Calibration
A general definition for calibration is the determination of the relationship between values indicated by a sensor and the actual corresponding values. The calibration of an astronomical instrument involves the procedures employed to remove the instrumental signature from the scientific data. The main goal of the calibration is obtain a complete characterization of the instrument that will be used for data analysis and processing. It is expected to gain a good understanding of the behaviour of the instrument, and the acquisition of calibration files.
The calibration of the instrument can we described into three categories: detector calibration, spatial calibration, including wavelength, and flux calibration
A Calibration Plan will be prepared before the start of the Commissioning.
This document will define the following items:
Al the operation modes of observations.
The formats for all the raw data that the instrument will produce.
Data and procedures required for calibration.
Observing programs required to acquire calibration data.
Procedures required for the verification of the instrument performance.
The final delivery will be the Calibration Plan Document. The results of the Calibration Plan will be used as the input for the parameters that uses the Pipeline for automatic data reduction.
Most of the calibration of the instrument will be done during the process of Assembly, Integration, and Verification (AIV) and before shipping to La Palma. Several features of the instrument will be characterized, as for example:
The Image quality as function of wavelength and position in the FOV.
The Focus position (offsets) for the different filters.
The throughput as a function of wavelength for the main optics.
Image stability as a function of temperature and orientation.
Scattered light tests.
Ghost images tests.
Here is a summary of the most important characterizations of the instrument (some of those test are performed during AIV, but should be repeated during commissioning):
Read noise (quiescent).
Read noise (motors moving).
Gain.
Linearity/full well.
Bad Pixel Map.
Dark Current.
Measure shutter open/close time.
Measure shutter open/close accuracy.
Determine error in flat fielding vs. exposure time.
Filter zero points in ugriz.
Night sky backgrounds in ugriz
Color terms for photometric transformations.
Filter passbands.
Dewar rotation (align detector columns with slitmask grid).
Image scale.
Astrometric image solutions, with all grisms, filters.
Vignetting characterization.
Measure global best focus for each filter.
Measure temperature variation of focus.
Determine spatial variation in optimal focus for each filter.
Measure flatfield characteristics and derive optimal procedures for creating flats.
Measure fringing in read filters.
Rotate grisms to get spatial direction parallel with detector columns.
Adjust grisms in the spatial direction to get the middle of the longslit in the middle of the detector.
Measure dispersion and resolution for all elements.
Measure grism efficiencies for all elements.
Measure dispersion spectra for all comparison lamps.
Determine dewar hold times.
Measure flexure.
Measure system throughput for imaging.
Measure system throughput for spectroscopy.
It is advisable that most of the observing modes are tested at the IAC before delivering the instrument to La Palma. This decision will require to build a telescope and star simulator, which can simulate observations of points sources and extended sources at any position in the sky by tilting and rotating the instrument.
Pre-commissining requeriments
The OSIRIS team will expect that GTC is commissioned and operational, including
Operational tracking.
Guiding nominal.
Operational pointing model.
Operational telescope focusing.
Telescope offsetting operational.
Commissioning
A approximate definition of commissioning is that of a systematic process of ensuring that a new build system perform interactively according to the documented design intent and the owner´s operational needs, and that specified system documentation and training are provided to the facility staff.
Commissioning of a piece of equipment e.g. an observing instrument, is intended to verify that the equipment performs as anticipated, to characterize all of its available modes of operation and their performances, and to demonstrate its capability to to science at the level it was designed.
Closely related with the Commissioning is the concept of System Verification (SV or ``end-to- end testing") is a term defined by the Gemini Observatory, as is defined as a test of the total Observing System, telescope plus instrument. SV is intended to demonstrate that the entire observing system is in place, the scientific observations with the commissioned instrument can be planned and performed, and the resulting data are of the quality expected and can be handled in the manners specified for use of the GTC user community. The SV is likely to be the last step in the Commissioning and Calibration of OSIRIS.
The commissioning is the phase following construction during which the capabilities of an instrument are demonstrated in its final operational configuration. During commissioning, both verification and validation tests are performed on the complete system to ensure that the instrument meets all its science requirements and is ready for operation.
The basic operational parameters of the instrument will be measured during AIV. We aim during commissioning to characterize completely the behavior of the instrument in all the operational aspects, in a stand-alone mode and integrated with the GTC control system. During the basic commissioning the main focus is not the science operation.
Commissioning time will be night-time and daytime. Several of the test proposed can be performed during daytime operation.
Results of the commissioning will be used for the calibration of the instrument in routine operations. The information obtained during this phase is useful as aid to future observers, as well as a fundamental reference for the baseline calibrations to be performed during routine operation in the observatory. Also, the data obtained during commissioning will be incorporated at the final version of the Users Manual and the observing tools available for observing planning and preparation.
The estimated time for Commissioning will be TBD nights, assuming that there are no major problems in hardware and software during this period.
Note that to this estimated time must be included the time required for the assembly and integration of the instrument at La Palma, as well as the tests with the GTC control.
Commissioning will be divided in two phases.
Commissioning Phase 1
In the first stage instrument tests will be performed to check the proper functioning of all the subsystems, to evaluate the performance parameters and to rehearse and optimize the observing modes. The Commissioning team will first proceed to unpack the instrument, checking for damage due to transport, re-assembly the instrument and install it at the telescope.
Instrument tests will be performed which could not or could not be fully executed during AIV in the IAC. For example, the interaction with the telescope, the observation modes, and on-sky performance tests. The basic performance parameters of the instrument will be measured, emphasizing those that were specified in the construction contract. Key observing scenarios should be verified.
Some of the basic test to be performed will be the image stability of the instrument, the image quality over the FOV, and the system throughput. Several of those test would be also performed during AIV.
After this period, a performance assessment phase should follow to evaluate the data obtained and correct hardware or software problems encountered during the initial Commissioning. A comparison must be done between the instrument performance to the predicted and designed values. A period of time should be available for the analysis of the test performed and for the preparations of the modification in the hardware and software.
Commissioning Phase 2
The second phase should be dedicated to a complete characterization of the instrument properties, to delivered input for the calibration plan and to test the pipeline. Time will be dedicated to train the GTC science and maintenance staff.
The phase will start with tests of subsystems modified during Commissioning 1, and end with the final acceptance tests.
It should be advisable to produce science data during this phase, in order to check that the instrument meets the science requirements for each observing mode.
The Phase 2 will end with the had-over of the instrument of GTC.
Examples of tasks to be performed during Commissioning
Below are of some of the test required in the commissioning of some of the observing modes for OSIRIS that will be commissioned.
Imaging: Standard Broad Band / Narrow Band
This is a basic Imaging mode. It allows the user to take image by using conventional standard broad band or interference filters. Direct Imaging can be performed over the whole FOV of the instrument. Several of the tested features during commissioning: science programs selection, acquisition of calibration files, telescope operation, data acquisition, data reduction. As saving time measure, seems convenient to make a image of the field that is needed for MOS.
Imaging: Tunable Filter Standard
Using TFs and conventional broad band or specially devised interference intermediate-band filters as order sorters. Inserting different TFs (one for the blue and another for the red spectral range) shall allow coverage of the full optical spectral range. Tunable imaging can be performed over the whole FOV of the instrument. In the standard mode the whole OSIRIS FOV is usable and only one wavelength range is tuned. The commissioning is similar to the Standard Broad Band / Narrow Band. Spectrophotometric flux standard observations are needed for each of the combinations of filter with Z distance (gap) for the Tunable Filter.
Spectroscopy: Long Slit Standard
The traditional spectroscopy shall be done selecting one of the possible long slits fixed width available in the slit loader to account for different seeing conditions. Several of the tested features during commissioning: target acquisition; selection of desired position angle; calibration stability (e.g. under flexure); image quality in dispersion and spatial directions; dispersed flat field quality; sky subtraction and systematic errors; limiting performance using signal/noise measurements.
Spectroscopy: Multi-Object Spectroscopy
As in the long slit mode, multi-object spectroscopy shall be possible using customised masks. Multi-object spectroscopy can be performed over the OSIRIS FOV in the spatial direction and in the part of the OSIRIS FOV parallel to the dispersion direction, depending on the resolution and spectroscopy image quality. In the standard mode slitlets are used for sky subtraction. Several of the tested features during commissioning: design of slit mask avoiding order contamination and slit overlaps; manufacture of slit masks; check of quality via flatfield; repeatability of mask positioning; acquisition of field via star holes,; astrometric accuracy check by imaging through mask; understanding of atmospheric refraction distortion; check signal/noise in sky-subtracted spectra.
Last update August 8, 2005, by Héctor Castañeda