On Day One OSIRIS will be installed at the GTC Nasmyth focus, and its
final destination will likely be the Cassegrain focus as soon as it is
available. OSIRIS will then be directly attached to the mechanical rotator,
with no additional floor support from the Nasmyth platform.
A key OSIRIS feature is the use of Tunable Filters (TFs), a technology already
tested in 4m class telescopes.
A Tunable Filter is essentially a Fabry-Perot
etalon working at low spectral resolution (equivalent to narrow band spacing),
with high performance coating optimized over the range of wavelengths
studied. TFs allow monochromatic imaging at any wavelength with a user-selected
bandwidth in the range 10--50 Å. At optical wavelengths it is possible
to avoid strong airglow lines selecting the appropriate centroid and bandpass
within clear atmospheric windows. OSIRIS will use two
tunable filters, optimized for the red and blue parts of the spectrum,
to cover the full optical range. Piezo-electric transducers allow varying
the plate spacing from 2 to 12 microns. Orders are blocked by broad band
filters and by an additional set of interference filters of 350--450 Å
FWHM. Several advantages for this instrumental approach are: (a) avoid
the large expense to manufacture large numbers of narrow band filters,
(b) uniformity and control of the instrumental response function for all
wavelengths and band-passes, and (c) the use of the same optical path
for continuum and emission line images. In addition to the TFs, Broad
band imaging in the SDSS
(Sloan Digital Sky Survey) filters will also be provided.
Low-resolution
spectroscopy (R = 500, 1500 and 2500) will be available for the entire
wavelength range, by using several grisms. Long-slit together with multi-object
spectroscopy will be possible with a variety of focal-plane masks.
The
CCD controller will allow to synchronized the TF switching frequency (up
to over 100 Hz) with charge motion over large areas of the detector. In
this form it is very straightforward to obtain simultaneously images of
object and continuum in a single exposure, or to carry out time-series
photometry in the same or in different wavelengths with timescales of
less than one second. Also, surveys for high redshift objects with emission
lines can be efficiently performed allowing, for example, a fast cluster
membership identification.
Galactic
and extragalactic astronomy studies will benefit greatly from instruments
with tunable filter technology at a large telescope such as GTC. OSIRIS
at the GTC will permit two dimensional studies of very faint emission
line objects (and relatively faint absorption systems) at a continuous
selection of wavelengths and redshifts. Together with its complementary
spectroscopic modes, its large field of view, and the image quality provided
by GTC, OSIRIS will be a very competitive tool of wide use for the GTC
astronomical community, and a prime instrument with a potential to attack
a wide range of classical and edge-front observational programs.
Acronisms
Global
Pictures
Index
Optical
Design
Mechanical
Design
Detectors
Software
Optical Design
Given the requirement to use Tunable Filters available by Day One, OSIRIS
needs a very small pupil (about 10 cm or less) compared to most instruments
in 8-10 meter telescopes. This makes the OSIRIS optical design very unique
and challenging. The OSIRIS optical concept is basically a focal reducer
with a collimator-camera configuration, that provides an internal pupil
about 80mm in diameter. The entrance aperture of OSIRIS is a squared 430
mm on a side at the GTC focal surface, covering the 8.53arcmin x 8.53arcmin
field of view that fills the OSIRIS detector. At this surface a mass loader
allows placing of a set of pre-cut cylindrical masks. To provide sufficient
unscrambled pupil space while maintaining good collimation and image quality,
the collimating mirror has an off-axis ellipsoidal figure (conic constant
-0.94, 1240 mm focal length, and a vertex 1290.64 mm away from the telescope
optical axis) that separates by 16.15deg the incoming and collimated beams
of the OSIRIS reference field, a field displaced 2.5arcmin from the telescope
optical axis. The collimator unit also operates as flexure compensator.
Between the collimator and the camera,
a flat mirror folds (folder mirror) the
optical path to fit the instrument within Cassegrain envelope of the telescope.
The dimensions and shape of this flat mirror were designed not to obstruct
at all the beams of an 8.53arcmin x 8.03arcmin FOV, at the expense of a pattern
of vignetting losses beyond this area.
For the rich set of different observation modes, OSIRIS allows for several
different combinations of optical elements along the collimated-beam pupil
space. The free pupil space provided by the OSIRIS optical design is tight,
but enough to adequately place a tunable filter or a grism, and three
tilted filters or masks before the TF/Grism.
The all-refractive OSIRIS camera consists
on 9 lenses defining 6 optical elements, all spherical surfaces. The last
lens is the dewar
window. The camera effective focal length of 181 mm provides the required
detector scale (0.125arcsec/pixel) on a flat focal plane tilted 1.83 degrees.
With the present element apertures, the camera does not vignette a telescope
FOV of at least 11 arcmin in diameter. The first doublet is allowed to
move to focus the camera for all combinations of pupil optics and for
assembly and integration purposes.
More
details about the optical design
Optical
Requeriments
Limiting
Magnitudes
Field-of-View
comparison with other intruments
Etalon
Pictures
Optics
Pictures
Collimator
Pictures
Mechanical Design
The mechanical design faces two main difficulties coming from the requirement
that OSIRIS shall operate at both Nasmyth and Cassegrain
GTC foci: 1) to accommodate all elements and structures within the Cassegrain
envelope (2 m diameter per 2 m length), and 2) to provide a stiff enough
structure in both foci. The latter requirement specially concerns the
collimator cell structure. Below we describe three of the more important
mechanical subsystems.
Main
Mechanics Requirements
More
details about the mechanical design
Flexure
control
Mask loader
A slit
loader with capacity for up to 13 masks allows to insert and remove
slits masks to and from the telescope focal plane. In addition to user-customized
masks for multi-object spectroscopy, a number of fixed width long slit
masks will be available. One extra mask for point-like fats photometry
and another for charge-shuffled continuum subtraction will be available
as well. The set of long slit, user and special purpose masks will be
charged into the mask loader depending upon the nightly schedule.
Each door
is made by an aluminum arm that rotates around a bearing located in
one of its ends. The geometry of the arm is designed so that during
its turn does not impact with the enclosure of the instrument. In open
position, it guides the mask entrance into the compartment.
The opening
of each revolving door of the mask cassette is carried out manually
and independently and it allows the entrance of exit of the mask of
the cassette. When it is closed it presses the mask on the mask retainer
unit. A small adjustable screw allows to regulate the correct pressure.A
security mechanism, made by a beam and a microswitch, allows to anchors
the door in each one of their extreme positions and prevents that the
cassette moves in Z, if it detects some open door.
Camera unit
The Camera unit consists of the Camera Barrel, the Shutter
and the Detector Unit (two CCDs mounted inside a liquid nitrogen cryostat).
The Camera barrel includes a barrel for the first -focusing- doublet,
and second barrel for tow singlet and two-doublet lenses. The last camera
lens is mounted to the detector unit as a cryostat window.
Wheels
The Wavelength
Selection Subsystem consists of four wheels. The first three wheels
provide space for conventional and order sorter filters (18 positions
available), and collimated beam masks. The last wheel (nearest to the
camera) allocates two tunable filters and seven grisms.
Shutter
Pictures
Wheels
Pictures
Detectors
The detector
consists of a mosaic of two (2k x 4k) CCDs (15\micron pixels). The OSIRIS
planned CCDs are the MIT/LL-CCID20
MBE. These detectors have excellent red sensitivity, and improved blue
sensitivity thanks to the Molecular
Beam Epitaxy (MBE) process. Preliminary tests show that MBE technique
is working and that LL detectors will become exceptionally sensitive.
The backup CCD detectors will be e2v
CCD 44-82.
The
Preliminary Design of the detector controller contemplates a commercial
SDSU-2
controller, using a timing board with parallel cable linked to a commercial
digital PMC frame grabber. The CCD controller will allow charge to be
shuffled up and down the detector in a synchronous mode with shutter operation,
TF tuning and filter selection.
Detector
Pictures
Software
The
OSIRIS software will be deployed on different machines executing both,
conventional and Real Time Operating System (RTOS).
These machines will be connected to the GTC
Control System (GCS) through different networks, mainly ATM.
An Ethernet (10/100
Mb/s) is also used for enginnering purposes, and are also available serial
and CAN type field buses. The OSIRIS Control Software is being developed
using a Use Case Method for requirements capture, and using a Distributed
Object Oriented Approach which is integrated into a major framework,
the above mentioned GTC Control System. The RUP
(Rational Unified Process) has been used as software process
framework.
Detector Control
The OSIRIS Detector Control System
(DCS) will be responsible for the image acquisition in the OSIRIS
instrument. The main purpose of the OSIRIS Detector Control System is
to perform the image acquisition of the OSIRIS instrument. The DCS will
command the SDSU controller in charge of the CCD control. The images will
be acquired through a RS-422
output connected to a PMC digital frame grabber, and then will be delivered
upon request to the Data Factory subsystem for processing.
Data Factory
An automatic set of procedures will be available to process the data acquired
from standard OSIRIS observing modes. At least (a) standard imaging, (b)
long-slit spectroscopy, (c) charge shuffling imaging and (d) fast photometry
observing modes will be processed using the pipeline. The processing
is done in two phases: a pre-process and a post-process. The pre-processing
procedures include standard CCD operations: bias subtraction, dark current
correction, flatfield correction, and cosmic-ray events removal. Post-processing
of data depends of the observing mode and will include wavelength calibration
and geometric distortion correction. The OSIRIS Data Factory is completely
included in the GCS Data Factory subsystem at the Operations Co-ordination
level. The GCS hardware and software standards will be used in the design
and implementation of the OSIRIS Data Factory.
Observing Program Management
The Observing Program Management Subsystem (OPMS) groups all the GTC facilities
for the creation and/or modification and submission of observing proposals.
The OPMS will give access to the tools that help to prepare the observation.
Two basic tools are required: The Mask Designer and the Exposure
Time Estimator. The Mask Designer is needed to define the
exact positions and shapes of the slits in the focal plane masks, in order
to perform multi-object spectroscopic observations. The second tool is
useful to calculate exposure times and/or signal-to-noise ratios for a
specific observing mode and astronomical target.
Sequencer
The OSIRIS Sequencer subsystem is integrated into the GCS Sequencer and is
responsible for the commands execution, decoding such commands, breaking
them down into smaller operations understable by the rest of the subsystems,
co-ordinating the concurrent and sequencial execution of the operations
between the different subsystems involved, and finally returning the results
to the emitter of the command. An operation is the most general
input to the Sequencer. An operation can be an observation, a calibration
procedure or a maintenance procedure.
Data
Factory Summary
Data
Factory Use-Case Model Survey
Mask
Designer Pictures
OSIRIS Home Page
Last update July 18, 2005, by Héctor Castañeda
