The ISS serves as a platform for both Earth and astronomical observational instrumentation.

Do routine movements of the crew or other objects ever cause problems for these observations through either vibrations or tiny rotations, either reducing the quality of the observations or requiring active correction by the instruments?

If so, could an example of the problem and some details about its solution be provided? Are we talking degrees or arcseconds or something in between for example?


1 Answer 1


NICER, on the ISS, uses a combination of passive damping and active control to achieve pointing within 66 arc seconds of the target 99% of the time. As far as I know, no ISS astronomical payload has achieved better pointing than this.

The NICER ISS payload consists of an X-ray timing instrument (XTI) as well as supporting hardware that points the XTI at celestial objects while maintaining an interface to the ISS and its infrastructure (Fig. 1). The XTI comprises an instrument optical bench (IOB) that holds an aligned collection of 56 X-ray concentrators (XRCs) and associated focal plane modules (FPMs). The XRCs are non-imaging optics that concentrate X-ray photons from a distant target onto the FPMs, with a field of view of approximately 30 arcmin. Within each FPM is a commercially available silicon drift detector (SDD) that detects individual X-ray photons, recording their energies with a precision of about 2% and their arrival times to better than 100 ns. Combined, the XTI provides an effective collecting area that peaks at nearly 1,900 cm2 for X-ray energies of 1,500 eV and ranges from 200 to 12,000 eV.

As the ISS orbits Earth, the XTI tracks celestial targets to an accuracy better than 66 arcsec more than 99% of the time. NICER’s pointing system uses a star tracker, with built-in microelectromechanical gyros and accelerometers, to provide an attitude solution at a 10 Hz rate. This high rate enables pointing control software running in NICER’s onboard computer to command an elevation-over-azimuth gimbal system to steer and maintain the XTI on target while overcoming vibrations from the ISS and the NICER payload itself. To minimize some of the vibrations, passive tuned mass dampers (TMDs) attached to corners of the XTI dissipate certain predicted mechanical modes.


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