SPTIME takes an input source spectrum, either a reference blackbody, power law, or a user spectrum, a background ``sky'', observing parameters such as exposure time, central wavelength, seeing, airmass, and moon phase, instrument parameters such as aperture sizes and detector binning, a description of the spectrograph, and desired output. The output consists of a description of the observation, SNR statistics, and optional graphs and tables of various quantities as a function of wavelength over the spectrograph wavelength coverage. The SNR is computed from the detected photons of the source and background, and from the instrumental noise characteristics.
Each of the components specify a transmission or related function giving the fraction of incident light transmitted as a function of some parameter or parameters. Except for the aperture, which is a function of the incident source profile (typically the seeing profile) relative to the aperture size, the transmissions of the components are a function of wavelength.
To make SPTIME easily configurable by observatories and users, all the component transmission functions are given in text files, called tables since they often contain an interpolation table. The data files may also include parameters, pointers to other tables, or defaults for the task parameters. The files are searched first in the users working directory and then using a list of directories. Thus, users may place files in their work area to override system supplied files and observatories can organize the data files in a database directory tree.
Many spectrographs provide a wide variety of wavelength regions and dispersions. For gratings (and to some extent for grisms) this means use of different gratings, orders, tilts, and possibly camera angles in the spectrograph. The transmission as a function of wavelength (the grating efficiency) changes with these different setups. If the transmission function is given as an interpolation table this would require data files for each setup of each disperser. The structure of SPTIME allows for this.
However, it is also possible to specify the grating and spectrograph parameters and have the task predict the grating efficiency in any particular setup. In many cases it may be easier to use the calculated efficiencies rather than measure them. Depending on the level of accuracy desired this may be adequate or deviations from the analytic blaze function can be accounted for in another component.
SPTIME may be called directly as an IRAF task. Because of it's generality there are many parameters. Also there will usually be many tables for different gratings and filters. Therefore it is desirable to provide a user interface which limits the parameters and selections for specific instruments. The can be done with IRAF scripts, IRAF GUIs, or a web interface. At NOAO such a web interface has been developed for specific instruments. The purpose of this interface is to support users' in preparing proposals. The web form can currently be found at http://www.noao.edu/scope/spectime/.
The output of SPTIME consists of text and graphs. The text output verifies the requested inputs along with some additional detail and the results of the observation at some requested wavelength. The results include throughput information, the signal-to-noise and exposure time. Figure 3. shows an example of the text output.
There is a selection of graphs which may be produced showing various quantities as a function of wavelength. These include the expected observation data numbers, signal-to-noise, and throughput, as well as the individual transmission functions. Figure 1 shows an example of some of the graphical output.