OSIP: Order-Sorting, and Integrated Probability

Grating/ACIS-S order-sorting is done by comparing CCD Pulse-Height (PHA) values to limits stored in a table. These clipping limits are determined from the CCD resolution and gain, which are both dependent upon CCD pixel, especially CHIPY on the Front-Illuminated (FI) devices. The OSIP tables, used by tg_resolve_events, are constructed for an a priori specified width about the center of the main peak. The widths are typically 3*sigma (where sigma is the Gaussian dispersion of the main peak's functional parameterization). For the outer CCDs of the ACIS-S array (S0 and S5), it is necessary to reduce the width so that order regions do not overlap.

Here are graphical examples of the order-regions. The horizontal axes are the grating m*lambda (tg_mlam; m is the diffraction order and lambda the wavelength in Angstroms) from the Level 1.5 events file. The vertical axis is the ratio of the grating wavelength (m*lambda) to the CCD "wavelength" (proportional to hc/PHA), and is a real-valued estimate of the diffraction order.

The position of each photon is used to look-up the limits in the OSIP table. These are plotted for each photon and form the envelope about the events. In addition, the integrated probability between those limits is plotted, but offset in y by order-1 (this is used by mkgarf in computing the effective area).

Features to note:

Events within the boundary are assigned to the nearest integral order
The MEG and HEG boundaries get very wide on the side where the order is imaged at high CHIPY where CTI degradation has made the CCD resolution lowest.
The escape peak affect is visible in the integrated probablility (central line through each order) as a glitch at about 6 A;
The integrated probability drops on S0 and S5 because of the discontinuous change to 2*sigma limits
Bifurcation is present in the order boundary (the OSIP limits have doubled values at the same x); this is due to the strong gradient in CCD resolution with CHIPY, and the dither images photons at different CHIPY for the same x.
In the observed data, there are still residual gain errors which shift the distribution away from the expected center and causes some clipping (e.g., MEG at tg_mlam=-17A).

Simulation, -110 C, MEG

Simulation, -110 C, HEG

Observation, -110 C, MEG (Capella, ObsID 1318)

Observation, -110 C, HEG (Capella, ObsID 1318)

The OSIP files are:

Temp           Filename Template
-90     acisD1999-07-22osipN0003.fits
-100    acisD1999-08-13osipN0003.fits
-110    acisD1999-09-16osipN0003.fits
-120    acisD2000-01-29osipN0003.fits

IDL code which made the figures above is here.

2000.08.23 d.p.huenemoerder dph@space.mit.edu