Narrowband solar calibration system for a radiometric system

1. A system for calibrating a broadband detector comprising:

a narrowband telescope for viewing a celestial body,

an earth viewing telescope,

a broadband detector selectively coupled to the narrowband telescope or the earth viewing telescope,

a narrowband filter selectively inserted in an optical path of the narrowband telescope or the earth viewing telescope, and

a processor configured to calibrate the broadband detector based on viewing the celestial body with the narrowband telescope or the earth viewing telescope, and selectively inserting the narrowband filter in the optical path.

2. The system of claim 1 wherein

the narrowband telescope includes a first narrowband filter in its optical train, and

the narrowband filter selectively inserted in the optical path is spectrally similar to the first narrowband filter in the optical train of the narrowband telescope.

3. The system of claim 2 including

a gimbaled platform for receiving the narrowband telescope and the earth viewing telescope,

wherein the gimbaled platform is configured to provide elevation and azimuth rotation of the narrowband telescope and the earth viewing telescope for viewing the celestial body.

4. The system of claim 3 wherein

the narrowband filter is fixed in elevation with respect to a housing of the system, and

the gimbaled platform is configured to rotate the narrowband telescope and the earth viewing telescope on an elevation axis, so that the narrowband filter is selectively inserted in the optical path of the narrowband telescope or the earth viewing telescope.

5. The system of claim 3 wherein

the gimbaled platform is configured to rotate the narrowband telescope and the earth viewing telescope on an azimuth axis, in which the narrowband telescope and the earth viewing telescope are effective in azimuth scanning the celestial body.

6. The system of claim 1 wherein

the processor is configured to integrate radiative data received by the broadband detector when

(a) the narrowband telescope views the celestial body without the first narrowband filter in the optical path,

(b) the narrowband telescope views the celestial body with the first narrowband filter in the optical path, and

(c) the earth viewing telescope views the celestial body with the first narrowband filter in the optical path.

7. The system of claim 6 wherein

the celestial body is the sun.

8. The system of claim 1 wherein

the narrowband telescope includes a field-of-view (FOV), and

when the narrowband telescope views the celestial body, the celestial body under-fills the FOV.

9. The system of claim 1 wherein

the narrowband telescope includes first and second narrowband telescopes having, respectively, first and second narrowband filters in its optical path for viewing the celestial body, and

the narrowband filter includes first and second narrowband filters;

the first narrowband filter is spectrally matched to the first narrowband filter of the first telescope, and

the second narrowband filter is spectrally matched to the second narrowband filter of the second telescope.

10. The system of claim 1 wherein

the narrowband telescope and the earth viewing telescope view the celestial body while orbiting the earth.

11. A method of obtaining radiation data comprising the steps of:

(a) obtaining first radiation data by scanning a first celestial body using a narrowband filtered telescope;

(b) obtaining second radiation data by scanning the first celestial body using an external narrowband filter inserted in an optical path of the narrowband filtered telescope;

(c) calibrating the external narrowband filter using steps (a) and (b);

(d) calibrating an imager by obtaining third radiation data by scanning the first celestial body using the external narrowband filter inserted in an optical path of the imager; and

(e) imaging a second celestial body using the imager.

12. The method of claim 11 wherein

scanning the first celestial body includes scanning the sun, and

scanning the second celestial body includes scanning the earth.

13. The method of claim 11 wherein

scanning the first celestial body includes raster scanning the celestial body while orbiting the earth.

14. The method of claim 13 wherein

raster scanning includes

scanning the celestial body by rotating, back and forth, the narrowband filtered telescope in azimuth, and

fixing the narrowband filtered telescope in elevation.

15. The method of claim 11 wherein

obtaining radiation data includes using the narrowband filtered telescope with an internal narrowband filter that is spectrally the same as the external narrowband filter.

16. The method of claim 11 wherein

step (c) includes obtaining a ratio between an integral of the first radiation data and an integral of the second radiation data.

17. The method of claim 11 wherein

scanning the first celestial body includes providing a field-of-view (FOV) for the narrowband filtered telescope which over-fills a radiative footprint from the first celestial body.

18. A method of scanning a celestial body comprising the steps of:

(a) scanning a celestial body using the following sequence:

(i) scanning the celestial body using a first optical path having a first narrowband filter,

(ii) scanning the celestial body using the first optical path having the first narrowband filter and a second narrowband filter,

(iii) scanning the celestial body using an imager with a second optical path including the second narrowband filter inserted in the second optical path; and

(b) scanning another celestial body using the imager with the second optical path and without the second narrowband filter inserted therein.

19. The method of claim 18 wherein

scanning the celestial body includes scanning the sun, and

scanning the other celestial body includes scanning the earth.

20. The method of claim 19 wherein

scanning includes raster scanning the sun using a FOV that over-fills a radiation footprint from the sun.