FUSE Simulator Tool Help Page

Version 1.3, Updated for NRA 2
The FUSE Simulator Tool takes user-defined inputs from a Web form and calculates a simulated FUSE spectrum. A variety of input spectra are available, including modifications for varying amounts of foreground extinction, neutral hydrogen absorption, and molecular hydrogen absorption. The simulator calculates the expected raw counts, the flux-calibrated data product, and the signal-to-noise (S/N) ratio in all four detector channels separately. Poisson-distributed noise is added to the simulated count and flux spectra. The user may view output of any of these quantities as well as sums of the SiC and LiF channels separately, or all four co-added.

The simulator tool does not provide a completely accurate description of the instrument performance. The "pixels" used by the simulator are user selectable, and they do not correspond to actual detector pixels. While the actual instrument resolution varies between 20,000 and 25,000 across the different spectral channels, the simulator assumes a uniform resolution of ~20,000 for point sources observed through the larger apertures. There is fixed pattern noise in the detectors (visible at S/N ratios of 20:1 or higher) that is not simulated. Instrument performance at very low flux levels is also not well characterized. The simulator tool is intended to give a first-order impression of the expected count rate and approximate resolution. It cannot be used to address feasibility in situations that stretch the performance of FUSE.

Once input parameters have been chosen, click the "Do the Simulation" box at the bottom of the page using the left-most mouse button. A typical simulation of the full 900-1200 Å range with 0.01 Å pixels requires about 1 minute.

For help with displaying the output products, see the section entitled Viewing the Output.

Other References:

User-supplied Parameters:

Wavelength Interval to Simulate: FUSE is sensitive in the wavelength range 900 Å to 1200 Å. You may choose any wavelength interval in this range for your simulation.

Simulation Bin Size: FUSE has a spectral resolving power of R=20,000 to 25,000 over the whole far-UV band. The ~6 µm FUSE detector pixels translate into a spectral pixel size of 0.006 Å. One resolution element corresponds to ~6 detector pixels. As a consequence, the simulator uses a minimum bin size of 0.01 Å for the simulation. Larger bin sizes can be chosen (this speeds up the calculation), but you should be careful that you do not undersample narrow emission or absorption features, or the simulation will be inaccurate.

Output Bin Size: Normally the number of output bins will equal the simulation bin size, but for faint targets where high spectral resolution is not the goal, you may choose to bin up the result to obtain increased signal-to-noise.

User Supplied Spectrum: To select this option, click the button on the left. This is one of two choices for supplying an incident spectrum to the simulator. A user-supplied spectrum should be a full pathname to a file residing on violet.pha.jhu.edu (or a disk accessible from violet). The file should be ASCII-format with two columns giving wavelength and relative flux in ergs cm-2 s-1 Å-1. The modeled flux is normalized to the Source flux at the specified wavelength. The normalization is done before any extinction or absorption is applied.

Users without accounts on violet can use anonymous ftp to place a model file in an accessible directory: 
	> ftp violet.pha.jhu.edu
	> login: anonymous
	> passwd: your_email_address
	> cd upload/fuse_sim
	> put filename
	> quit
The pathname you should then enter on the fwebsim input form is 
	/home/violet/ftp/upload/fuse_sim/filename
Model Spectra: To select this option, click the button on the left. To see the full selection of model spectra available for use, use the scroll bar on the right-hand side of the window. To choose a particular model, click on the desired spectrum so that it is highlighted. The flat spectrum is a constant in F(lambda). The power laws are also in flambda. The remaining spectra are stellar models from the small grid computed by Tom Brown with temperature, surface gravity, and log abundance as shown. These versions have been convolved with a rotational line profile using V sin i = 100 km s-1.

Source Flux: The selected input spectrum is normalized to this flux at the specified wavelength. The normalization is done before any extinction or absorption is applied.

E(B-V): The input spectrum can be attenuated by foreground extinction. The extinction curve used is a Cardelli, Clayton, and Mathis (1989, ApJ, vol. 345, p. 245) model assuming RV =3.1.

Neutral Hydrogen Absorption: A selection of foreground neutral hydrogen columns is available by scrolling through the window with the scrollbar on the right. All current models are for an assumed Doppler parameter of b=10 km s-1. The models use Voigt profiles for all Lyman lines up through n=50.

Molecular Hydrogen Absorption: A selection of foreground molecular hydrogen columns is available by scrolling through the window with the scrollbar on the right. All current models are for Voigt profiles with an assumed Doppler parameter of b=1 km s-1. Only the ground-state vibrational level is assumed to be populated. Upper rotational levels are assumed to be in thermal equilibrium at a temperature of 80 K.

Slit: Three slits are available for observations. To select one, hold down the left-most mouse button and release it on the slit of your choice. The 30" aperture is the default for most observations as it gives the best assurance that the target will remain in the slit throughout the course of an observation. It also provides the best grasp on diffuse sources. The 4.0" aperture nominally gives 95% throughput. For nominal pointing characteristics, the instrumental resolution is not degraded for observations of point sources. The 1.25" aperture ensures the best instrumental resolution, but its throughput is only 65% (for nominal pointing characteristics of the spacecraft). For both the 4.0" aperture and the 1.25" aperture, special peakups and scheduling are required to assure that the target remains in the slit for all four channels.

Day/Night: These buttons permit the user to choose between orbital day or orbital night for the simulation. Airglow line intensities appropriate to the selection are then used in the simulation. During orbital night, airglow is mostly in the Lyman lines. During orbital day many other lines from the earth's upper atmosphere also become visible. The line intensities are intended to be representative. In reality they are a smoothly varying function of solar zenith angle and the local viewing direction. Scattered Lyman alpha also contributes to the background in the larger apertures. The daylight airglow spectrum assumes a Lyman alpha intensity of 20 kR; this is reduced to 1.5 kR during orbital night.

Point/Diffuse: Point sources are assumed to have an instrumental resolution as represented by the 4.0" aperture, roughly R~20,000. Diffuse sources are assumed to fill the aperture, and the resolution is degraded depending upon the chosen aperture size.

Viewing the Output

When the simulation is completed a new web page appears that permits the user to select the output format. The results can be viewed either as an ASCII file or plotted as a PostScript file. The user is free to select the displayed wavelength and intensity ranges for the plot. Multiple views of different wavelength ranges or intensity ranges of the same output product are possible. To conserve disk space, however, files older than 30 minutes are deleted every half hour, so all output viewing for a single simulation run must be completed within 30 minutes.

Once input parameters have been chosen, click the "Do the Print/Plot" box at the bottom of the page using the left-most mouse button. After the printed file or the plot has been viewed, click the "back" button on your Web browser to return to the FUSE Simulator Tool Results page to continue viewing the output.

Select an output product to display: A total of 21 output files are generated by the simulator: raw counts in each of the four detector channels, summed raw counts in the SiC and LiF channels separately, and summed raw counts from all four channels. A similar set of 7 files is available showing the data in flux-calibrated form and also as S/N vs. wavelength. To choose a particular file for display, use the scroll bar on the right side of the inset window to view the possibilities, then click the desired file type with the left-most mouse button. The selected file should appear highlighted.

Displayed Wavelength Range: Enter the minimum and maximum wavelengths to be shown in the plot. If either value is "-1", the range will be autoscaled.

Displayed Intensity Range: Enter the minimum and maximum values to be displayed on the y axis. Raw count files are in units of counts per output bin size, flux files are in units of ergs cm-2 s-1 Å-1, and S/N files are in units of the S/N per output bin. If the value for the maximum intensity (the second input box) is "-1", the range will be autoscaled.

Print or Plot: Use the left-most mouse button to click whether you want to view the ASCII file or to make a plot.



gak@stsci.edu