Hi Maybe I missed something here. It would hardly be the first time.
If the objective is to come up with a sub 1 ms resolution on observing the object. And we have chosen this all so indeed we get "fast" changes. Isn't a 1,000 second integration going to get in the way? If we need the integration to simply "see" the signal, then determining it's "center" within the integration time to less than 1 ppm seems unlikely. On a hand waving basis that's sort of a 60 db signal to noise. As I said, I may be missing something. Bob On Mar 20, 2011, at 2:31 PM, Bruce Griffiths wrote: > jimlux wrote: >> On 3/19/11 10:41 PM, Bruce Griffiths wrote: >>> Bruce Griffiths wrote: >>>> jimlux wrote: >>>>> >>>>>> >>>>>> A 10-12m diameter dish is probably close to the minimum feasible >>>>>> aperture. >>>>>> A 4m dish can be made to work in conjunction with a mauch larger dish >>>>>> (eg 30m). >>>>>> >>>>> >>>>> The original speculation was for measuring the small change in earth >>>>> rotation rate, for which some sort of interferometric measurement of >>>>> a stellar source could be used. >>>>> >>>>> The source has to be bright (so you can detect it with a practical >>>>> antenna.. not everyone has a 30m dish in their back yard) >>>>> The source has to be small angle (or at least something that you >>>>> could accurately determine the centroid of) >>>>> The source has to be "not moving" (which I think leaves out using >>>>> something like jupiter) >>>>> The frequency of measurement has to be somewhere that the atmosphere >>>>> won't dominate the uncertainty (leaving out optical, I think) >>>>> >>>>> >>>>> SO what's the brightest small angular radio source out there? >>>> >>>> 3C273 >>>> >>>> RA 12:29.1 DEC 02:03.1 >>> Its flux density is around 30 Jy in the waterhole region. >>> ie about 3E-17W per square meter for a 100MHz bandwidth. >>> The radio spectrum is relatively flat due to the synchroton nature of >>> the blazar source. >> >> >> Ok, so lets say our ambitious amateur has a 3 meter diameter dish.. that's >> about 7 square meters. Knock that down to 4 square meters to make up for >> illumination and feed issues. So we're looking at 12E-17 W >> or 1.2E-13 mW or -130dBm, in 100 MHz BW. >> >> Say we want the "signal" to be comparable to the noise power, what do we >> need for a noise temperature.. kTB = -130dBm. kT = -174dBm/Hz for 300K, B = >> 80dBHz. (so at room temp, kTB would be -94dBm.. we need to drop noise >> power by at least 40 dB, so T needs to be down in the "sub 1 K" area, which >> is totally impractical. >> >> Looks like we need a bigger antenna.. >> Unless there's some clever correlation scheme. >> >> >> >> >> > With 2 or more antennas and integration times of 100sec to 1000 sec its > routine to image objects well below the thermal noise level. > The fluctuations in the source signal correlate whereas the thermal noise in > a receiver/dish pair do not. > > Modeling of the relative drift and frequency (and phase) offset (even if they > are hydrogen masers) of the 2 sampling clocks involved is sometimes necessary. > > Bruce > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
