RA = sinΘ' (Θ' = exit angle)
RA ∼ 1 ⁄ 2N (where N = f/number)
So, for a given optical system and object, the intensity of the image is a function of the f/number (ref. 6). Without very bright, high-contrast targets a very long integration time is necessary with the sensor. The integration time necessary is precluded by the inability of the tracker to hold still a target with any appreciable angular rate in the very narrow FOV. The apparent motion of these targets and long integration times causes an unacceptable blurring of the image. The requirement to image targets of lower intensity and/or faster angular rates necessitates a shorter focal length system. An alternate secondary mirror configuration was designed that reduces the focal length to 150 inches (381 cm) and f/12. This increases the light-gathering ability and will reduce the motion-based blur of faster moving targets. The 12.5-inch (31.75 cm) telescope used during the ISAFE program was also dual-configured as a 300-inch (762 cm) f/24 and a 150-inch (381 cm) f/12 system.
The finder scope (a modified PVPAEO TH-2) is used for target acquisition and most of the optical tracking tasks. The ability to rapidly move between a wide FOV (acquisition) and narrow FOV (tracking) is necessary to successfully acquire and smoothly track targets.
Targets other than reentry vehicles will likely require different optics to meet individual requirements. For image acquisition of aircraft at much closer ranges and higher angular rates than those of reentry vehicles, a wider FOV system is preferred. Also, the ability to vary FOV is desirable since some types of targets tend to vary apparent size significantly during a tracking task. A CMC Night Falcon II demonstration unit was made available for evaluation of tracking and imaging aircraft at medium to long range (still much closer than reentry targets). This unit has several features that make it ideal for this application. The Night Falcon II is MWIR sensitive (InSb sensor) with a similar size focal plane array (256 x 256 array, 30µ pitch) to that of the Radiance HS used in the large telescope. Sample images from the CMC unit are shown in figures 13 and 14.
Figure 13. IR image of commercial aircraft at 15.5 miles (25 km) with CMC Night Falcon II.
Figure 14. IR image of helicopter at close range with CMC Night Falcon II.