2.2. Data reduction and analysis
The data reduction has been performed in the usual manner including bias level correction, flat-fielding, background correction, registering of the stars (alignment of the R-band image with respect to the H image), scaling of the R-band image, and final continuum subtraction. In almost all cases the H images (split into two individual exposures for cosmic ray removal) have been combined. All these reduction steps have been performed using various IRAF packages.
For the northern survey the bias correction was performed by using the IRAF colbias task, taking the information of the overscan region into account. This was appropriate, since the bias exposures showed diagonal stripes. For the southern hemisphere objects the bias correction was done using a masterbias frame (one per night) obtained from median combining individual bias frames of each night. The flat-field correction was done using a normalized flat-field image created from combining the individual flat-field exposures for each filter separately. Final background correction was done by subtracting the mean sky countrates from the flat-field corrected object frames. These values were estimated from at least three regions on each object frame that were neither contaminated by cosmic-ray hits nor populated by stars. The R-band frames were aligned and shifted with respect to the H exposures before combining each of the two object exposures. Finally the R-band image was scaled to the H image. The scaling factor has been determined from the ratio of the countrates of individual foreground stars in the R-band and H images. Then the scaled R-band image was subtracted from the H image. Due to the given bandwidth of the used H filters also the adjacent [NII] emission is covered. For the sake of brevity we therefore always refer to the emission of H + [NII] when we speak of an H image. More details on the continuum subtraction are given in Sect. 3.3.