IXO/CAT: Performance

 

CAT Performance, Characteristics

Data for the following plots was derived from the response files, and for convenience, some is tabulated here: ixocat_performance_D180.tbl. This is the "high throughput" scenario.

Performance using a Project goal of 3000 cm^2 effective area is given below following the high throughput section (the official requirement is 1000 cm^2).

The grating performance here is for a 1000 A period grating. Below we compare to the 2000 A grating, which has been fabricated and has a higher technological readiness.


CAT Effective area vs. diffraction orders 1-5 (color), and their sum (gray)

CAT effective area vs. order, energy scale CAT effective area vs. order, wavelength scale

Each of the smooth, single order curves covers the same spatial region; the sharp drops at order ends represent the physical edges of the detector readout array.


System effective area: CAT orders 1-5 (black), zeroth+direct (red), no-grating (gray)

The CAT orders 1-5 (black) and zeroth+direct (red) are obtained simultaneously on different detectors. The CAT gratings have some efficiency in zeroth order, and become practically transparent (transmission > 99%) at high energies; only the thick support structures lead to small reductions in efficiency. The innermost of the 3 sets of mirror annuli is not covered by gratings and contributes fully. The red curve is the CAT zeroth order plus the central clear mirror contribution, which are imaged onto the calorimeter (XMS). The red and black curves represent the combined CAT instrument effective area. The gray curve is for comparison to the no-grating case.

System effective area


Resolving Power; Individual orders (thin,colors); effective composite (thick gray)

CAT resolving power, energy scale CAT resolving power, wave scale

The effective resolving power was measured empirically by fitting the FWHM of the composite counts of overlapping orders. This is only one way to define the effective resolution.


System resolving power; CAT (black); microcalorimeter (red)

Note: These would be obtained simultaneously with the CAT instrument.

system resolving power


A Figure of Merit [sqrt(A*R)]: CAT (black), zeroth+direct (red), no-grating (green)

The CAT orders 1-5 (black) and CAT zeroth plus direct beam (red) are obtained simultaneously. The no-grating case (green) is shown for comparison with the full-open mirror and XMS-only.

system figure of merit



Area and Figure of Merit for the IXO Project Requirement Configuration

We have also computed responses for the baseline effective area of 3000 cm^2, resolving power of 3000. These are the relevant figures for that lower area. Data for the following plots is tabulated here: ixocat_performance_D54.tbl.

CAT Effective area vs. diffraction orders 1-5 (color), and their sum (gray)

CAT effective area vs. order, energy scale CAT effective area vs. order, wavelength scale

System effective area: CAT orders 1-5 (black), zeroth+direct (red), no-grating (gray)

System effective area
A Figure of Merit [sqrt(A*R)]: CAT (black), zeroth+direct (red), no-grating (green)
system figure of merit

Comparison to 2000 A Period CAT Grating

The 2000 A period grating has a higher technological readiness and small facets have been fabricated. Overall efficiency is similar to the 1000 A grating, but is different in detail. Instead of 5 orders, 10 are required to cover the same wavelength range. The detector array is also shorter.

The following graph compares the efficiencies.

2000 A period CAT efficiencies compared to 1000 A period

The solid red curve is the sum of 10 orders for the P=2000A case. The thick solid green curve which is for the 1000A CAT follows it fairly closely; it is higher at the lowest energies (but here CCD efficiencies will be rather low). Hence, simulations done with the P=1000 A model are also an adequate represention of expected performance for the 2000 A configuration.

Here are alternate views of the model, showing individual orders, and showing the total efficiency on a wavelength scale.

2000 A period CAT efficiencies, all orders 2000 A period CAT efficiencies, wavelength


This page was last updated Jan 8, 2010 by David P. Huenemoerder. To comment on it or the material presented here, send email to dph@space.mit.edu.
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