1. SummaryRay 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 imagesNote 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:
3. Ray Tracing in the EXOSAT LE TelescopesEXOSAT 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:
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 DistortionsSince 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.
5. ConclusionAgreement 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)
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