SPRAT drawing

SPRAT

Introduction


SPRAT (SPectrograph for the Rapid Acquisition of Transients) is a low resolution, high throughput spectrograph. It employs a VPH grating and prism ("grism") assembly to give a straight-optical path. The grism and slit are removed from the beam for acquisition, and then placed in the beam for spectroscopy.

SPRAT was fitted to the telescope at the start of September 2014, and a programme of detailed characterization is underway. At present the slit is set fairly wide in order to aid robotic acquisition. This may be adjusted once we have better experience. The following preliminary specification has been determined:

Specification


  • Camera: Andor iDus 420 Series, 26.6 x 6.6 mm/1024 x 255 pixel CCD.
  • Field of view in imaging (acquisition) mode 7.5 x 1.9 arcmin.
  • Wavelength Range 4000 - 8000 Angstroms.
  • Order blocking filter cuts out all light below 4000 Angstroms.
  • Dispersion 4.6 Angstroms/pixel.
  • Slit width 1.8 arcseconds, giving a resolution of 18 Angstroms (4 pixels), corresponding to R=350 at the centre of the spectrum.
  • Spatial Pixel Scale 0.44 arcsec/pixel.
  • Slit length 95 arcseconds
  • Approximate sensitivity for one photon per second per angstrom at 5500 Angstroms is V=16.5. An exposure time calculator is available here .
  • Adjustable grating option. The grating may be set to two different configurations which are optimized for "blue" or "red" throughput.
  • Xenon arc calibration lamp.
  • Tungsten flat field lamp.

(last update: 23 Jan 2015)

Sensitivity vs wavelength


Our plan is to eventually provide absolute sensitivity curves for the two grating modes. In the meantime the example spectra below and the derived ratio plot show the difference in throughput between the two modes as a function of wavelength. Switching between the modes takes < 10 seconds.

Comparsion of blue and red grating options Comparsion of blue and red grating options

(Click on graphs to enlarge)

Acquisition


Acquisition with SPRAT uses the same concepts of "WCS FIT" or "BRIGHTEST" as implemented for FRODOspec . However, unlike FRODOSpec acquisition imaging is done on the same detector as used for spectroscopy. This eliminates a source of error in changing instruments, and allows us to acquire onto the (relatively) narrow slit. This means you must have accurate coordinates (better than 1 arcsec) for your targets.

The acquisition process moves the target onto a "magic pixel" on the SPRAT CCD. The location of this pixel may vary slightly over time, though it should be close to 480,181. Targets should thus be located on or very close to this pixel in the final acquisition image obtained just before spectra are acquired. Note that all acquisition images are made available to users.

On a well-populated field the acquisition process usually takes 4-5 minutes. This includes time to take an image of the slit after acquisition but just before the first spectral data are acquired.

Data Reduction Pipeline


An adaptation of the FRODOSpec pipeline is being used to provide reduced and wavelength calibrated 1d and 2d spectra to users. Access to the raw data is also provided.

SPRAT Xenon arc spectrum

(Click above to enlarge)

Wavelength and Flux Calibration


We recommend that all users include a Xenon arc observation at the end of their group (see the SPRAT Phase2 guidelines for details). This arc will be used by the pipeline to wavelength calibrate spectra extracted from your data. A simple arc map is shown to the right.

Although all spectral images obtained with SPRAT are flat-fielded and a sky-subtracted spectrum is created by the pipeline (and included in all reduced multi-extension FITS data), at the present time we do not apply any sort of flux calibration or telluric correction to pipeline-processed data. Users may therefore wish to add a standard star observation to their SPRAT observing sequence so that their calibration observations are matched in time and airmass with the science data; see the SPRAT Phase2 guidelines for details on how to do this using the Sequence Builder. Links to tables of spectro-photometric standard stars are provided by ESO here.

Users should be aware that there will be some level of uncertainty associated with flux calibrated SPRAT data due to inaccuracies in the acquisition process (the standard and/or the science target may not be perfectly positioned on the slit) and or course due to slit losses. However, instrumental and atmospheric absorption does have a rather extreme affect on SPRAT data: see for example this comparison of a pipeline-processed spectrum before and after telluric correction.

Phase 1 info


SPRAT is available for common user applications from Semester 2015A onwards. You should be aware of the following overheads:

  • Acquisition Time (slew plus accurate acquisition): 4 minutes
  • Readout Time: 10 seconds
  • Grating Mode (blue/red) change: 10 seconds
  • Xenon Arc Exposure (recommended at start or end of exposure): 10 seconds
  • [Tungsten Lamp Exposure (not routinely obtained or usually required): 10 seconds ]

Phase 2 Info

Guidelines on how to prepare observations are given in the Phase 2 web pages. Note in particular the instrument-specific User Interface Instructions. The phase 2 "Wizard" should always be used to prepare observations. Groups that can not be prepared with the wizard should be discussed with LT Phase 2 support. Note that LT staff do not routinely check observing groups, and that observing groups are potentially active as soon as they are submitted. Please do contact us if you have any questions about your observations.

RTML and Automated Spectral Followup

SPRAT can accept observation requests formated in Remote Telescope Markup Language (RTML) which allows you to automatically trigger certain observations from within your own autonomous software. We provide a command line based utility which can format RTML packets for you, making it easy to integrate observing requests into a shell script or other data processing pipeline. For some use cases it might also be easier for users to upload all their observing requests from the command line instead of using the phase2ui. The facility is available to all users who have TAG-awarded time allocations. Please contact us if this sounds of interest and we can provide full user instructions.

Standards

A single spectrophotometric standard observation is performed on any nights that are anticipated to offer photometric clarity. This provides routine monitoring of overall system throughput and stability and the exposures are available to all observers. A single standard observation is not sufficient for precision spectrophotometry and will not, for example, allow derivation of a night-specific atmospheric absorption. Users requiring a higher level of photometric or atmospheric calibration must schedule their required standards from their own time allocation.