# Interferometry of distant sources

1. Feb 26, 2016

### naima

When you try to get fringes, the worse is when the source is extended and the light has a large energy spectrum.
With a star, the source seems pointlike. Is there another advantage as regards with bandwidth?

2. Feb 26, 2016

### Drakkith

Staff Emeritus
What do you mean?

3. Feb 28, 2016

### naima

Suppose that an optical polychromatical signal is emitted at a given pointlike place during T. It has a coherence length there. When the signal propagates its shape changes and its Fourier transform too. If we measure the coherence length at a far distance of the souce, Is it the same?

4. Feb 28, 2016

### naima

I read that the coherence length of the light coming from a star is $c/ \Delta \omega$
What is the definition of this Delta? How is it measured?

5. Feb 28, 2016

### Drakkith

Staff Emeritus
I'm sorry, I don't know much about this topic, but I'll try to help as best I can.

If we're talking about broad-spectrum light, then I'm not sure there is a coherence length since the phase varies randomly.

I think delta is given by the bandwidth of the emitted light. I'm not sure how this applies to the light emitted from a star, though.