Telescope commissioning progress



icon 7th Nov 2011

Subject: Changes in FITS keywords

Recently a new software release of the GTC Control System was implemented and as a result a number of changes to the FITS keywords took place. Here you can find the listing and description of these modifications. Further changes and extensions of the FITS keywords are anticipated, in particular in relation to new instruments and as telescope functionality expands. We are working on a full keywords dictionary.

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icon 23rd June 2011

Subject: Astronauts visit GTC

On 23 June 2011 GTC was honoured by the exceptional visit of an impressive number of well-known astronauts, cosmonauts, and scientists as part of the STARMUS Festival event (see, celebrating the 50th anniversary of the first man in space, Yuri Gagarin.

In the afternoon a live broadcast of a round-table discussion was organized from within the GTC telescope dome.

Visitors to the GTC included Neil Armstrong, Alexei Leonov, Bill Anders, Jim Lovell, Richard Dawkins, George Smoot, Leslie Sage, Charlie Duke, Jill Tarter, Francisco Sánchez, Garik Israelian and, last but not least, Brian May.

The visit happened to coincide with commissioning activities of the CanariCam instrument, so the visitors were able to see from the GTC control room a movie of the Moon one of them had walked on, but this time at the invisible mid-infrared wavelengths. Undoubtedly a memorable event for all!

Round table live broadcast with the telescope in the background

Visitors in the GTC control room, with from front to back: Jill Tarter, Jim Lovell, Alexei Leonov, Neil Armstrong and Francisco Sánchez.

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icon 31st May 2011

Subject: A fish-eye's view of the sky above the GTC

For astronomical observations from the ground it is important to know the current condition of the atmosphere during the night. While the scientific team at night is working with the telescope from the control room, it may happen that clouds drift in and cover part of the sky without the observers being aware of this in time, thus affecting the observations. If the scientific team would know where the clouds are they could adapt their program accordingly, pointing the telescope at a clear patch of sky, or changing the type of observation. For this reason it was decided to install an automatic camera that takes regular pictures of the full sky. This all-sky camera is located close to the telescope building, working day and night. Besides being a tool for the astronomers at night, the camera generates pretty pictures of the sky, such as the one shown below. Live pictures can be found on here. In the future we intent to generate movies that will show the dynamically changing conditions of the sky above the observatory.

The GTC all-sky camera, attached to the roof of the services building.


Example of a single shot of the complete night sky.

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icon 24th May 2011

Subject: Status of GTC dome

Since the start of operation of the GTC there have been persistent problems with the dome enclosure. Although the main shutter has been reliable in operation, the most important problem still persist: for reasons of safety of the equipment the main shutter is normally not fully opened. This implies that targets observed by the telescope at an elevation of more than 72 degrees are vignetted by the dome shutter. Vignetting of the telescope beam reaches about 40% at an elevation of 85 degrees. The observing team at night, when planning the observations ensures that this problem does not affect the observations. However, it does preclude taking of long time series under photometric conditions of objects with a declination between 10 and 46 degrees. GRANTECAN is working towards resolving this problem.

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icon 14th July 2010

Subject: M3 re-aluminized

The tertiary mirror, M3, of the GTC was re-aluminized in April during a brief stand-down of three days. M3 as well as the primary mirror had become quite dirty due to the ongoing work of completing and commissioning the telescope. Now that we have entered in the operational phase of the telescope, having clean mirrors is a priority.

Recoating of M3 has implied aproximately a 16% increase in reflectivity. From an initial reflectivity of around 76 % (for wavelengths of 470, 530, 650 nm) and around 73 % at 880 nm the reflectivity has improved to around 91 % and 88%, respectively.

Apart from the reflectivity also scattering losses were measured. Scattering measured at specular reflection angles has reduced by two orders of magnitude and is now close to zero, sign of having a very clear mirror surface.

The secondary mirror had already been re-aluminized during 2008, and along the next few months the segments of the primary mirror will be recoated.

The 36 segments of M1 will be gradually replaced in small batches during the coming months. This will require only a minimum of night-time calibration to model the optimal position of each mirror segment. Reflectivity of the M1 mirror segments varies from 76 to 85%, depending on how long they have been in the telescope. The worst segments will be recoated first, in order to achieve the best possible performance as quickly as possible.

In any case, from the moment the first segments are being recoated the full mirror will be cleaned using CO2 snow. The expectation is that this will bring a gain of 5% in reflectivity for the mirrors with the old coating. Snow cleaning with CO2 will be a regular occurrance.

the oval tertiary mirror just after having been recoated.

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icon 10th Nov 2009

Subject: GTC primary mirror reflectivity

After traveling for billions of years through the universe, the photons from distant galaxies arrive at the GTC where they are happily reflected into focus by the 75.7 square mereters worth of M1 primary mirror surface. Keeping the 36 mirrors segments clean is an important and continuous task that has occupied GRANTECAN personnel for a significant fraction of their time.

Since the start of the year a total of 14 segments have been replaced by freshly re-aluminized units. This process started with the segments that were most affected by dust due to their long presence in the telescope. As soon as the segments are mounted in the telescope they start to collect dust that will reduce reflectivity and increase scatter. Therefore, we have indicated a regular cleaning activity to keep dust off the mirrors using CO2 snow. By spraying CO2 snow over the vertically inclined mirror surface the dust particles come off the surface and float down. This is a well-known and effective method used at many telescopes around the world. But an added complexity at the GTC is the size of the primary mirror which renders a seemingly straight forward task into a logistically complex and time-consuming exercise.

At the moment mirror reflectivity on average is 86%.

Keeping the primary mirror reflectivity in optimal condition will be a continuing effort following a programme of regular CO2 snow cleaning activities combined with occasional re-coating of batches of segments. In this way we will ensure that every square centimeter of mirror surface counts and that no ancient photon is lost for science.

The huge size of the GTC primary mirror makes the CO2 snow cleaning activity a complex and time-consuming task. Using a mobile hydraulic lifting platform the operator can reach every part of the mirror.



Close-up of the primary mirror where the operator is preparing for spraying CO2 snow on the primary mirror segments.

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Last modified: 13 February 2013

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