Simulations

Response files can be downloaded here:

Instrument	Response Files
X-ray grating	Tarball (693 MB)
Far-UV spectrometer	Tarball (1.2 MB)

An exposure time calculator for the FUV spectrometer can be found here.

A Word About X-ray Cross Talk
Please note that cross talk between orders (order overlap) affects all orders, and should not be ignored, as doing so will lead to significant errors. What this means is, if you want to simulate a spectrum in, say, the -6 order, you need to load the responses for the -6 order plus the contributing responses for the -5 and -7 orders. Arcus relies on the CCD intrinsic energy resolution to sort geometrically overlapping spectral orders. There are different ways to assign photons to the different spectral orders which have an effect on the amount of cross talk there is: With very conservative PHA filtering, the cross talk is held to a minimum — but this reduces the effective area, so you will need to compensate with a longer exposure time. The flip side of this is to increase range of PHAs sorted into any one order; this will give you a larger effective area and reduce the exposure time, but it will increase the amount of cross talk. (A more in-depth discussion of order sorting can be found here.)

These two approaches are included in the X-ray response files. The "osiptouch" directory corresponds to the maximum extraction area; Arcus's full effective area is used, but cross talk can be very substantial (up to ~20%). The "osip60" directory corresponds to a very narrow extraction region; cross talk is minimal, but this comes at the cost of a ~40% reduction in Arcus's effective area.

All in all, simulating spectra with Arcus can get complicated rather quickly if you are not yet familiar with it. To address this, the Arcus team has made a guide for how to simulate spectra using PyXSPEC, ISIS, XSPEC, and SPEX. A guide on how to use the response files in Sherpa can be found here.

A Closer Look
People searching for more detail on Arcus's optical layout or other technical specs will probably find what they are looking for on Dr. H. Moritz Günther's pages. These include results from ray-trace simulations to get a sense for what Arcus data would look like and performance metrics. There are also interactive 3D overviews of the lightpath, as seen below, to make it easier to visualize the components and layout of the CCDs. The first overview is for a point source at a single energy; the second is for a point source with a flat spectrum. Quoting from Dr. Günther's website:

Point source at single energy

"Rays start in the aperture, which consists of four rectangles located above the SPO channels. Each channel has a number of SPO modules, shown in green. Photons bounce off the mirrors twice in a Wolter type I geometry. However, in this simulation the SPOs are somewhat simplified such that the reflection actually happens in a single plane. Behind the SPOs are the CAT gratings (white). For each channel the gratings are arranged to follow the shape of the Rowland torus. This can been seen best when the figure is rotated to look at the 'side' of the arrangement. CAT gratings are slightly tilted such that the photons hit the gratings bars on the 'side', but this blaze angle is < 2 degrees and is hard to see by eye. SPOs and gratings are held by a support structure in the forward assembly, which is not shown. The forward assembly and the focal plane are connected by an extensible boom (black). Some SPOs are located outside the boom, and the light needs to pass though the sides of the boom, as shown in the boom study. Detectors (yellow) are again arranged on the surface of the Rowland torus, zooming in on the detectors shows that they follow a circle.

"Note that the Rowland torus for each channel is slightly different, see the Rowland torus specifications for details.

"The figure shows a ray-trace for an on-axis point source with monochromatic emission at 0.5 keV = 24.8 Å. Arcus has four channels positioned in two pairs. The optical axes are different for each channel to reduce the impact of chip gaps. Two channels disperse left to right, the other two right to left, also shown in the explanatory images of the signal on the detector. In the simulation, the bundle of rays from each channel forms a separate trace on the detectors, rays from different channels do not mix. Rays are colored by channel, i.e. all rays in the same channel have have the same color."

Point source with a flat spectrum

"This simulation is very similar to the above. However, here the spectrum is continuous instead of just a single energy, and thus four the traces on the detector can be seen better. The rays are colored according to their grating order. Orders overlap spatially (e.g. order 5 for 20 Å photons gets dispersed to the same position as order 4 for 25 Å photons) and need to be separated using the energy resolution of the CCDs. In the ray-trace, we can thus see rays of different color at the same spatial position."

Questions? Concerns? Contact us!
People with questions are strongly encouraged to contact Dr. H. Moritz Günther ( hungther [at] mit [dot] edu ) or Dr. David Huenemoerder ( dph [at] space [dot] mit [dot] edu ) for more information.