from AO to NGAO (part I)

Once again, it has been a long time I did not write anything on this Blog. I am about to fail on my decision to write 2 posts per week in average, so I need to catch up

In January, I mentioned to you that I was involved in a new instrument designed for the Keck telescope called Next Generation Adaptive Optics or NGAO (we love acronyms in our field). What project is all about is the point of this post.

Adaptive optics is a relatively new technique which allows to correct in real time the effect of the atmospheric turbulences. The light coming from a star, planet or galaxy must cross our atmosphere before reaching the large aperture mirror of a telescope. This path is not simple, in fact our atmosphere is composed of horizontal layers in motion at the junction of which turbulent motion (whirlpool) formed. Because of this motion and because the temperature of our atmosphere varies with the altitude, the indice of refraction of layer junctions also change with time and also spatially. Basically before reaching us, the phase of the light is modified significantly to a way that is unpredictible. That’s the reason for which astronomers love to build telescopes on the top of high mountains where the atmosphere layer is thinner so the atmospheric turbulence effect lower. In practice when one watches a star, an “unresolved” source (eq. to a very small source) with a telescope, we should see something like a point source or at least an Airy figure (the point spread function of the telescope if you want to know the details). In reality, we see a “blob” moving with time (see the animation attached). We have a way to estimate the quality of a site, that we call “seeing” which correspond to the size of the blob. Typically from the ground at sea level the typical “seeing” is 4 to 6 arcsec in visible. From Lick observatory, located a ~1300 m altitude the seeing is in average of ~1-2”, so a significant gain compared to a telescope located in San Francisco. However, Mauna Kea which is an exceptionnal site for astronomy (at 4200 m altitude) as an average seeing of 0.6”!

But that’s not enough…. Theoretically a 10m-size telescope like Keck could have an angular resolution of 0.01” in visible (we use the physical law Resolution(in arcsec) = wavelength/Aperture size *206265= 0.5E-6 (visible light)/10m *206265.). When we observe through our atmosphere, we get only an angular resolution 0.6” (the seeing), 60 times lower than what we should get theoretically. That also means that we the Keck telescope we could see as much detail as someone observing with a 17 cm aperture telescope!

It is obvious that solutions should be found and one of the most expensive astronomical project, the Hubble Space Telescope (HST) is an answer to this problem. Our atmosphere is blurring our images? Let’s put a telescope above it, in a satellite orbiting at 480 km (300 miles). This project costs obviously a lot of money but bring spectacular images and science results. The HST angular resolution is 0.05” in visible (2.2m aperture telescope). Hubble total budget (design, launch, service missions, new instruments_ is already over several (~10) billion of dollars and we have only one of them, so the competition to get telescope time is extreme.

The idea of Adaptive Optics (AO) came from Horace Babcok who published in 1953 “The Possibility of Compensating Astronomical Seeing”, PASP 65, 229-36 (1953) a paper describing how we could correct the effect of atmospheric turbulences, analyzing the phase distorsion (with a wavefront sensor), and correcting it using a deformable mirror.

I will continue this post tomorrow… in the mean time, you may calculate yourself what is the angular resolution power of the human eye, and the angular resolution of the future TMT (Thirty meter telescope).

See you tomorrow.

F.

observation of star without and with adaptive optics (ADONIS at L Silla-3.6m)