Low profile multi-axis star sensing
A small, ultra-light-weight star tracker for space applications uses an out-of-field rejection filter to reduce the physical size and mass of the star tracker by effectively eliminating the need for a sun shade. The out-of-field rejection filter combines a converter for converting randomly polarized light to p-polarized light with an angular selectivity filter that can reject out-of-field p-polarized light. The underside of the angular selectivity filter can be used to reflect a calibrated light source into the optical path.
1. A star tracker comprising:
an out-of-field rejection filter comprising:
a converter to convert randomly polarized light to p-polarized light; and
an angular selectivity layer to select in-field p-polarized light and reject out-of-field p-polarized light;
telescopic optics; and
a focal plane array to resolve the selected light.
2. The star tracker of claim 1 , further comprising an adaptive optical element (AOE) arranged to reduce centroiding error by adjusting focus of, or reducing aberrations in, selected light.
3. The star tracker of claim 1 , wherein an interior side of the angular selectivity layer is reflective to s-polarized light and is arranged to reflect light from a calibration source within the star tracker to be resolved on the focal plane array.
4. The star tracker of claim 1 , wherein at least a portion of the telescopic optics are diamond-turned optics.
5. The star tracker of claim 1 , wherein the star tracker does not comprise, and is not coupled to, a sun shade.
6. The star tracker of claim 1 , wherein the star tracker does not comprise, and is not coupled to, a sun shade longer than twenty centimeters.
7. The star tracker of claim 1 , wherein the focal plane array is an active pixel array.
8. The star tracker of claim 1 , wherein the converter and the angular selectivity layer are arranged at an angle of about 55° with respect to each other.
9. The star tracker of claim 1 , wherein the angular selectivity layer are arranged to be flush with each other.
10. A multi-axis star tracker package comprising:
a base;
at least three star trackers mounted to the base, each having a boresight, each star tracker comprising:
an out-of-field rejection filter comprising:
a converter to convert randomly polarized light to p-polarized light; and
an angular selectivity layer to select in-field p-polarized light and reject out-of-field p-polarized light;
telescopic optics; and
a focal plane array to resolve the selected light;
wherein the boresights of the star trackers are arranged such that the star trackers have substantially non-overlapping fields of view.
11. The multi-axis star tracker package of claim 10 , wherein the package comprises exactly three star trackers arranged to have an azimuth difference of about 120° between each star tracker.
12. The multi-axis star tracker package of claim 11 , wherein each star tracker has a field of view of ±4.5° about a 55° angle relative to the base.
13. The multi-axis star tracker package of claim 10 , wherein the package comprises exactly four star trackers arranged to have an azimuth difference of about 90° between each star tracker.
14. The multi-axis star tracker package of claim 10 , wherein the package does not comprise any sun shade having a length greater than ten centimeters.
15. The multi-axis star tracker package of claim 10 , wherein an interior side of an angular selectivity layer of at least one of the star trackers is reflective to s-polarized light and is arranged to reflect light from a calibration source within the star tracker to be resolved on the focal plane array.
16. A method of star tracking comprising:
admitting light to a star tracker having an attitude;
converting randomly polarized light in the admitted light to p-polarized light;
angularly selecting the p-polarized light to reject out-of-field light;
resolving the selected light on a focal plane array; and
determining the attitude of the star tracker based on the resolved light.
17. The method of claim 16 , further comprising:
sensing that the Sun is in the field of view of the star tracker; and
based on the sensing, activating at least one sun shutter in the star tracker to protect the focal plane array from damage.
18. The method of claim 16 , further comprising:
sensing that the Sun is in the field of view of the star tracker; and
based on the sensing, adapting at least one adaptive optical element (AOE) in the star tracker to defocus direct sunlight to protect the focal plane array from damage.
19. The method of claim 16 , further comprising reflecting calibration light off of an interior side of a filter used to angularly select the p-polarized light, the calibration light being resolved on the focal plane array.
20. The method of claim 19 , wherein the selected light imaged on the focal plane array forms an uncalibrated image, the method further comprising performing focal plane calibration based on the calibration light illuminated on the focal plane array to derive a calibrated image from the uncalibrated image.