[HN Gopher] A simple 11.2 GHz radio telescope (Hardware) (2020)
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A simple 11.2 GHz radio telescope (Hardware) (2020)
Author : _Microft
Score : 104 points
Date : 2021-02-09 15:44 UTC (7 hours ago)
(HTM) web link (physicsopenlab.org)
(TXT) w3m dump (physicsopenlab.org)
| _Microft wrote:
| I'm completely out of my depths here but hear me out: could
| pulsars be observed with such a thing? That would definitely be
| the icing on the cake.
|
| A way to precisely point the dish would be needed and an
| equatorial mount to compensate for Earth's rotation. If the
| signal was too faint, maybe some lock-in amplifier magic by
| creating a synthetic trigger signal with the expected rotational
| frequency of the targeted pulsar and then shifting the phase
| until the signal is maximized?
| privong wrote:
| > I'm completely out of my depths here but hear me out: could
| pulsars be observed with such a thing? That would definitely be
| the icing on the cake.
|
| In principle, yes. But I think in practice it would be
| difficult at best. Pulsars become fainter at higher
| frequencies, and this dish looks relatively small. So I suspect
| that combination of factors would mean that there aren't many
| (if any) pulsars that one could detect with this setup.
|
| > expected rotational frequency of the targeted pulsar and then
| shifting the phase until the signal is maximized?
|
| A technique like this is one way to search for pulsars. Though
| you don't use trigger signals since you don't a priori know the
| pulsar's spin period. So you record data for a period of time,
| then try many timesteps over which to fold the data and see if
| there's a pulse at that period. I'm sure there's better ways
| now, for it to be done.
|
| PRESTO is one of the major pieces of software used to search
| for pulsars: https://www.cv.nrao.edu/~sransom/presto/
| scionthefly wrote:
| Yes, ONE telescope may not be able to adequately resolve a
| pulsar at that frequency.
|
| But a /network of/ these telescopes, appropriately
| coordinated, that's a different story. And seems like a
| logical next step. You could use GPS conditioned timekeeping
| and standardized directional setup to coordinate data from
| multiple telescopes. We do that in some distributed physics
| projects like cosmic ray studies.
| privong wrote:
| I'm not sure what you mean by "resolve"; are you referring
| to resolving the pulsar's pulse in time?
|
| I was speaking more about the flux detection limit of such
| a dish (implicitly assuming the receiver could make
| sufficiently short measurements to enable folding of the
| data to detect pulses in the time-folded dataset). One
| could try to coherently sum the measurements from a number
| of telescopes to increase the signal to noise.
|
| Alternately, one could also try to detect the pulsar by
| averaging over the pulse profile, but that still requires
| that the telescope+receiver sensitivity is better than the
| period-averaged flux density. But then you're risking
| confusion of other, continuum, radio sources in the beam.
|
| It's still the case that most pulsars are much fainter few
| GHz frequencies than they are 1 GHz (e.g.,
| https://arxiv.org/abs/1302.2053). Though there are likely
| selection effects (discussed in the linked paper), most of
| the pulsars we know about can be expected to be ~40x
| fainter at 10 GHz than they are at 1 GHz. The linked paper
| cites a 6.5 GHz survey that identified 18 pulsars (compared
| to > 1000 detected in the ~1.4 GHz survey).
|
| I suppose it depends on what one's aims are, though.
| Someone wanting to only detect pulsars in general, it'd be
| easier to do at lower frequencies. But there's certainly
| some science to be done by observing them at higher
| frequencies.
| petschge wrote:
| If you want to do that at 140MHz, sure. But at 11GHz
| interferometry is -- despite being conceptually the same --
| quite hard.
| jcims wrote:
| Doesn't sound like you're out of your depth. It's been done
| with a worse receiver - https://www.rtl-sdr.com/detecting-
| pulsars-rotating-neutron-s...
| _Microft wrote:
| Well, I am a physicist, so this wasn't purely techno-babble
| but since I never had anything to do with astronomy or radio-
| observations a caveat seemed warranted.
|
| Thanks for the link btw!
| petschge wrote:
| The "lock-in magic" you are describing is called "phase
| folding" in the pulsar community and can be done in software.
| The expected rotational frequency would be taken from an
| ephemeris file and there is tools to search for it, if unknown,
| as well. If the telescope is sensitive enough, is a good
| question. I don't see any results in the article that shows
| sensitivity numbers.
| michelpp wrote:
| An excellent next step would be to use a distributed mesh of
| these and do interferometry to resolve spatial details on the
| sky.
|
| Observations could be distributed with bittorrent and coordinated
| over a blockchain. There exists off the shelf open source
| interferometry packages used by professional astronomers today
| that can be used for the analysis.
|
| Accurate positioning is needed but can often be achieved to
| within a couple of meters using google maps and a sat image of
| your back yard.
|
| EDIT: Do the downvoters hate interferometry? Oh no, it's just
| because I used the world blockchain. Chill people this is a
| thread about a blog post about turning a piece of trash into a
| fun day of amateur astronomy.
| petschge wrote:
| Absolute positioning is not that important as long as the
| relative positions are constant to within a small fraction of
| the wavelength, in this case to within a millimeter. Which is
| feasible if you mount it to the side of a building and there
| isn't too much wind.
|
| The much bigger problem is that you need a very accurate clock.
| Much better than what you can get from a GPS receiver.
|
| And lose the blockchain bullshit.
| toomuchtodo wrote:
| Are there any crystal or photonic oscillators that are up to
| the task for this time keeping? Or is RF interferometry bound
| to something like a chip atomic clock [1] (~$4k in low
| quantities)?
|
| [1] https://coverclock.blogspot.com/2017/05/my-
| stratum-0-atomic-...
| ajford wrote:
| It's been almost ten years since I last done any of the
| back end math for this kind of thing, but you could
| probably get within the ballpark using COTS rubidium
| oscillators like those used for cell towers. They often
| show up on auction sites at affordable prices.
|
| Use the pulse output to lock a station clock for your local
| time needs (like syncing your data recording to wall time)
| and the sine output to lock your mixing oscillators.
| scionthefly wrote:
| I suspect that you can get a start by looking at how cosmic
| ray studies conducted at different sites are coordinated:
|
| https://ieeexplore.ieee.org/document/1351816
| petschge wrote:
| Except you need closer to 0.01ns for this use case
| instead of the 55ns you get out of the GPS receivers. And
| half your data would have more, possibly much more,
| deviation than that anyway. If you don't know which half,
| that will lead to loss of sensitivity and possibly
| artifacts in the reconstructed signal. Interferometry is
| actually hard.
| brandmeyer wrote:
| GPS receiver accuracy relative to the constellation's
| time base is much much better than 55n. I'm working with
| a cheap (< 100 USD in prototype qty) OCXO that's good to
| a few parts in 10^-12 over 10 second periods. The GPS
| timebase varies from the TAI time base by up to a couple
| dozen nanoseconds, but it is very slowly varying.
|
| A good GPS-disciplined OCXO can hold that accuracy over
| much longer intervals.
|
| To take advantage of that accuracy you have to share the
| master reference oscillator between the telescope's clock
| tree and GPS receiver's clock tree. A commercial
| chipset's PPS output isn't going to cut it, but a
| dedicated amateur could definitely build a GPS-tagged
| radio telescope off of a common crystal like that.
| [deleted]
| [deleted]
| privong wrote:
| > Accurate positioning is needed
|
| Accurate timing is also needed to shift and combine the signals
| from the various dishes.
|
| > often be achieved to within a couple of meters using google
| maps and a sat image of your back yard.
|
| Position accuracy requirements are fractions of the wavelength
| being observed. So for this 11.2 GHz dish (26.8 mm), the
| position of the dish needs to be known to much better than 2mm.
|
| In practice this can be done by observing a bright source with
| a well-known sky position and then solving for the antenna
| positions. Aspects of this are discussed in Memo 503 from the
| Atacama Large Millimeter Array Memo series.
| http://legacy.nrao.edu/alma/memos/html-memos/alma503/memo503...
| tlb wrote:
| Yes, super-duper accurate timing. A nanosecond of timing
| error means your focal point moves by an angle equal to 1
| foot divided by the distance between dishes.
|
| You can calibrate out static timing errors, like due to
| different lengths of wire. But if the delay is varying for
| any reason your focal point will be slewing all over the
| heavens.
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