Previous: Grisms and Filters
Up: The LDSS-2 Instrument
Previous Page: Grisms and Filters
Next Page: LEXT
The throughput of the instrument is very high, peaking near 60% for a wide wavelength range as shown in Fig.3 which summarizes the performance measured in the laboratory. The whole system including the atmosphere, telescope and detector efficiencies gives an overall throughput of around 20%, as shown in Table 4.
Overall system throughput is shown in Fig. 3(d)
These figures mean that in an 1800 second integration with the medium grism, a galaxy with will produce 63 detected photons per pixel in the wavelength direction near 450nm. The corresponding counts from dark sky will be 178 detected photons per pixel, so the sky noise dominates the read noise. Assuming that all of the light from the object goes through the slit and is spread over two pixels in the spatial direction, the signal to noise will be 3 per pixel in the wavelength direction. In practice it has been found that a total integration time of 4 hours will give signal to noise adequate to measure redshifts for 80% of a sample of field galaxies with
Note that acquiring a field is a relatively complex and time-consuming process, and even with the aid of the LEXT package will take about 20-30 minutes for each new mask. This overhead means that it becomes inefficient to use the instrument for total integrations much shorter than 60 minutes.