The images were reduced following the standard procedures of bias
subtraction and flat-fielding using the IRAF
(4)
task CCDPROC. The dark current in both detectors
used was negligible (
1count/hour/pixel) and no dark
subtraction was applied. In those cases where three or more frames in
one band were obtained for the same object the cosmic rays were
rejected using the IRAF task IMCOMBINE. When only one or two
images were available we removed the cosmic rays interactively using
the IRAF tasks COSMICRAYS and CREDIT
within the CRUTIL package.
Considering the relatively small number of nights that were
photometric (12 out of 49) we decided to use the following strategy in
order to flux-calibrate our images. During the non-photometric nights
at Palomar and Las Campanas observatories we obtained very deep
exposures in R and
H
. During the
photometric nights the
adopted follow-up strategy was slightly different at each site. At
Palomar we took short (600-900s) B and R exposures of the same
objects already observed in R and
H, along with images of a
large number of photometric standards
(Landolt 1992a).
Then, using the
fluxes of the field stars in the calibrated R-band, deep R and
H images along with the
filter+detector response functions
(see Figure 2) we cross-calibrated
our deep R and
H images (see
Appendix A). At Las Campanas,
during the photometric nights we observed both objects and
spectrophotometric standards
(Hamuy et al. 1992;
Landolt 1992b)
through all B, R and
H filters. During these
nights we also took short B and R exposures of the objects
already observed
in R and H during
previous non-photometric nights. The flux
calibration of the H
images taken during non-photometric
conditions was carried out in the same manner as the Palomar case (see
Appendix A). The coefficients obtained
from the calibration are given in Table 4. This
strategy has allowed
us to make optimal use of telescope time, achieving relatively small
photometric errors (see Table 5). In
order to check the reliability of the
H calibration based on
the use of R-band
calibrated data, we compared the results obtained from this method
with those resulting from the observation of spectrophotometry
standards through the H
filters during the Las Campanas
photometric nights. The calibration results agreed to within 5%.
| Night | Telescope | Band |
C |
k |
k,B-R |
rms | N | Notes |
| (mag) | (mag/am) | (mag) | ||||||
| (1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) | (9) |
| 12/04/01 | P60 | B | 23.760 | 0.264 | +0.035 | 0.03 | 49 | |
| P60 | R | 23.787 | 0.120 | -0.051 | 0.01 | 34 | ||
| 12/07/01 | P60 | B | 23.769 | 0.250 | +0.050 | 0.05 | 11 | (a) |
| P60 | R | 23.796 | 0.110 | -0.050 | 0.04 | 12 | (a) | |
| 01/20/02 | P60 | B | 23.717 | 0.225 | +0.052 | 0.03 | 25 | |
| P60 | R | 23.762 | 0.104 | -0.046 | 0.02 | 22 | ||
| 01/21/02 | P60 | B | 23.674 | 0.202 | +0.069 | 0.04 | 15 | |
| P60 | R | 23.793 | 0.116 | -0.050 | 0.08 | 12 | (b) | |
| 02/08/02 | C100 | B | 24.960 | 0.298 | +0.050 | 0.01 | 10 | |
| C100 | R | 24.942 | 0.192 | -0.050 | 0.06 | 6 | (b) | |
| C100 | H
6570 |
21.903 | 0.110 | 0.01 | 3 | (a) | ||
| 03/05/02 | C100 | B | 24.390 | 0.187 | +0.050 | 0.05 | 4 | (b) |
| C100 | R | 24.341 | 0.074 | -0.050 | 0.01 | 4 | (b) | |
| C100 | H
6600 |
21.446 | 0.110 | 0.02 | 4 | (a) | ||
| 03/06/02 | C100 | B | 24.491 | 0.250 | +0.050 | 0.04 | 3 | (a) |
| C100 | R | 24.398 | 0.110 | -0.050 | 0.03 | 3 | (a) | |
| C100 | H
6600 |
21.448 | 0.110 | 0.03 | 3 | (a) | ||
| C100 | H
6630 |
21.512 | 0.110 | 0.02 | 3 | (a) | ||
| 03/09/02 | C100 | B | 24.342 | 0.225 | +0.050 | 0.03 | 4 | (b) |
| C100 | R | 24.335 | 0.110 | -0.050 | 0.01 | 4 | (a) | |
| C100 | H
6600 |
21.430 | 0.117 | 0.04 | 5 | (b) | ||
| C100 | H
6630 |
21.472 | 0.089 | 0.05 | 4 | (b) | ||
| 03/14/02 | P60 | B | 23.745 | 0.287 | +0.074 | 0.04 | 32 | |
| P60 | R | 23.755 | 0.114 | -0.035 | 0.05 | 37 | ||
| 05/15/02 | P60 | B | 23.718 | 0.266 | +0.054 | 0.03 | 29 | |
| P60 | R | 23.745 | 0.114 | -0.059 | 0.02 | 30 | ||
| 07/07/02 | P60 | B | 23.886 | 0.286 | +0.047 | 0.03 | 30 | |
| P60 | R | 23.829 | 0.113 | -0.055 | 0.03 | 28 | ||
| 07/08/02 | P60 | B | 23.853 | 0.347 | +0.080 | 0.05 | 37 | |
| P60 | R | 23.825 | 0.154 | -0.042 | 0.05 | 40 | ||
Note. - (a) Both the extinction coefficient and the color term are fixed. (b) The color term is fixed. Columns stand for: (1) Date corresponding to the civil time at the start of the observing night. (2) Observatory and telescope (P60: Palomar Observatory 60-inch; C100: Las Campanas Observatory du Pont 100-inch). (3) Band. (4) Photometric zero point in magnitudes. (5) Extinction coefficient in mag/airmass. (6) B - R color coefficient. (7) RMS of the calibration. (8) Number of standard stars used in the fit. (9) Notes. |
||||||||
The continuum-subtracted
H images of our sample
were obtained from the R-band and line+continuum
H images making use of
the equations given in Appendix A.
Briefly, we compared
the flux in counts of a large number of field stars ( > 30) both in
the R-band and H
images. Then, using the IRAF task GAUSS the
image with the best seeing was convolved with a gaussian kernel to
match the seeing of the worst-quality image. Finally, we divided the
R-band image by the corresponding scaling factor and subtracted it
from the line+continuum H
image. Residuals in the
continuum-subtracted images due to the presence of very bright field
stars were removed interactively using the IRAF task
CREDIT within the
CRUTIL package. For more details about this procedure and the
flux-calibration of the resulting continuum-subtracted
H image
the reader is refereed to Appendix A.
4 IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation. Back.