Christensen, Finn Erland1; Joensen, K. D.3; Gorenstein, P.3; Priedhorsky, W. C.3; Westergaard, Niels Jørgen Stenfeldt1; Schnopper, H. W.4
1 National Space Institute, Technical University of Denmark2 Astrophysics, National Space Institute, Technical University of Denmark3 unknown4 Danish Space Research Institute
It is shown that compact designs of multifocus, conical approximations to highly nested Wolter I telescopes, as well as single reflection concentrators, employing realistic graded period W/Si or Ni/C multilayer coatings, allow one to obtain more than 1000 cm2 of on-axis effective area at 40 keV and up to 200 cm2 at 100 keV. The degree of concentration is defined by a focusing factor i.e., the effective area divided by the half power focal area. For the cases studied, this is 400 at 40 keV and 200 at 100 keV for a 2 arcmin imaging resolution. This result is quite insensitive to the specifics of the telescope configuration provided that mirrors can be coated to an inner radius of 3 cm. Specifically we find that a change of focal length from 5 to 12 m affects the effective area by less than 10%. In addition the result is insensitive to the thickness of the individual mirror shell provided that it is smaller than roughly 1 mm. The design can be realized with foils as thin (≤0.4 mm) as used for ASCA and SODART or with closed, slightly thicker (∼1.0 mm) mirror shells as used for JET-X and XMM. The effect of an increase of the inner radius is quantified on the effective area for multilayered mirrors up to 9 cm. The calculated Field of View (full width at half maximum), ranges from 9 arcmin at 1 keV to ≥5 arcmin at 60 keV. Finally, the continuum sensitivity of the design assuming a signal to noise ratio of 5 and a 10% energy bandwidth has been calculated. For a balloon flight observation of 104 sec. with a telescope having 2 arcmin imaging resolution the point source sensitivity is ∼3 · 10−6 photons/cm2/s/keV up to 70 keV for a W/Si coated telescope and up to ∼100 keV for a Ni/C coated telescope. For a satellite observation time of 105 sec and an imaging resolution of 1 arcmin the sensitivity is ∼10−7 photons/cm2/s/keV which demonstrates the great potential of this hard X-ray imaging telescope in the energy range up to 100 keV.
Experimental Astronomy, 1995, Vol 6, Issue 4, p. 33-46