When talking about telescopes, size is important. Due to its sheer size, walking inside the dome of this telescope is truly a humbling experience. Built up of 36 hexagonal segments, the 10.4 m segmented mirror makes GTC a photon collecting monster capable of detecting the weakest of signals, which is exactly what is needed when studying exoplanet atmospheres. Having been inaugurated in the summer of 2009 this telescope has been of great use to exoplanet astronomers in part due to the unique tunable filter capabilities of the OSIRIS instrument.
The GTC has had its fair share of issues, or should I say, the dome has. The motors used for opening the hatch do not pull equally which results in a lot of strain being put on one of the motors. Thus to make sure the telescope is always able to close the dome’s upper hatch, the observer is restricted to observing below about 70 degrees from the horizon. The side ports are also soldered shut for reasons which are unclear to me. Despite this, the instruments and the adaptive optics systems are working very well.
During my visit I was shown around the whole complex and got to sit in on a night with observing. The staff were extremely friendly and eager to show me the telescope. The observation done the night of my visit was that of an exoplanet transit and the instrument used was OSIRIS with its tunable filter. The tunable filter has the unique capability of becoming a variable narrowband filter which the observer can tune to specific wavelengths and thereby choose the filter’s peak transmission wavelength. This especially comes in handy when looking for alkali metals in the atmospheres of hot Jupiters, when one can tune the filter to the specific wavelengths where one expects to see absorption. The tunable filter then does away with skylines and other features caused by the Earth’s atmosphere. Two great papers describing the technique in more detail are Cólon et al. (2010) and Sing et al. (2011).
As more tunable filters get commissioned and the new instruments CIRCE followed by the second generation instrument EMIR are inaugurated, the GTC will undoubtedly have loads to offer in the quest to study exoplanet atmospheres.