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2. The instrument

The following set of scientific and technical guidelines were adopted as basis for design of the instrument. (i) It should be capable of observing faint extended objects like reflection nebulae, accretion disks, dusty active galaxies etc., with accuracy limited by photon noise and resolution limited by seeing. This demands a typical field of view of a few arcminutes, an optics which is well-matched with the telescope to minimize loss of light and a detector of sufficient sensitivity. Also, the effects of atmospheric scintillation should be eliminated and instrumental polarization minimized so that such measurements of low level polarization could be made. (ii) Since the objects of interest are very often quite faint, the instrument should have an acquisition and guidance (A&G) system which allows pointing the telescope with an accuracy of a few arcseconds and tracking to better than 1tex2html_wrap1239 over long periods. (iii) Observations should be possible in various wavelength bands in the optical and near-IR regions. (iv) The entire instrument consisting of the optics, A&G unit, associated electronics etc. should be a self-contained unit which can be easily mounted on the telescope, with only electrically isolated communication links to the computers for instrument control and data acquisition. (v) The cost of the instrument should be minimized by using standard optical and electronic components and by avoiding over-specification as far as possible.

2.1. Principle of the instrument

Figure 1 (click here) is a schematic representation of the optical arrangement of IMPOL. The basic idea behind this arrangement is to use a Wollaston prism with its axis normal to the optical axis of the system, as the analyzer to convert the linear polarization in the incoming light into relative intensity of two orthogonally polarized beams (the ordinary and the extraordinary), separated by a small angle of 0.5tex2html_wrap1285. This measurement is sufficient to define one of the Stoke's parameters Q or U. A half-wave plate with its fast-axis normal to the optical axis of the system, is kept before the Wollaston prism on a rotatable mounting. When the half-wave plate is rotated through an angle tex2html_wrap_inline1247, the plane of polarization rotates through an angle tex2html_wrap_inline1249. At this new position of the half-wave plate another measurement on the orthogonally polarized beams can be made to determine the second Stoke's parameter as well. It is easily seen that, for this arrangement the intensities of the extraordinary and ordinary beams tex2html_wrap_inline1251 are given by
where tex2html_wrap_inline1253 are the unpolarized and polarized intensities respectively; tex2html_wrap_inline1255 are the position angles of the polarization vector and the half-wave plate fast-axis respectively, with reference to the axis of the Wollaston prism. Since the angle tex2html_wrap_inline1257 is conventionally measured with respect to the celestial north-south axis (tex2html_wrap_inline1259 towards north celestial pole and increasing counter-clockwise), the axis of the Wollaston prism is kept aligned to it.

Figure 1: Schematic of the IMPOL optical layout. The field lens and half-wave plate are from Karl Lambrecht (part Nos. 322305 and WPAC 2-22-BB400/700 respectively). The Wollaston prism is from Bernard Halle (PWQ 30.30) and the camera lens is Nikkor AF Telephoto 85 mm, f/1.8

We define the ratio
where tex2html_wrap_inline1261, is the fraction of the total light in linearly polarized condition. This ratio reduces to the normalized Stoke's parameters tex2html_wrap_inline1263 and tex2html_wrap_inline1265 for tex2html_wrap_inline1267 and tex2html_wrap_inline1269. In practice, additional measurements are made at two more values of tex2html_wrap_inline1247, namely 45tex2html_wrap1285 and 67.5tex2html_wrap1285, for reasons explained in Sect. 5.

The half-wave plate and the Wollaston prism are placed in between a field lens-camera lens combination, which reimages the telescope focal plane on to the main CCD with a reduction factor of about 3.8; the field lens reimages the telescope aperture on the half-wave plate and the light reaches the camera lens without any vignetting. As shown in Fig. 1 (click here) each point in the telescope focal plane produces two images on the CCD, corresponding to the ordinary and the extraordinary beams. In order to avoid overlap of the images of adjacent points, for observations of extended objects, a grid of parallel obscuring strips is placed at the focal plane of the telescope. The width of and spacing between the strips are chosen in such a way as to avoid overlap of the ordinary and the extraordinary images on the CCD. Four 0.3 mm diameter holes are provided at the four corners of the grid and are used to focus the surface of the grid, which coincides with the focal plane of the telescope, onto the surface of the CCD. The grid is made of black dielectric material to avoid polarization of the stray light arising due to reflections from its edges. Besides, the edges are made slanted (Fig. 1 (click here)) to prevent vignetting of the telescope beam.

The A&G unit has a field of view of about 2tex2html_wrap1297 and can be positioned anywhere within a tex2html_wrap_inline1279 area at the focal plane of the telescope close to the main field. This area has been chosen on the basis of the requirement that three stars brighter than mv = 15 should be available with 95% confidence level towards the poles of the Galaxy.

Figure 2: Various blocks of the IMPOL control system. The block marked "T" represents the XY stages of the A&G unit

2.2. Instrument control and data-acquisition

A block schematic of the IMPOL control system is shown in Fig. 2 (click here). The electronics assembly of the instrument consists of three parts - (i) the positioning systems for the half-wave plate and the A&G unit probe, (ii) the CCD camera for the A&G unit and the associated electronics and (iii) the main CCD camera with its own control electronics and host computer (PC 486) for exposure control and data-acquisition. The important parameters of this CCD camera, which was also developed at IUCAA, are listed in Table 1 (click here) (for more details refer to Deshpande & Gadre 1994).


Parameters Value
CCD make EEV CCD02-06 series
CCD chip size 385(H)tex2html_wrap_inline1301578(V)
Pixel size tex2html_wrap_inline1303
Active area tex2html_wrap_inline1305
No. of amplifiers 1
Quantum efficiency blue: 20%
yellow: 50%
red: 60%
Readout speed tex2html_wrap_inline1307 per pixel
Acquisition & Display time 12 s for full frame
Read noise (total) tex2html_wrap_inline1309 rms
Gain tex2html_wrap_inline1309 / ADU

Table 1: The parameters of IUCAA CCD camera


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