GEMINI NEAR INFRARED SPECTROGRAPH
INTEGRAL FIELD UNIT
DOCUMENTATION
FOR THE
CRITICAL DESIGN REVIEW
(23 May, 2000; at the University of Durham)
17 May, 2000
Compiled by:
C.M. Dubbeldam,
Senior Mechanical Engineer,
for the GNIRS Project Team
Reviewed by:
J.R. Allington-Smith,
Work Package Scientist
Approved by:
D.J. Robertson,
Work Package Manager
University of Durham
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GNIRS IFU CDR DOCUMENTATION
EXECUTIVE SUMMARY
The Gemini Near InfraRed Spectrograph (GNIRS) currently under development at NOAO and
scheduled for delivery in the summer of 2002, will include a powerful and innovative Integral Field
Spectroscopy (IFS) capability. The design, integration and test of the GNIRS Integral Field Unit (IFU)
are the responsibility of the University of Durham’s Astronomical Instrumentation Group. Its design is
based on the Advanced Image Slicer concept developed as a result of research conducted under the
auspices of the Durham Instrumentation R&D Program. A slicer-based system has many advantages
over fibre-based designs, especially for cryogenic instruments such as GNIRS.
The GNIRS IFU consists of two fully independent, self-contained modules offering different spatial
resolution and field size. Both modules are mounted inside the GNIRS slit slide mechanism. This slide
mechanism is employed to select the required spectroscopy mode by sliding the respective module
into the instrument’s optical path. The High Resolution (HR) mode, which provides a field of view of
1.0”  1.5” with a sampling resolution of 0.04” over 972 spatial elements and a spectrum length of
1024 pixels, is optimised for use with fully adaptively corrected images, while the Low Resolution (LR)
mode provides a larger field of view of 3.2”  4.4” with a sampling resolution of 0.15” over 625 spatial
elements and a spectrum length of 1024 pixels, will be used with tip-tilt corrected images. They will be
used in conjunction with the long and short cameras respectively. The instrument will be able to
change remotely between the conventional aperture spectroscopy (AS) mode, in which light enters
the spectrograph main optics via a slit, and each of the IFS modes. Location of the IFU modules at
the telescope focal plane inside the slit slide mechanism allows for a rapid changeover of observing
mode by simply inserting or withdrawing the appropriate module from the beam.
Summary of GNIRS IFU System Parameters
MODE
Low Resolution
High Resolution
0.15”  0.15”
0.04”  0.04”
1.0  2.0
1.25  2.5
21
26
3.15”  4.46”
1.04”  1.50”
625
972
Spectrum Length
1024 pixels
1024 pixels
Slice Dimensions
7.4  0.5 mm2
9.4  0.5 mm2
GNIRS Camera Employed
Short Camera
Long Camera
Spatial Sampling
Magnification (Parallel  Perpendicular to Slice)
Number of Slices
Field of View
Number of Spatial Elements
A crucial feature of the IFU’s optical design is the excellent image quality, which has been optimised
to take full advantage of the outstanding images provided by the Gemini telescopes. The mechanical
design ensures that all optical components remain accurately aligned and that the IFU’s performance
remains predictable and maintainable under all environmental conditions. These challenging goals
have been achieved in a module, which is sufficiently compact to be easily accommodated inside the
GNIRS instrument’s slit slide mechanism.
University of Durham
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GNIRS IFU CDR DOCUMENTATION
TABLE OF CONTENTS
EXECUTIVE SUMMARY ........................................................................................................................ 2
1. INTRODUCTION................................................................................................................................ 4
2. CDR DOCUMENTATION .................................................................................................................. 4
The following documents form an integral part of the CDR Documentation:
1. FUNCTIONAL AND PERFORMANCE REQUIREMENTS DOCUMENT
2. OPERATIONAL CONCEPTS DEFINITION DOCUMENT
3. INTERFACE CONTROL DOCUMENT
4. DESIGN REPORT
5. ASSEMBLY, INTEGRATION AND TEST PLAN
6. MANAGEMENT PLAN
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GNIRS IFU CDR DOCUMENTATION
DOCUMENTATION FOR THE GNIRS IFU CRITICAL DESIGN REVIEW
1. INTRODUCTION
The Gemini Near InfraRed Spectrograph (GNIRS) currently under development at NOAO and
scheduled for delivery in the summer of 2002, is one of the initial phase instruments for Gemini. In its
baseline form it provides a powerful capability for low to medium resolution spectroscopy over the 1 to
5 m wavelength range with two different cameras giving image scales of 0.05 and 0.15 arcsec per
pixel. With the addition of the Integral Field Unit (IFU), it will gain a powerful and innovative integral
field spectroscopic capability. The design, integration and test of the GNIRS IFU are the responsibility
of the University of Durham’s Astronomical Instrumentation Group. Its design is based on the
Advanced Image Slicer concept developed as a result of research conducted under the auspices of
the Durham Instrumentation R&D Program. A slicer-based system has many advantages over fibrebased designs, especially for cryogenic instruments such as GNIRS.
The contract to build GNIRS has been awarded to the National Optical Astronomy Observatories
(NOAO) of the USA. This was before the requirement for an Integral Field Spectroscopy (IFS) mode
had been clearly defined. The priority for the inclusion of an IFS mode in GNIRS was subsequently
identified by the Gemini Project. Building on the development of the Advanced Image Slicer carried
out at the University of Durham’s Astronomical Instrumentation Group (AIG), the Gemini Project
asked Durham to propose an IFU design for GNIRS. As a result of this work the AIG have been
awarded a Gemini Work Package (through the UK Gemini Project Office - UKGPO) to design and
build an IFU for the GNIRS.
The project has now progressed to its Critical Design Review (CDR); the information presented in the
CDR documentation will give a detailed description of the proposed optical, mechanical and thermal
design of the IFU, and will enable the CDR Review Board to fully evaluate the current status of the
program. At this review, we present details of the Low Resolution (LR) Mode only. Additional work is
required to develop the design of the High Resolution (HR) Mode; this work will be completed after
this CDR. This will give Durham the opportunity to apply to the HR Module the experience gained
during the design and production of the LR Module. In our project plan we have included provisions
for a further review of the HR Mode.
2. CDR DOCUMENTATION
The following documents form an integral part of the CDR Documentation:
1. Functional and Performance Requirements Document (FPRD)
2. Operational Concepts Definition Document (OCDD)
3. Interface Control Document (ICD)
4. Design Report
5. Assembly, Integration and Test (AIT) Plan
6. Management Plan
The IFU’s system-level functional and performance requirements are specified in the FPRD.
Operational issues, such the generic observation procedure, system calibration, data reduction, etc.
are discussed in the OCDD. All interfaces (mechanical, electrical, thermal, optical, etc.) with the
GNIRS instrument are controlled by the ICD. A detailed description of the IFU’s optical, mechanical
and thermal design is presented in the Design Report. The optical design section includes the
specification (prescription data) of all optical surfaces, and the results from the performance analysis
(image quality, throughput, etc.), diffraction analysis and tolerance analysis. Note that completion of
the stray light analysis has been postponed until after this CDR. The mechanical design section
includes the analysis of the tolerances produced by the tolerance analysis of the optical design, the
structural analysis using finite element models, and a discussion of the thermal aspects of the design.
The design report will also include detailed manufacturing and assembly drawings. The AIT plan
presents an overview of the integration and test strategy, and gives a specification of the applicable
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GNIRS IFU CDR DOCUMENTATION
test requirements and essential test equipment. The management plan presents an overview of the
project organisation (including details of key management, scientific and technical personnel), a
discussion of the quality control issues and a detailed project plan (schedule) and cost breakdown.
An introduction to the principles or Integral Field Spectroscopy and a review of the current state of the
technology is given in the following paper by Dubbeldam et al.: “An Integral Field Unit for the Gemini
Near InfraRed Spectrograph”, SPIE Proceedings, Vol. 4008-136, 2000.
University of Durham
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