Circular Polarisation
- Make sure you have requested the quarter-wave plate in your NOT NIRF.
Staff will then install it.
- With this plate installed there are, in fact, two observing modes
available: 1) Circular polarimetry and 2)
Simultaneous circular and linear polarimetry.
- Circular polarimetry:
In the PP program MODE menu choose Circular Polarisation. Then the
quarter-wave plate is rotated in steps of 90 degrees. Also in this mode
there will be 8 integrations per "measurement", which gives a redundancy
of 4. Use the REDUCE menu for on-line averages, but note that the sign in
the polarisation degree will be wrong (i.e. left-handed and right-handed
circular polarisation is swapped here). It will come out right in the
off-line reduction program (CPOLAR), however.
Note that for historical reasons the output counts in this mode will not
be divided by the integration time as in the other two modes. This will
not affect the results, of course, and the numbers in channels A and B
on the PP display give counts per second as usual. (Thus, no need to
worry that the numbers in the data file are larger than expected!)
There are no real circular polarisation standards, but the following
is a relatively stable reference object for circular polarisation.
| Name |
RA (2000) |
DEC (2000) |
Sp. T |
B |
V |
P (in B) |
| Grw +70 8247 |
19:00:10 |
+70:39:36 |
DA:w |
13.24 |
13.19 |
~ -4 % |
- Simultaneous circular and linear polarimetry:
In the PP program MODE menu choose Linear Polarisation. Then the
quarter-wave plate is rotated in steps of 22.5 degrees. The efficiency
obtained is about 0.7 for circular and 0.5 for linear polarisation. The
off-line reduction program (CIRLI)) takes these factors
into account and lists the correct values for linear and circular data
in the respective output files. The on-line reduction facility using
REDUCE,
however, assumes that the half-wave plate is in, and therefore the linear
polarisation values on the screen have to be multiplied by a factor of 2
to get the true values.