A Parallel Grid Imaging Proportional Counter Optimized for Detection of Low Brightness Stellar XUV-Sources

We have developed a position sensitive proportional counter with good efficiency and spatial resolution at long wavelengths (¿ > 100 Å), to be used as a focal plane image detector in conjunction with grazing incidence optics for the study of low brightness celestial objects in the 10-250 Å (XUV) band. The detector features two orthogonal wire grids with associated electronic read-out system providing two-dimensional position resolution and good linearity over an aperture of several centimeters. The read-out system, based on the charge division technique, has been optimized for minimum inherent noise. The position resolution obtained with this read-out system is limited by electron diffusion only over the full wavelength range for the gasmixtures investigated (9:1 Ar/CH4 and 7:3 Ar/CH4) and pressures (0.4-1.1 bar). Typical values for the rms position resolution obtained with a test counter, with an absorption depth of 1.7 mm at 0.6 bar, are 50 ¿m at 13.3 Å and 100 ¿m at 44 Å.