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Recommendations for planning observations with AF2

Observing with AF2 requires careful advance planning, and on this page we offer advice about three critical issues:

In particular, do not underestimate how long it will take to produce the configuration files. Start to prepare the files well in advance of the run (e.g. a month), contact your support astronomer for advice on how to plan the observing run and send her/him the configuration files as soon as you can for checking them in the AF2 observing system.
A wise observer would also arrange to arrive at the telescope at least one day in advance of their observing as configurations may have to be re-done at short notice due to unforeseen circumstances (e.g. the loss of a fibre).

Astrometry / choice of fiducial stars

  • The astrometry of the targets has to be of the highest accuracy obtainable to minimize the offset errors in the fiber positioning. Astrometric catalogues as UCAC3 and SDSS have proved to provide good results. The astrometry should also account for proper motions of the fiducial and science target stars. Even those stars with small proper motions, e.g. 20 mas/year, might have coordinate differences of 0.3 arcsec after 13 years, given that the astrometric coordinates are usually referred to Epoch 2000.

  • The astrometry of the fiducial stars and the science targets used in the configuration files should be from the same source. If they are from different catalogues, there is a risk that although an acquisition is good enough and fiducial stars are well centred in the fiducial fibres, the relative offset in the astrometry of the science targets can result in them not being well centred in the science fibres.

  • At least 3 acquisition (fiducial) stars are required for an aceptable acquisition, the more the better, but to be safe we suggest more than 4. They should be well distributed over the AF2 field of view, e.g. some fiducial stars in the centre of the field, and some in the outer areas around 20 arcminutes from the centre. Avoid using fiducials beyond the 20 arcmin radius, as the distortion are stronger there and might bias the field acquisition.

  • The coherent fiducial bundles, allow one to see, in the acquisition/guiding camera, stars as faint as magnitude V~16 with a nearly full moon, good seeing (1~arcsec), and some integration. However, you should select stars a bit brighter than this limit (V≤15), to account for possible poor weather conditions. Ideally the fiducial stars should all be bright enough (V < 15) and, if possible, of similar brightness, in the range 13 < V < 14.

  • Always make sure that you have the most up-to-date versions of files small_fibres.dat and wht_prime.dat.

  • Using a buffer size of 1.5 when creating the configuration file with af2_configure is mandatory, otherwise your setup will likely be rejected by the AF2 system computer. Even if such a configuration is accepted, the risk of the fibre-positioning mechanism losing a fibre rises significantly for smaller buffer sizes, leading to potential mechanical problems which could take hours to fix, or, even worse, could cause permanent damage to the fibres.

  • Remember to set the parameter FIBRES SMALL in your field configuration file (.fld)

  • Note that no more than 99 "spare" fiducial stars should be contained in the output file (.cfg) ("spare" fiducials are those listed at the end of the file and numbered starting from 901), or the setup command given at the Instrument Control System (ICL) may fail.

  • Fibres can be positioned over a field of 1 degree diameter; although there is some vignetting at radius >20 arcmin, and image quality outside the 25 arcmin radius is poor, the PSF showing a some elongation that causes light to be lost for targets in this outer area. We thus recommend users to avoid putting science fibres in the annulus outside the 25 arcmin radius, and consider there there will be some moderate light loss due to the optics also at radii between 20 and 25 arcmin from the centre.

  • Try to avoid to assign the important science targets to the bad fibres, with the lowest throughput and the worst image quality.

  • It is not necessary to provide distinct configuration files for a range of hour angles. For a given configuration, the appropriate sidereal time is input when the field is configured at the telescope; this overrides the hour angle specified in the configuration file.

  • It can be useful to determine in advance if a rotator limit will be reached during the observations of a given fibre configuration. This can help to plan the observations during the night, as there is a blind spot of 3 degrees (the mount PA has a range from -163.37 to 193.63 degrees), and when it is reached the rotator has to be moved back into limits, with the subsequent loss of observing time. We can determine the time when the limit will be reached from the sky PA in the configuration file or, alternatively, we can avoid to choose a range of sky PA for which the rotator will reach the limit in the period of time when the fibre configuration will be observed. See instructions on how to determine the rotator limits at here.
  • If many different fibre configurations are going to be observed during a night, it is recommended to take sky twilight flats in circle configuration, in the evening or morning, hence they can be used with all the configurations when reducing the data. Circle configuration flats are also useful to calculate the relative fibre througput.

  • Sky substraction is a tricky issue and should be addresed in different ways depending on the science program.
    When observing bright objects, the sky substraction should be good enough if just assigning some 'sky' fibres next to the target fibres and then calculating a median sky spectra to be substracted.
    For faint targets, the sky substraction need to be more accurate, hence the best strategy is to move the telescope back and forth between objects and neighboring sky (beam-switch), so that the sky spectra are measured on the same fibres as the science objects.
    The spectral range of interest is also very important when deciding the best strategy. At the very red end, the sky emission lines are very strong and also vary very quickly, hence the 'beam-switch' strategy might not be the best option, and the sky should be measured simultaneouly with the science spectra.

  • AF2 suffers from ghosting caused by undispersed light. Its intensity depends mainly on the source brightness and the exposure time. The recommendation to keep a low level of ghosting is to avoid mixing in a same fibre configuration objects with a wide range of magnitudes, but create e.g. a configuration file for bright targets and another one for faint objects.

  • It is recommended not to observe fields at very low elevation, as the fibres would need frequent re-configurations to account for the differential refraction in the field. It takes around 20 min each time, which would increase substantially the overheads.

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Contact:  (AF2 Instrument Specialist)
Last modified: 12 September 2016