Image Polarimetry with ISIS
- Configuring the spectrograph for image polarimetry
- Configuring the telescope
- Aquaring objects and taking data
- Some useful numbers
1. Configuring the spectrograph for image polarimetry
ISIS is designed to work as a
spectrograph, but by replacing the grating
for a flat mirror and by taking away the spectrograph entrance slit we
create an imaging system. With some additional optical components ISIS
can
be made into an imaging polarimeter by making use of the existing
waveplates
that are an integral part of ISIS. To facilitate imaging polarimetric
observations, apart from changing the grating for a mirror, a wide
calcite
analyzer needs to be inserted, together a Dekker comb mask, and a
passband
filter. Finally, focus adjustments need to be made. This documents
describes
in detail how this is accomplished. It requires several modifications
of the setup that have to be done by the SA early in the afternoon.
1.1- Mount the Savart (calcite) analyzer (see Fig. 1) in the
multislit position of
the ISIS slit carriage. You may have to remove the image slicer.
This operation should only be done by the support
astronomer.
First put the dichroic out
of the beam for easy access:
SYS@taurus> bfold 0
Then protect the slit unit with the
dekker:
SYS@taurus> dekker 1
Then move the multislit unit to the
light path
SYS@taurus> mslit
Then unlock the slit door:
SYS@taurus> slit_door open
You can open the slit door located on
top of the red cryostat. Inside
you'll have access to the the slit unit, dicroic, filters and dekker.
Carefully slide out the image slicer (or whatever unit in the multislit
unit) and put it in its box and store it in the WHT
cabinet.
Take the analyzer unit out of it's box and carefully slide it in
and lock it's pin holder.
If you want to change filters (see next section) proceed in a similar
way with the filter units, just pull them out, put
the units back to its box and introduce the other ones in the unit. Be
careful to arrive to the end and that the filter units are locked.
Close the slit door by hitting it's lockers. Finally lock the slit door
in the control window:
SYS@taurus> slit_door close
Fig. 1 - The analyzer unit for image polarimetry. The unit should
be
mounted in the multislit mask unit of
ISIS. The calcite is below the field mask (in yellow).
1.2 - Mount the mirror at the
appropriate position in place of the
grating. There is a flat mirror mounted in a ratting mount. Just
replace the ratting with the flat mirror unit in the same way as
changing grating. This operation should only be done by the support
astronomer or
telescope operator (if trained).
SYS@taurus> setgrating red R1200R
to change the red grating to the
mirror one, any valid name of a ratting works fine with the mirror as
it will be used in order 0. The same for the blue arm.
Then
answer yes to access the grating door. The doors are then unlocked and
can be open manually. To release the gratings use the open/close
buttons. Lock the
grating door by hitting the lockers. Back at the console set the
central wavelength to 0.
SYS@taurus> cenwave red 0
The same if you plan to use the blue
arm.
Notice that
there is only one flat mirror unit, so imaging polarimetry can only be
done with one arm at a time.
1.3- Mount the filters (colour and/or neutral density) in the
modified
ISIS below slit filter slides (see Fig. 2). This operation should only
be done by the
support astronomer. The filter slides are then mounted in their
position below the ISIS slit unit following the same procedure used to
mount the analyzer unit (in fact is a good idea to change the analyzer
and filters together). Only two filters can be mounted on both
RED and BLUE arm, but remember that only one arm can be used.
Fig. 2 - Upper image is one of the two filter units of the RED arm.
Other two can be mounted in the blue
arm. Lower image is the R filter mounted in the filter unit. Notice
that black tape at the two sides of the
5x5cm square filters are needed to avoid stray light.
Normal filter slides holds two
filters, while the modified one holds
only one. Due to the positions of the filters in the modified
slides, it vignettes the beam in any of the two positions of the normal
filter unit. To overcome this problem and minimize vignetting, it is necessary
to insert the filter slide to a
position
4 which is between normal positions
2 and
3
(in position
1 filter slide is OUT). To use a filter mounted in
BFILTB slide type
SYS@taurus> bfiltb 4
You can use filters in the following slides: RFILTA, RFILTB, BFILTA,
BFILTB.
1.4 - Put the half-wave plate in
the beam in preparation for
observing by typing HWIN (HWOUT takes it out of the
beam again).
SYS@taurus> hwin
1.5 - Setup the CCD by setting
an
appropriate window and readout speed,
as advised by your support astronomer
SYS@taurus> window red 1
"[730:2148,1350:2150]"
SYS@taurus> window blue 1
"[700:2148,1600:2500]"
1.6 - Setup ISIS to take a lamp
exposure
SYS@taurus> agcomp
SYS@taurus> complamps w
SYS@taurus> compnd 2 (or
whatever neutral density filter is required).
This setup produced flats like the
one presented in Fig. 3
Fig. 3 - This is the smaller window that include all the field and the
bias columns of the CCD (left end of the image).
The double pattern of slits correspond to the ordinary and
extra-ordinary image of the comb mask as produced by
the calcite block.
1.7 - Focus the spectrograph
by
moving the collimator. First set the
collimator to a rough focus using the commands
SYS@taurus> rcoll 10000
(for the red arm)
SYS@taurus> bcoll 5500
(for the blue arm)
Then step in units of
500 collimator units about this nominal focus taking a GLANCE
exposure
at each setting until the edges of the dekker are at their sharpest (as
determined e.g. with imexam). The spectrograph will now be in focus.
1.8 - Check the polarization optics by putting a polaroid in the
beam
using the
SYS@taurus> mainfiltc MF-POL-PAR
Now you should check zero angles of the
half-wave plate by taking images around expected zero angle and looking at
the difference in the intensity between the ordinary and extraordinary
beams. The zero angles measured on August 3 2012 for the blue arm were 6,
51, 28.5 and 73.5 degrees instead of 0, 45, 22.5 and 67.5 degrees.
At zero angles around 6 and 51 degrees, the intensity difference between the
ordinary and extraordinary beam should be maximal, and at angles around 28.5 and 73.5
degrees the intensity difference should be minimal.
These angles must be introduced in the imaging polarimetry observer's script
at /home/whtobs/impolscript. Also, make sure that the camera name is updated
in the script.
Remember to
remove the
polaroid with the MAINFILTC
OUT command.
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2. Configuring the telescope
2.1 - Determine the rotator
centre on the direct-view mirror by typing AGCOMP and asking the
telescope operator to determine the rotator centre on this
mirror. Put a marker on the DS9 display. It is also a good idea
to write down the
TV coordinates of this point in case the marks are erased.
2.2 - Perform a calibrate about this
point on the direct-view
mirror (the telescope operator will do this for you).
2.3 - Crudely refocus the telescope by subtracting 0.60 mm
from its ISIS slit value (typically 97.95 mm) using the ICL command
FOCUS 97.35 (or whatever). This will enable you to take exposures and
view the star roughly in focus on the CCD and on the slit-viewing
mirror. To ensure that the star is also in focus on the direct-view
mirror, set TVFOCUS 15000 in TVSCALE 5 at ICL.
2.4 - Determine the position of the centre of the central
dekker on the direct-view mirror:
- switch to AGSLIT mode, point the telescope to a bright star and take
a glance image with ISIS.
- identify the star in the image, ask the TO to apply offsets until the
star appear centered on the central dekker
- switch to direct view using
AGCOMP and take an image with the direct view camera.
- Mark the position of the star on the
direct-view display and note the coordinates of
the point in case the marks are erased. This position will be
used to identify and roughly center the targets with the direct view
camera.
2.5 - To center the targets just use the direct view mode, identify the
target and the with the help of the TO, center it in the marker done in
2.4. Then remove the direct view mirror using AGSLIT, and take a glance
image in ISIS. Then do a final tuning by asking the TO to introduce the
offsets to center the target in the dekker mask.
2.6 - Refocus the telescope accurately using the usal focus
procedure. First, setup the spectrograph for observing on-sky by typing
AGSLIT and ROT SKY 0. Then acquire an (of order) 11th magnitude star
and centre it in the centre of the central slot of the dekker mask as
found in the previous step. Take a test exposure using GLANCE to
determine the optimum exposure time and use FOCUSRUN command in the
usual way.
2.6 - Refocus the autoguider by subtracting 2000 from its
ISIS slit value using the ICL command AUTOFOCUS 2000 (or whatever). You
are now ready to observe.
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3. Acquiring objects and taking data
3.1 - Acquire objects by typing
SYS@taurus> agcomp
and positioning the object of
interest on the desired slot, as determined by the marks drawn on the
TV screen.
Then type
SYS@taurus>
agslit
and the object should now fall directly onto the desired dekker slot.
Hence this acquisition procedure does not require one to move between
LSLIT and MSLIT, which would compromise the flat-fielding (since the
positions of the dekker mask edges as projected onto the detector would
continuously be changing).
3.2 - Update a target name and exposure time in /home/whtobs/impolscript and
take data by typing
SYS@taurus> source impolscript
This will take 4 images at zero angles determined earlier. Fig. 4
is an example of an image of a 0 polarization standard.
Fig. 4 - image of a 0 polarization standard star.
Back to the top
4.1 - There are 7 slots in
the dekker mask and hence 14 slot images on the CCD (due to the action
of the calcite block).
4.2 - Each slot is approximately 6 arcseconds wide and the
throw of the calcite is approximately 7 arcseconds. Hence there is a
gap of width 1 arcsecond between the o and e rays. The masked spaces
between the slots are approximately 11 arcseconds in width.
4.3 - The image of the calcite block obtained on the CCD is
approximately 132 by 111 arcseconds in size. Each pixel is
approximately 0.22 by 0.22 arcseconds in size with the REDPLUS, and 0.2
by 0.2 arcseconds with the EEV12.
4.4 - If the rotator position angle on the sky is zero, the
slots of the dekker are aligned approximately east-west.
