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:
TO> bfold 0
Then protect the slit unit with the
dekker:
TO> dekker 1
Then move the multislit unit to the
light path
TO> mslit
Then unlock the slit door:
TO> 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:
TO> 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).
TO> 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.
TO> 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. Unfortunately, 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 and minimize vignetting, it is necessary
to step the filter slide to a
position between normal positions 1 and 2. At
present this operation should be
done by the telescope operator and/or the support astronomer, since it
involves using engineering control
rather than the ICL. The support astronomer should also be able to
advise
on which
position each slide needs to be driven to. If, for whatever reason, it
is
necessary for the observer to move the slide, the following list of
commands
should be used (carefully) at the engineering
console.
- Initialize the filter slide by typing TO> isis_init -m
REDFILTERA (or REDFILTERB, or BLUEFILTERA, or BLUEFILTERB). This sets
the step
count of the corresponding filter unit to zero.
- Goto the engineering desk opposite the observing console and set
the engineering control selector switch to CASS INST.
- On the engineering terminal (marked TERMINAL A), type BFB TT (if
you want to move the BFILTB slide, or BFA, RFA, RFB in case of the
other mechanisms). For other slides, consult the list
of pneumonics in the table below.
- Now type UNSEAL. Note that the characters are not echoed to the
terminal.
- Type three question marks in a row (???). The last column of the
resulting output shows the position of the slide in encoder units. This
should be zero at this stage.
- Type, for example, 5100. 4 INDEX (be carefull that RFA and BFA
use 3 instead of 4, see table below). This will move the BFILTB slide
(SMDM number 4 - see the table below) 5100 encoder units from its
current position to approximately three-quarters of the way between
filter positions 1 and 2. This is the optimum position for this slide -
for other filter slides consult the table below for the number of
encoder units to move.
- Type ??? and check the result in the last column. It should now
say 5100 (or whatever).
- Type 4MS to return to normal engineering control (at which you
can then type BFB SEE at any time to check the position of the
BFILFB filter slide, for example). Note that the ICL mimic will
(wrongly) show that the filter is out of the beam.
SLIDE PNEUMONIC SMDM IMPOL
POSITION
----------------------------------------
RFILTA RFA
3 5500
RFILTB RFB
4 5500
BFILTA BFA
3 ?
BFILTB BFB
4 5100
NOTE: when using the RED arm this is
not needed as the position in the ICL rfilta 3 or rfiltb 3 produce
almost the same vignetting, and the filters can then be easily
moved. The procedure when using the BLUE arm needs to be reviewed
in a future test.
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).
TO> hwin
1.5 - Setup the CCD by setting
an
appropriate window and readout speed,
as advised by your support astronomer
TO> window red 1
"[730:2148,1350:2150]"
1.6 - Setup ISIS to take a lamp
exposure
TO> agcomp
TO> complamps w
TO> compfilta 2 (or
whatever neutral density 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
TO> rcoll 10000
(for the red arm)
TO> bcoll 30000
(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
TO> mainfiltc pol-par
take a series of images at 0, 45,
22.5 and 67.5 degrees using the lpol command
TO>lpol red <exp time>
check that the peak counts have now
switched beams between 0 and 45, and 22.5 and 67.5.
Remember to
remove the
polaroid with the MAINFILTC
OUT
Back to the top
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.
Back to the top
3. Acquiring objects and taking data
3.1 - Acquire objects by typing
TO> agcomp
and positioning the object of
interest on the desired slot, as determined by the marks drawn on the
TV screen.
Then type
TO>
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 - Take data by typing
TO>
lpol red <exp time> "comment"
or just lpol and follow the prompts. The procedure will take four
exposures at half-wave plate angles 0, 45, 22.5 and 67.5 degrees. Fig.
4 is an example of an image of a 0 polarization standard. If, for
whatever reason, you decide to abort LPOL, type ctrl-c.
Fig. 4 - image of a 0 polarization standard obtained with LPOL
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.
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ISIS Instrument Specialist
Last modified: 24 June 2011 |
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