How to look at polarimetry data to see if the half-wave plate motion is OK

Its easy.  Just go to the directory with the data in it, and type

commandp `pwd` indexer

(note the funny backwards apostrophe - it is found at upper left of keybd)

Here is an example of how i used this on some data that Paolo took at the end
of March:
_________________
BEGIN PASTED TEXT FROM CONTROL SESSION
_________________

[viper@control 030328.sparo]$ ls
rcw57_013715  rcw57pol_015410  rcw57pol_020756  rcw57pol_022142  rcw57pol_023527  rcw57pol_024913
rcw57_014234  rcw57pol_020103  rcw57pol_021449  rcw57pol_022834  rcw57pol_024221
[viper@control 030328.sparo]$ cd  rcw57pol_022142
[viper@control rcw57pol_022142]$ ls
030328022231  frame_053310.jpg  frame_053958.jpg  hwp  indexer  pointing  sparo
[viper@control rcw57pol_022142]$  commandp `pwd` indexer
--------------------------------------
Device name: 'indexer'
Device has: '1' channel
Channel 0 Name: 'pos' 
Channel 0 Type: 'd' 
Scan  List Reads:
0 |
--------------------------------------
--------------------------------------
Device name: 'indexer'
Device has: '1' channel
Channel 0 Name: 'pos' 
Channel 0 Type: 'd' 
Scan  List Reads:
0 |
--------------------------------------
Response: 'NEWFILE'
Command: 'tell indexer move -159.953708'
Response: 'Indexer says: done with move'
Command: 'tell indexer move 10'
Response: 'Indexer says: done with move'
Command: 'tell indexer move 31.874607'
Response: 'Indexer says: done with move'
Command: 'tell indexer move 29.097079'
Response: 'Indexer says: done with move'
Command: 'tell indexer move 29.667416'
Response: 'Indexer says: done with move'
Command: 'tell indexer move 30.746067'
Response: 'Indexer says: done with move'
Command: 'tell indexer move 30.492584'
Response: 'Indexer says: done with move'
Command: 'tell indexer init'
Response: 'Indexer says: initialization complete'
Command: 'tell indexer tag NEWFILE'
[viper@control rcw57pol_022142]$ 

_______
END OF PASTED CONTROL SESSION
_______

Here is what is happening, in a nutshell:

since you start at 150 after a successful polarimetry file, you need to
first move back to zero, which is done by moving -160, then +10.  Then
there follow five 30 degree moves that take you through the whole cycle
of half-wave plate positions.  to summarize:

-160
+10
+30
+30
+30
+30
+30

if all the moves are within a degree or two (as they are in the above
example) then you are happy.  occasionally, the first 30 degree move will
be off by more than 2 degrees but i have not seen it be off by more 
than 3 degrees.


Here is what is happening, in full detail:

wave-plate starts at approx. 150 degrees.  so the first thing is a move
from that starting place to zero.  this is a move of approximately -150
degrees.  it is not exactly -150 degrees because the half-wave plate gets
measured before the move (by the hwp device).  in the above example, i
can infer that it started at 149.95 degrees. so the move would be -149.95
degrees so that we get back to zero, or as close as possible to zero.  
in reality this will be a move of -159.95 and then a move of +10.  (this
overshoot is to overcome the backlash that is typical in such systems.)

next move is from 0 to 30 
then 30 to 60
then 60 to 90
then 90 to 120
then 120 to 150

none of these 30 degree moves are exactly 30 degrees because in each case
the hwp device returns the measured wave-plate position before each move,
and then the indexer attempts to move the wave-plate from the measured
position (that will not be exactly right) to the desired postion (that
will be an exact multiple of 30 degrees).

it is good to keep an eye on these angles for any sign of trouble.

- Giles