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SPIRAL Basics


This page of notes is intended for users with AAOmega experience, but who are new to SPIRAL. It is not intended as a full manual for the SPIRAL system, but rather a page of useful notes. Please mail questions or comments to Sarah Brough at sb@aao.gov.au.

Contents:

Target Acquisition


A readily identifiable target can be acquired with the Acquisition and Guiding (A&G) camera and accurately centred on the IFU within 5 minutes at the <0.7arcsec single lens level.

When the target is ill-defined in an acquisition image (it may, for example, be too faint for a short exposure, extended with no clearly-defined feature or perhaps an emission-line source not easily seen in a broad band image) then the user will define a nearby bright (<18mag) acquisition star. The acquisition star is then acquired using the A&G camera and the telescope guiding started. The observer then supplies clearly defined offsets from the acquisition star to the well-defined position in the target object.

This process requires:

Finding charts

The SDSS CasJobs, STARLINK Gaia, ESO skycat or ds9 image tools all produce adequate charts using DSS or similar images. The user may need to add adequate annotation to these images.

Calibration data


Dithering on source


Small dithers on source allow one to recover dead IFU pixels, and to mitigate the effects of the bad CCD columns on the Blue CCD. Use integer IFU lens offsets (i.e. 0.7 arcsec) dither steps North or South (for the default PA=0) as this moves the spectra into the next bank of 32 fibres on the CCD, whereas a lens dither East or West would move the spectra up a fibre within the current fibre bank, which does not move away from the bad columns. Integer IFU lens offsets (factors of 0.7 arcsec) make stitching the mosaic back together much easier.

Mosaicking a large target


When trying to mosaic a large target, consider how much overlap one will give between the adjacent pointings. Typically I use East-West offsets of 21.0 arcsec on the sky, which gives an overlap of two lenslets, and 10.5 arcsec offsets North-South which gives a 1 pixel overlap in this shorter IFU dimension. This allows mosaics to be properly scaled when stacking the data later on, to allow for transmission variations with time.

Data Reduction


SPIRAL data reduction is also done using 2dfdr. The 2dfdr page provides all necessary links and information for the data reduction task including information about the file format and fixing the wavelength solution if necessary.

Spatial and spectral scales


The lens scale of the SPIRAL IFU is 0.7 arcsec square pixels.
The spectra project to a FWHM of ~2.4 pixels at the CCD.
The resolution of the SPIRAL system is x1.5 that of MOS mode due to the smaller fibres, altough the PSF is less well sampled (FWHM~2.4pixels)

Sarah Brough (sb@aao.gov.au)