years (Maeder 1981) As a result, our approximation is probably as good as any given our present knowledge

of W-R stars.

The final HII region D227 contains a G2la star which was treated in the same manner as the W-R stars.

CHAPTER 7

115

7.1. The Instrumentation

Hydrogen-alpha surface photometry observations of 17 L.M.C. HII regions were obtained during the .1979/80 and 1980/81 Magellanic Season (October to February). These observations were done with a S.E.C. Vidicon T.V. 2-D

photometer/Reducteur Focal combination, placed at the f/18 focus of the Siding Spring Observatories' one metre telescope. A summary of the observations is given in Table 7.1. which

lists the date of observation, name, R.A. and Dec and ex­ posure time for each object.

The instrumentation is best described by sub-dividing into three distinct sections, the T.V. photometer, the optics and the filter transmission characteristics.

7.1.1 The S.E.C. Vidicon T.V. 2-D Photometer

The T.V. photometer used for our observations was a 25 mm electrostatically focussed class II Westinghouse S.E.C. Vidicon camera. For a detailed description of the specifications and performance of this instrument the reader is referred to the papers by Rodgers e t . al. (1980) and Da Costa et.al. (1977)

The major attributes of this system include; its large field size (a 10' x 10' unvignetted field at f/3.4), low dark current and excellent linearity over the full digit­ ized intensity range of 0 to 256 (each increment corresponds

TABLE 7.1

CURRENT

DATE NAME OF R.A D.E .C . EXPOSURE COMMENT

OBJECT -* _{TIME (SEC)}

hr min sec deg min sec

25/10/79 D31 04 55 20.3 -67 13 40 1080 D199 05 26 35.1 -68 51 21 900 Sum of two D192 05 25 47.9 -67 31 11 600 exposures N4 9 05 26 00 -66 05 59 180 N4 9 05 26 00 -66 05 59 180 14/01/80 N49 05 25 55.7 -66 05 47 360 Sum of two D121 05 16 49.5 -67 21 05 300 exposures D13 8 05 20 29.6 -66 47 41 180 IC418 05 26 45.1 -12 42 53 3 IC418 05 26 45.1 -12 42 53 3 D311 05 45 21.3 -69 46 43 300 D 2 8 1 05 40 44.9 -70 03 04 600 IC418 05 26 45.2 -12 42 48 3 N49 05 25 55.7 -66 05 47 180 N49 05 25 55.7 -66 05 47 180 IC418 05 26 45.0 -12 42 53 3 15/01/80 IC418 05 26 45.0 -12 42 37 3 IC418 05 26 45.0 -12 42 37 3.5 D229/231 05 33 05.5 -67 43 43 300 D145/149 05 21 32.3 -69 41 16 240 D301 05 43 14.6 -67 51 00 1500 N49 05 25 55.4 -66 05 44 180 N49 05 25 55.4 -66 05 44 180 16/01/80 IC418 05 26 43.9 -12 42 25 150 Saturated IC418 05 26 43.9 -12 42 25 5 N49 05 22 55.1 -66 06 19 180 IC418 05 26 44.0 -12 42 21 2 G.F. N49 05 25 57.9 -66 05 41 180 N49 05 25 57.6 -66 05 37 180 IC418 05 26 45.9 -12 42 31 5 IC418 05 26 45.9 -12 42 31 2 G.F. C o n t 'd

TABLE 7.1 (cont'd) C U R R E N T DATE NAME OF OBJECT hr R . A .

min sec deg

D.E. min C. sec EXPOSURE TIME (SEC) COMMENT 27/11/80 N1535 04 13 18 -12 48 12 30 N1535 04 13 18 -12 48 12 300 G.F. N1535 04 13 18 -12 48 12 300 G.F. N49 05 25 56 -66 06 64 180 D137/South 05 20 36 -65 31 30 1800 N49 05 25 56 -66 06 54 160 D137/North 05 20 39 -65 28 18 2100 N1535 04 13 21 -12 48 00 30 N1535 04 13 21 -12 48 00 240 G.F. IC418 05 26 32 -12 43 30 2 D196 05 25 05 -67 36 42 1200 D25 04 53 36.5 -70 02 16 1800

116

to approximately 11 photoelectron equivalent) (Dopita 1977). These qualities make the T.V. photometer an ideal instrument for doing surface photometry of L.M.C. HII regions.

The target of the T.V. is read over a 15 x 15 mm area, producing a 256 x 256 data array of roughly 60 x 60 pixels. The spatial variations in the gain over this target area consist of three components. The first is a low

frequency modulation which results in a 35% reduction in gain in going from the centre to the edge of the target. Super­ imposed on this is an area of decreased target sensitivity resulting from overexposure of a coude spectrum which cuts diagonally across the centre target (Rodgers (1980)). Thirdly there is a high spatial frequency component produced by pixel to pixel gain variations. In theory, it is possible to remove all of these variations by dividing the raw T.V. data, point by point, by the image read when the camera is exposed to a

uniform light source (flat field). In practice, the inefficient magnetic shielding of the tube.allows the Earth's magnetic field

to both rotate and distort the images, making it impossible to fully remove pixel to pixel gain variations. The majority of the noise left, after a frame has been flat-fielded, is produced by the pre-amplifier in the T.V. (about 30 photo­

electron equivalent) except at high signal levels where photon noise becomes important.

117

absolute fluxes for planetary nebulae to within an uncertainty of 6% using the T.V. photometer. He also showed that

absolute surface brightnesses determined from a properly exposed and calibrated T.V. frame (under good observing conditions) should be reliable at the 10% level.

7.1.2 The Optics

Figure 7.1 shows the whole of the optical train for the T.V. photometer/Reducteur Focal system. The two beams shown in this diagram are those originating at a point on the entrance pupil of the telescope, and those originating at a point at infinity (i.e. a point in the field of view).

If we consider the rays originating from the pupil one sees that the role of the wave processor corrector lens is to render the beams telecentric (i.e. all principal axes of ray cones in the field are made parallel). The Aeroekta

Imaging lens, serves to re-image the pupil. At this pupil is placed the 35mm Leitz Summulux f/1.4 object imager lens,

which produces an image of the sky on the 15 x 15 mm T.V. target.

The last two lenses in the optical train, the Aeroekta Imager and the Object Imager lens, combine together to form an inverse teleconvertor br reducteur focale) which changes the incoming f/18 beam to f/3.4, the reduction factor being in the approximate ratio of the focal lengths.

The front corrector lens was removed from this optical system for our observations as strict telecentric operation was unnecessary over the field of view used and

Figure 7.1

This figure shows the optical train of the T.V.

Photometer/Focal Reducer combination, used for the

hydrogen-alpha surf ace-photometry observations. Two beams,

one corresponding to a point at infinity (the F/18 beam)

and the other corresponding to a point on the primary

(the Aperture beam), are shown in this diagram. A des­