User manual

The NIRWALS Simulator is a browser-based tool, which you can access online at https://simulator.salt.ac.za/. It should run in any recent version of Firefox, Chrome or Safari.

The simulator page has three tabs.

Generate Spectrum: A tab for defining the source spectrum and non-instrument and non-exposure related properties, and for generating plots of the source and sky background spectrum.
Configure NIRWALS: A tab for defining the instrument setup, and for generating a plot of the throughput.
Make an Exposure: A tab for defining exposure related properties, and for generating plots of the expected electron counts and signal-to-noise ratios.

Let's see how these tabs can be used to simulate a NIRWALS observation.

Generate Spectrum tab

We start on the Generate Spectrum tab, which conveniently is selected already when you first open the Simulator page in your browser.

Defining a spectrum

In order to define a spectrum, choose one of the spectrum types from the respective drop-down list and click the Add button. A new form section will appear, which lets you specify parameters related to your spectrum, as outlined in the following sections. Multiple spectra can be defined in this way, and all these spectra are added together. So, for example, you could simulate a blackbody with an emission line by first adding a blackbody and then an emission line. You can delete a spectrum by clicking on the cross in the top right corner of its parameter form, as shown below.

Blackbody

The spectrum type "Blackbody" denotes an (ideal) blackbody spectrum. The following parameters can be set.

Parameter	Description
Apparent magnitude	The apparent magnitude in the J band.
Temperature	The blackbody temperature, in K.

Galaxy

The spectrum type "Galaxy" denotes a galaxy spectrum. The following parameters can be set.

Parameter	Description
Apparent magnitude	The apparent magnitude in the J band.
Type	The morphological classification of the galaxy according to the Hubble sequence. The available values are E, S0, Sa, Sb, Sc and Sd.
Age	The age of the galaxy. The available values are Young and Old.
Redshift	The redshift of the galaxy.

The galaxy spectra are based on GALEV evolutionary synthesis models.

Emission line

The spectrum type "Emission line" denotes a Gaussian emission line. The following parameters can be set.

Parameter	Description
Central wavelength	The central wavelength, i.e. the wavelength for the line maximum, in Å.
FWHM	The full width at half maximum, in Å.
Flux	The total flux in the line, in erg / (cm² s) (for a point source) or erg / (cm² s arcsec²) (for a diffuse source).

Source magnitude

See the section on flux normalisation on the simulator physics page for an explanation of how the J band source magnitude is used.

Choosing the source extension

You have to select whether your source is a point source (the default) or a diffuse source. Diffuse sources are assumed to be completely homogeneous and infinite in size. For such sources the magnitude or flux must be given per square arcsecond.

Earthly items

You also need to specify a couple of parameters which are not related to the source.

Parameter	Description
Target ZD	The zenith distance of the source, i.e. the angle between the lines from the telescope to the zenith and to the source, in degrees.
Effective mirror area	The effective mirror area (see below), in cm².
Seeing	The atmospheric seeing in the direction of the zenith, in arcseconds.

As part of the SALT design, the pupil (that is, the view of the mirror as the tracker sees it) moves during the track and exposures, thereby constantly changing the effective area of the telescope. You should choose an average value for your planned observation.

The pupil for three different tracker positions. The grey areas are non-illuminated parts of the mirror.

Viewing the source spectrum and the sky background

You can view plots of the source spectrum and of the sky background by clicking the Show Spectrum button. These plots are not updated when parameter values change; you have to explicitly click the button again to get updated plots.

Warning

The current version of the Simulator does not take solar or lunar parameters into account when calculating the sky background.

Configure NIRWALS tab

Once you are happy with your spectrum (and the other parameters on the Generate Spectrum tab), you can select the Configure NIRWALS tab, on which you can specify the instrument setup abd estimate the throughput.

Instrument setup

You have to specify the following parameters for the instrument setup.

Parameter	Description
Mode	The instrument mode. The only option is Spectroscopy Mode.
Filter	The filter to use for your observation. The available options are Clear Filter and LWBF (Long Wavelength Blocking Filter).
Grating	The number of grating lines per millimeter. The only available option is 450.
Grating angle	The grating angle, i.e. the angle between the grating normal and the incident light rays, in degrees.

Viewing the throughput

You can generate a plot of the estimated throughput for your chosen parameters by clicking on the Update Throughput button. The throughput shown includes the following factors:

The atmospheric transmission.
The mirror efficiency.
The fibre throughput.
The filter transmission.
The grating efficiency.
The detector quantum efficiency.

This plot is not updated when parameter values change; you have to explicitly click the button again to get an updated plot.

Make an Exposure tab

Once the instrument is configured, you can proceed to the Make an Exposure tab.

Detector setup

You have to specify the following detector parameters.

Parameter	Description
Gain	The gain, i.e. the number of electrons per ADU.
Read Noise	The noise for a single detector readout, as a number of electrons.
Full Well	The full well capacity, i.e. the number of electrons a CCD pixel can hold before it saturates. At the time of writing, this parameter is not used.
Sampling mode	The sampling mode. The available optionns are Fowler and Up the Ramp.
Number of samples	The number of samples taken in a single exposure.

Regarding the gain, read noise and full well capacity you can select a predefined set of values suitable for a faint source, select a predefined set of values suitable for a bright source, or specify your own custom values.

Calculating the signal-to-noise ratio

If you want to calculate the signal-to-noise ratio (SNR) as a function of wavelength, you have to select the Solve for SNR tab near the bottom of the page (unless it is selected already, which it is by default) and specify the following parameters.

Parameter	Description
Exposure Time	The exposure time for a single exposure, in seconds.
Detector Iterations	The number of exposures to take.

Afterwards you can click the Solve for Signal to Noise button to view a plot of the electron count for a spectral bin as a function of wavelength, as well as a plot of the SNR for a spectral bin, again as a function of wavelength. These plots are not updated when parameter values change; you have to explicitly click the button again to get updated plots.

Calculating the exposure time

On the other hand, if you want to calculate the SNR at a given wavelength as a function of exposure time, you have to select the Solve for Exposure Time tab and specify the following parameters.

Parameter	Description
Requested SNR	The SNR value you need. This is used to decide what SNR range to include in the plot (see below).
Wavelength	The wavelength at which the SNR should be calculated.

You may use the Central Wavelength link to populate the wavelength input field with the midpoint of the considered wavelength range.

Note

Currently the considered wavelength range is fixed to be the full range available to NIRWALS, i.e. [8000 Å, 17000 Å].

By clicking on the Solve for Exposure Time button you can then view a plot of the electron count for a spectral bin as a function of wavelength, as well as a plot of the SNR for a spectral bin as a function of exposure time. The latter plot covers SNR values from 0 to twice the requested SNR. The plots are not updated when parameter values change; you have to explicitly click the button again to get updated plots.

Is there a Python library?

We currently do not offer a Python library for carrying out the simulations. You are of course welcome to have a look at the Python code pages or at the GitHub repository, but be warned - the data files required for running the code are not publicly available at this point in time.