time. Therefore only targets with fluxes higher than the background flux contribution (in the beam) can be measured with sufficient polarisation accuracy - of order 1%. Polarisation errors of better than 0.5% cannot be achieved. For C_160 targets that are as strong as the background we recommend to perform a pointing centered on one pixel instead on the centre of the C200 array. For each on-target pointing the observer is obliged to perform a polarisation observation on the local background. This measurement will also be used by the Science Operations Centre to increase the accuracy of determination of instrumental polarisation.

Since the new ISOPHOT polarisation capabilities have been drastically reduced, we request you to revisit your proposal and send us via helpdesk an updated target list and observing modes. You may wish to start with a few pointings and finalize you programme at a later stage. Some practical details are given below. Please provide us with the following information:

Tips and notes:

* The 256 sec integration time per polariser splits into two cycles of 128 sec per polariser. An integration time of 128 sec is needed to achieve high S/N differential signals. The signal reproducibility in the different cycles is used as a measure of the signal uncertainty.

* The maximum S/N mentioned above refers to the source+background emission. As a consequence, background subtraction reduces the effective S/N of the source.

* Polarisation of targets with a polarisation degree of less than 2% cannot be determined.

* Per pointing the following OTTs are required: 

-----------------------------
filter  time per        OTT
        polariser (sec)
-----------------------------
C_160   256             1300
C_160   512             2000
P_25    256             1400
P_25    512             2400

* If you cannot find a suitable target from your original proposal, you may select an alternative source which is visible and with sufficient fluxdensity.

* You can check the visibility of a target on: http://www.iso.vilspa.esa.es/dbob/sdbob/sdbob.html

* Minimum flux estimates for sources for 1% polarisation error:

C_160: Fnu > 2 Jy/pixel for a 10 MJy/sr background level
P_25: Fnu > 5 Jy/aperture for a 50 MJy/sr background level

* It is not necessary to compute the new pointing position in case you want to center your source on an array pixel; this will be done on our side.

* Signal evaluation can be done in PIA (PHT Interactive Analysis) in the usual way. A special fortran programme has been developed to derive all polarisation parameters including errors.