The instrument mechanics are driven by very stringent requirements on image stability in imaging and spectroscopic modes, all of them fullfilled successfully.
The main requirements are:
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An image movement on the detector smaller than 1/5 of the smallest FWHM (GTC plus seeing plus instrument) per hour and spectral stability better than 10% (goal 5%) of the nominal resolution in one hour.
Errors due to slit fabrication and assembly, errors in spectral and focus directions due to positioning, flexures, temperatures variations, etc., will not contribute to long-slit flux calibration uncertainties by more than 3.7 % during a whole night.
Overheads, intended as changes of observing mode or change of elements within the same mode, smaller than the nominal detector read-out time (40 seconds). The largest overhead in the current design is of 24 seconds in the worst situation.
The following elements should be loaded simultaneously in the instrument: SDSS filter set, order sorters for low resolution tunable imaging and spectroscopy, masks to calibrate tunable filters and spectrograph, low- and intermediate-resolution grisms. With the current wheel design, it is possible to accommodate all tunable filters order sortes, all spectroscopic order sorters, the SDSS set, and the grisms for low and intermediate resolutions.
These requirements, together with the need to be able to work at Nasmyth and Cassegrain foci of the GTC, make the mechanical design quite challenging. To fulfill them, detailed error budgets using Finite Element Analysis (FEA) models and Zemax for analysis have been developed to control the contribution of each component to the image movement and considering to collimator as compensator of flexure residuals in open loop.
Last update July 28, 2005, by Héctor Castañeda