Date of latest modification : September 02 1999


Frequently Asked Questions
(and Answers)

See also: SAX Observer's Handbook and its Addendum (return to home page and check out the AO information)
  • How do these cameras work ?
  • Their imaging properties derive from the coded mask principle. For an explanation, see the pertaining pages at the SRON site. Click here for images of 1 WFC or even 3 WFCs with their masks pointing at you!
  • What does the coded mask through which the photons enter, look like ?
  • Click here for a picture of the coded mask. The mirror symmetry around the diagonal (bottom-left, top-right) is clearly discernible.
  • What kind of images do the Wide Field Camera's (WFCs) produce ?
  • There are two kinds of images, i.e.,

    1. spatial detector images (example for diffuse sky background), which are the sum of the projections of the detectormask by all X-ray sources in the field of view

    2. spatial sky images

    A. Cygnus field raw image with Cyg X-1 and X-3
    B. Galactic centre
    1. raw image
    2. manipulated image with source identification,
    3. image made with a more refined algorithm.

    These celestial images are produced by deconvolving the detector images with the pattern of the detector mask.
    Detector images give no more than an indication of the presence of sources in the field of view, e.g. a strong source at an off-axis position will project the mask pattern on only a part of the detector. On the other hand, sky images can be directly interpreted and for example overlayed with celestial maps of known sources.
  • How large is the field of view ?
  • The Field of View (FOV) is 40 degrees by 40 degrees, in principle. However, as projection is the imaging principle, a source near the borders of the FOV can be difficult to recognise in the deconvolved image.
    Nevertheless, on occasion we have found Cygnus X-1 and GX9+9 within 10 pixels of the borders of the FOV. Due to the triangular response of the camera's, the FWHM (full width half maximum) of the FOV is 20 degrees by 20 degrees.
  • How many pixels does the detector boast ?
  • The mask contains 256 x 256 elements of 1 square mm (i.e. 1 x 1 mm^2)each, while the detector has 768 x 768 pixels of 0.141 square mm ( 3/8 x 3/8 mm^2), so that the mask pattern is oversampled.
    The sky image contains 510 x 510 sky pixels.
  • What is the detector background ?
  • Background levels were verified with the observation of an empty field observation at high galactic latitudes. The diffuse cosmic background was found to be 102 +/- 2 c/s for both WFCs in the whole energy range for the entire field of view. Detector background levels are time-dependent, with the lowest values at about 40 c/s. Summing up, the total background is about 140 c/s, so 10 c/s higher than specified in the original SAX Observer's Handbook (from Addendum to SAX Observer's Handbook).
  • What is the detector effective area ?
  • The geometric area of the detector consists of the sum of the surfaces exposed to the incoming detected photons - see some detector images (first light, diffuse background, same picture as above or galactic center with many sources). As the mask whithin the frame has 4 quadrants which each comprise 5x5 blocks of 2.3 cm x 2.3 cm, we find:

    Geometric Area = 4 x 5 x 5 x 2.3 x 2.3 = 529 cm^2

    However, to enhance the contrast in the projected mask image, only one third of the pixel area is open (2/3 is closed), while the pixel frame also occupies area. Of each open mask pixel of size 1.0 mm x 1.0 mm the rectangle of 0.9 x 0.9 mm^2 is actually open.
    Open mask fraction = window x open area fraction = (0.9 x 0.9) x 1/3 = 0.27

    Summing up,

    Effective Area = 529 x 0.27 cm^2 = 142.8 cm^2

    Another way to check the consistency of this number is to use the instrument definition files in the WFC software distribution (wfc1.ins and wfc2.ins, extension WFCcf). Reading out the values for the open and closed areas for the central pixels (around [x-pix,y-pix] = [255.5,255.5]), we find:

    maximum illuminated Open Area = 175.6 cm^2

    maximum illuminated Closed Area = 351.8 cm^2

    together 527.4 cm^2 which is consistent with the geometric area found above - the difference is caused by the averaging over the central pixel. We see also that in fact the open area fraction is 1/3 indeed.

    (At some places in the WFC literature, the effective area confusingly is quoted as "430 cm^2"; this is equal to the geometric area x the windowfactor = 529 x (0.9 x 0.9) cm^2, not taking into account the open fraction of 1/3).

    Figure 8 of the article describing the WFCs gives the full dependence of the effective area on the photon energy (Jager, R. et al., The Wide Field Cameras onboard the BeppoSax X-ray astronomy satellite, Astron. Astrophys. Suppl. Ser. 125, 557-572 (1997)).
  • What is the resolution with respect to angle, time and energy ?
  • 4.86 arcminutes (FWHM) corresponding to 1 pixel, 0.5 ms, and 18% at 6 keV, respectively. After complete calibration of the WFCs, it is expected that an angular precision of 1 arcminute can be attained in the localisation of sources. A deconvolved sky image contains 510 x 510 pixels.
  • What are the pointing directions of the WFCs in the frame of the satellite ?
  • The coordinate frame of the SAX satellite is by definition taken to be:
    +X : direction to the Sun,
    +Y : pointing direction of WFC2, the second wide field camera, and
    +Z : pointing direction of the NFI (Narrow Field Instruments).

    As the WFCs point in opposite directions with respect to each other, WFC1 observes the -Y axis.
  • How are the WFC instrument axes (x,y,z) defined with respect to the satellite axes (X,Y,Z) ?
  • The axes of the skyimage for the WFC1 are defined as:
    x1 = - Z
    y1 = - X
    z1 = - Y
    and those of the skyimage of WFC2 as:
    x2 = Z
    y2 = - X
    z2 = Y
    Refer to the StageI document for a detailed discussion.
  • How do sources contribute to the background in the sky image ?
  • The background in a sky image is 1/3 of the total sum of counts from all sources, due to the projection principle of the coded mask. This entails, that a single strong source in the field of view (FOV), e.g., Sco X-1, can affect the imaging of other sources.
  • What is the countrate of, say, Cyg X-1 or the Crab ?
  • On June 25, 1996 WFC1 observed the variable source Cyg X-1 for 1800 seconds.
    The countrate was then found to be 210 counts/second, while the average photon flux in the energy range of 1.8 - 30 keV amounted to 1.65 photons/cm^2 s. The measured flux of Cygnus X-1 was 2.0 e-8 erg/cm^2 s.
    In August 1996, a raster scan was performed on the Crab. When on axis in the centre of the field of view, a countrate of 297 cnts/s was found for Crab.
  • Which sources have been observed up to now ?
  • Thousands, too many to list here. Refer to the wfc observation log to search for a sky region or a time period.
  • According to the Sax Observer's Handbook, it is possible to observe mCrab sources in long exposures. Can this really be done ?
  • With some caveats: yes.
    The limiting sensitivity S of the WFCs at high galactic latitudes can be written in theory as S[Crab] = 0.2 / sqrt(t[sec]) so that a source of 3 mCrab, e.g., should just be visible in a 4 kilosecond exposure within a sky region with not too much noise.
    Sources of about 10 mCrab are seen routinely in WFC images at the Quick Look Analysis facility for data of one orbit (i.e., in 3ksecs).
    Furthermore, in the crowded galactic centre sources up to arond 7 mCrab have been seen in 51 ksec (see galactic centre image, also referred to above). The detection limit in this field is especially difficult to predict, as many sources vary in intensity and contribute to Poisson noise at other locations in the field of view.
    One should keep in mind that the statistics of mCrab sources will be poor and in consequence significant spectra as a rule cannot be extracted from these data - even after heavy rebinning. The following table gives a sample of weak sources detected up to now.
    Sensitivity limit detections far from the galactic plane in the 2 - 8 keV energy band

    Exposure time (ksec) SourceIntensity (mCrab)SigmaDetection limit (mCrab,5 sigma)
    5.5Tycho SNR8.49.03.2
    Gamma Cas6.05.23.0
    16 EXO0748-6766.67.52.3
    Eta Car4.48.11.5
    36AM Her2.77.61.5
    74AM Her1.56.70.9
    (Addendum to SAX Observer's Handbook by Jean in 't Zand, SRON, Utrecht)

  • How can I convert energy channels to keV ranges ?
  • See the preliminary conversion table.
  • What does the star tracker configuration code mean ?
  • See startracker explanation.
  • Where can I ask a real question ?
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    Questions or comments - even complaints - can be addressed to

    This page is maintained by Hans Muller (SAX/SDC - SRON)