RESULTS OF THE TELESCOPE COMMISSIONING

• LATEST NEWS
• Changes in FITS keywords
• A fish-eye's view of the sky above the GTC
• Status of GTC dome
• GTC primary mirror reflectivity

Latest News

7^th 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.

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 http://www.gtc.iac.es/pages/multimedia/webcams-en-el-orm.php. In the future we intent to generate movies that will show the dynamically changing conditions of the sky above the observatory.

24^th 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 73 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 11 and 46 degrees. GRANTECAN is working towards resolving this problem.

10^th 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 meters 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 iniciated 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.

Caption 1: 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.

Caption 2: Close-up of the primary mirror where the operator is preparing for spraying CO2 snow on the primary mirror segments.