Measured and modelled zeroth-order 1D projections, Jan.'00

Back to HETG TechNotes


The 1D projection of the zeroth-order image of an HETGS observation is a key measure of the performance of the non-grating system components (aspect system, HRMA, ACIS, and ground processing).

The zeroth-order image is generally piled up in its core and hence not an accurate indicator of the core PSF. The un-piled 1D projection of the PSF core can be obtained however by using the events in the ACIS frame-transfer streak.

Note: In the plots included here the x-axis represents distance along the TDET X direction; the peaks of the distributions have been artificially offset to 15.0 Angstroms and the x-axis scale is that of the MEG dispersion (~0.0111 A = 1 pixel.)

The plot at right shows in yellow the 1D projections from the zeroth-order and streak events for the Capella Obsid 1318 observation. The severe effects of pileup are clear in the depressed core of the zeroth-order curve (lower yellow curve.)

The same analysis was performed on a MARX simulation (which does not include pileup) and yields the purple and blue curves for zero-order and streak events respectively. The relative scaling of the streak and zeroth-order events has been properly taken into account as demonstrated by the internal agreement of these two simulated curves.

Comparing the Capella observation to the simulated data shows very good qualitative agreement in the overall profiles. The biggest difference is that the real observation's wings are perhaps a factor of two higher than the MARX model's.

The green curve shows the zeroth-order 1D projection from Mkn 412 (obsid 457.) It has been scaled to match the Capella wings and shows that the Mkn 421 core is also piled up with the core's peak at roughly 1/3 the expected level. (The Mkn 421 streak events are consistent with the expected level but not show for clarity.) The measured zeroth-order event rate for the Mkn 421 observation is of order 1 event per ACIS frame time.

[ click for full plot ]

Processing and Fits to the Curves

The histograms are of the non-dispersed zero-order and are binned in bins having left-right edges specified in detector or sky pixels running from 1.0 to 199.0 pixels with the peak of the distribution centered at 100.0. In order to have these projections look like dispersed data for analysis convenience, the center is mapped to 15.0 Angstroms and the MEG conversion from pixels to wavelength is applied.

ISIS software was used to fit each of the curves by a Gaussian in the core region and by a Lorentzian in the wings. Plots of the curves and text output of parameters of the fits are given in the following table.
Description Products
Capella zeroth-order plots , text
Capella streak plots , text
MARX Simulation zeroth-order plots , text
MARX Simulation streak plots , text
Mkn 421 zeroth-order plots , text


Histograms of the zero-orders were created with the HAK code, version 1.3 using an updated

The various plots and fits shown above were created using ISIS version 0.7 with the following ISIS scripts.

Please send any comments to Dan Dewey at

MIT Accessibility