New Horizon Cameras Options

[Guest_vjkane2000_*]

I've been wondering what the focal lengths are for the two New Horizon cameras (Ralph and Lorri). Curious to know how much Alan et al. were able to squeeze into their weight budget.

[MarsInMyLifetime]

At risk of reopening what appears to be a dead thread (and mods may want to move this post for that reason), my search for LORRI optical characteristics led to this thread where the original question about Ralph and LORRI focal lengths was never answered. That information has been published elsewhere but the thread needs closure for the sake of other seekers who arrive here, and I have a follow-on question that is not answered anywhere else, so this thread may yet be useful.

The optical characteristics of the Ralph visible/NIR/IR camera are described in this paper, principally in Table 2 on page 6:
http://www.boulder.swri.edu/pkb/ssr/ssr-ralph.pdf
(TL;DR:
an unobscurred, off-axis, three-mirror anastigmat design;
Telescope Aperture: 75 mm
Focal Length: 657.5 mm
f/#: 8.7)

The optical design of the LORRI camera is described in these slightly variant resources, in Table 6 on page 10 and in section 3.1.2 Optical Design on page 12:
http://lanl.arxiv.org/ftp/arxiv/papers/0709/0709.4278.pdf
http://www.boulder.swri.edu/pkb/ssr/ssr-lorri.pdf
(TL;DR from the abstract:
a narrow angle (field of view=0.29°), high resolution (4.95 µrad pixels), Ritchey-Chrétien telescope with a 20.8 cm diameter primary mirror, a focal length of 263 cm, and a three lens field-flattening assembly)

I gathered that the LORRI primary and secondary mirrors are made from low thermal expansion silicon-impregnated silicon carbide. The field-flattening lenses are of fused silica. This design was based on monochromatic imaging (color via filters) therefore the refractive components are not achromatic (they can't be, being all of the same refractive index). This goes a long way in explaining some of the transmissive properties of the system. Yet two details don't seem to be mentioned anywhere:

1. What reflective coating was used on the mirrors (aluminum? something more exotic?)? Was the reflective coating hardened or overcoated in any way (such as silicon monoxide commonly applied to terrestrial mirrors)? Is deep space tarnishing even an issue?

2. Were the optical components coated in any manner? In a monochromatic environment, I suppose that spectral multicoating may be meaningless, but how durable are fused silica surfaces? And aren't internal reflections (ghosts) still a design problem for a multiple element field group?

The papers are otherwise helpful about nearly all other questions one might have about these camera systems. But even if coatings were not needed, that design point was not clear to me in the papers. Thanks for any... erm... illumination on this.

[Gerald]

...http://lanl.arxiv.org/ftp/arxiv/papers/0709/0709.4278.pdf
..
2. ... And aren't internal reflections (ghosts) still a design problem for a multiple element field group?

P.24:

The ghosts are dominated by out-of-field illumination at the red extreme of the LORRI passband, depending on the radiance distribution over field angles just outside the FOV up to approximately 0.37° off-axis. The ghosts are strongly dependent on source spectrum and will be characterized extensively with Jupiter observations

An application I've been pondering has been, whether this effect could be used to retrieve color information from LORRI images.

[MarsInMyLifetime]

An application I've been pondering has been, whether this effect could be used to retrieve color information from LORRI images.

If each image is filtered, won't any out-of-band ghost artifacts likewise be removed? Moreover, if they are off-FOV, isn't that the same as "not imaged?" I'm not sure where in the recorded field this unfiltered color information could get in.