3. Observations

The photometric measurements were performed using the 1.0-m telescope at the South African Astronomical Observatory (SAAO) at Sutherland (Cape Province), during the period 15-22 January and 1-8 February 1994. An RCA CCD was employed which offers a detecting array of pixels (Walker et al. 1984) and, combined with the available glass filters, this approximates to the Cousins photometric system. At the Cassegrain focus a pixel (which is 27m square) corresponds to 0.388 arcsec on the sky. We orientated the long axis of the CCD to be north-south and allowed for a overlap between adjacent fields to assist with their registration, etc. This provided an effective non-overlapping image area of  arcmin of sky per frame. During this period, we imaged 36 fields in all four colours that were located around the core of the cluster. Care was taken to avoid fields that contained stars with magnitudes brighter than approximately V=9, as it was found that the transfer of excess charge from saturated objects along the geometry of the CCD precluded any useful photometric measurements. The coordinates of the field centres are presented in Table 1 (click here) and the area of sky that has been imaged is shown relative to the brightest stars in Fig. 1 (click here).

 

1 08 39 17.4 -53 07 07

2 08 39 29.7 -53 07 07

3 08 39 42.0 -53 07 07

4 08 39 54.3 -53 07 07

5 08 40 06.7 -53 07 07

6 08 40 19.0 -53 07 07

7 08 40 19.0 -53 10 04

8 08 40 31.0 -52 46 28

9 08 40 31.0 -53 07 07

10 08 40 31.0 -53 10 04

11 08 40 42.6 -52 43 31

12 08 40 42.6 -52 46 28

13 08 40 55.3 -52 43 31

14 08 40 55.3 -52 46 28

15 08 40 55.3 -52 52 22

16 08 40 55.3 -52 55 19

17 08 40 55.3 -52 58 16

18 08 40 55.3 -53 01 13

19 08 41 07.6 -52 46 28

20 08 41 07.6 -52 49 25

21 08 41 07.6 -52 52 22

22 08 41 07.6 -52 55 19

23 08 41 07.6 -52 58 16

24 08 41 07.6 -53 01 13

25 08 41 07.6 -53 04 10

26 08 41 20.0 -52 46 28

27 08 41 20.0 -52 49 25

28 08 41 20.0 -52 52 22

29 08 41 20.0 -52 55 19

30 08 41 20.0 -52 58 16

31 08 41 20.0 -53 01 13

32 08 41 20.0 -53 04 10

33 08 41 32.5 -52 55 19

34 08 41 32.5 -52 58 16

35 08 41 32.5 -53 01 13

36 08 41 32.5 -53 04 10

 

  
Figure 1: A general view of the core of the open cluster, IC 2391, indicating those stars with visual magnitudes brighter than V=13.5. The area of sky imaged for each of the 36 fields is outlined by the boxes; the identification numbers corresponding to those given in Table 1 (click here). Cluster members previously identified by Stauffer et al. (1989) in this field are encircled and are labeled SHJM

Typical exposure times were 600 : 300 : 120 : 120 seconds for the B : V : R : I frames respectively. Fifteen E-region standard stars (Menzies et al. 1989) were observed for standardization purposes. These allowed an in-depth determination of the instrumental colour transformation to the standard system. In addition, 2-3 E-region stars were obtained throughout the night at similar air-masses to the programme fields for the determination of extinction coefficients and zero points. Furthermore, sky, flat, preflash and bias calibration frames were obtained at the beginning and end of each night.