LOng Range Reconnaissance Imager (LORRI) on the New Horizons spacecraft is a panchromatic (black and white) camera with a large telephoto lens. It uses a lens with 262 cm focal length and an 8.2-inch (20.8-centimeter) aperture to focus visible light onto its sensor. To put the size of its aperture in perspective, if LORRI’s focal length was 35 mm, a common focal length in DSLR cameras, then the f-number or f-stop for LORRI would be f=35/208 which is 0.168. A typical f-stop for common DSLR cameras is 2.8 when their shutter is wide open. As we know from basic photography 101, a lower f-number allows more light to reach the film or image sensor.
Not having any color filters provides a bigger space for each pixel on LORRI’s 1024 x 1024 pixel CCD sensor. This makes LORRI as sensitive as possible for imaging objects in the Pluto system, where light levels are 1000 times lower than at Earth. According to the New Horizons team, LORRI has a 0.29° x 0.29° field of view, and each pixel subtends 0.00028° x 0.00028° (1.02 arcseconds x 1.02 arcseconds). LORRI can also capture images in a lower resolution mode (256 x 256 pixels) by co-addition of every 4x4 pixels into one pixel.
The resolution of images may seem very low compared to current camera resolutions but keep in mind that the New Horizons was launched in January 2006 and LORRI began snapping shots of the Pluto system almost 9 years later at the start of 2015. Perhaps there were energy consumption and other factors that resulted in designing such a small sensor by today’s standards.
LORRI has been developed at Johns Hopkins University Applied Physics Laboratory to study geology on Pluto and its moons. Its unprecedented high-resolution images of Pluto system could be used, among other applications, to count the number of craters on each surface and measure their size to reveal the history of impacting objects in that distant region.
This is a documentray about the New Horizons by NASA (~59 minutes)
To see more photos taken by LORRI, please visit its page on NASA website