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.