EUROPEAN SOUTHERN OBSERVATORY
La Silla - Science Operation Department
Title of the page
|Version: 1.0 2004-11-05 Paul Le Saux, Gaspare Lo Curto|
This study reports on tests performed with the TIMMI2 template TIMMI2_pol_tec_minAngle
which allows, by using the polarized filter N-filter+pol
and the rotating polarimeter INS.ROT1.POS
, under the light of the SIMULATED_SKY
of INS.OPTI1 function to determine a "zero" angle of the polarimeter,
i.e. when it is orthogonal to polarized filter.
Moreover the MIDAS-PCO interface had to be generated in the TIMMI2 instrument machine (wtimmi) in order to allow analysis of the data directly by the template.
A new entry had to be put in the modelist table to define the working paramters for the "N+pol" unit. After some tests the paramters were good enough to keep maxima and minima well inside <sensitivity - saturation> range .
Some of the first results presented below refer to the maximum of the polarization curve, all the other ones to the minimum. This is because the different tests were performed in different phases of the optimization of the paramters (DIT). This has however no influence on the conclusions reported.
The template fields are:
INS.POL.LOWANGLE.LABEL "Polarimeter test lower angle"; # Label used in P2PP INS.POL.UPPANGLE.LABEL "Polarimeter test upper angle"; # Label used in P2PP INS.POL.STEPNUMBER.LABEL "Polarimeter test step number"; # Label used in P2PP INS.POL.BACKLASH.LABEL "Polarimeter angle hysteresis correction"; # Label used in P2PP INS.POL.START.LABEL "Polarimeter start angle between to tests"; # Label used in P2PP SEQ.PASS.NUMBER.LABEL "number of pass along the lowerangle upperangle interval"; # Label used in P2PP SEQ.LOOP.NUMBER.LABEL "number of times the test is executed"; # Label used in P2PP SEQ.INIT.FLAG.LABEL "reinitialize ROT1 before next iteration"; # Label used in P2PP SEQ.INIT.PERIOD.LABEL "number of iterations bewteen two reinitializations"; # Label used in P2PP SEQ.INTERLOOP.DELAY.LABEL "number of seconds bewteen two test iterations"; # Label used in P2PP
Therefore the template allows to measure the behaviour of ROT1 rotation
or initialization according to the
value FALSE or TRUE of
-rw-r----- 1 plesaux users
52505 Nov 7 09:56 25-41min32.8hyst0.ps
-rw-r----- 1 plesaux users 52698 Nov 7 09:56 25-41min30.6hyst-10.ps corrected by -10 degrees
Two things can be noticed in the two plots above:
1) the curves are different in the two cases;
2) the drift of the position of the minimum is minor when the backlash is applied.
It is not clear however whether the difference between the two cases (with and without backlash) is significative.
The three previous examples plus another one not registered may lead
to think that maxima can fall randomly
in two areas : around 60 or around 120 degrees.
Later on it was not possible to find again the 60 deg. maximum thru
6 consecutive initializations of the polarimeter
rotation wheel or carriage, but it is clear that between two consecutive wheel restarts
the minimun position moved up to more than 10 degrees.
Out of suspicion we decided to iterate the test with a delay between
each other iteration :
here are the curves representing the drift of minimum position angle
On the good side it has to be said that initializing the polarizer carriage,
or moving in and out does not seem to add any more troubles to the ones
The scientific usefulness of the polarizer in this conditions is at best doubtful. Time and manpower must be found for it before the next polarimetric run. There seem not to be any polarimetric run until the end of period 74.