GPI 3rd commissioning run — Astrometric calibration with a little help from MagAO

Astrometric calibration is critical for GPI: When we see a faint dot near a star, the best way to check whether it is a planet orbiting that star, versus whether it is a background star along the same line-of-sight, is to compare the astrometry at a later date. Astrometry means measuring the stars — measuring the exact position in arcseconds and angle from North. But to figure out the size of our pixels on the sky, and the orientation of our camera and which way is North, we have to observe known groups of stars and measure their separations and angles. Then we compare our measurements to those from other instruments and tie that back to basic calibrations done in the lab with pinhole masks to create a common reference frame. This is how we calibrate astrometry.

A handful of faint stars clustered around a bright guide star makes for an excellent astrometric calibration field. These are images of the same field with MagAO/VisAO in z’, MagAO/Clio in H-band, and GPI in H-band. VisAO images courtesy Jared Males, Clio images courtesy KM, and GPI images courtesy Jason Wang.

But the field of view of GPI is very small, and it is hard to find a group of stars that are very close together, that also have a bright enough guide star for the AO system.

Tour of the Telescope

Yesterday, we had a chance to see the telescope in all of its glory. And it is HUGE! It really makes you appreciate the amount of equipment you need to directly image these faint extrasolar planets that are orbiting other stars. Andrew, the telescope operator, then pointed the telescope down so that we could get