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AF2 Enhancements 2012-2013

The demand for AF2 had been decreasing in the course of the years, with observers often reporting an overall throughput well below that expected from the ING exposure time calculator. Given the strategic importance of multi-fibre spectroscopy for ING's future (see ING's strategy page), during 2012 and 2013A ING staff carried out an end-to-end analysis of the reasons for the loss of photons. This showed that poor centering of the fibres on the targets was the dominant contribution to the low throughput. A number of improvements were implemented in AF2 to address this and other issues: a new automatic acquisition and guiding tool, a better geometric distortion map, a new fiber centroiding algorithm, a new detector, and new comparison lamps were commissioned. This page summarises the enhancements implemented.

AF2 performance has improved very significantly as a result of these enhancements, and we believe that, as a wide-field multi-object spectroscopy facility at the WHT, it is ready to perform as a useful precursor and science test-bed for WEAVE surveys.

Target-acquisition improvements

Analysis of the target centering problem covered the dominant contributors to fibre positioning: astrometry issues, focal plane geometric distortions, differential refraction, mechanical accuracy of the AF2 robot, fibre centroiding algorithm, and field acquisition. Improvements were needed in field acquisition, in the fibre centroiding algorithm and in the distortion map. We give details in the subsections below.

With these improvements, working with fields with favorable acquisition conditions (8-10 fiducials uniformly distributed over the field of view; homogeneous astrometry for fiducials and targets), AF2 acquisition is excellent (fibre offsets below 0.4 arcsec for 90% of the fibres). New recommendations for planning the acquisition of AF2 targets are provided in the AF2 acquisition page.

New acquisition and guiding tool

From semester 2013B we offer a new tool (fiberguider) for field acquisition, which automates the acquisition process by finding, and applying to the telescope control system (TCS), the translation and rotation offsets that optimally centre the reference stars on the fiducial fibres. The new acquisition tool allows one to minimise the acquisition errors throughout the whole AF2 field of view.

The acquisition process had previously been done manually by the telescope operators, who visually estimated the telescope offsets to be applied to the TCS. This was a time-consuming, subjective and non-repetitive task that could contribute significantly to the acquisition error.

The guiding algorithm has been revamped together with the acquisition. Guiding is now done on all or a user-selected subset of the fiducial stars, and, importantly, the guider controls both telescope pointing and instrument rotation. In the past, AF2 only allowed to guide with one fiducial star, leading to guiding errors for targets far from the guiding star.

During semester 2013A, the new acquisition and guiding tool was commissioned, with excellent results. Residual acquisition errors of the fiducial stars are ~0.15 arcsec in translation, and negligible in rotation. As well as optimising the acquisition, the tool reduces the acquisition overheads, from typically ~15 min when done manually to ~5 min with the tool.

The acquisition accuracy on science fibres depends strongly on the number and distribution of the fiducial stars on the field of view, as well as on other factors such as astrometric errors, as discussed in the AF2 acquisition page.

Updated distortion map

A new distortion map of the AF2 focal plane has been determined empirically, and has been implemented in the AF2 observing system.

The new mapping parameters yield a focal plane geometry that is accurate to better than 0.25 arcsec at 20 arcmin radius field of view and to better than 0.05 arcsec over the central 10 arcmin radius.

Improved fibre centroiding algorithm

A new algorithm has been implemented to determine the centroids of the fibres used by the AF2 robot as part of the iterative fibre placement sequence. The new algorithm replaces the Gaussian profile used earlier with a top-hat profile when fitting the image of the back-illuminated fibre. This change yields more accurate and stable centroiding, especially with damaged or dirty fibres which do not present nicely symmetric shapes.

Fibre inspection

All fibre input-end microprisms were examined with a microscope to inspect and inventory the quality of each fibre, which can be degraded by possible damage to prisms, accumulated dirt and debris, increased opacity of optical cements, etc.
We measured the relative fibre throughput from sky spectra frames and checked the degradation in the spectral resolution caused by each fibre.

A new list of bad fibres (15% of the total fibres) has resulted from this study. More details can be seen in the AF2 Fibres page.

New Red+4 detector

The current AF2+WYFFOS blue-sensitive detector has degraded cosmetically and exhibits fringing of ~30% at 850 nm.
ING has purchased an e2v 231-84 4k x 4k, red-sensitive, fringe-suppression CCD, Red+4, which has been successfully commissioned in semester 2013A. The majority of AF2+WYFFOS users will be benefit from its ultra-low fringing (<1% at 850 nm), and 25-60% better sensitivity in the red.

Throughput measurements

Throughput measurements of the overall AF2+WYFFOS system: atmosphere, prime focus corrector, fibres, spectrograph, and detectors (the old WHTWFC and the new Red+4) have been carried out. The results yield an improvement in throughput with the Red+4 detector from 4500 Å to the red end. Further information regarding the throughout measurements are reported at the AF2 Throughput web page.

The new throughout results were updated in SIGNAL Exposure Time Calculator.

Calibration lamps

The WYFFOS calibration unit, mounted at the WHT broken Cassegrain focus, traditionally contained Helium, Mercury and Neon arc-calibration lamps, and a Tungsten continuum lamp.
During semester 2013A a set of new lamps has been commissioned, aiming to improve the flat-field and wavelength calibration capabilities.

A Quartz-Tungsten-Halogen (QTH) lamp has replaced the Tungsten lamp as a flat-field light source. It enhances the flux in the extreme blue by 1.7 times the old Tungsten lamp flux, which improves fibre tracing toward the blue end and reduces the exposure times needed for obtaining flat field maps.

For wavelength calibration, a new Thorium-Argon hollow-cathode arc lamp, with its rich line spectrum, improves radial velocity precision in the red by as much as a factor of two. And Cadmium and Zinc lamps have been acquired to improve the calibration capabilities in the two bluest echelle orders (orders 6 and 7). From semester 2013B onward we offer users the possibility of choosing among the traditional set of arc-lamps (Mercury, Neon and Helium) and the new set of Thorium-Argon, Cadmium and Zinc arc lamps.

The upgraded calibration capabilities are described in the AF2 calibration page.

Reduction pipeline

An IDL-based pipeline has been developed to perform full reduction of AF2+WYFFOS data, including fibre-to-fibre sensitivity corrections and optimal extraction, with provision for quick-look, real-time analysis.
Version v1.02 of the pipeline is available for AF2 users to download at AF2 data reduction page.

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Contact:  (AF2 Instrument Specialist)
Last modified: 14 August 2013