The Isaac Newton Telescope (INT) saw first light in 1967 in Herstmonceux, United Kingdom, the site of the Royal Greenwich Observatory. At the time, the INT was the 5th largest telescope in the world (acc. to wikipedia).

It soon became clear that better astronomical weather conditions would benefit the astronomical use of the telescope. Such conditions were found in the late '70s at the Roque de Los Muchachos Observatory (ORM) on La Palma, and it was decided to move the telescope to there. The INT resumed operations in 1984, as part of the Isaac Newton Group (ING). More information about the INT can be found on the public information web pages.

The telescope has a polar disc/fork type equatorial mounting. Current telescope operational range is as follows:

• Zenith distance < 70°
• - 6 h < hour angle < + 6 h (above pole)
• Declination > -30° 09' 30"

Note that the lower shutter (or windshield) causes vignetting for zenith distances > 57 degrees and it can be raised for such observations. This sky visibility plot shows the INT visibility. Another old plot shows the INT visibility when the lower shutter is raised for low-altitude observations.

A more detailed description of the optics and the mounting of the INT is available in the Observers' Guide. The INT's Telescope Control System runs on a DEC Alpha machine. For more information please see the INT TCS manual.

The instruments offered at the INT starting from Semester 06B are

• the Wide Field Camera (WFC)
• the Intermediate Dispersion Spectrograph (IDS)
  

Detailed description of how to operate the telescope and the instrument is contained in the above pages and online manuals.

For applying for observing time and preparing observations, please visit the ING Astronomy web pages. Note that the WFC and the IDS will be scheduled in minimum blocks of 4 weeks, as the ING can only support a small number of instrument changes every semester. For this reason, we invite the community to present large programmes to make the best use of the INT (smaller proposals asking no less than 4 nights are still accepted however).

As of August 2003, the INT is fully operated by visiting astronomers and no telescope operator is available. A detailed introduction to the telescope is given by ING students, who stay at the telescope until midnight of the fist night of an observing run. ING staff astronomers will be available for contact prior to the observations, and will be at the telescope in the afternoon of the first day of each run.  Being alone at the telescope for most of your observing run, it is very important that your read carefully the documentation on Health and Safety Issues for Visiting Observers.

The INT median seeing is about 1.2 arcsec (based on a recent INT seeing and image quality internal report using summer 2011 data). More information on local seeing can be found at the ING seeing measurements web pages. Weather at ORM is clear most of the time (INT weather downtime), being clouded only 20-25% yearly. The INT technical downtime is around 2% only, comparable with the WHT. The dark-of-moon ORM sky brightness at high ecliptic latitude is similar to that at other good dark sites, V~21.9 mag arcsec².

IMPORTANT: When the laser is working at the WHT, the INT observers should monitor the "TELESCOPE SUMMARY" in the Laser Traffic Control System web page, in order to check if there is any limitation in the INT pointing because of collisions with the laser.