Presentation on theme: "SXC meeting SRON, July 19-20, 2007. 2 SXC meeting 19-20/07/2007 Alignment Positioning of mirror with respect to detector (internal). Positioning of total."— Presentation transcript:
2 SXC meeting 19-20/07/2007 Alignment Positioning of mirror with respect to detector (internal). Positioning of total instrument on spacecraft (with respect to eRosita) (external) Degrees of freedom: translation (x,y,z), rotation (x,y) z-telescope axis x,y perpendicular to telescope Take mirror as single unit, properly aligned internally. Assume a perfect lens - Mirror translation in x,y translates to image translation x,y on detector - Mirror translation in z will cause defocus - Mirror rotation has no effect for small rotations (within field size 40 arcminutes) Alignment constraints: - Keep tolerances within 0.5 pixel = 0.4 mm (0.85 arcminutes) ( 1 pixel -> 815 µm (?) )
3 SXC meeting 19-20/07/2007 Translation (x,y) - Internal stability should be within 0.5 pixel (0.4 mm (in x,y)) - Absolute alignment -> within central field of eRosita (total field 40 arcmin) -> 10 arcminutes (?) -> 5 mm tolerance (in x,y) ( Total error budget: internal SXC + spacecraft mounting + eRosita ) Translation (z) - Defocus (broadening of PSF) of 0.5 pixel (0.4 mm) -> 1.8 mm tolerance (in z) Tolerances - If we can keep total mechanical tolerances within 1 mm, there is no need for adjustment mechanisms for alignment. - Total error budget: - detector/dewar coordinate system + tolerances (?) - mirror coordinate system + tolerances (?) - telescope tube + mounting (?) - stability (temperature range) (?) - spacecraft mounting (?) + eRosita (?) (need for total error budget)
4 SXC meeting 19-20/07/2007 Effects of temperature Focus: If telescope tube is made of aluminium 1 m tube, 10 K temperature range -> 0.4 mm z-translation -> 0.1 mm PSF broadening -> Make telescope tube for predefined operational temperature (?) within 10 K temperature range -> adds 0.4 mm to error budget (In case of carbon fibres, thermal expansion can be designed to be zero. In that case there are no constraints on temperature) (Needs same sort of data for dewar/detector) Temperature gradient over mirror shell: measurements: 1 K gradient -> 0.05 mm PSF broadening -> 1/16 pixel if linearly with temperature -> 4 K gradient over mirror shells is ok
5 SXC meeting 19-20/07/2007 Focus Do we want exact focus (PSF is 1/40 of pixel) or proper sampling on sky?. To have better sampling (PSF at least more than 1 pixel) we need to defocus (at least about 5 mm). Proper (Nyquist) sampling requires PSF of 2 pixels. Do we want this ?? Other solution single pixel PSF and dithering on the sky (is that possible ??) What is the pointing stability and pointing reconstruction accuracy ??
6 SXC meeting 19-20/07/2007 Alignment Plan - No adjustment mechanisms (mechanical tolerances < 1 mm) need coordinate system definitions (alignment cubes/corner cubes) on different components (dewar, mirror, mounting) - Optical verification (alignment cubes/corner cubes) - X-ray verification on long beam facility - verification at spacecraft level (?) (SXC internal; versus eRosita) alignment cubes
7 SXC meeting 19-20/07/2007 Alignment verification Verification in long beam facility. - Source at finite distance -> focus position at different distance. (e.g. 2.16 cm off at Panter) - At nominal focus position (infinite source distance) there will be a defocused image of the mirror shells. - Check defocused image size and position to verify alignment - To verify proper position of defocused image with respect to focus distance, mirror has to be partially blocked.
8 SXC meeting 19-20/07/2007 Blocking X-ray sky background Sky outside mirror can potentially add to X-ray background. need 1.2 mm of aluminium in line of sight, to block sky background sufficiently visible to sky detector mirror support ring telescope base plate (or filter container) dewar
9 SXC meeting 19-20/07/2007 fortunately, natural support structure of mirror combined with diaphragm in base plate, appears to block the sky sufficiently. detector mirror base-plate diaphragm base- plate diaphr agm base-plate diaphragm must be 1 mm larger than beam ( 1 mm tolerance) diaphragm in base plate or filter container ? Distance to detector ? mirror support ring 30 cm 30 cm