We measured the transmission of the two neutral densities (ND_short, ND_long) installed in NaCo for the J, H, Ks, L′, and M′ filters using datasets gathered from the ESO archives where bright targets were observed consecutively with and without a neutral density (see Table A.1). Each dataset was reduced with the ESO Eclipse package (Devillard 1997). Eclipse carried out sky and/or dark subtraction, the division by the flat field, bad pixel flagging, and interpolation. The scripts finally shifted and added dithered images to produce a post-cosmetic frame of the sources. We performed aperture photometry on the sources for each setup and compared them to retrieve the transmission factor (different integration radii were considered depending on the dataset). We repeated the procedure for different targets for given filters to estimate an error on the transmission factor. Values are reported in Table A.2. We considered in addition the transmission factor reported in (Boccaletti et al. 2008) for the ND_short neutral density for the computation of the error in the Ks band. To conclude, we only got a single measurement of the ND_long neutral density transmission in the M′ band. We considered a higher error for this transmission based on the impact of the level of fluctuations of the background around the target.
Observations log for the calibration of the neutral density transmission.
Transmission of the neutral density filters.
Properties of young companions and binaries used as comparison objects.
© ESO, 2013