New Horizons Jupiter Encounter |
New Horizons Jupiter Encounter |
Guest_AlexBlackwell_* |
Jan 10 2007, 09:47 PM
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Jan 26 2007, 05:36 PM
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Administrator Group: Admin Posts: 5172 Joined: 4-August 05 From: Pasadena, CA, USA, Earth Member No.: 454 |
I read in "Calibration of the New Horizons Long-Range Reconnaissance Imager" by Morgan et al. that LORRI has "a PSF FWHM of 1.5 pixels with little variation across the FOV..."
PSF = Point Spread Function; FMHM = Full width at half maximum. And from the instrument info on the Cassini PDS disks: "The NAC point spread function (PSF) was designed to be approximately the same physical size as a pixel in the near-IR. The full width at half maximum (FWHM) of the PSFs of the NAC through the clear filters is 1.3 pixels." Why would the FWHM be wider through the clear filters than the IR? To compare the masses, the mass of both Cassini cameras is about 57.83 kg. Anyone have a guess as to what fraction of that the NAC makes up? LORRI is only 5.5 kg! --Emily -------------------- My website - My Patreon - @elakdawalla on Twitter - Please support unmannedspaceflight.com by donating here.
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Jan 27 2007, 01:49 PM
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Dublin Correspondent Group: Admin Posts: 1799 Joined: 28-March 05 From: Celbridge, Ireland Member No.: 220 |
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Jan 27 2007, 04:59 PM
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Senior Member Group: Members Posts: 3648 Joined: 1-October 05 From: Croatia Member No.: 523 |
Cassini NAC is a reflector telescope and I don't see any obvious refracting elements in its design (such as field flattening optics) so where would chromatic aberration come from? Maybe the design simply optimizes for focus using the IR filters (they do, after all affect focus ever-so-slightly) so using no filter (clear frame) produces a slight defocus. Diffraction/wavelength effects would make longer wavelengths more blurred, contrary to this behaviour, right?
I know the WAC uses a special "clear" filter that improves focus over the visible wavelengths since it's basically an old Voyager WAC optics design. EDIT: Correction, there appear to be field corrector optics and the window of the CCD package in the NAC. Chromatic aberration might be the sole reason for the PSF variation. ----- Back on topic... A question for John or Alan: What are the maximum exposures for LORRI without saturating various targets at Jupiter, at these approach phase angles? I see a typical exposure is 3 ms. The difference between 4 ms and 2 ms seems to be quite a bit less noise in the former case. Is the only reason such low exposures are used to avoid overexposure (say at Europa / Io)? This post has been edited by ugordan: Jan 27 2007, 08:22 PM -------------------- |
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