OGIP Calibration Memo CAL/GEN/92-022

(OBSVERSN = 1992a)
Ian M George & Ron S Zellar
Mail Code 668,

Version: 1994 Aug 08


This document describes the standard format adopted by the OGIP for the storage of the obscuration factor (a.k.a. collimator response) of an instrument as a function of energy, and position.
Intended audience: primarily OGIP programmers & hardware teams.
Log of Significant Changes
Release Sections Changed Brief Notes
1992 Jul 24 First Draft (within memo CAL/GEN/92-003)
1993 Oct 04 All Separation from CAL/GEN/92-003
1993 Nov 24 All Added HDUCLASn info
1994 Aug 08 All General Review/up-dates


The following documents may also be of use:

1  Introduction

Within the OGIP caldb, the term Obscuration Factor refers to the energy-independent reduction in the collecting area of an instrument as one moves off-axis (normalized to unity on-axis) due purely to the geometry of the collecting surface(s) and surrounding support structures. Elsewhere, the Obscuration Factor is also sometimes referred to as the 'Collimator Reponse' or 'Geometric Vignetting Function'.

1.1  Storage Options

In the general case, an Obscuration Factor calibration dataset consists of a 2-dimensional grid, ObscFact, with the axes defining the position relative to the optical axis (i.e. off-axis & azimuthal angles θXMAXMA), in detector coordinates (e.g. θXMA, φXMA), or potentially any other spatial coordinate frame. Below we assume the former for convenience only. There are no other major storage options.

1.2  Dataset Origins & Storage Recommendations

The construction, format used (within the limitations discussed here) and delivery of the data to the HEASARC (including any updates) is the responsibility of the h/w teams and/or GOF. However, below, are the recommendations of the HEASARC calibration team based on their experience.
An Obscuration factor dataset is required for scanning, collimated detectors. Virtual calibration files are generally prefered.
Prior to launch, the effects of vignetting and obscuration at off-axis positions are usually measured at a (limited) number of photon energies during ground calibration experiments and/or combined with theoretical (e.g. ray-tracing) models to produce the off-axis correction factors. It is recommended that the Vignetting function and Obscuration factors be stored separately in the form of CCNMxxxx = VIGNET (see CAL/GEN/92-021) and CCNMxxxx = OBSCFACT datasets.
The Vignetting function (either alone, or including the effects of obscuration) alone cannot be measured in-orbit. Instead, observations of standard cosmic sources (e.g. the Crab) combined with spectral modelling enables the Spectral Response of the instrument (i.e. the effective area of the optics multiplied by the vignetting function, the transmission of any filters & windows and by the detector efficiency as a function of energy) to be determined. The results of such calibration observations should be stored as a CCNMxxxx = SPECRESP dataset. However, should such measurements reveal that a discrepancy with previous calibrations which is identified with (or interpreted as) a mis-calibration of the obscuration factor dataset, h/w teams are urged isolate and also supply an updated obscuration factor dataset/algorithm to the HEASARC.

1.3  Dataset vs Task Summary

It is often fairly straightforward to parameterize the obscurration factor of an instrument. As a result such a calibration dataset may often be more easily and economically storted as a virtual calibration file, and an associated standalone s/w task (see CAL/GEN/92-003). This is recommended wherever possible.

1.4  Software Considerations

Data Files:
Interpolation between the spatial coordinate grids (eg thetaXMAXMA) grid points is usually required. By default, downstream software will use a simple 2-dimensional linear interpolation when calculating the Obscuration factor between θXMAXMA grid points. Thus the θXMAXMA grid should be of sufficient resolution to enable this to be reasonable approximation.
Virtual Files:
No specific issues.

1.5  Relationships to Other Calibration Datasets

Downstream s/w should assume further calibration input is required for an Obscuration Factor dataset under the following conditions:
An Obscuration Factor dataset is used in the construction of the following calibration datasets:

2  Data File Formats

The dataset file formats currently allowed are:

2.1  The Obscuration Factor Extension (OBSVERSN = 1992a)

One extension for each telescope in a BINTABLE format with a single row of 3 columns: two containing the spatial position grid points, and one containing the Obscuration Factors (relative to an on-axis value of unity).
Extension Header
Beyond the standard FITS keywords required, the following keywords/values are mandatory: and the following keywords/values are mandatory for CIF purposes (see CAL/GEN/92-008): and the following mandatory to supply further information:
Data Format:
The data within the extension is organised as a BINTABLE with the following columns:
  1. θXMA, a fixed-length REAL vector (array, each element within which is 4-byte) containing the off-axis angles.
    The FITS column name is THETA (but see below).
    The recommended units are arcmin.

  2. φXMA, a fixed-length REAL vector (array, each element within which is 4-byte) containing the azimuthal angles.
    The FITS column name is PHI (but see below).
    The recommended units are arcmin.

  3. ObscFact, a fixed-length REAL vector (array, each element within which is 4-byte) containing the obscuration factor at each θXMAXMA grid point.
    The FITS column name is OBSCFACT.
    The order of data storage is ObscFact (θXMAXMA)

These are summarized in Table 1.
Table 1: Summary of the OGIP format for Mirror/Collimator Assembly Obscuration Factors (OBSVERSN = 1992a).

to (filename).(ext)

HDUVERS1: 1.0.0
HDUVERS2: 1.1.0
EXTNAME : OBSFACT (suggested, not required)
Description: Energy independent obscuration factors for an XMA or Collimator as a function of spatial position (eg off-axis & azimuthal angles).
An alternate spatial coordinate frame may also be used (see text).
Optional columns containing the statistical and systematic error arrays are not shown.
1 23
Off-axis Azimuthal Obscuration
anglesangles Factors
θXMA φXMA ObscFact
format of each column
4-byte 4-byte 4-byte
real real real
array array array
total number of elements per row
i j i×j
column name

Points to Note & Conventions

3  Virtual File Formats & Allowed Standalone Tasks

Standalone tasks to perform the following tasks are currently allowed:

3.1  VCF Requirements

See CAL/GEN/92-013.
Extension Header
Beyond the standard FITS keywords required, the following keywords/values are mandatory: along with those keywords/values mandatory for CIF purposes as given in within the appropriate sub-section of Section 2, with the exception of: those required for all virtual files listed in CAL/GEN/93-013, and the following mandatory to supply further information:
Data Format:
See CAL/GEN/92-003 and CAL/GEN/92-013. The number and type of parameters specified depends solely on the requirements of the associated standalone task.

4  Related Software

The following list of subroutines/tasks are available:

5  Example FITS headers

... section incomplete


George, I.M. & Zellar, R.S., 1992. OGIP Calibration Memo CAL/GEN/92-003. f
George, I.M. & Arnaud, K.A., 1993. OGIP Calibration Memo CAL/GEN/92-002a
(addendum to CAL/GEN/92-002). f
George, I.M., Arnaud, K.A., Pence, W. & Ruamsuwan, L., 1992. Legacy, 2, 51.
(CAL/GEN/92-002 f)
George, I.M., Zellar, R.S. & Pence, W., 1992. OGIP Calibration Memo CAL/GEN/92-011. f
George, I.M., Pence, W. & Zellar, R.S., 1992. OGIP Calibration Memo CAL/GEN/92-008. f
George, I.M., Zellar, R.S. & White, N.E., 1992. OGIP Calibration Memo CAL/GEN/92-013. f
George, I.M. & Zellar, R.S., 1992. OGIP Calibration Memo CAL/GEN/92-019.
George, I.M. & Zellar, R.S., 1992. OGIP Calibration Memo CAL/GEN/92-022.
George, I.M. & Angelini, L., 1993. OGIP Memo OGIP/93-013. f
f available on-line from the anon ftp account on legacy.gsfc.nasa.gov.

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On 8 Apr 2004, 10:59.