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Specifications & Interfaces

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This page describes key specifications and interfaces for Gemini instruments including interfaces for ISS-mounted instrumentsconsiderations for bench-mounted instrumentssystem services available, and the Observatory environments. Key interfaces are described in interface control documents (ICDs).

Gemini Specifications and Interfaces for Instruments

Document ID Document Title Revision/Date
download INST-REQ-0001 Science and Facility Instruments Common Requirements Specification C 08/20/2018
download ICD 1.5.3/1.9 Instrument Support Structure to Science Instruments ICD E 07/18/2018
download ICD 1.9/2.7 Science Instruments to Facility Handling Equipment ICD E 04/27/2016
download ICD 1.9/3.6 Science and Facility Instruments to ISS System Services ICD H 08/30/2018
download ICD 1.9/5.0 Science and Facility Instruments to Transport, Observatory and Operational Environments ICD D 08/06/2018
download ICD G0014 Optomechanical Coordinate Systems B 09/08/2011
download ICD 1.1.13/1.9 Interlock System to Science Instruments ICD A 06/16/1997
(one package)
GIAPIBuilderReq-01302009 GIAPI Software Requirements for Instrument Builders 04 11/18/2014
GIAPIUse-08292006 GIAPI Design and Use 08 11/18/2014
GIAPIC++ICD50-02042008 GIAPI C++ Language Glue API 10 01/28/2014
ICD 20 Synchro Bus - Node/Page Specifications D 10/11/2013

Overview of Interfaces

Instruments mounted on the telescope Instrument Support Structure (ISS) must adhere to the following requirements and guidelines. The ISS webpage provides an overview of the ISS, telescope and observing floor.

Mechanical Interfaces 

See ICD 1.5.3/1.9 and ICD 1.9/5.0 for more details. 

Space Envelope: Instruments must satisfy the space envelope requirement as specified in drawing 89-GP-1000-0004 and ICD 1.5.3/1.9. Extension into the ISS (such as to add an entrance shutter) will require a waiver of the space envelope.

Mounting: Instruments must support mounting via use of Gemini ballast weight structures or using the standard Gemini physical interface specified in ICD 1.5.3/1.9. The ballast weight structures are empty frames upon which to mount instruments and add weight. 

Ports: There are three ports on the ISS available for science instruments: `side looking' or 'side ports' on Faces 3 and 5, and an `upward looking' or 'bottom port' on Face 1 (when the telescope is pointing towards zenith).    

Mass: Instrument weight on the ISS science instrument port faces must not exceed 2000kg. 

Thermal Control: It is preferred for instruments to minimize heat released to the dome environment and into the ISS structure. The threshold requirement for facility (non-visiting) instruments is 100W and 50W respectively.

Orientation: ISS-mounted instruments are normally subjected to changes in orientation as the telescope points from zenith to horizon and the Cass Rotator turns through its full range of 540 deg.

Vibration: Gemini telescopes are vibration-sensitive environments. Instrument builders should take care both to minimize induced vibrations, and to minimize the impact of vibrations coming from other sources. Gemini has experience with various cold head types and vibration mitigation techniques. External sources of vibrations include other instruments and other parts and components of the telescope system. This set of dome floor vibration measurements at GN is one characterization of the vibration environment, but it will vary depending on the site and on the instruments mounted on the ISS. Gemini has several sets of reports and data for various instruments. 

Flexure: ISS-mounted instruments will experience internal flexure due to variant gravity vectors, and may require passive or active flexure compensation. While every instrument will experience flexure in its own unique way, one characterization is given in two GPI papers on flexure modeling and flexure compensation.   

Optical Interface

See the Optics webpage for more details (telescope prescription, coatings, specifications, etc.)

Port: Instruments must be able to use Gemini's optical interface for the instruments, which is a circular port 400mm in diameter through which the telescope or calibration beam can be directed. The port is central to the ISS face and the beam emerges perpendicular to the interface plane. A machined land is provided to make a light tight seal with the instrument. Fastener detail for mounting a baffle component can be provided as needed. The nominal telescope F/16 focus is 300mm from the mechanical interface of the ISS. The plate scale at focus corresponds to 1.611 arc seconds per mm. The ports are detailed in ICD 1.5.3/1.9.

Guiding: Fast guiding is typically required at Gemini for good image quality. This is usually provided by a peripheral wavefront sensor (PWFS-2) that performs M1 active optics contract and M2 tip/tilt correction. Please see the Guiding and WFS webpage for more information. 

Flexure when guiding: Instruments using the PWFS for guiding may also notice flexure. The NIRI and GMOS webpages on guiding describe each instrument's experience using PWFS. Characterization of the NIRI-PWFS2 flexure was performed in 2008, plotting the change in pixels (also here) as the CRCS rotated from 180, to -180, back to 180 in 45 degree increments. 

Pointing Accuracy: Current blind pointing accuracy across the entire sky is +/- 6-8 arcsec, with average blind pointing errors +/- 2-3 arcsec. Use of A&G system puts a science target on any instrument within a positioning error induced by mapping (+flexure) of the guider. That error is +/- 1 arcsec for PWFS2 or +/- 0.5 arcsec with OIWFS. 

Calibration: GCAL is the facility system for flat field and wavelength calibrations. Please see the Calibration webpage for more information. 

Bench-Mounted Instruments

Gemini's Pier Lab has previously hosted BHROS, and will host GHOST, both bench-mounted fiber-fed spectrographs. The Pier Lab is located ~15m below the dome floor and having a ~800mm diameter shaft connecting the Lab to the dome floor. Lab geometry is described here.

  • Pier Lab internal usable diameter: 8,000 mm
  • Pier Lab nominal access door width (subtracting door thickness): 3,000 mm
  • Pier Lab nominal maximum height (subtracting structure and crane): 3,000 mm
  • Pier Lab ceiling hole: 800 mm
  • Uniform stationary Live Load capacity is 600 lb per square ft or 2929 kg per square meter.

System Services

See ICD 1.9/3.6 for more details. 

Gemini provides the following services to instruments via the Services Panel. For bench-mounted instruments, Gemini may provide the same or subset of Systems Services to another location on a case-by-case basis. Ideally instruments will have a patch panel to facilitate hook up to the service panel, and the instrument should come with equipment used in typical setup including interconnecting hoses, cables, etc. 

Coolant Dowtherm SR-1, 40 % by volume and water
Helium Closed Cycle Coolant GM Cooler Types
Compressed Air
Communications Wired ethernet only (GN) / Wired or wireless ethernet (GS)
Power UPS and MAINS
AC power frequency 60Hz (GN) / 50Hz (GS)

Gemini Observatory and Operational Environments

See ICD 1.9/5.0 for more details. 

This set of environmental measurements for both sites provides temperature, pressure and humidity data collected over two years. The data was taken from an external weather station near each telescope and the data sampling is not strictly uniform.

Altitude 4200m (GN) / 2700m (GS)  
Ambient air temperature  -15C to +25C
Ambient air temperature rate of change up to 1.3C/hr
Relative humidity 0% to 100%
Gravity component limits
(components vary continuously in discrete steps)
X axis +/-1g
Y axis +/-1g
Z axis -1g to 0g
Vibration  Varies between sites

Gemini Observatory Participants