FRODOspec


Part of FRODOSpec on optical bench before shipping to site.
© 2007 A.Scott. (click for bigger [85KB])

Introduction

FRODOSpec (Fibre-fed RObotic Dual-beam Optical Spectrograph) is a SRIF collaboration with the University of Southampton to develop a multi-purpose integral-field input spectrograph for the Liverpool Telescope. As the name implies it is a dual beam design with the beam split before the entrance to the individually optimized collimators. Two resolution options are available on each arm. With low resolution selected on each arm, the entire spectrum from the blue cutoff of the optical fibres (around 3800 Angstroms) to the red limit of the detectors at around 1 micron can be obtained in a single shot. The low resolution is implemented using conventional transmission gratings.

The higher resolution option for each arm is provided using a VPH grating bonded to a prism in order to throw the beam to the same angle as that obtained using the diffraction grating at low resolution. The central wavelengths in this configuration are fixed (centered around features of astrophysical interest).

The spectrograph is bench mounted on the observing floor of the telescope enclosure and is fed using a fibre bundle from the Cassegrain focus of the telescope. An integral field unit using a microlens input array feeds the fibre bundle.

Current Status

Updated September 2010: The science grade array is now fitted into the blue arm. The red arm remains fitted with an engineering grade chip until a source of contamination in the dewar can be removed.

Adjustments were made to the baffling on site of the blue arm on 27th November 2009 removing the stray light contamination that had been affecting that arm. A new baffle design is scheduled to be installed on both arms late 2010.

Specification & Capability

Parameter
Blue arm
Red arm
Low Resolution (Grating) Wavelength Range (Å)
3900-5700
5800-9400
High Resolution (VPH) Wavelength Range (Å)
3900-5100
5900-8000
Resolving Power
(low dispersion)
2600
2200
Resolving Power
(high dispersion)
5500
5300
Integral Field Input Module: 12x12 lenslets each 0.82 arcsec on-sky

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Sensitivity

The following sensitivities have been measured using the engineering arrays, and so provide a conservative estimate of the final sensitivity of the instrument suitable for preparing proposals for the next Semester. The sensitivity figures below are zeropoint magnitudes, calculated in the AB magnitude system to get one detected photon/second/Angstrom.

Wavelength Red VPH Red Grating Blue VPH Blue Grating
4000 - - 12.9 12.5
4500 - - 14.5 14.2
5000 - - 13.8 14.2
5500 - - - 13.3
6000 13.8 14.2 - -
6500 15.4 15.1 - -
7000 15.3 14.9 - -
7500 15.0 14.9 - -
8000 14.6 14.3 - -
8500 - 13.8 - -
9000 - 13.0 - -

Twilight and Tungsten flats for tracing fibres

Starting 9th June 2010, tungsten lamp flats have been obtained automatically every night and are used for all level 2 pipeline reductions.

Prior to June 2010, pipeline reductions were performed using the twilight sky flat fields which are available here to download. The provided flats are a mean of 5-7 frames taken on the same night and have been passed through the same data reduction pipeline as the science data, so have been cropped in the same way. These provide a means of tracing the fibre positions in the 2D spectra for early data which are only avaiulable in the data archive as level 1 reductions.

Valid from 20th March 2010 to July 2010:

Valid from 26th November 2009 to 12th March 2010:

Valid from 2nd October 2009 to 24th November 2009:

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Acquisition

Two modes of acquisition onto the spectrograph are provided:

"WCS FIT"

In this mode you must provide accurate (within 1-2 arcsec) J2000 coordinates for your target. Just picking the coordinates listed by SIMBAD will often not be good enough. If you do not have recent high quality astrometry already, consider checking it with a brief RATCam image of your field first before investing your time allocation in long FRODO integrations. An acquisition frame using RATCam will be automatically obtained when the observation is executed, and a WCS fit carried out to that frame against the USNO B2 catalogue. Assuming the fit is successful (i.e. there are sufficient, and bright enough, reference stars found), then the telescope will be offset to move the supplied coordinates onto the reference pixel, and another RATCam image taken to confirm the offset was correctly applied. It is important to note that for targets which are brighter than around 8th magnitude in the USNO catalogue (i.e. not for outbursting objects which are normally fainter than this) the USNO catalogue will contain an exclusion zone around the target which means that this method will fail. You must supply J2000 coordinates.

"BRIGHTEST"

In this mode an acquisition frame will be obtained using RATCam and the telescope offset to place the brightest object on the frame at the reference pixel and another RATCam image taken to confirm the offset was correctly applied. This mode is generally used in situations where the WCS would fail due to catalogue incompleteness due to bright sources in the field. For this to work, your target must be the brightest source within one full field-of-view of the acquisition camera. I.e., if using RATCam then there cannot be any brighter source within 5arcmin of your target.

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Data Reduction Pipeline

Data taken by FRODOSpec is reduced by two sequentially invoked pipelines. The first pipeline, known as the L1, is a CCD processing pipeline which performs bias subtraction, overscan trimming and CCD flat fielding. The second pipeline, known as the L2, performs the processes unique to IFS reduction. The L2 became operational on 9th July 2010, with the second version released in May 2011. The description in the following sections conform to the second version of the pipeline.

L2 Pipeline

L2 data products are eight part multi-extension FITS files with each extension containing a snapshot of the data taken at key stages of the reduction process. The lowest tier of reduction product available to the user is the L1 image. The output data product format is shown below.

 
HDU Index EXTNAME Details
0 L1_IMAGE L1 image
1 RSS_NONSS Non sky subtracted row stacked spectra
2 CUBE_NONSS Non sky subtracted datacube
3 RSS_SS Sky subtracted row stacked spectra
4 CUBE_SS Sky subtracted datacube
5 SPEC_NONSS Non sky subtracted 1D spectrum
6 SPEC_SS Sky subtracted 1D spectrum
7 COLCUBE_NONSS Non sky subtracted collapsed datacube image
 

If the sky-subtraction process is unsuccessful, the corresponding HDUs (*_SS) will be blank. One-dimensional spectra (SPEC_*) are constructed using only the flux from the brightest five fibres.

Data that has been processed successfully by both pipelines will have a filename ending in "_2.fits".

Pipeline in Detail

The L2 pipeline reduction process uses three files, the "target" frame, the "arc" frame and the "continuum" frame. The pipeline can be visualised using the following schematic:

FRODOspec L2 reduction pipeline flow chart

  1. Fibre tramline map generation using the continuum frame and polynomial tracing
    frfind, frclean, frtrace
    The positions of the fibre profile peaks are determined and polynomial traces determined for each.


  2. Standard aperture flux extraction
    frextract
    Using the tramline mappings, the flux is extracted from each fibre in the target frame, continuum frame and arc frame using a 5 pixel aperture.


  3. Wavelength mapping on a fibre-to-fibre basis
    frarcfit
    Candidate lines are found in the arc RSS frame and their positions compared with a reference list containing known arc line pixel positions and corresponding wavelengths. Lines are then matched and pixel to wavelength calibrations determined for each spectrum.


  4. Fibre transmission correction
    frcorrectthroughput
    Using a continuum RSS frame, fibre-to-fibre throughput differences in the target RSS frame are normalised.


  5. Rebinning of data to a linear wavelength calibration
    frrebin
    In order to obtain a single wavelength solution applicable to all fibres, the flux from each spectrum in the target RSS frame is rebinned using linear interpolation to a linear wavelength scale with the same starting/ending wavelength positions and pixel scales.


  6. Sky subtraction (if applicable)
    frsubsky
    If the routine can successfully identify sky-only fibres, the sky flux contribution is removed for all spectra in the target RSS frame.


  7. Formatting of output data product
    frreformat
    A data product with the output format shown above is constructed.

Extraction using the Starlink software

Starlink users may extract the appropriate extension using the CONVERT:FITS2NDF command, appending the filename with the extension they wish to extract in brackets. The result is a single extension .SDF file.

For example, to extract the raw image from the multipart FITS file r_e_20100311_1_1_1_2.fits use: 

 > convert 
 > fits2ndf "r_e_20100311_1_1_1_2.fits[0]"

Or alternatively, you can access the required HDU by using the corresponding EXTNAME key: 

 > fits2ndf "r_e_20100311_1_1_1_2.fits[L1_RAW]"

Extraction using DS9

DS9 users may view the available extensions by adding -multiframe on the terminal command line: 

 > ds9 -multiframe r_e_20100311_1_1_1_2.fits

A frame extraction can be achieved by adding the -frame and -savetofits parameters, specifying the frame to be extracted (frame 1 corresponds to the primary HDU): 

 > ds9 -multiframe r_e_20100311_1_1_1_2.fits -frame 1 -savefits output.fits -quit

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Wavelength Calibration

FRODOspec Xenon arc in blue grating
Xenon arc in FRODOspec blue grating (bigger version (10.4kB))

FRODOspec Xenon arc in red grating
Xenon arc in FRODOspec red grating (bigger version (10.4kB))

FRODOspec contains two different calibration light sources that perform different functions:

  • a xenon arc lamp is used for wavelength calibration
  • a tungsten continuum lamp is used for fibre tracing, to calibrate the built-in spatial distortion in the system

Use of the lamps is not compulsory, but it is recommended because at present we do not know the long-term stability of the FRODOspec system. We therefore recommend a Xenon arc exposure is obtained every time the gratings are moved.

Movement of the grating plus arc and science exposures must be made in the correct sequence, not only to obtain correct calibration, but also to avoid the risk of the arc light causing loss of the guide star. The sequence should be:

  1. select grating
  2. obtain science data
  3. obtain xenon arc exposure

Although simultaneous observations of science or arcs in both red & blue arms are possible, it is NOT allowed to take an arc in one arm while obtaining science data in the other. This is governed by the robotic control system, which if necessary will wait for science observing to finish in one arm before beginning an arc calibration in the other. This must be borne in mind both when calculating telescope time in phase 1 and scheduling observations in phase 2.

Phase 1 Information

Overheads for compulsory and recommended FRODOspec actions are:

  • Acquisition time (slewing plus moving the target onto the fibre bundle): 4 minutes
  • Each readout time: 20 seconds
  • Grating change: 20 seconds
  • Xenon arc exposure (recommended for wavelength calibration): 60 seconds
  • Tungsten lamp exposure (recommended for fibre tracing): 60 seconds

Phase 2 Information

For information on how to program FRODOspec observations using the Phase 2 UI, see the FRODOspec Phase 2 Instructions.

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