ON-SITE PREPARATION

Contents :

• Overview of the things to be done

• Observation scenarios : defining TIGER configurations

• Exposure types obtained during a run

• Minimal set of service exposures needed to reduce an object exposure

• Read (or review) carefully the detailed Observer's Manual to familiarize yourself with the Pegasus environment created for the OASIS + AOB instrumental configuration.



• The instrument set up should be prepared according to your needs by the CFHT staff. This means installing grisms, filters, CCD, and controlling the relative orientations of the lens array, the grism dispersion, and the CCD columns. The OASIS configuration files are then updated using the Engineering mode. To make sure that it's done, it is strongly recommended to fill the pre-run preparation form available at the CFHT web site and send it to the staff well in advance of the observing run. Before starting your observations, verify with your Support Astronomer that everything is installed and ready for your run.



• Using the OASIS PEGASUS interface, define the scenarios to be used for observing the astrophysical targets scheduled.



• Prepare a list of the exposures to be obtained, including at least the minimal set of service exposures which is mandatory for the subsequent data reduction process.



• Plan carefully your observations in order to minimize the number of AOB beamsplitter changes during your nights. At minimum, 15 minutes will be lost on the sky for each change.
  
  
• If you are not already familiar with these tools, practice the Image, IQE and Graph Pegasus functions for image analysis.



• Become familiar with the AOB control windows.



• Become familiar with the Offset function if you are planning to work with the f/8 mode.

• Principles
  
  Within some limits, the TIGER mode of OASIS is fully adaptable to the astrophysical target. You may want to perform fairly high resolution spectroscopy of nuclear regions of nearby galaxies, or low resolution spectroscopy of z~3 remote Ly-alpha emitters. You may be interested in wide field 3D spectroscopy of an HII region, or in a finely sampled narrow spatial field centered on the nucleus of M31. And, as possible with TIGER, you may want to define a spectroscopy configuration and an imagery configuration, both to be used on the same class of objects. This is what is called a Scenario : a full set of TIGER optical parameters, and some rules to use what and when. There are two classes of scenarios : imagery, and spectroscopy, and their definition is made with the help of the standard interface described below.
  


• Defining scenarios through the PEGASUS interface
  
  The various function accessed through this interface are described in the PEGASUS section of the Observing procedures chapter.
  Scenario definition is described here, as it is part of the on-site preparation of observations. It uses the Create New Scenario sub-function of the Observing assistant, detailed here :
  
  
  
  
  
  • Imagery scenario
    
    
    
    
    • Scenario name : the first line invites the user to give the name he wants to use during the whole run for the scenario to be defined. Twenty alphanumeric characters are allowed, no more; the window is not scrollable. It is good practice to give informative names; something like QSO IMA Ly alpha LR2 is obviously better than QSO001...
    
    
    • The next (blue) line reminds that OASIS is used in Imagery mode. The following parameters are requested :
    
    
    • Focus : here, the user chooses from the pull-down menu to use either the F/20 AOB focus, or the direct F/8 Cassegrain focus.
    • Field of view : choose from the pull-down menu among the two possibilities offered; the choice range varies with the focus and the sampling selected, of course. The field of view displayed is for a full frame CCD; if the user choose to use a sub-raster, the field will be reduced accordingly.
    • Sky sampling : choose from the pull-down menu. The effective sampling value is proportional to the binning factor; if you choose to read the CCD in 2x2 pixel blocks, the sampling value will be twice the one you get with no binning, and so on... The user has to make a trade-off between sampling and speed of CCD readout : higher sampling arcsec/pixel value means coarser images, but shorter CCD readout time. The final spatial resolution in arcsec will be roughly twice the value selected here.
    • Wavelength range : choose here the filter to be used, and thus the spectral domain for the images to be obtained. It is often useful to define a special wide field, "no-filter" imagery scenario to be used for pure detection.
    • Beam Splitter : for scenarios using the AOB, a beam splitter is used to extract (sorry...) some flux from the science channel, to be used by the corrective system. The transmission displayed to the right is the one to the science channel. Choose your favorite splitter, after looking at the AOB@ manual. If you plan to use a beam splitter with some wavelength cut-off, look at its position relative to your wavelength range !
    • CCD raster : you may here choose to use not a full-raster (full frame) CCD reading, but a sub-raster, usually to speed-up a series of test imagery exposures on an already well-centered object.
  
  
  • Spectroscopy scenario
    
    
    
    
    • Scenario name : the first line invites the user to give the name he wants to use during the whole run for the scenario to be defined. Twenty alphanumeric characters are allowed, no more; the window is not scrollable. It is good practice to give informative names; something like QSO Ly alpha LR2 is obviously better than QSO0012...
    
    
    • The next line (black and blue) reminds that OASIS is used in TIGER mode.
    
    
    • The next (blue) line means that you are entering the zone used to define the optical parameters of the spectroscopic exposures of this scenario.
      
      • Spatial : in TIGER mode, the spatial sampling of the spectroscopic data is performed by the micro-lens array, and is defined in terms of arcsec-per-lens. The effective spatial resolution on the final reconstructed images will be roughly twice the spatial sampling defined here. Choose in the pull-down menu the spatial sampling you plan to use. Note that some (lower values, for high spatial definition) use the AOB, and that the observable field is proportional to the value of the sampling used, as expected...
      • Spectral : the spectral sampling depends on the grism used which is from the CFH MOS/SIS set. Choose in the pull-down menu the sampling you want; the final spectral resolution will be roughly twice this value.
      • Wavelength range : the spectral domain is limited by an interference filter to avoid spectra overlap on the CCD. Choose in the pull-down menu the filter you want. It may be a standard OASIS filter, or some custom filter. In that last case, the parameter file filter.data must have been updated accordingly, and the OASIS status updated (giving this filter position in the filter wheel) through the engineering function of the PEGASUS interface; all that must be done by CFHT staff only.
      • Beam Splitter : for scenarios using the AOB, a beam splitter is used to extract (sorry...) some flux from the science channel, to be used by the corrective system. The transmission displayed to the right is the science channel one. Choose your favorite splitter, after looking at the AOB@ manual. If you plan to use a beam splitter with some wavelength cut-off, look at its position relative to your wavelength range !
    • The next (blue) line tells you that you are entering the zone used to define field acquisition (field check) exposure modes. These are the image exposures made through the same filter as the spectroscopy exposures of this scenario; they are used to check image object centering, to offset to some nice position, to obtain deep reference images. On such images, for instance, you will get later an estimate of the image quality. As the filter has already been set, there is only one free parameter to be specified :
      
      • Spatial sampling : in imagery mode, the spatial sampling of the field is performed by the CCD itself, as the lens array is not used. Choose the value you prefer from the pull-down menu; the final spatial resolution will be roughly twice this value

• Astrophysical object exposures



• Imagery
  Classical direct imagery of the object.


• Spectroscopy
  The exposure which will produce the 3D [alpha,delta,lambda] TIGER data cube.




• Service exposures



• Wavelength calibration
  GUMBALL spectral lamp exposure.


• CCD calibration
  Bias and dark exposures for CCD frames preprocessing.


• Pupils
  Pupil exposures are mandatory for spectra pattern initial recognition, as well as for the first step of wavelength calibration.


• Flat-field continuum spectroscopy
  Flat field continuum exposures are mandatory for the spectra pattern recognition which leads to the building of the spectra extraction mask, as well as for the photometric correction of reconstructed images.


• Flat-field imagery
  Used for photometric corrections of imagery exposures.


• Test
  Any exposure which is not supposed to be kept, as it \ was obtained to evaluate exposure time, offset, seeing, guide star characteristics, etc...

• Scenario-related exposures
  
  
  • Pupil exposures
    One for each spectroscopy scenario defined.
  • Flat field continuum exposures
    One for each spectroscopy scenario defined.
  • Flat-field imagery exposures
    One for each imagery scenario defined.
  • Flux calibration exposures
    One for each scenario (imagery and spectroscopy) defined. In both modes, be careful to use an object whose image fits entirely on the CCD, even if it is a standard star spread by a not-so-good seeing.
  • Wavelength calibration exposures
    One for each service spectroscopy exposure (flat continuum, flux) obtained.


• Object-related exposures
  
  
  • Wavelength calibration exposures
    One for each spectroscopy exposure. It is better if the object exposure is flanked by two calibration exposures obtained just before and just after, just in case.


• Summary
  
  In order to be able to reduce the data, the observer must obtain :
  
  
  
  • For each imagery scenario :
    
    • A flat field exposure, obtained with the GUMBALL.
    • A flux calibration exposure.
  
  
  • For each spectroscopy scenario :
    
    • A pupils exposure, obtained with the GUMBALL.
    • A flat field continuum exposure with its associated wavelength calibration exposure, both obtained with the GUMBALL.
    • A flux calibration spectroscopy exposure of a standard star, with its associated wavelength calibration exposure obtained with the GUMBALL.
    
    
  • For each object spectroscopy exposure :
    
    • A wavelength calibration exposure. A couple, before and after the object exposure, is safer.

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