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Sharper than Hubble: Large Binocular Telescope achieves major breakthrough
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"Mike Dworetsky" writes: Yes, but it reduces the amount of mass budget available for telescope and detector equipment if the antenna has to be larger. Also, you can save a small amount of delta-V by not having to go into orbit around the sun. Other way around, I think: less delta-V to solar orbit. (The vehicle has to "slow down" at L2.) I expect that more than compensates for a bigger HGA or transmitter. Yes, in solar orbit the available communication rate decreases with time. The question is whether it's still adequate at the end of the observatory's lifetime (which I think for JWST is set by hydrazine supply). Communication rate depends on transmitter power and antenna gain, so you have to size those for end of mission. That's bigger than needed for L2, but as noted above, you need lower delta-V. Spitzer just passed the 1 AU distance mark, and it's still operating. Even in solar orbit, you need something for dumping momentum. That can be hydrazine, in which case you need a whole lot less than for station keeping at L2, or it can be cold gas, which avoids any risk of contamination. I don't see why round-trip communication time is an issue. These observatories aren't operated in real time. The program for a week or so gets uploaded, and the data acquired are stored onboard and downlinked when convenient. I wonder if there is a technical spec somewhere that explains why L2 vs solar orbit. There must have been a tradeoff study, but I haven't seen it. I did see the one for Spitzer, which concluded solar orbit was better. (As I recall, LEO and HEO were also considered, but neither was even a viable second choice.) JWST has a higher data rate than Spitzer (because of larger detector arrays), but it isn't obvious to me that outweighs the contamination issue. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
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