There are two different requirements that drive the mechanical design of OSIRIS: The instrument top-level requirements and the GTC mechanical constraints. The general design criteria are listed in the following paragraph
The main top-level requirements more directly affecting the mechanical design are included in Table 1, whereas a resume of GTC interface constraints is included in the table below.
| Top-level Requirement | Value |
| Observing Modes | Imaging (with conventional and
tunable filters)
Long-slit spectroscopy using grisms Multi slit spectroscopy using grisms |
| FOV | 8.53' x 8.67' useful for imaging
(equivalent to 422 x 429 mm in the telescope focal plane)
8.53' x 5' useful for spectroscopy (equivalent to 422 x 247 mm in the telescope focal plane) |
| Image quality in long slit spectroscopic mode | The FWHM shall be better than 0.6 arcsec in a FOV of 5' x 5' |
| Instrument stability in imaging mode | The image shall move less than ± 7.2 m m (goal ± 3.6 m m) in one hour exposure |
| Instrument stability in spectroscopic mode | The image of the slit shall move less than ± 3.6 m m (goal ± 1.8 m m) in the spectral direction in one hour exposure |
| Spectrophotometric accuracy in long-slit mode | The flux through the long slit shall vary less than 3.7% during all the night, assuming changes of slits |
| Spectrophotometric accuracy in MOS mode | The flux through the MOS mask shall vary less than 6.7% during all the night, assuming changes of slits |
| Slit orientation accuracy | The slit shall be oriented perpendicular to the grism rulling with an accuracy better than 0.5o |
| Slir Repeatability | The positional repeatability of the slit in the focal plane shall be better than ± 33 m m |
| Focus operation | OSIRIS shall be designed to operated at Nasmyth and Cassegrain focus |
| Mask loader mechanism | OSIRIS shall have at least 13 different focal plane masks for spectroscopy available without interruption of the instrument operation |
| Wavelength Selection Subsystem | 14 conventional filters, 3 collimated beam masks, 2 tunable filters and 6 grisms shall be available simultaneously at OSIRIS without interruption of the instrument operation |
| Interface constraint | Value |
| Total mass | Total mass is limited to 2000 Kg at the Cassegrain position and 2400 Kg at the Nasmyth position |
| Geometrical envelope | Envelope shall be according A.4 for Nasmyth and A.5 for Cassegrain. The external diameter of 1990 mm in the Cassegrain envelope is not enough for OSIRIS and a new envelope of mm diameter is being negociated |
| Maximun size of each module | The maximum size of each instrument module shall be 3.25 x 3 x 3 m |
| Position of CoG | The z coordinate of CoG shall be located within the range (+385,-1115) mm in Nasmyth (NGCS) and within the range (+385,-615) mm in Cassegrain (CGCS) |
| Unbalanced force | Maximum moment to rotate a unbalanced instrument is 1000 N.m |
| Nasmyth mechanical attachment | The mechanical interface between the Nasmyth rotator and OSIRIS shall consist of a flat circular hollow flange according A.6 |
| Cassegrain mechanical attachment | The mechanical interface between the Cassegrain rotator and OSIRIS shall consist of a flat circular hollow flange still undefined |
Design Guidelines
The basic design guidelines, derived directly or indirectly from the OSIRIS functional requirements, the interface constraints and the optical design, as well as from manufacturing, integration, alignment, operation and maintenance criteria, are included in the following list and will be spreaded through the specifications documents. Guidelines shall be used as discriminating criteria to select the instrument technical baseline from the available options.
OSIRIS is functionally divided in 7 mechanical subsystems:
The Support Structure (IN-OS-SS)
The Support Structure shall be the main structural part of the instrument. It also provides the interface between the instrument and the telescope instrument rotator. The support structure consist of an optical bench (made of a front and back steel panels joined transversally with stiffeners) that supports the slit, the FM, the WSS (filter and grism wheels), the camera barrel, the shutter and the cryostat. The FM is joined to the bench by a cantilevered beam. A similar beam is used to join the camera barrel, the shutter and the cryostat to the bench. A hyperstatic beam structure supports the collimator cell. The optical bench is joined to the Cassegrain and Nasmyth interface flanges through three support trusses. A conical adapter is used to attach the instrument to the Nasmyth rotator A thermal envelope encloses the instrument to protect it and control the temperature fluctuations.
The cabinets containing the electronics modules could be mounted on the rotator flange (structurally decoupled from the OSIRIS support structure) or outside the rotator (attached to the primary cell in the Cassegrain position, or in the floor of the Nasmyth platform) if the mass budget exceeds the telescope limits.
The Slit Subsystem (IN-OS-SL)
Some observing modes (Imaging with Charge Shuffling, Fast Photometry and the different options in Spectroscopy) require that some part of field coming from the Telescope be blocked. The Slit Subsystem has an array of different masks. Each one with different kind of slits. The masks are situated together in a mask cassette, and a mechanism select one mask and remove it from the cassette to place it on the GTC focal plane.
The Slit Subsystem consists of the Mask Selection Mechanism that allows to select one observing mask, the Mask Loader Mechanism that removes the Mask from the Cassette and insert it in the GTC focal plane and the Masks itself. Multi Slit Masks are manufactured in site and mounted on a rectangular Frame. Long Slit, Fast Photometry and Charge Shuffling Masks are manufactured in the same frame structure.
Masks can be removed from OSIRIS during normal day maintenance wiithout dismount
The Collimator Subsystem (IN-OS-CO)
The Collimator Subsystem consists of the Collimator Mirror and the Cell that supports the mirror and allows to attach it to the OSIRIS support structure
The Collimator is an off-axis ellipsoid having an optical clear aperture of 533x531 mm. A lightweight substrate having an aspect ratio of 6:1 and a minimum mass reduction of 40% has been used to fulfil the error budget requirements for image movement.
The Cell is isostatically attached to the OSIRIS Support Structure using three interface zones defined in the upper part of the hexagonal Collimator Interface. This hexagonal interface is joined to the optical bench using a beam structure. The Cell has to provide safety stops to prevent the falling of the mirror in the case that the main support components fail.
The design of the Collimator Subsystem has not been developed. The design, fabrication, assembly and tests of this unit shall be subcontracted at the specification level.
The Folder Mirror Subsystem (IN-OS-FM)
The Folder Mirror Subsystem consists of the Folder Mirror, the Cell, the Positioning Mechanism to tilt the mirror and the FM Support that allows to attach it to the OSIRIS support structure.
The Folder is a flat mirror having an optical clear aperture of 264x185 mm. The Cell can be tilted in two axis situated in the mirror plane using a Positioning Mechanism attached to the FM Support. This mechanical support is attached to the OSIRIS Support Structure using TBD interface zones in the Folder Interface. The Cell has to provide Safety Stops that prevent the falling of the Mirror in the case that the main support components fail.
The design of the Folder Mirror Subsystem has not been developed. The design, fabrication, assembly and tests of this unit shall be subcontracted at the specification level.
The Wavelength Selection Subsystem (IN-OS-WS)
The following optical components shall be inserted in the optical beam by the WSS: Tunable Filters Grisms Conventional filter Collimated beam masks The WSS shall allow to insert/remove any of these elements from the storage position to its nominal position. All these operation shall be driven by remotely controlled motorised mechanisms.
The Wavelength Selection Subsystem consists of three wheels to mount the wavelength selection optics, a tilt mechanism to rotate the conventional filters about a radil axis, the WSS Support that provides the interface between the wheels and the OSIRIS Support Structure and the WSS optics (filters, masks and grisms). The WSS shall be mounted between the FM and the Camera Barrel.
The Camera Subsystem (IN-OS-CA)
The Camera consists of the Camera Optics, the Shutter and the Detector Unit (the two CCDs mounted inside a liquid nitrogen cryostat). The Camera Optics consists of the Camera Barrel - that includes the Barrel #1 with the first doublet (D1), the Barrel #2 with the singlets S1 and S2 and the doublets D2 and D3, and the Focus Unit that moves axially the Barrel #1 for focusing - and the Cryostat Lens with the singlet S3
The Support Elements Subsystem (SE-OS)
Last update July 28, 2005, by Héctor Castañeda