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The LFOSC was installed at the Guillermo Haro Observatory (GHO) through the project "Optical Identification of ROSAT X-ray sources" started in 1991. The description of the project and LFOSC can be found in Zickgraf et al. (1997).
For direct imaging it is possible to use Johnson B,V , Cousins R,I and H filters. A limiting magnitude of 23m in R can be reached within 900s.
For the spectroscopic observations it is possible to used masks with circular holes of 3" projected diameter in the focal plane of the telescope. These masks can be produced directly from the CCD images using a computer controlled drilling device. In addition to the object holes each mask contains holes producing spectra of the sky background near the objects. Alternatively, a slit mask can be inserted in order to perform normal long slit spectroscopy. The spectra are oriented in E-W direction.
Two different grisms giving reciprocal linear dispersions of 250Å/mm and 360Å/mm, respectively, are available. For the position of the long slit, the spectral intervals covered are 4000-7200 Å and 4200-8800 Å, respectively. With a hole diameter of 3" the spectral resolution is about 13 Å and 18 Å, respectively, i.e. a resolving power of at central wavelengths.
Tests showed that with these standard stars a spectral classification with an accuracy of better than 5 subclasses can be obtained for objects brighter than about V= 19m with 40min exposure time. Therefore, even the optically faintest X-ray luminous coronal emitters in the, INAOE-LSW project, subsample were identified with LFOSC. Counterparts with emission lines, as e.g. AGN, can be identified as faint as to V=21m.
The layout of LFOSC is shown below.
|EEVP8603 Device Specifications|
|Bias Mean level||
|Software gain||Gain (e-/ADU)||RON (e- /pix)|
The quantum efficiency curve for the EEV P8603 chip is shown below:
Data acquisition and storage
The PC that controls CCD is equipped with
a DAT and an EXABYTE drive. Backup of the images is done through the software
available in the PC by selecting the correct option on the shell. The data
are stored in fits format. In order to read them on the Workstation, it
is necessary to used procedures developed by Serrano(ping), these are BAJANOM
|lines/mm||Interval covered wavelength (Å)||Dispersion (Å/pixel)||Resolution (slit)|
Wavelength calibration: It is possible to take comparison exposure at each new position in order to avoid shifts due to flexions of the instruments with the built-in comparison lamps (He,Ar, Xe). If you are satisfied with shifts of a pixel or two, then a single comparison exposure in the beginning of the night is good enough. There is another comparison source available by using the screen on the dome. The lamps available are called azul and roja.
Flat-field calibration: Flat-field calibration is needed to remove the pixel-to-pixel sensitivity variations on the chip. LFOSC has another built-in halogen lamp in order to do it. You can take a flat-field image after each exposure or use the screen on the dome and take several images at the beginning of the night.
Holemasks: If you are using holemasks, perpendicular to the direction of dispersion, i.e. in N-S direction, it is necessary a minimum separation of the holes of at least 5pixels in order to avoid overlapping spectra.
|AB magnitude that gives 1 ADU/s/Å at standard gain (4? e-/ADU)|
|Wavelength (Å)||AB mag|
Examples of observations obtained with LFOSC are shown:
R band image of the field around the X-ray source RXJ1207.7+3148. The 90% SASS error circle (radius 35 ) is indicated. Many faint and diffuse objects, most likely distant galaxies, are visible. The brightness of the faintest visible objects is R~23. For the two brightest objects near the source position, designated by "A'' and "B'', spectra were obtained. The remaining objects are too faint for spectroscopy. Object "A'', whose spectrum is shown in the lower panel, appears to be a galaxy with R~ 20.3m and B~ 22.3m. Absorption features most likely due to MgI b and NaI D with a redshift of z~0.23 are visible. Residuals of the night sky lines are marked by "ns'', "x'' is a cosmic. Object "B'' is a 16th magnitude F-type star. This star is visually too faint to be a plausible counterpart. On the B image no object brighter than about 22 was visible making a QSO as counterpart unlikely. The galaxy "A'' is visually too faint to be a plausible counterpart of the X-ray source . Hence, the most likely identification of this X-ray source is a distant cluster of galaxies
R image of the X-ray source RXJ0747.3+6822. The 90% SASS error circle is indicated (radius 44" ). With two exposures spectra of all objects within 60" radius around the RASS position could be obtained. "r'' denotes reflex images of bright stars in the field. In the lower panel the spectrum of object "A'' is displayed which is the likely counterpart of the RASS source. It is a Sy 1 galaxy with V~ 18m at redshift z = 0.120
R image of the position of the X-ray source RXJ0403.5+0837. In the the 90% SASS error circle (radius 44" ) several possible candidates for the optical counterpart are visible. The objects observed spectroscopically are designated by S1, "2'', "5'', and "8''. The bright object S1 is a 13th magnitude G to K-type star. Object #5 with V~18.4is a QSO with H+[OIII ] and MgII at redshift z = 0.589. H partly falls into the atmospheric band at 7600Å. Each of the two objects could be the X-ray source or at least contribute to the observed X-ray flux (see text). The remaining objects are faint stars which can be excluded for being the counterpart of the X-ray source
For more information you
can consult the following:
Back to the Observatorio Astrofísico Guillermo Haro
Back to the Astrophysics Department
Back to the INAOE