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6. Commissioning of IMPOL

The first phase of commissioning involved laboratory tests to determine various parameters like linearity, gain and readnoise of the detector, vignetting in the optics etc. Measurements were also made with a 3000 K filament source to estimate depolarization and instrumental polarization in tex2html_wrap_inline1527 bands, and a wide band defined by a 3 mm thick KG3 glass. A 100% polarized beam showed that the depolarization is about 0.4% in the B and V bands, less than 1% in the R band, and less than 6% in the broad band. There was no discernible rotation of the position angle between these bands. For R and the wideband, the accuracy of measurements is limited by the performance of the polarizer used. Measurements made with a Lyot depolarizer introduced in front of the artificial star showed that the instrumental polarization is about 0.03% in the B-band and less than 0.06% with the wideband filter.

The second stage of tests were conducted when the instrument saw the first light in February 1996, at the tex2html_wrap1219 1.2 m Gurushikhar Infra-Red Telescope facility on Mt. Abu, Rajasthan, operated by the Physical Research Laboratory at Ahmedabad, India. Table 2 (click here) contains the results of some of the observations of standard stars in the V-band. Column 1 of the table gives the identification number of the star. Columns 2-5 contain the measured values and tex2html_wrap_inline1557 errors of the fractional polarization p and the position angle tex2html_wrap_inline1257. Column 6 gives the error tex2html_wrap_inline1331 based on photon statistics alone, derived as illustrated in Sect. 3. Columns 7 and 8 list the published values of p and tex2html_wrap_inline1257.

The first two stars in the table are polarized standards while the last two are unpolarized ones. The four measurements of star HD 39587 are made at different corners of the CCD frame in order to verify the uniformity of response. Comparing the measured and published results for the polarized stars we see that there is no indication of depolarization.


Star p tex2html_wrap_inline1331 tex2html_wrap_inline1257 tex2html_wrap_inline1575 tex2html_wrap_inline1577 p0 tex2html_wrap_inline1581
(1) (2) (3) (4) (5) (6) (7) (8)
HD % % tex2html_wrap1285 tex2html_wrap1285 % % tex2html_wrap1285
43384 3.05 0.04 172 0.3 0.03 3.0 170
154445 3.65 0.03 90.5 0.2 0.02 3.7 90
102870 0.03 0.01 64.4 7.8 0.02 0.017 162
39587 0.03 0.03 140 27.5 0.02 0.013 20
0.05 0.02 159 12.7 0.02
0.09 0.02 34 6.7 0.02
0.05 0.02 167 11.3 0.02

Table 2: Observations of standard stars


Neither is there any rotation of the position angle beyond a couple of degrees. From Col. 3 and Col. 6 for the unpolarized stars, we see that the error in the measured polarization is comparable with that expected on the basis of photon statistics alone.

In Fig. 4 (click here), the values of tex2html_wrap_inline1589 are plotted against tex2html_wrap_inline1591 for all the unpolarized standards observed in the V-band. The extremely low correlation coefficient between q and u, is indicative of the low instrumental polarization. The average values of q and u, give a value of polarization tex2html_wrap_inline1603. These observations show that for accuracies tex2html_wrap_inline16050.05% in the measurement of polarization p, the performance is still limited by photon noise and the instrument polarization floor has not yet reached.

Figure 4: Normalized Stoke's parameter q is plotted against u for a number of unpolarized standard stars. The correlation coefficient of the points is about 0.06 and the average values of q and u give a value of tex2html_wrap_inline1603

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