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Overview

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.
 
 
 

CCD

EEVP8603  Device Specifications
Format	385X578
Pixel size	22 m
Operating temperature	-30o C
Bias Mean level	21 ADU

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 and RENOM.
 
 

Grisms available

 

Grisms details
lines/mm	Interval covered wavelength (Å)	Dispersion (Å/pixel)	Resolution (slit)
G1	4000-7200	5,5	13
G3	4200-9000	8,2	18

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Filters

Observing Strategies

Calibration Frames

Bias frames:  Zero level is very stable. The averge value is 21 ADU. Therefore, 5 to 7 of these frames at the beginning and at the end of the night are enough, they can be combined into a median bias frame by using imcombine.

Dark frames: The dark current on all the chips is negligible. During cloudy nights it was tested with exposures up to 4 hours, the dark level of the frames was 21, hence dark frames are not usually taken.

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.

Flux Calibration: Calibration to flux (and removal of atmospheric and instrumental wavelength effects) can be readily achieved by observing one or more spectrophotometric standards during the night. We recommend using the ``Kitt Peak Spectrophotometric Standards". These are 25 mostly line-free stars calibrated at 50Å intervals from 3200Å to 8100Å (Massey et al. 1988 ApJ 328, 315); 11 of these have been calibrated out to 1 micron (Massey et al. 1990, ApJ, 358, 344). Finding charts can be found in each of the domes, as well in the 1988 paper cited above. The spectrophotometric extinction curve for Cananea is not available, but considering its proximity to Kitt Peak, we recommend you to use the relatively well-known curves for Kitt Peak available in IRAF. It is desirable that you observe spectrophotometric standards at airmasses similar to that of your objects.

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.

Guiding Camera

Intensified CCD camera

Calibration and Efficiency

Examples

Examples of observations obtained with LFOSC are shown:
 
 
 

II

Other sources
 

        For more information you can consult the following:
 

• INAOE Technical Reports 166 and 167
• Cookbook at the telescope dome
• Zickgraf et al., 1997, A&A 123, 103

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