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Calibrations required for FOCAS polarimetry
Observers should take the following data for a full calibration of
their polarimetric data. Not all data need to be taken for every object
observed, although it is recommended that observers confirm those
polarimetric properties of the instrument at least once in an
observation run. Especially, the instrumental polarization should
be checked at the same CCD position as the target.
1. Observation of polarimetric standard stars
- Unpolarized standard star for linear polarimetry
- FOCAS has an almost stable instrumental polarization of order
p=0.05% at any position within field of view (FOV). However,
the level of instrumental polarization highly depends on the
position in FOV. Also, it is slightly wavelength-dependent.
The degree of polarization, p_ins, and its position angle, PA_ins,
are expressed as p_ins ~ 1.2x10^(-9) r^4 (%) and PA_ins ~ PA_FOV
at B, V and R-bands, where r is the radial distance from
the center of FOV in arcsec and PA_CCD is the position angle
of the position in FOV. The instrumental polarization pattern
near V band is shown in Fig. 4. For the correction of it, you
should obtain polarimetric data of unpolarized standard stars
at the same slit position where you obtain those of your target(s)
during your observation run. For imaging polarimetry of a diffuse
object, you should take data of unpolarized stars at many position
in FOV and construct an instrumental polarization map at each
band used for target observation.
Figure 4. Sample of instrumental polarization pattern
- Strongly polarized standard star for linear polarimetry
- To know the zero point of the position angle of polarization in the
celestial plane, the observer should obtain data of strongly polarized
standard stars.
- Standard stars for circular polarimetry
- Although the FOCAS members are not fully familiar
with circular polarimetry, it is recognized that
no standard star observation is necessary for
circular polarimetry with 0.2-0.3 % accuracy.
If you expect a circular polarimetry with higher
precision, you are asked for arranging your own
calibration method. We welcome any your comment.
Table 1. Commonly Used Standard Stars for FOCAS Linear Polarimetry
Strongly Polarized Standard Stars (V-band data)
Star |
RA2000 | DEC2000 | Mag. |
Sp. |
Pol. (%) |
PA(deg) |
Ref. |
Comment |
BD+64d106 | 00 57 36.7 | +64 51 27 | 10.3 | B1V | 5.69+-0.04 | 96.6+-0.2 | Sch92 | |
HD 7927 | 01 20 04.9 | +58 13 54 | 5.0 | F0Ia | 3.32+-0.04 | 92.1+-0.2 | Wol96 | |
BD+59d389 | 02 02 42.1 | +60 15 27 | 9.1 | F0Ib | 6.70+-0.2 | 98.1+-0.1 | Sch92 | |
HD 19820 | 03 14 05.4 | +59 33 48 | 7.1 | O9IV | 4.82+-0.03 | 115.4+-0.3 | Wol96 | |
HD 25443 | 04 06 08.1 | +62 06 07 | 6.8 | B0III | 5.15+-0.03 | 135.1+-0.2 | Wol96 | |
HD 251204 | 06 05 05.7 | +23 23 39 | 10.3 | B0IV | 4.04+-0.07 | 147. | Tur90 | |
HD 43384 | 06 16 58.7 | +23 44 27 | 6.3 | B3Ib | 2.94+-0.04 | 169.8+-0.7 | Hsu82 | |
HD 154445 | 17 05 32.2 | -00 53 32 | 5.6 | B1V | 3.67+-0.05 | 88.6+-0.7 | Wol96 | |
HD 155197 | 17 10 15.6 | -04 50 03 | 9.2 | A0 | 4.38+-0.03 | 103.2 | Tur90 | |
HD 161056 | 17 43 47.0 | -07 04 46 | 6.3 | B1.5V | 4.00+-0.01 | 66.3+-0.3 | Wol96 | |
Hiltner 960 | 20 23 28.4 | +39 20 56 | 10.6 | B0V | 5.66+-0.02 | 54.8+-0.1 | Sch92 | V~14 companion star at ~6" NW |
VI Cyg #12 | 20 32 40.9 | +41 14 26 | 11.5 | B5Ia | 8.95+-0.09 | 115.0+-0.3 | Sch92 | |
HD 204827 | 21 28 57.7 | +58 44 24 | 7.9 | B0V | 5.34+-0.02 | 58.7+-0.4 | Wol96 | |
Unpolarized Standard Stars (V-band data, except for Tur90)
Star |
RA2000 | DEC2000 | Mag. |
Sp. |
Pol. (%) |
PA(deg) |
Ref. |
Comment |
Beta Cas | 00 09 10.7 | +59 08 59 | 2.3 | F2III | 0.04+-0.02 | 72.5 | Sch92 | |
HD 12021 | 01 57 56.1 | -02 05 58 | 8.9 | B7 | 0.08+-0.02 | 160.1 | Sch92 | |
HD 14069 | 02 16 45,2 | +07 41 11 | 9.0 | A0 | 0.02+-0.02 | 156.6 | Sch92 | |
HD 21447 | 03 30 00.2 | +55 27 07 | 5.1 | A1IV | 0.05+-0.02 | 171.5 | Sch92 | |
G191B2B | 05 05 30.6 | +52 49 54 | 11.8 | DA1 | 0.06+-0.04 | 147.7 | Sch92 | |
HD 94851 | 10 56 44.2 | -20 39 52 | 9.2 | B9 | 0.06+-0.02(B) | --- | Tur90 | |
GD 319 | 12 50 04.5 | +55 06 03 | 12.3 | DA | 0.09+-0.09 | 140.2 | Sch90 | |
Gamma Boo | 14 32 04.7 | +38 18 30 | 3.0 | A7III | 0.07+-0.02 | 21.3 | Sch92 | |
HD 154892 | 17 07 41.4 | +15 12 38 | 8.0 | F8V | 0.05+-0.03(B) | --- | Tur90 | |
BD+32d3739 | 20 12 02.1 | +32 47 44 | 9.3 | A6V | 0.03+-0.02 | 35.8 | Sch92 | |
BD+28D4211 | 21 51 11.1 | +28 51 52 | 10.5 | oP | 0.05+-0.03 | 54.2 | sCH92 | V~15 companion star at ~4" N |
HD 212311 | 22 21 58.6 | +56 31 53 | 8.1 | A0V | 0.03+-0.02 | 51.0 | Sch92 | |
Zeta Peg | 22 41 27.7 | +10 49 53 | 3.4 | B8III | 0.05+-0.02 | 40.0 | Sch92 | |
(*) Note that the comments for the presence of nearby companion star are not complete.
References
Hsu82: Hsu & Breger (1982; ApJ, 262, 732),
Sch92: Schmidt, Elston, & Lupie, (1992; AJ, 104, 1563),
Tur90: Turnshek, et al. (1990; AJ, 99, 1243), and
Wol96: Wolff, Nordsieck, & Nook (1996; AJ, 111, 856).
2. Observation of dome flats through fully-polarizing filter
From an observation through fully-polarizing filter, we obtain (1) the
wavelength dependence of the equivalent optical axis of the multi-layer
half-wave plate, and (2) the depolarization factor of FOCAS. Although the
latter (2) should be negligible (dP/P < 0.05) for most cases, the former
(1) should be corrected. Although we will provide a correction table for
(1), those who need higher accuracy and reliability should take dome flats
with fully-polarizing filters. FOCAS has two such filters, one for the
optical wavelength region and one for the near IR region.
Figure 5. Spectropolarimetry sample of dome-flat image taken through
fully-polarizing filters.
3. Distortion
In the case of long-slit spectropolarimetry or imaging polarimetry for a
diffuse (extended) object, the observer should know the distortion pattern
in the 2D-spectral or field images. We will provide the distortion pattern
table (ready to use for IRAF/geomap). Those who need higher
accuracy for the distortion correction, they need to obtain the data with
the special masks for the distortion measurement using arc lamps and/or
dome flat lamps. If the object is point-like (i.e., the observer does not
need to know the spatial information on the image), one does not have to
obtain the distortion calibration image.
4. Arc lamp for wavelength calibration
This should be taken in the same manner as for non-polarimetric spectroscopy.
5. Normal dome flats for flatfield
These should be taken in the same manner as for non-polarimetric
spectroscopy/imaging observations. However, you must obtain
flat-field images at each wave plate position (i.e., PA=0.0,
45.0, 22.5, and 67.5 for linear polarimetry and PA=45.0 and
135.0 for circular polarimetry) for each FOCAS optics configuration
to obtain flat-field image of synthetic un-polarized light
(after combining the set of images).
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