Grating properties

Two plane transmission gratings, manufactured by Bausch & Lomb, of 150 gr/mm (110 mm x 110 mm) can be used for slit-less spectroscopy at the 1.23m telescope . In the table are listed their properties.

Grating		Blaze angle		Blaze Lambda		Max. effic.
Blue		3.05 deg		3550 Å		78%
Red		6.30 deg		7300 Å		78%

 

They can be mounted (one each time) on a rotator which is controlled remotely to set the grating orientation. The rotator is placed in the f/8 beam of the telescope after the filter wheel. The distance to the focal plane is around 44 mm, being the final linear reciprocal dispersion of 1500 Å/mm. For the Site#2b 2048 x 2048 pixel of size 24 microns, the dispersion is 36 Å/pixel and the field of view 17.2 arcmin square when using the 100mm diameter filters (Johnson B,V,R,I). The standard 50mm filters also can be used in combination with the grating but in this the field of view get smaller.

Observations and first results Last spring we had the opportunity to commision the gratings in combination with the 2kx2k CCD camera at the 1.23m during technical time. In our test observations we took slit-less spectra of spectrophotometric standard stars as well as some interesing objects to show the performance of the gratings. To illustrate the excellent results obtained during this run we show in Figure, a zoom image of a QSO with previously known spectra. This QSO, has a magnitude of 19.1 and a redshift of 3.125. The image and extracted spectrum shown in the figure correspond to an individual 1200 sec exposure with the Blue grating and no filter with a seeing of 1.3 arcsec. As you can clearly see the main features of the QSO are present, as Lyalpha 1216 Å, NV 1240 Å, SiIV 1394 Å + OIV 1402 Å. The emission lines of the quasar spectrum along with the Balmer series absorption lines of the standard star HZ4 allowed us to determine the linear reciprocal dispersion. In the grey-scale image the zero order, i.e. the direct image without dispersion, is also present. It corresponds to wavelength null and can be used in the dispersion determination and as reference to obtain redshifts. Moreover it can be used to obtain good astrometry for the objects.

Star forming galaxies in the local universe C.E. García-Dabó 1, J. Gallego1, J. Zamorano1 (1)  Departamento de Astrofísica, Universidad Complutense de Madrid, The red grating and a Johnson R filter are being used by members of the Departamento de Astrofísica, de la Universidad Complutense de Madrid for the search of Halpha emission-line galaxies. The UCM objective-prism survey was carried out with the Calar Alto Schmidt telescope using photographic plates. This is a natural continuation of the UCM survey because this instrumental setup, with a CCD as detector, allows to detect very faint emission-line galaxies. The UCM team intend to determine the low-end of the luminosity function of the star forming galaxies of the local universe. Some promising test images have been taken by F. Prada and the first run have been carried out recently. Processing of the spectroscopic images is in progress. The search for candidates is made with a custom made package called OPERA . This software contains a bunch of tools for the automatic analysis of objective-prism astronomical images.

A list of references relevant to our project is given below,

- Alonso et al. 1994, ApJS 122,415
- Gallego et al., 1995, ApJL 455, L1
- Gallego et al. 1997, ApJ 475, 502
- García-Dabó et al. 1999a, ApSS 263,99
- García-Dabó et al. 1999b, ApSS 263,95
- Zamorano et al. 1994, ApJS 95, 387
- Zamorano et al. 1996, ApJS 105, 343