P Gran Telescopio CANARIAS
Observer's toolbox


This web page pretends to assist potential users of the GTC with planning and preparing their observations by providing links to various tools.

We welcome suggestions to expand this page.

icon Index


icon GTC's geographic location

The GTC is located at the Roque de los Muchachos Observatory on the island of la Palma. GTC coordinates are: Latitude: 28º 45’ 24’’ N and Longitude: 17º 53’ 31’’ W, at an elevation of about 2300 meters above sea level.

See larger map


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icon Finding chart generator

There exist a variety of tools to generate finding charts. Here we provide a link to the ESO finding chart generator and to the SDSS finding chart generator.

For finding charts to be useful they must:

  • Cover a reasonable field commensurate with the instrument's field of view (say 7 x 7 arcmin in the case of OSIRIS imaging observations, and 3 x 3 arcmin for OSIRIS longslit observations).
  • Be of sufficient resolution.
  • Indicate the scale.
  • Be taken in a visible wavelength band (g' or r' band, for instance).
  • Indicate North and East directions.
  • Clearly indicate any sources of interest, i.e. the target(s), the possible blind offset star, slit location.

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icon Blank fields

Blank fields containing few (bright) stars are useful for obtaining sky flat fields. For GTC a selected list of blank fields is given here.

This list was compiled using the TESELA tool developed under the umbrella of the Virtual Observatory and where you can also make your own searches. Full details about the search tool for GTC blank fields can be found in Jiménez-Esteban et al. (2012, MNRAS, 427, 679).

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icon Standard stars


For queue-scheduled observing GRANTECAN employs a set of spectro-photometric standards. This is a rather limited set of stars selected to serve a wide range of science projects. The selection criteria suitable for general-purpose standard stars implies making trade-offs and hence may not present the optimal choice for each specific program. Therefore users can always propose to observe specific calibration targets, but these would have to be defined as observing blocks for which the time will be charged to the program.

The current list of spectrophotometric standard stars: queue observing spectroscopic standard stars
And photometric standard stars: queue observing photometric standard stars

Other sources of standard stars can be found at the following links:


GRANTECAN welcomes suggestions to improve our selection of standard stars.

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icon Atmospheric conditions summary

This sections summarizes some of the key sky-quality parameters that may be relevant to planning observations with GTC. In summary:

  • About 75% of the night time is usable for science observations.
  • Median seeing is about 0.8 arcseconds at 500nm and unit airmass.
  • Typical extinction is of order 0.12 mag in V per unit airmass.
  • Sky brightness is 22.7, 21.9 and 21.0 mag/arcsec2 in B, V and R under dark sky conditions.
  • Median percipitable water vapor levels are 7.3 mm.

Clearly the average values vary between seasons. For further details we refer to the IAC's sky quality group web pages and this brochure.

The average monthly weather losses experienced during the first years of operation of the GTC are shown in the following plot:

timeUse graph


A typical La Palma night-sky spectrum is shown below, taken with the OSIRIS R1000R grism, under dark sky conditions:

timeUse graph

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icon Seeing monitors

The ORM site hosts three Differential Image Motion Monitors (DIMM), operated by the IAC, the TNG and ING, respectively. A DIMM monitor provides an estimate of the semi-instantaneous seeing. The on-line data in real time can be found through the following links:

Interpreting seeing result from a DIMM has to be done with certain care as its measurements should not simply be equated to seeing observed with, for instance, the OSIRIS instrument. The actual image quality depends on a large number of factors such as wavelength, zenith angle, direction in the sky, outer scale of turbulence and possible local effects within or outside the dome.

The DIMM seeing measurement gives a representative measure for the true seeing at a certain instant in time, at a wavelength of 500nm, and calibrated towards the zenith. Hence for practical purposes when observing a target at lower elevation or at a different wavelength then the seeing will be different.

Seeing scales with wavelength to the power -0.2, and with airmass to the power 0.6. The following tables indicate the effect of these scaling factors.

wavelength (nm) 400 500 700 900 1200 1600 2200
relative seeing 1.04 1.00 0.93 0.89 0.84 0.79 0.74


airmass (Elevation) 1.00 (90°) 1.03 (75°) 1.15 (60°) 1.41 (45°) 2.00 (30°) 2.92 (20°)
relative seeing 1.00 1.02 1.09 1.23 1.51 1.90


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Last modified: 16 December 2020

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