3 Polarimetric calibration

For the purpose of calibration, we observed five different polarized and one unpolarized standard stars taken from a list by Sekowski (1975). The results for the standard stars both "polarized'' and "unpolarized'' are reproduced in Tables 2 and 3 respectively. For the standard polarized stars, the observed polarization (p), error in polarization ($E_{\rm p}$) and position angle ($\theta$) are listed in Cols. 7, 8 and 9 of Table 2. The error in position angle (PA) i.e. $E_{\theta}$ though not listed in the table, can be easily calculated by the relation (Serkowski 1974):

$$% E_{\theta} \simeq {{E_{\rm p}} \over{p}} \times 28.65 ^{\circ} %$$ (2)

for $E_{\rm p} \ll p$. The Cols. 3, 4 and 5 represent the maximum polarization value, corresponding position angle and wavelength as listed by Serkowski (1975). A small discrepancy within the two sets of values can be due to the fact that, Serkowski (1975) values are at a particular wavelength where polarization becomes maximum. Whereas our observations were carried out either in white light (W) or V filter ( $\lambda= 0.55~ \mu$m). Since the interstellar polarization varies with wavelength, what we should observe in the W filter is an observed value of p which is somewhat lower than $p_{\rm max}$. Further, if $\theta$ also slightly varies with wavelength, the observed $\theta$ will be different from $\theta _{\rm max}$. However, this seems to be more unlikely keeping in mind the constancy of $\theta$ for interstellar polarization (Coyne 1975).

At this stage without going into the details of wavelength dependence of interstellar polarization, we conclude from Table 2. that, the observed polarization values are matching with those enlisted by Serkowski (1975) within errors. However, the PA ($\theta$) values show an average offset $\rm d \theta$of $-6.9^{\circ}$ for three nights in December 1997 and $-10.3^{\circ}$ for April 01, 98. The instrument was remounted on Apr. 02, 1998 due to technical reasons and we observe a different $\rm d \theta$ of $-7.1^{\circ}$ on April 02, 1998.

Now from our observations for the unpolarized standard star HD 114710 (in Table 3), we can conclude that our instrumental polarization is less than 0.05%.

 

 

Table 3: Obervation of Unpolarized Standard star (Ref. 1 indicates Serkowski, 1975 and Ref. 2 indicates our present observations)

Date	UT	star	$p_{\rm max}$	$E_{\rm p}$	p	$E_{\rm p}$	Filter
		HD #	(%)	(%)	(%)	(%)
			(Ref. 1)		(Ref. 2)
Apr. 01, 98	18:45	114710	0.018	0.014	0.020	0.021	V