The Web-based WFC3 Spectroscopy Simulator aXeSIMweb

We offer the simulation of WFC3 slitless spectrscopic images as a web application. The user can either prepare and upload a catalogue with many simulation objects or specify the parameters of a single simulation object directly in the web form interface.

After submission, the simulation is scheduled and executed. The simulation results are written to a URL and the user is notified by email where to download his data. The simulations are available on this URL for the period of three weeks.

The web application uses the slitless simulation software aXeSIM to compute the simulated images, offering a subset of the options available in aXeSIM. User who would like to conduct extensive simulations or use their own spectral templates are encouraged to work with aXeSIM directly.

A set of templates is available to describe the spectrum of the simulation objects.

Run aXeSIMweb

Disclaimer

aXeSIMweb was developed independently of the STScI WFC3 spectroscopic Exposure Time Calculators (ETC). While every effort has been made to ensure that aXeSIMweb provides similar estimates of detected counts as the ETC, differences are bound to result. The STScI ETC is to be taken as the official tool and should be used to determine exposure times for the purposes of critical time justification in HST Phase I and II proposals. The WFC3 ETCs can be found here.

Input Fields

The name of most input fields correspond to parameters of the aXeSIM task simdata, and users are referred to the aXeSIM manual for a thorough description of the various parameters and the simulation process..

Object catalogues prepared by the user are uploaded with the input field incat (2a), which is a file selector. Alternatively, the parameters of a single object are given in the fields 2b (X_IMAGE, Y_IMAGE ... SPECTEMP). Every field in 2b corresponds to a column in an object catalogue.

Mandatory fields are marked with a * (as in the web application).

*1 email

Email address of the user. After the simulation the notification email is send to the address given here.

*2a incat

The catalogue with simulation objects provided by the user. It corresponds to the Model Object List used in the aXeSIM software package. The format of the object list is described below.

*2b X_IMAGE

The X-position of the simulated object in [pix]. This position refers to the direct image that accompanies the simulated slitless image.

*2b Y_IMAGE

The Y-position of the simulated object in [pix]. This position refers to the direct image that accompanies the simulated slitless image.

*2b A_IMAGE

The major axis extent of the simulated object in [pix]. As the corresponding object parameter in SExtractor, it is the simga of the object extent, not the FWHM.

*2b B_IMAGE

The minor axis extent of the simulated object in [pix]. As the corresponding object parameter in SExtractor, it is the simga of the object extent, not the FWHM.

*2b THETA_IMAGE

The major axis angle of the simulated object in [deg], measured with respect to the positive X-axis.

*2b MAG

The brightness of the simulated object in [mag_AB]. An object with a spectral template is normalized to this brightness level in the total passband given by tpass_flux. For an object with a flat f_λ "spectrum", the f_λ value is calculated using the brightness value and the pivot wavelength of the total passband given in tpass_flux.

*2b Z

The redshift of the simulated object

*2b SPECTEMP

The spectral template of the the simulated object. The available spectral templates are described and illustrated here.

*3 config

The slitless configuration, hence instrument and grism, to simulate. For the UVIS/G280 mode, only CHIP #1 can currently be simulated.

*4 tpass_flux

The total passband to use for normalizing the spectral templates or determining the flat f_λ values. Total passbands (including HST mirrors, entrance window and so on) for all WFC3 broad band filters are available.

5 exptime_disp

The exposure time of the simulated dispersed image. If given, noise is added using a simple noise model (noise from readout, object and sky background noise). In all cases the simulated dispersed image is given in [e/s].

6 bck_flux_disp

The background flux for the dispersed image in [e/s]. Typical avalues are 0.089 e/s for the UV/G280 grism, 1.0 e/s for the IR/G102 grism and 1.5 e/s for the IR/G141 grism. The background flux is added to the dispersed image only if an exposure time is also given.

7 tpass_direct

The total passband for simulating the direct image that accompanies the dispersed image. is simulated if the input field is left empty. Total passbands (including HST mirrors, entrance window and so on) for all WFC3 broad band filters are available. The position of the direct image with respect to the slitless image mimics closely real data.

8 exptime_direct

The exposure time of the simulated direct image. If given, noise is added using a simple noise model (noise from readout, object and sky background). In all cases the simulated dispersed image is given in [e/s].

9 bck_flux_direct

The background flux for the direct image in [e/s]. The background value is added to the dispersed image only if an exposure time is also given.

User Provided Catalogue

With a pre-prepared catalogue several objects can be simulated. There is a limit for the maximum number of objects to be simulated in one run, which is set to 100.

Catalogue Format

The catalogue of simulation object is a simple ASCII list, and each row contains the parameters of one object to be simulated. It corresponds to the Model Object Table in the aXeSIM task simdata. The format is identical to the SExtractor object table format.

Mandatory columns are:

• NUMBER: unique obejct identifier; int
• X_IMAGE: X-position of object [pix]; float
• X_IMAGE: Y-position of object [pix]; float
• A_IMAGE: Major axis rms [pix]; float
• B_IMAGE: Minor axis rms [pix]; float
• THETA_IMAGE: Major axis object angle with respect to the X-axis [pix]; float
• MAG_?<number>*: AB-magnitude at wavelength <number> [nm]; float

In this format all object "spectra" are flat in f_λ with the f_λ value derived from the AB-magnitude and the wavelength <number>. To use the template spectra provided in the the web application, following columns must be added:

• Z: redshift of the object; float
• SPECTEMP: index of the spectral template; int

The entry in the column SPECTEMP which corresponds to each spectral template (e.g. "10" for a F0V star and "30" for a Sc galaxy) is given in the pull-down menus for the input field SPECTEMP and in the spectral template description. The entry "0" marks an object with a flat f_λ "spectrum".

Example

As an example, we give a catalogue with 10 simulation objetcs. Objects #1 and #10 have a flat f_λ "spectrum", all others spectral templates at various redshifts.

# 1  NUMBER
# 2  X_IMAGE
# 3  Y_IMAGE
# 4  A_IMAGE
# 5  B_IMAGE
# 6  THETA_IMAGE
# 7  MAG_F438W
# 8  Z
# 9  SPECTEMP
     1  500.0   100.0  1.0  1.0  90.0    25.0  0.0   0
     2  500.0   200.0  1.0  1.0  90.0    25.0  0.1  26
     3  500.0   300.0  1.0  1.0  90.0    25.0  0.2  27
     4  500.0   400.0  1.0  1.0  90.0    25.0  0.3  28
     5  500.0   500.0  1.0  1.0  90.0    25.0  0.4  29
     6  500.0   600.0  1.0  1.0  90.0    25.0  0.5  30
     7  500.0   700.0  1.0  1.0  90.0    25.0  0.6  31
     8  500.0   800.0  1.0  1.0  90.0    25.0  2.0  32
     9  500.0   900.0  1.0  1.0  90.0    25.0  2.5  32
    10  500.0  1000.0  1.0  1.0  90.0    25.0  0.0   0