Optical and NIR observations were carried out in two rounds at the Las Campanas Observatory (LCO), Chile. The telescopes used were the 1.0m f/7 Swope telescope and the 2.5m f/7.5 Du Pont telescope of the Carnegie observatory. Table gives a summary of the two observing runs. The filters and detectors used are listed in Tables and respectively.
|40'' TEK#||40'' TEK#||100 IRCAM|
|25 Jan 1995||0434-225||17 Feb 1996||0420-263||27 Feb 1997||0503-284|
|26 Jan 1995||0446-206||18 Feb 1996||0354-263||1251-289|
|1251-289||20 Feb 1996||0354-263||1346-252|
|27 Jan 1995||0349-278||0952-224||29 Feb 1996||0344-291|
|28 Jan 1995||0453-206||21 Feb 1996||0420-263||0446-206|
|1354-251||22 Feb 1996||0354-263||1048-238|
|29 Jan 1995||0344-291||0938-205||1103-244|
|1126-246||23 Feb 1996||0938-205||1215-215|
|: TEK#1, TEK#5, IRCAM are the CCDs and the IR camera used at LCO.|
|Details are in Table .|
A large number of twilight flats were taken, both in the evening and in the morning. During the day dome flats were also taken, but these were not used eventually since the quality of the twilight flats was acceptable. An overscan bias region was present on the CCD. Still, a few bias frames were obtained at the beginning of the night and also every few hours during each observing session, just to ensure that the bias level remained constant for the length of the exposures used. Dark current is very low in modern CCD detectors and was neglected. No dark current contribution was later discernible in the data.
Object scheduling was done so that objects were closest to zenith when observed. As far as possible, objects were sandwiched between observations of standard stars. Standard stars from the Standard Areas (SA) 98 and 104 were observed at different air masses for effective calculation of the apparent magnitudes. Though all objects in the sample were meant to be observed during the runs, when only one of two objects close to each other could be observed, the brighter one was preferred. Because of this, a bias in redshift selection of the sample might have crept in with preference having been given to the nearer (brighter) object. It has already been demonstrated (Figure ) that no radio flux density selection bias has been introduced.
|K' is a varient of Johnson's K.|
|Detector||Size||Read noise||Pixel size||Scale|
|Visible||TEK#1 CCD||1024 1024||8||24||0.7|
|TEK#5 CCD||2048 2048||8||24||0.7|
|NIR||HgCdTe array||256 256||40||40||0.348|
Multiple frames of each object were taken with exposure time adjusted to avoid saturation. Each object was also moved around in the frame during successive exposures, so that bad pixels do not affect any particular object in all the frames. The typical total exposures in the filters were 60 and 20 minutes respectively. The standard stars were observed in focus with exposures of few to several seconds, and also out of focus with longer exposures so that the bright peak gets distributed into a torus. This is useful for standard stars since one is interested in total magnitudes and not in radial profiles of the stars. The longer exposures also help avoid the effect of false structures being introduced in the image due to the opening and closing of the shutter. The stars were also moved about in the CCD frame.
The procedure adopted was slightly different in the case of IR observations. The sky is brighter in IR so that exposures have to be much shorter to avoid saturation. Exposures of 35 seconds were therefore used. Seven such exposures were taken with the object at the same point within the array. The telescope was then moved a little so that the object shifted within the array and seven more exposures were taken. Several such sets were taken per object. In case the object covered a large part of the IR array, separate sky frame sets were taken between every two object sets. The dark current for the IR detector is not negligible and every day a large number of dark exposures of durations 2, 3, 5 and 35 seconds were taken.
The standards used for standardization were HD 38921, Gl 347a and HD 161743 from Elias (1982). These were observed several times at different airmasses throughout the night.