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Notes on the steps
I use in the polarimetric analyses of Mimir data. Interested users are
free to download software and follow these instructions (or not). Caveat
emptor: the programs are not guaranteed to be perfect or even correct.
Users must develop their own tests and confidence metrics.
Last update: 1/9/2006
Analysis of polarimetric
data is currently different for standards and target stars, as the standards
are so bright that defocussed observerations have been necessary to date.
Hopefully this will be changed as a result of our recent tuning of the
detector arrays wells to allow brighter signals (more ADUs).
Standard
Star Processing
Instrumental
Calibration & Polarization Determination and Findings
Target
Star (and field) Processing
Link
to the Software Page
Standard Star Processing
- Step_0:
Create a new directory, typically called "Step_13_Pol_Standards"
to hold the analyses
- Create subdirectories
for the major steps:
- POL_Step_1_Images
- POL_Step_2_Input_Lists
- POL_Step_3_Photometry
- POL_Step_4_Polarimetry
- Step_1:
In the "Images" directory, collect all processed images
(linearity, dark, flat corrected) into subdirectories named after
the night designations (e.g., "0519")
- Step_2:
With reference to the observing logs, run ls.pro
for each group of observations of a standard star, collecting the
input file names from the "Images" directories and putting
the results into the "Input_Lists" directory, with names
like "0519_E25a.dat" to signify the data come from night
0519, object Elias 25, and the first ("a") set of polarimetric
observations.
- Note, for
the F/5 camera setting, two or more standard stars may appear
in the same images. For those cases, duplicate the output ".dat"
file from the above step, giving the new file a name reflecting
the other standard star(s) in the same images.
- Examples
include: C14+C4+C21; C7+C24; C10+C11; C22+C24; E25+E22+E14
- Step_3:
For each input file in the "Input_Lists" directory, run
defocus_star_phot.pro
to produce an output photometry file in the "Photometry"
directory. The program is set up to make the ouput name reflect the
input file name (e.g., "0519_E25a_phot.dat") by default.
- The program
will start by displaying the first image in the input file and
ask the user to select the center of the star of interest
- A zoomed
piece of the image will then appear and the user is again asked
to select the center of the stellar image. The program will then
use this rough center as the starting point to identify the correct
star in all of the images, establish the right aperture to use
to obtain good S/N and will proceed to performing aperture photometry
for a number of apertures.
- Step_4:
For each file in "Photometry" directory, run pol_std_analysis.pro
to produce one summary polarimetry file for each input file. By default,
the output file names reflect the input file (e.g., "0519_E25a_pol.dat").
This program will use the photometry file to compute the instrumental
U and Q (and P and PA) and uncertainties using the Qi-Ui analysis
method, the folded-flux Q-U method, and the folded-flux sinusoid fitting
method. Only the results from the Qi-Ui method are stored in the output
files.
Instrumental Calibration
& Polarization Determination and Findings
The polarization
results from the standard star observations must be regressed against
the known values to be able to ascertain the polarization efficiency,
the instrumental Stokes Q and U contributions, and the HWP offset angle.
A complete analysis includes fitting these across the field of view
of the instrument to determine field dependencies of these quantities.
- Make a directory
to save results to, say "POL_Step_5_Calibration"
- Build
summary text-type data files for the Whittet et al. (1992) standards
that were observed. and the set of standard star observations performed.
- An example
of this "whittet_short_names.dat"
- Build a file
containing the pathnames to all the summary polarimetry files from
Step_4 above using ls.pro
- An of this
"Mimir_observations_file_list.dat"
- Use the routine
pol_matrix_calibration.pro
to regress the observations against the standard values. This program
can operate in several different modes and will produce different
fit orders. It is probably best to read the code to see what it is
trying to do.
- Outputs consist
of the polarization efficiency (91.2+/-2.5%), the polarimeter
offset angle (33.7+/-0.8 deg) and images of the instrumental
polarization and its position angle. These may be downloaded by
clicking the representative images below.
 |
 |
| Image
of the instrumental polarization values across the Mimir FOV
for the 10x10 arcmin field of view. Central contour corresponds
to 0.20% and the contour step increases from there in steps
of 0.20%. Click on the image to download the FITS file. |
| Image
of the position angle of the instrumental polarization across
the Mimir FOV for the 10x10 arcmin field of view. Central contour
corresponds to a PA of -10 deg and contours steps increase in
steps of 2 deg. Click on the image to download the FITS file. |
- Alternatively,
if one does not wish to correct for the variation of the instrumental
polarization across the chip, the "zero order fits" can
be used:
Efficiency:
89.7+/-2.0%; Offset Angle: 32.5+/-0.6 deg; Pinst: 0.35+/-0.02%;
PA of Pinst: 4.8+/-2.0
Target Star (and
field) Processing
Analysis of target
star or star fields consists of organizing the images, finding and removing
any remaining sky background variations across the field of view, coadding
into single HWP images, star finding, photometry, then grouping photometric
results by HWP angle for each star, and determination of polarizations,
including correcting for all instrumental effects.
- Step_0:
Create a new directory, typically called "Step_14_Target_Polarimetry"
- Create subdirectories
for the major steps:
- TPOL_Step_1_Directory_Lists
- TPOL_Step_2_Coadd_Lists
- (note that an "Images" directory is not made,
as the images are assumed to reside elsewhere)
- TPOL_Step_3_Combined_Images
- TPOL_Step_4_Find_Stars
- TPOL_Step_5_Photometry
- TPOL_Step_6_Polarimetry
- TPOL_Step_7_Combine_Fields
- Step_1:
Using the ls.pro program, make
a complete list of all the target polarization images for the observing
run and put the summary file in the "Directory Lists" directory,
with a name like "full_list.dat". To do this, create one
(or more) ls-type summary file(s) for each night, then merge the files
into the one file.
- Step_2:
Use the prepare_polar_science_for_coadd.pro
routine on the full list to create summary files for each object name
and each HWP angle. Direct the output to the "Coadd_Lists"
subdirectory
- Create subdirectories
in "Coadd_Lists" for each major project or object studied
(e.g., "L204" or "GPIPS") and move all files
for each project into their own subdirectory.
- Create
a master list of all of the file lists of FITS images for each
object, using the ls.pro
routine. As input, select all of the list files for an object
in its "Coadd_Lists" subdirectory. Direct the output
to the same directory with a name reflecting the object (e.g.,
"L204_listfile_list.dat").
- Step_3:
Combine all images for each object/field with the same HWP angle (dithered
images). This step consists of several sub-steps: (1) determination
of the mean sky distributions across the FOV for each object, to allow
performing a second order correction to cleans up structure left by
the flat field corrections; (2) determination of the pixel shift offsets
between the images; and (3) coadding the images, using the pixel shift
information. Because of the large numbers of images involved, this
is best done using batch-type processing.
- Create subdirectories
under "Combined_Images" for each major project or object,
as for Step_2
- For each
object, use the ls_to_batch.pro
routine to set up a batch file. The program to be listed in the
batch file is tp_image_combine.pro
and the input list of files are those generated in Step_2, part
2. Name the resulting batch file something like "L204_batch.dat"
and leave it in the "Combined_Images" subdirectory for
that object.
- Set up a
journal file:
- type
"jfile = dialog_pickfile()" in the IDL dialog window
- Go to
the "Combined_Images" subdirectory for this object,
and create a file with a name like "20060104_L204_batch_journal.dat"
- type
"journal, jfile" in the IDL window
- Run batch.pro
on this batch file.
- Close out
the journal file: type "journal" in the IDL window
- Examine the
journal file for any problems arising during batch processing
- Repeat Steps
2-7 for all objects/projects
- Step_4:
- TBD
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