FIRST@LICK: Two nights hunting for the fringes
As I mentioned before, the Grail of our experiment called FIRST, is to record patterns of interference fringes. Using these “images” we should be able to reconstruct an image with a high dynamic at proximity of the star, allowing us to explore the close vicinity of a nearby star. This is an interesting area, unknown to astronomers and where exoplanets form. To validate this technique we mounted FIRST on the Shane-3m telescope. As predicted this hunt for fringes was a long and tedious task.
On July 25 at 5am, after successfully setting up and aligning FIRST on the telescope, we collected the first photons coming from a star. This was not yet the successful first light of the instrument since we merely acquired data on the camera control. This sub-system allows us to refine the pointing of the telescope by aligning the light of the star where the fibers are located. The following step is to inject the light in the fiber and hopefully record interferometric fringes.
As we expected for this kind of experiment, the path that we chose was not straightforward. We had to face several technical issues. Below a brief summary of my notes:
– On July 26 UT (the second night on the sky) we decided to remove the prism on the system to maximize the flux on the detector. We observed several bright stars at the zenith such as Vega, Eta Cygni and Deneb. We discovered that we could inject successfully the flux of the stars in 3 fibers but somehow several of them were unresponsive. We also discovered that the stability provided by the AO system is necessary to be able to properly optimize the injection in the fiber… No fringes yet.
– On July 27 UT (the third night on the sky) the entire system was realigned, each fiber was cleaned out. To be able to use the AO system, we replaced the mirror of the periscope system by a beam splitter to send part of the light to the adaptive optics analysis. Guy Perrin built in a few hours in the machine shop of the Lick Observatory an “unorthodox” mount for this cube (image below) and we installed it on the AO bench.
We had to face poor atmospheric conditions with a very turbulent atmosphere but the AO system provided a consistent correction with an FWHM on the star of 0.5″ (for an exterior seeing of ~2″). We once again observed Vega and Deneb. At 3am, because of the absence of fringes, we concluded that the prism had to be reinstalled to help disentangle the fringes which are confounded in broad band wavelength range. We also found out that the detector camera was not used in EMCCD mode explaining the lack of sensitivity.
We are optimistic about the outcome of the coming fourth night…