SPRAT
SPRAT
Band
State
Description
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 is in routine use, available to all observers. The adjustable slit is set fairly wide at 1.8arcsec yielding a resolution of approximately R=350 (18Å).
Technical Data
- Camera: Andor iDus 420 Series, 26.6 x 6.6 mm/1024 x 255 pixel CCD.
- Read noise: 2.34 ADU (5.7e)
- Gain: 2.45
- Approximate sensitivity for one photon per second per angstrom at 5500 Angstroms is V=16.5. An exposure time calculator is available here .
- 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
- 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.
Results
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. Coordinate errors in large catalogues frequently exceed this, especially for moderate proper motion stars. We strongly recommend obtaining your own astrometry from a recent image wherever possible.
The acquisition process moves the target onto a "magic pixel" on the SPRAT CCD. The location of this pixel is maintained by operations staff and may vary slightly over time, though it should be close to 480,180. 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, before the first spectral data are acquired.
Though relatively complicated, most users have no need to get involved in the acquisition process beyond providing accurate target coordinates. The "Wizard for SPRAT" option in the phase2ui automatically creates the optimum observing sequence.
NB: It is also possible to use IO:O to provide an acquisition image instead of SPRAT. In such a case the LT will adjust the telescope to place the target on the "magic pixel" in IO:O's field of view. Then it moves the science fold to select SPRAT and put the target on the slit. See "Using IO:O as the Acquisition Instrument" on the main SPRAT page for full details, and the implications of setting this up.
