Serb. Astron. J. } 170 (2005), 123 - 125
UDC 521.95
Preliminary report
ASTROMETRY TEST OF MSCRED IRAF SOFTWARE PACKAGE
R. Popescu, P. Popescu, P. Paraschiv and A. Nedelcu
Astronomical Institute of the Romanian Academy
Str. Cutitul de Argint 5, RO-040558 Bucharest, Romania
(Received: March 16, 2005; Accepted: April 13, 2005)
SUMMARY: Astrometric properties of images obtained using MSCRED IRAF
software package are investigated. CCD observations of M35 open cluster were
taken in an observation run between October 10 and 17, 2004 at Belogradchick
Observatory (Bulgaria) using the 60cm Cassegrain telescope endowed with a 1kX1k
Apogee 47P CCD camera, ensuring to a field size of 6.16 square minutes, with
a scale of 0.722arcsec/pixel in 2X2 binned mode. Dithered images are resampled
and reansambled into a single larger image for which the World Coordinate System
(WCS) solution is recomputed and investigated.
Key words. Astrometry – Reference Systems
1. INTRODUCTION
IRAF (http://iraf.noao.edu) is the Image
Reduction and Analysis Facility, a general purpose
software system for the reduction and analysis of astronomical data. The main IRAF distribution includes a selection of programs for general image processing and graphics, and a large number of programs for the reduction and analysis of optical and
IR astronomy data, available as NOAO package. The
IRAF core system is the basic IRAF system and does
not include the suite of reduction packages known as
the NOAO package. The IRAF core system provides tools for reading and writing data in the FITS
format, interactive graphics, image arithmetic, image statistics, image frames combining while NOAO
package provides basic CCD reductions and flat fielding routines, spectra analysis, photometry and astrometry routines. These tasks are available in both
batch and interactive modes.
Other external or layered packages are available for applications, such as data acquisition or
handling data from other observatories, and wavelength regimes such as the Hubble Space Telescope
(optical), EUVE (extreme ultra-violet), or ROSAT
and AXAF (X-ray). The external packages are supplied separately from the main IRAF distribution,
and can be installed as needed. One such external
packages is MSCRED (Valdes 1998), the CCD mosaic data handling software. It includes functions for
reduction of mosaic exposures and fringe pattern removal together with routines for combinig dithered
or rastered exposures with respect to a common coordinate system. An image of this type is first resampled into an image in a celestial coordinate system
that can be shifted by integer amounts along both
image axes before combining.
2. OBSERVATIONS AND
DATA PROCESSING
The observations were carried out using a
60cm F/12.5 Cassegrain telescope at Belogradchick
Obervatory, Bulgaria under good sky condition. The
detector was an Apogee 47P CCD camera with a
MAC47-10 chip the dimension of which were 1024
X 1024 pixels, 13 microns each, and was used in
2X2 binned mode leading to a 6.16square arcmin
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R. POPESCU, P. POPESCU, P. PARASCHIV and A. NEDELCU
size Field of View (FOV) with a pixel scale of 0.722
arcsec/pixel. No filter was used. During this observations run dedicated to CCD imaging of optical counterparts of International Celestial Reference
Frame (ICRF) radiosources several fields with high
stars density were imaged. One of these regions was
M35, the core of which was covered by 3x2 overlapping frames.
The software used for image reduction and
analysis was IRAF, both for bias, dark and flat corrections, and for the astrometric computations. Positions of stellar objects in CCD images are extracted
using daofind routine. Catalog data (USNO B1.0,
UCAC2, 2MASS) for the selected area were obtained
using batch mode scripts from VizieR web service,
then ccxymatch was used to match stelar positions
from the catalogs with the positions extracted from
CCD images by daofind. Finaly, with the matched
star list from ccxymatch, ccmap computed the plate
solution and added WCS informations to the FITS
headers for each image. Fake matching induced by
bright stars near the image edge were rejected by
choosing a matching tolerance of 0.5 pixels.
The catalog chosen for the reference stars was
UCAC2 (Zacharias et al. 2004), an astrometric catalog containing 48,330,571 stars from declination -90
to +40. It resulted from R CCD observations, between 7.5 and 16 magnitude. The positional errors
were at 20 mas in the 10th to 14th magnitude interval, and at 70 mas in the limiting magnitudes.
Having WCS headers for all the frames, one
could use mscimage routine that produced images
which were sampled on an even grid of pixels in the
sky. By using the same grid for dithered or rastered
sets of observations, the images could then be combined using only integer pixel shifts in the two image
axes. The goal was to have only a single interpolation
of the data. In the next step, imcombine used coordinate system produced by mscimage to shift and then
combine dithered or rastered observations.
Fig. 1. WCS residuals in right ascension and declination for reference stars from the six original frames.
Fig. 2. WCS residuals in right ascension and declination for reference stars from the constructed mosaic.
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ASTROMETRY TEST OF MSCRED IRAF SOFTWARE PACKAGE
Table 1. Standard deviations of WCS residuals in α and δ for the original frames and the constructed
mosaic
Original Frames
MSCRED Mosaic
σO−C α (arcsec)
0.116
0.045
To investigate the mosaic image, we applied
the same procedure of astrometric computation described above, in the same manner as for the original
frames. Fig. 1 shows the WCS mapping status for
all the original frames, and Fig. 2 gives the mosaic
constructed with the numerical data given in Table
1.
3. RESULTS AND CONCLUSIONS
Table 1 shows an improvement in quality of
WCS solution for the MSCRED mosaic, as compared with any of the original CCD frames. The
explanation is that now, having a wider FOV, one
can choose reference stars labeled as ”astrometrically excellent”, with direct impact on reliability of
computed plate solution. The method can be applied to the deep astrometry (high-precision positions and absolute proper motions), to contribute to
a comprehensive photometric, astrometric, and spectroscopic database for selected fundamental open
clusters. Also, in the field of astrometry of optical counterparts of ICRF sources, the method allows
σO−C δ (arcsec)
0.090
0.052
a direct connection with reference stars previously
determined by photo plates programs as CONFOR
(Babenko et al. 2003) and, thus, to overcome some
of the drawbacks of the narrow field astrometry.
Acknowledgements – The work was performed in
collaboration with Bulgarian colleagues: Alexander Antov, Lubomir Iliev, Renada KonstantinovaAntova and Rumen Bogdanovski. We’d like to thank
them for their collaboration. The observational program was supported in the framework of UNESCOROSTE project.
REFERENCES
Valdes, F.:1998, ADASS VII, ASP Conf. Series 145,
ed. R. Albrect.
Zacharias, N., Urban, S. E., Zacharias, M. I., et al.
2004, Astrophys. J., 127, 304.
Babenko, Y., Daniltsev, A., Vertypolokh, A., Maigurova, N., Pinigin, G., Protsyuk, Y., Shulga,
A., Dementyeva, A., Ryl’kov, V., Bocşa, G.,
Popescu, P.: 2003, Rom. Astron. J., 13, 77.
ASTROMETRIJSKI TEST KORISTEI PROGRAMSKI PAKET MSCRED IRAF
R. Popescu, P. Popescu, P. Paraschiv and A. Nedelcu
Astronomical Institute of the Romanian Academy
Str. Cutitul de Argint 5, RO-040558 Bucharest, Romania
UDK 521.95
Prethodno saopxtenje
U radu su ispitivane astrometrijske
osobine CCD snimaka koristei programski
paket MSCRED IRAF. Snimano je otvoreno
jato M35 od 10. do 17. oktobra 2004. godine
na opservatoriji Belogradchick (Bugarska) ko-
ristei 60 cm Cassegrain teleskop sa Apogee
47P CCD kamerom 1kx1k. Polje vida je bilo
6.16 arcmin sa skalom od 0.722 arcsec po pikselu i binovanjem 2x2 piksela. Vixe snimaka
je ukljuqeno u jednu veliku sliku za koju je
raqunat i ispitivan sistem WCS.
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