4 ms·

Fascinating! I'd always wondered how they made it work with 60s-era technology, but when I searched I couldn't find much info on it. This answers some of my que

Fascinating! I'd always wondered how they made it work with 60s-era technology, but when I searched I couldn't find much info on it. This answers some of my questions. Any pointers on where I can read more?

labcomputer 5y ago
http://www.prc68.com/I/StellarTime.shtml#P http://www.prc68.com/I/StellarTime.shtml#P The Northrop patents are probably the most interesting. IIRC, if you trace the prior art, you’ll find some patents from the early days of WWII. How they work: * Telescope on an alt-az mount, preferably on the ground or a gimbaled platform linked to an INS. * Optical wedge (prism) behind a telescope to nutate the image around the optical axis * A rotating shutter behind the wedge, centered on the telescope’s optical axis (often a starburst pattern, but several of the patents propose different shutter patterns to mitigate the effect of background luminosity gradient). The shutter is phase locked to the prism. * Ground glass screen at the telescope’s focal point * Single-pixel optical sensor (PMT in the 1960’s, but you’d use a photodiode today) sensitive to the entire screen * lock-in amplifier synced to the shutter (note, however, that the patents do not describe it as such) The lock-in senses the output from the PMT. Phase and magnitude from the lock-in operate the azimuth and altitude servos of the telescope to center on the tracked star. When the tracked star is centered in the telescope, it makes a circular pattern on the screen, modulated by the shutter. When the star is off-axis the circular path is offset, so the instantaneous modulation frequency depends on the phase of the prism (it helps to see the figures in the patents). They also put an IR-pass filter somewhere in the system to cut down on scattered light from the atmosphere, which helps to improve the SNR. I’m not sure the filter is strictly necessary because the LIA should give you dozens of dB of processing gain and navigation stars are visible to the eye in a telescope during daytime—but it seems like “free” SNR.
numpad0 5y ago
I couldn't find the picture so spoke from memory and the details were quite wrong, but looks like "canopus sensor"[1] on Mariner spacecrafts was what I had in mind. 1: http://petermasek.50megs.com/starplot.jpg http://petermasek.50megs.com/starplot.jpg (mirror: https://images.app.goo.gl/9xAn7ATPMSKYLEHs8 https://images.app.goo.gl/9xAn7ATPMSKYLEHs8)

labcomputer 5y ago

http://www.prc68.com/I/StellarTime.shtml#P http://www.prc68.com/I/StellarTime.shtml#P The Northrop patents are probably the most interesting. IIRC, if you trace the prior art, you’ll find some patents from the early days of WWII. How they work: * Telescope on an alt-az mount, preferably on the ground or a gimbaled platform linked to an INS. * Optical wedge (prism) behind a telescope to nutate the image around the optical axis * A rotating shutter behind the wedge, centered on the telescope’s optical axis (often a starburst pattern, but several of the patents propose different shutter patterns to mitigate the effect of background luminosity gradient). The shutter is phase locked to the prism. * Ground glass screen at the telescope’s focal point * Single-pixel optical sensor (PMT in the 1960’s, but you’d use a photodiode today) sensitive to the entire screen * lock-in amplifier synced to the shutter (note, however, that the patents do not describe it as such) The lock-in senses the output from the PMT. Phase and magnitude from the lock-in operate the azimuth and altitude servos of the telescope to center on the tracked star. When the tracked star is centered in the telescope, it makes a circular pattern on the screen, modulated by the shutter. When the star is off-axis the circular path is offset, so the instantaneous modulation frequency depends on the phase of the prism (it helps to see the figures in the patents). They also put an IR-pass filter somewhere in the system to cut down on scattered light from the atmosphere, which helps to improve the SNR. I’m not sure the filter is strictly necessary because the LIA should give you dozens of dB of processing gain and navigation stars are visible to the eye in a telescope during daytime—but it seems like “free” SNR.

numpad0 5y ago

I couldn't find the picture so spoke from memory and the details were quite wrong, but looks like "canopus sensor"[1] on Mariner spacecrafts was what I had in mind. 1: http://petermasek.50megs.com/starplot.jpg http://petermasek.50megs.com/starplot.jpg (mirror: https://images.app.goo.gl/9xAn7ATPMSKYLEHs8 https://images.app.goo.gl/9xAn7ATPMSKYLEHs8)