Worked Example

The possible polarisation of a quasar shall be measured at 90m with the C100 detector. A chopping observation is desired in order to simultaneously use the background signal as a reference. For this observation the solid angle is , and the point spread function factor is .

The expected fluxes are:

EXPECTED SOURCE FLUX : 2.7
UNCERTAINTY IN SOURCE FLUX : 0.5
MAXIMUM BACKGROUND FLUX : 16

The user input on polarisation is:

EXPECTED POLARISATION : 10.0 %
POLARISATION ERROR : 0.15 % (=0.0015)

With the given source and background flux the effective polarisation becomes 7.0 deg (Eq. 33) and thus, with Eq. 32, the error in polarisation angle is 0.6 deg.

In order to derive the integration time per polariser with Equ. 45 the terms of Equ. 36, 38, 43, and 44 have to be calculated using the constants C1 and C2 from Tab. 10 and and from Tab. 15, respectively.

This yields an on-source integration time per polariser setting of 30sec. The next higher value being a power of 2 is 32sec.

Since the chopper is used the integration time per polariser setting is 64sec. For all three polarisers and, additionally, 2 non-polarised (also chopped) measurements of 32sec each, the total exposure time becomes 192sec + 64sec = 256sec.

The overheads which have to be added are:

time for target acquisition and slewing : 180 sec
time for instrument set-up : 15 sec
time for FCS exposures : 16 sec
time for wheel positioning (1 filter + 3 polarisers + no polariser): 50 sec
time for stabilization of heated detectors : 90 sec

The total observing time for this polarimetric measurement is: 607sec.