WFPC2 Instrument Science Report 95-01
1
We outline several strategies for WFPC2 polarization observations, and summarize their various advantages and disadvantages. Apertures and useful fields of view are described for various rotations of the polarizer. Two apertures are found to be problematic: POLQN18 will be relocated elsewhere on
WF2, and we recommend against using POLQP15P. Finally, we summarize the Cycle 4 polarization calibration plan.
1. Introduction.
Imaging polarization observations may be carried out using a dedicated
‘‘polarizing quad” filter described in the WFPC2 Instrument Handbook (Version
2.0, Burrows et al. 1994). The polarization quad filter (POLQ) contains four elements, each of which illuminates one of the four CCD cameras. The polarization angles of these elements are oriented at 45 degrees to one another, and are effective as polarizers over the wavelength range from approximately 2500 to
7500 Angstroms.
Additional polarization angles can be obtained over limited fields of view by rotating the filter wheel to three off-nominal angles. These rotations are selected by requesting filters POLQP15, POLQN18, or POLQN33, which correspond to rotations of the filter wheel by +15, -18, and -33 degrees, respectively. These rotations are usually used in conjunction with special aperture names listed in Table 1.
Observers should also be aware that the FOC also has an imaging polarization capability.
1. Copies of this report may be obtained from the Science Support Division, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore MD 21218, by e-mail to etkins@stsci.edu, by anonymous FTP to stsci.edu in files instrument_news/WFPC2/Wfpc2_isr/ wfpc2_pol_*.ps, and by WWW at http://www.stsci.edu/ftp/instrument_news/WFPC2/ wfpc2_bib.html.
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Table 1: Standard Filter / Aperture Combinations for Polarizer Quad
POLQ
POLQ
POLQ
POLQ
POLQ
POLQ
POLQ
POLQ
POLQP15
POLQP15
POLQN18
POLQN18
POLQN18
POLQN33
Filter Name
Aperture
Name
PC1
WF2
WF3
WF4
PC1
WF2
WF3
WF4
POLQP15P
POLQP15W
POLQN18
POLQN18
POLQN18
POLQN33
POS TARG
(arcseconds)
+8,+8
-12,+12
-12,-12
+12,-12
-22,-89
WF4
PC1
WF2
WF2
WF2
WF3
WF2
PC1
WF2
WF3
WF4
PC1
WF2
WF3
CCD
Polarization
Angle
1
Clear
FOV diam.
135 + ORIENT none
0 + ORIENT
45 + ORIENT
41”
34”
90 + ORIENT 34”
135 + ORIENT 17”
0 + ORIENT
45 + ORIENT
59”
55”
90 + ORIENT
15 + ORIENT
55”
3”
15 + ORIENT 30”
117 + ORIENT none
117 + ORIENT 29”
72 + ORIENT 34”
102 + ORIENT 33”
Comments
2
Not recommended, CAL4
CAL4
CAL4
CAL4
Maximized FOV
Maximized FOV
Maximized FOV
Maximized FOV
Not recommended
CAL4
Old definition (<2/20/95)
New definition (>2/20/95)
Non-standard (>2/20/95)
CAL4
1
The position angle on the sky of the polarization E-vector is approx. “polarization angle”+135 degrees
.
2
CAL4 indicates that exposures of external polarization calibrator targets have been taken or are planned in this filter / aperture combination during Cycle 4. VISFLATS are planned for all filter / apertures combinations.
2. Strategies for Observation.
There are three basic strategies for observation with this filter:
1.) Place target sequentially in all for CCDs to obtain four polarization angles 45 degrees apart.
2.) Place target on only one CCD (WF2) and use partial rotations of the pol. quad filter to obtain four polarization angles at approximately 0, 15, 102 and 117 degrees.
3.) Place target on only one CCD and use the spacecraft roll angle (ORIENT special requirement) to vary the polarization angle.
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Each of these strategies has several variations, along with various advantages and disadvantages which we now describe in more detail:
Strategies:
1.a.) Place polarizing quad in nominal rotation (filter=POLQ) and obtain exposures of the target in each camera with the same ORIENT (spacecraft roll angle).
See Table 2.
Pros:
Optimal polarization angles -- two sets of orthogonal angles giving direct measurement of stokes parameters.
Widest field of view.
Simple scheduling.
Cons:
Only WFC resolution.
Need separate guide star acquisition for each image (unless target is very small and is carefully placed near pyramid apex just outside cross-talk region on each CCD; see Figure 1).
Uncertainties due to flat-fielding errors between cameras.
Uncertainties due to spatial undersampling and registering WFC resolution images.
Differences in optics of the four relays (slight focus differences).
Table 2: A Set of Filter / Aperture Combinations for Strategies 1.a. and 1.b.
Filter Name
POLQ
POLQ
POLQ
POLQ
Aperture
Name
PC1
WF2
WF3
WF4
POS
TARG
(arcsec.)
+8,+8
CCD
PC1
WF2
WF3
WF4
Polarization
Angle
135 + ORIENT 17”
0 + ORIENT
45 + ORIENT
90 + ORIENT
41”
34”
34”
Clear
FOV diam.
Comments
Maximized FOV
1.b.) Obtain exposures of the target in each camera with the same ORIENT using the polarizer in its nominal rotation (filter=POLQ). For each camera obtain an
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exposure with and without the polarizer. Due to light loss in polarizer, the exposures without the polarizer need only be ~1/3 as long as those through the polarizer, to obtain the same signal.
Pros:
Optimal polarization angles -- two sets of orthogonal angles giving direct measurement of stokes parameters.
Widest field of view.
Eliminates most flat-fielding errors.
Simple scheduling.
Cons:
Only WFC resolution.
Need separate guide star acquisition for each image (unless target is very small and is carefully placed near pyramid apex just outside cross-talk region on each CCD).
Uncertainties due to spatial undersampling and registering WFC resolution images.
Differences in optics of the four relays (slight focus differences).
Requires extra exposure in each camera (~33% more exposure time plus additional overheads).
2.a.) Place target on WF2 and use partial rotations of the pol-quad to obtain images with different polarization angles. Use filter/aperture combinations shown in Table 3. This, in effect, moves the target to different locations around WF2.
Table 3: A Set of Filter / Aperture Combinations for Strategy 2.a.
Filter Name
POLQ
POLQP15
POLQN18
POLQN33
Aperture
Name
WF2
POLQP15W
POLQN18
POLQN33
POS
TARG
(arcsec.)
CCD
WF2
WF2
WF2
WF2
Polarization
Angle
0 + ORIENT 41”
15 + ORIENT 30”
117 + ORIENT 29”
102 + ORIENT 33”
Clear
FOV diam.
Comments
New definition (>2/20/95)
Pros:
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Cons:
Flat-fielding errors between CCDs are eliminated.
Only a single guide star acquisition is needed.
Simple scheduling.
Only WFC resolution.
Uncertainties due to spatial undersampling and registering WFC resolution images.
Non-optimal partial rotation angles. If spacecraft ORIENT is held fixed, the relative angles are 0, 15, 102, and 117 degrees, respectively, for POLQ,
POLQP15, POLQN33, and POLQN18. These angles would lead to correlated uncertainties in measurement of the Stoke parameters.
Flat-fielding errors between different regions of WF2.
2.b.) Place target on WF2 and use partial rotations of the pol-quad together with different spacecraft ORIENTs to obtain images with optimal polarization angles.
For example, using POLQ, POLQN33, and POLQN18, with ORIENTS 0,-42, and
+3, respectively, will produce a set of images with polarization angles optimally spaced 60 degrees apart. This, in effect, moves the target to different regions of
WF2. See Table 4.
Table 4: A Set of Filter / Aperture / ORIENT Combinations for Strategy 2.b.
Filter Name
POLQ
POLQN33
POLQN18
Aperture
Name
WF2
POLQN33
POLQN18
POS
TARG
(arcsec.)
CCD
WF2
WF2
WF2
Polarization
Angle
ORIENT
ORIENT+60
ORIENT+120
41”
29”
33”
Clear
FOV diam.
Comments
ORIENT=X; X=any angle
ORIENT=X-42
ORIENT=X+3
Pros:
Flat-fielding errors between CCDs are eliminated.
Only a single guide star acquisition is needed.
Optimal set of polarization angles.
Cons:
Only WFC resolution.
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Uncertainties due to spatial undersampling and registering WFC resolution images.
Flat-fielding errors between different regions of WF2.
Additional guide star acquisitions required.
More complex scheduling. May need to observe at different times of year.
Some regions of celestial sphere may not be schedulable at all.
2.c.) There is a small region of WF2 where all four filter rotations can be used at a single location on the CCD. The target is placed here and partial rotations are used to obtain images with different polarization angles. This mostly eliminates concerns about flat-fielding errors. See Table 5. Another possible variation might follow strategy 2.b. and use different ORIENTs to obtain more optimal polarization angles.
Table 5: A Set of Filter / Aperture / POS TARG Combinations for Strategy 2.c.
Filter Name
POLQ
POLQP15
POLQN18
POLQN33
Aperture
Name
WF2-FIX
WF2-FIX
WF2-FIX
WF2-FIX
POS TARG
(arcseconds)
+12.5,-12.5
WF2
+12.5,-12.5
WF2
+12.5,-12.5
WF2
+12.5,-12.5
WF2
CCD
Polarization
Angle
0 + ORIENT 10”
15 + ORIENT 10”
117 + ORIENT 10”
102 + ORIENT 10”
Clear
FOV diam.
Comments
Non-standard aperture
Non-standard aperture
Non-standard aperture
Non-standard aperture
Pros:
Flat-fielding errors eliminated.
Only a single guide star acquisition is needed.
Simple scheduling.
Images aligned to first order.
Cons:
Only WFC resolution.
Non-optimal partial rotation angles. If spacecraft ORIENT is held fixed, the relative angles are 0, 15, 102, and 117 degrees, respectively, for POLQ,
POLQP15, POLQN33, and POLQN18. These angles would lead to correlated uncertainties in measurement of the Stoke parameters.
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Small field of view.
3.) Place target at desired location in camera (e.g. center of WF3) with pol-quad in nominal rotation. Use only spacecraft roll angle (ORIENT) to obtain images of target with different polarization angles. Since maximum permitted spacecraft roll angle is 30 degrees off-nominal, several visits at different times of year will be required. See Table 6.
One possible issue for this method is long-term variations in the camera throughputs. We note that bi-weekly photometric monitoring is done at the PC1 and WF3 aperture positions in nine filters spanning F160 to F814. In principle, this monitoring data could be used to remove long-term variations, thus making the PC1 and
WF3 apertures preferable for this strategy.
Table 6: A Set of Filter / Aperture / ORIENT Combinations for Strategy 3.
Filter Name
POLQ
POLQ
POLQ
Aperture
Name
WF3
WF3
WF3
POS TARG
(arcseconds)
CCD
-12,-12
-12,-12
-12,-12
WF3
WF3
WF3
Polarization
Angle
45 + ORIENT
45 + ORIENT
45 + ORIENT
55”
55”
55”
Clear
FOV diam.
Comments
ORIENT=X, X=any angle
ORIENT=X+60
ORIENT=X+120
Pros:
Cons:
Only means of getting PC resolution.
Minimizes flat-fielding errors.
Optimal set of polarization angles.
Wide field of view (for WFC).
Several visits required at different times of year with additional overheads.
Pixel sampling problems occur since image rotates on CCD.
Long-term variations may introduce errors (intrinsic variations, OTA focus changes, contamination changes at short wavelengths). Photometric monitoring data might be used to remove some changes for PC1 and WF3.
Scheduling may be difficult due to guide star problems. Some regions of celestial sphere may not be schedulable at all.
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3. Aperture Locations and Fields of View.
In this section we describe the aperture locations and useful fields of view for the polarizer quad in both its normal and rotated positions.
Figures 1 through 4 illustrate the WFPC2 focal plane with the polarizer filter at various rotations. The upper portion top of each figure shows the ratio of a mosaicked on-orbit F555W VISFLAT taken through the pol. quad to one taken without the pol. quad. The contrast is set to show only the unvignetted regions near full intensity. The lower portion of each figure is a diagram showing the mosaicked
WFPC2 focal plane with the pol. quad superposed and aperture positions indicated. The (V2,V3) coordinate system is also indicated, along with the directions of x and y POS TARG offsets from the aperture locations.
Two important features are the regions where ‘‘cross-talk’’ occurs between different facets of the quad, and regions which are vignetted by the edge of the filter.
The ‘‘cross-talk’’ region is shown by cross-hatching, and this is where the OTA beam passes through more than one quad of the filter. The diameter of the OTA beam is approximately 33 arcseconds at the polarizer, and hence targets within about 17 arcseconds of the pyramid apex will have their light passing through multiple facets of the quad. Images in the region will appear normal, but will have corrupted polarization properties and will not be calibratable.
A second effect is vignetting by the edge of the filter at its rotated positions.
The vignetted area is indicated by the stippled region in the figures. The inner edge of the vignetted region is denoted by a heavy line, the dashed line corresponds to 50% light loss, and the outer edge corresponds to 100% loss (e.g. see
Figure 2, etc.).
The shape, size, and approximate locations of the cross-talk and vignetted regions were determined from the camera design, but the exact placements in the field of view were determined from measurements on the VISFLAT ratio images.
This was done by making plots of intensity vs. CCD row and column in the VIS-
FLAT ratios, and then noting the locations where vignetting became apparent.
These diagrams should only be regarded as approximate (position uncertainties
2 arcseconds), due to dif ficulties in judging the onset of vignetting and cross-talk in the VISFLATS.
We caution that, as with all WFPC2 observations, the regions within 2 arcseconds of the pyramid vertices should be avoided. These regions are located along the mid-lines of the CCD overlaps in Figures 1 to 4. The OTA beam is spherically aberrated at the pyramid, and hence a single target in this region will appear in multiple cameras.
We also caution that we have ignored the rounded corners of the quad filter elements when constructing the diagrams below. Observations within ~5 arcseconds of the quad element corners should be avoided.
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Figure 1 illustrates the field of view for the polarizer quad rotated to its normal position (filter=POLQ). The top portion of the figure shows the ratio of a mosaicked on-orbit F555W VISFLAT taken through the pol. quad (data set
U2M70C01T, available from the HST archives) to one taken without the pol. quad
(U2JP0V03N). The ‘‘cross-talk’’ regions are just discernible in the top panel. As the lower panel shows, cross-talk will corrupt targets at the default PC1 aperture, as well as 3/4 of the PC field of view, and considerable regions of the WF CCDs. If the PC1 aperture is used, we recommend specifying POS TARG=+8,+8, as this will center the target in the clear field of view. For large targets (diameter>30 arcsec.) on the WF CCDs, a 12” POS TARG is recommend in both x and y, again to improve centering in the clear field of view. For example, POS TARG=-12,-12 would be used for the WF3 or WF3FIX apertures. Table 1 lists clear fields of view and recommended POS TARGs for many filter / aperture combinations.
We note that analysis of the VISFLAT in Figure 1 indicates the center of the pol. quad is not coincident with the pyramid apex. The pol. quad appears to be offset relative to the pyramid apex by about 2 arcseconds in the direction of WF3 and WF4. The lower panel of Figure 1 includes this effect.
Figure 2 shows the field of view for a positive 15 degree rotation of the filter (filter=POLQP15). (VISFLAT U2M70M02T was used.) The current definition of the
POLQP15P aperture lies perilously close to the cross-talk region. Given that the distance from the cross-talk region (~1.5 arcseconds) is comparable to the accuracy of our position measurements and the HST pointing uncertainty, use of this aperture is not recommended.
Figure 3 shows the field of view for a negative 18 degree rotation of the filter
(filter=POLQN18). (VISFLAT U2M70I02P was used.) The current definition of the
POLQN18 aperture lies in the cross-talk region and will thus have uncalibratable data. Around 2/20/1995 we will move this aperture to the position shown by the circle (V2=-32”,V3=-17”). This will provide clear field of view ~29” in diameter with polarization angle 117 degrees. We also note that polarization angle 72 degrees can be obtained with POS TARG=-22,-89 to WF3, although there are no immediate plans to calibrate this setting with observations of external targets.
The corner of PC1 near the pyramid apex in Figure 3 shows an anomalous brightening. This effect is real and this region of PC1 should probably be avoided, though it is already far from any of the standard apertures. This feature appears only rarely in science images. Its exact cause is unknown, but is probably due to a light leak or stray reflection in the PC re-imaging optics.
Figure 4 shows the field for a negative 33 degree rotation of the filter (filter=
POLQN33). (VISFLAT U2M70E02T was used.) At this large rotation the filter wheel 11 ‘‘clear’’ position can be seen entering the beam on PC1, WF2, and WF4, but this should have little impact on observations.
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4. The Polarization Calibration Proposal.
The Cycle 4 polarization calibration proposal, number 5574, is currently executing and should complete observations in April. Completed exposures may be accessed via STARVIEW. A detailed listing of the proposal can be obtained on the
WWW under STScI, Presto, HST Program Information (http://presto.stsci.edu// cgi-bin/get-proposal-info), 5574. The proposal performs the following observations:
1.) Obtains images of an unpolarized star (G191B2B) at a range in wavelengths through each segment of the polarizer quad with the spacecraft in the same roll angle. This will enable an estimate of the instrumental polarization introduced by the pick-off mirror and other internal differences in the relays.
2.) Obtains images of a polarized standard star (BD+64D106) at a range of wavelengths through each segment of the polarizer quad with the spacecraft at the same roll angle. This will enable an estimate of the effectiveness of the polarizers as a function of wavelength, and check the absolute orientation of the polarizer quad position angles.
3) Obtain images of a polarized standard target (R Mon) through each segment of the quad using each camera at several wavelengths to enable a more accurate estimate of the absolute orientation of the polarizers. This will also test the actual ability to image the polarization distribution of a highly polarized target.
4.) Obtain images of a polarized standard target (R Mon) on WF2 using the partial rotations of the polarizer (method 2.a. in section 2). This will enable a better estimate of the absolute orientations of the partially rotated polarizers. This will also test the actual ability to image the polarization distribution of a highly polarized target using this method.
5.) Internal flat-fields (VISFLATs) with the polarizing quad in its nominal and rotated positions.
For each target (items 1 - 4) exposures are made at the PC1, WF2, WF3,
WF4, POLQP15W, and POLQN33 apertures with a single ORIENT. In effect, both strategies 1.a. and 2.a described above are used. In addition, VISFLATS are taken at all four filter rotations (including POLQN18). Each target or flat is exposed in five filters: F336W, F410M, F555W, F675W, and F814W.
With these measurements we expect to quantify the attainable accuracy of polarization information as a function of target brightness and wavelength for each of the methods described above in section 2. We anticipate, given the expectation of substantial instrumental polarization from the large angle reflection of the pickoff mirror, to reach a limit in polarization accuracy at the few percent level.
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We acknowledge assistance from Sylvia Baggett, Colin Cox, and Paul Scowen in compiling this report.
5. References.
Detailed Specification for Optical Filter, Wide Field / Planetary Camera Flight
Equipment, 1990, JPL Part Specification CS514376 Rev. D.
Instrument Description and User Handbook for Wide-Field Planetary Camera
II, 1993, JPL publication 751-34, Rev. A, (SE-01), JPL D-11212.
Wide Field and Planetary Camera Handbook, Version 2.0, Burrows, et al.,
1994. Available from STScI Science Support Division.
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