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 firstname.lastname@example.org.
- Target Acquisition
- Calibration data
- Dithering on source
- Mosaicking a large target
- Data Reduction
- Spatial and spectral scales
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:
- A bright (<18mag) acquisition star which is clearly identifiable, i.e.:
- Not a binary
- Well separated from bright neighboring stars, since the night assistant will not know which star to use.
- Have a small, or well defined proper motion
- Come with a finding chart when ever possible.
- Well defined offsets
- Make one
- Even if it's only a DSS image from Gaia
- Really, go make one
- A printable PS/PDF file is the preferred format. FITS file are not recommended.
- Black and white please. If colour, please make a black and white copy as well and ensure it prints clearly.
- The A&G camera works in the V-band so the chart MUST resemble the V-band image to be of use (please, no IRAC 8micron image finders).
- Label your target
- An IFU overlay on the image can be useful
- Clearly mark the acquisition star if one is to be used
- And add the chart scale (make it about 5'x5)
- Give the PA (if required)
- And indicate North and East
- A long and complex text explanation accompanying a poorly constructed finding chart is of no value. A single A4 chart can and should accurately convey all of the relevant information succinctly.
Take Bias frames. The dome lights need to be off, so these are best done in the afternoon. Take 30 or so every few days.
Take Twilight flats each night. These need to be done pretty much the moment the sun hits the horizon.
Take standard stars, the ESO list is pretty good. These will probably need to be quite long exposures (5mins).
Consider taking a few blank sky exposures during the run, perhaps 10-20 mins on a blank patch in a gap between targets. This will provide a high quality sky spectrum for sky subtraction later on and can usually be easily slotted into a program.
Take standards even in poor conditions, since there are some transmission structures in the SPIRAL fibres, and you will want to get a relative calibration even if an absolute calibration will not be possible in cloud.
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.
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.
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.
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 (email@example.com)