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IMAGE OBSCURATION IN THE LE TELESCOPES

1. Summary

Ray tracing studies have been performed to clarify the origin of unexpected structural features observed in the images of both EXOSAT low energy telescopes. A comparison of observed and simulated images covering the total field of view suggests that the ME flap has opened by an angle of 94.5 ± 0.5 degrees, thereby obstructing the aperture of LEIT1 and LEIT2 and reducing the effective telescope throughput to 0.721 of its nominal value for on-axis sources. Several other possible causes of the image obscuration have been investigated and rejected, viz: LE flap not fully opened, transmission grating ring not entirely in "out" position, filter wheel position shift, selective degradation of mirror coating material, field stop position shift and entrance aperture position shift.

Although 'over-opening' of the ME flap can explain the anomalous features observed in the LEIT images, MBB (the satellite prime contractor) comment that there is no evidence that the mechanical design was in any sense marginal and that qualification tests in vacuum demonstrated in a fully representative fashion tne performance of the hardware and its conformity with design.

2. Diagnosis of observed images

Note that two different coordinate systems are commonly used in describing the EXOSAT system: the spacecraft coordinate system and the LE image coordinate system. +y of the image system is practically equivalent to -y of the spacecraft system and +x of the image system to -z of the spacecraft system. Unless stated, reference is made to the image system coordinates.

An analysis of a number of images at different positions in the field of view of both telescopes has been carried out using data from a raster scan observation of Cyg X-2 (day 186, 1983). 40 positions in the LEIT1 FOV and 7 in the LEIT2 FOV have been used. Fig. 1 shows typical images (LEIT1) in each quadrant at approximately 25 arc min from the centre of the FOV.

A number of conclusions can be drawn from an analysis of the images:

  1. For all field positions outside a radius of about 15 arcmin around the centre, the image shows a cut-out wedge of nearly zero brightness.
  2. Each wedge is symmetric with respect to the y-axis and opens towards the +y direction.
  3. The opening angle of each wedge is between 95° and 120°.
  4. Images at opposite y positions tend to show a change in opening angle but in general the characteristics of the wedges in each of the four quadrants of the field of view are identical.
  5. Images with a relatively high surface brightness reveal a wedge not of zero brightness but containing counts clearly in excess of the ambient background.
  6. LEIT1 and LEIT2 images show identical wedges for each field of view position.
  7. Close to the centre of the field-view, ie. at a radial distance of less than about 15 arcmin, the images appear elongated, being longer in the x than in the y direction.

3. Ray Tracing in the EXOSAT LE Telescopes

EXOSAT carries two identical low energy imaging X-ray telescopes of the Wolter type 1 configuration, ie. a confocal and co-axial mounting of a grazing incidence paraboloid and hyperboloid. Each telescope comprises two nested systems. The telescope optical axis is oriented in the +x direction (in spacecraft coordinates). The field of view is limited by field stops in the paraboloid and hyperboloid section. For stray light baffling, an aperture plate is mounted in front of the telescope. It has a free opening slightly larger than would be required for the field of view. Flaps, which covered the ME detectors (ME flap) and both telescopes (LE flap) during the ground calibration and launch phases, were opened in flight to act as telescope and star tracker light baffles and are positioned towards +y and -y respectively (spacecraft coordinates) with their surface planes parallel to the x-z plane.

The following items have been specifically modelled in the program:

  • mirror surface geometry.
  • size and position of field stops in the paraboloid and hyperboloid sections.
  • size and position of entrance aperture plate.
  • position of grating hinges.
  • size and position of grating ring.
  • size and position of filter wheel opening.
  • position of LE flap hinges.
  • size and position of LE flap.
  • position of ME flap hinges.
  • size and position of ME flap.
A dedicated ray tracing program has been used to simulate the X-ray images formed by the EXOSAT LE telescopes. The code generates X-ray events randomly distributed across the aperture area for any given angle of incidence and determines their paths to the focal plane, checking for the proper intersection requirements with the mirror surfaces and for any obstruction.

Because perfect mirror surfaces introduce additional features in the images, the program includes local axial slope errors of the surfaces, according to the measurements on the EXOSAT qualification model (P. de Korte et al. App.Optics, 20, 1080 (1981)).

4. Origin of the Image Distortions

Since the wedge in the X-ray images is always in the +y direction, the location of the obscuring object is in one specific hemisphere. Extreme ray paths for the inner and outer shells of the telescope system define the envelope of the ray bundle within which rays can be focussed by the telescope. Figure 2 gives a close-up view of this envelope in the region of the focal plane and shows that rays entering the telescope at the highest y- values are imaged into a region of lowest y-values and vice versa. This property is important because any image distortion can then be attributed directly to the relevant hemisphere of the telescope and the origin of the distortion investigated along the ray path in this particular hemisphere.

Because the wedges appear at +y, the obscuring body has to be in the -y hemisphere as seen from the telescope's optical axis. This conclusion is valid (see figure 2) if the detector is located close to the nominal focal plane. (Only a shift of more than 7 mm towards the mirror system would give the reverse signs. Such a de-focus,ing would however produce a significantly broadened onaxis image - which is not observed). The following items can therefore be excluded as possible obstructions of the ray path because they both are located in the +y hemisphere.

  • the transmission grating ring (not fully in "out" position).
  • the LE flap (not fully opened).
4 candidate obscuring mechanisms, all associated with the -y hemisphere, have been studied and rejected as follows
  • A shift of the filter wheel in the y direction - obscuration at +y and -y cannot be reproduced.
  • Deterioration of the gold coating of the mirror surfaces over a sector angle of about 100 degrees, such that the reduced reflectivity produces a wedge of lower surface brightness in the X-ray image - a significant flux would be expected even if the gold were totally removed and this is not observed.
  • A shift of the field stops inside thc mirror assembly in the y direction~- it is not possible to produce the observed image structure.
  • A shift of the entrance aperture plates, located about 100mm in front of the telescopes - the blocking effect is asymmetric with respect to the y direction.
The effect of a rotation of the ME flap of more than 90° (88° is the nominal opening angle), such that the upper part of the flap shadows the entrance aperture of both telescopes by the same amount, has been studied. A series of simulated images has been produced at a variety of different source positions within the field of view. Figure 3 shows typical EXOSAT and simulated images for the same FOV position. A comparison of all observed and simulated images suggests that the ME flap is within the field of view of LEIT1 and LEIT2. Furthermore, under this assumption each property of the structural features listed in Section 2 can be reproduced in the simulated images.

5. Conclusion

Agreement between the observed and simulated images has been found by setting the opening angle of the ME flap at 94.5° ± 0.5° , 6.5° greater than its nominal value.

Based upon the value of 94.5° the effective throughput of the telescope is 0.721 of its nominal value for on-axis source positions, a figure in good agreement with the value of 0.735 ± 10% determined from calibration measurements of the Crab Nebula and currently used by the Observatory software for flux determination.

(This article is a summary of a report prepared by Dr B. Aschen- bach of MPE, Garching, describing work carried out by him under contract to MBB, Munchen. - D. Andrews)


Figure 1: Typical EXOSAT LEIT1 Images showing image obscuration

fig 1 description above

Figure 2: Off-axis X-ray light path in the outer shell of the nested mirror system

fig 2 description above

Figure 3: Comparison of observed and simulated images at same FOV position

fig 3 description above


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Last modified: Thursday, 26-Jun-2003 13:48:32 EDT