Literature

Resources for researchers and hints or guides

Radar astronomy with the XC crosscorrelators

The XC crosscorrelators offer various opportunities for research and study.

One of these opportunities is to turn a radio antenna or a radio antennas array to a single or multichannel radar to study the moon or other NEOs.

To do this there are two possible configurations: one is to connect a transmitter to the external sampling rate output, the second is to connect the transmitter to the frame end output.

The transmitter will then emit a pulse each frame end or each sampling pulse synchronized with the readout clock of each antenna input.

The auto and cross correlation degree will then be relative to the reflection of the transmitted pulses from the object observed.

Using a single antenna will be very useful to determine the distance of the target of the observations, whilst in interferometer configuration there’s the possibility to draw a detailed map of the surface of the moon, for example or the shape of a smaller NEO like an asteroid or a meteor field during starfalling crossing dates.

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Stellar interferometry FAQ

What is stellar interferometry?

Stellar interferometry is an observational technique that makes use of more telescopes to obtain a picture of a star or a stellar object.

A stellar interferometer measures the coherence ratio between couples of telescopes (single baselines) and builds or fills a plot with the coherence ratios obtained by changing distances between the telescopes.

Aperture synthesis parenthesis

The telescopes can be placed without being moved, in this way a perspective projection calculation for the position of the coherence ratio measured into the plots is needed. This method is widely used in astronomical stellar interferometers.

This technique, called aperture synthesis (synthesis of an aperture) does a calculation of the perspective distance between the telescopes, adjusts the light arrival time delays between them, and takes advantage of earth rotation to change this distance, since the rotation of earth takes about 24 hours to complete a 360 degrees rotation, the time needed to fill the plot with the coherence ratios is often very long.

What is coherence ratio and why it is useful?

I wrote here about the coherence ratio. What is it? Coherence ratio is the similarity between two fluxes or elements taken for comparison. In stellar interferometry a high coherence ratio between two fields of view means that the stars in a field are mostly the same as into another one. Intensity interferometry, for example measures the intensities only, so the mean luminosity of an observed area is compared with the mean luminosity of another one.

More advanced intensity interferometers compare or count the arrival time of intensity fluctuations and count the synchronously measured peaks read out as voltage pulses. The XC correlators behave like these kind of interferometers.

How images are obtained?

The coherence ratios map then shows only the coherence ratios, so the similarities in various distances. The more the pixel is far from the center the more wide the field should be to see bright pixels.

This means that there are no direct pictures shown in these plots (maps). Only a count of symmetries in the space domain.

The answer is to try an inverse Fourier transform of the map, so to obtain the raw picture that generated these coherence which depend on the place where they were taken.

These maps, however, are not always fully plotted. They need some models that complete them.

What are models and how to use them?

Since most of the maps (Fourier planes) are still incomplete after many observations also, models are used to complete them. A good model is a complete map of the Fourier transform of the object observed picture.

Models should be mixed to the observation for estimation of the complete picture and aimed to obtain a nice and realistic inverse transform.

They can also be generated by an estimation of the missing parts of the map, and AI and machine learning are used to improve the results in a more realistic manner here.

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Superetherodyne reception with an XC correlator

The XC correlators offer reception bandwidths up to 200MHz. This feature allows them to obtain spectra and crosscorrelations with greater accuracy in comparison with other products.

A special feature of the XC correlators is the capability to work in superetherodyne mode by applying an oscillation signal to the external amplifier.

The superetherodyne mode can be used in radio quantum receivers, photomultiplier tubes, avalanche photodiodes, silicon photomultipliers, nanowire superconducting detectors and any quantum detectors connected to an XC correlator.

The standard oscillation frequency is set to 400MHz. The intermediate frequency remains the same as their normal bandwidth but the observed frequency is in offset by 400MHz, reaching out 600MHz and 1.67 nanoseconds timing resolutions on products like the XC8.

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Hint: observe an exoplanet with an XC correlator using reflex motion

Exoplanets orbiting around a star are observable in various ways.

Schematic example of a reflex motion observation

Typically astronomers use techniques like photometry to determine if a planet transits in front of a star by observing the light curve of a variable star.

An instant drop-off of the luminosity of the star, and a drop-on of the self, mostly indicate that a planet transits in front of the observed star, expecially if this is observed periodically during an amount of time.

Another technique is called “reflex motion”:

This technique is often used on pulsars in radio astronomy. A pulsar emits very strong magnetic emissions in the radio wavelength regime.

A planet orbiting around a pulsar, when this is in the opposite side of the star (that means the pulsar is in middle way between the planet and Earth), reflects parts of the radiation of the pulsar.

These reflected radiations arrive delayed to an earth observer.

This event can be observed by an autocorrelator.

The XC series crosscorrelators can work in three modes: quantum counters, quantum autocorrelators and quantum crosscorrelators

An autocorrelator like the XC series cross-correlators in autocorrelation mode can reach temporal precisions down to 5 nanoseconds.

The XC series crosscorrelators can decrease the temporal resolution time up to 65µs each step, so to coarse precision but acceptable in astronomy.

Light and radio emissions travel at the speed of light and each 3 nanoseconds travels to around 1m.

The autocorrelator shows the total emissions during an offset time, so if a planet reflects the light or radio emissions from a star, you can grab the actual perspective distance from the star itself to the planet with an extreme precision.

This obviously isn’t restricted to radio astronomy: a measurement can be done with optical sensors like photomultiplier tubes, avalanche photodiodes or silicon photomultipliers.

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