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Commissioning of FIES in its new location

Around the change of year 2005/2006 the high-resolution FIbre-fed Echelle Spectrograph (FIES) was moved from the observing floor to a dedicated building next to the telescope. This new location provides a stable environment to enable high-accuracy measurements to be obtained with FIES.

This page is targeted at (potential) users of FIES, and provides the current status of the development and commissioning of FIES in its new environment.

Papers describing FIES
FIES lightpath overview
Fiber units
Calibration units
Wavelengths, dispersion, etc.
Inter-order distances and PSF
Wavelength zero-point stability
Shutter limitations
Exposure meter
Simultaneous-sky mode
Modal noise
Fiber shaker

Technical papers describing FIES

The FIES spectrograph report to STC (Lindberg & Frandsen, Sept 1998)

FIES technical report (Lindberg, Lens-Tech, Skellefteaa, Sept 1998)

FIES: a high resolution FIber fed Echelle Spectrograph for NOT (Frandsen & Lindberg, in Astrophysics with the NOT, page 71, 1999)

CCD13 pre-commissioning report (A. Sørensen, 2003)

Design of the the Fiber bundles B and C (Lindberg, 2011)

FIES: The high-resolution Fiber-fed Echelle Spectrograph at the Nordic Optical Telescope (Telting et al. 2014, AN335, 41)

FIES lightpath overview

The light from the telescope is picked off in the telescope adapter by an approximately 45 degree mirror that can be moved into the telescope beam at any time. The light from the sky is imaged onto small adjacent mirrors on the fiberhead. Each mirror has a central hole, behind which a fibre is located. The fibres are running from the telescope adapter to the spectrograph: one 200 micron fibre (low-res), and three 100 micron fibres (med-res sky, med-res and high-res).

For target acquisition the standby imager StanCam is used. The light reflected from the small mirrors of the fibrehead is imaged onto StanCam, to allow the telescope to be tweaked until the target is centered onto one of the fibres.

As of 2010 the atmospheric dispersion corrector (ADC) is installed under the 'roof' of the telescope adapter, and can be swung into the telescope lightpath before the pickoff mirror.

FIES detector

FIES is equipped with an E2V 2k x 2k detector. See CCD13 for details, and the pre-commissioning report (A. Sørensen, Oct 2003).

Detector characteristics:
After disconnecting the pressure sensor, the read-out noise in all amplifier modes is about 4 electrons. Full-frame read-out time with a single amplifier is about 90 seconds. AB-amplifier mode reads out twice as 'fast' but the central order of the echellogram is lost.

FIES spectrograph

The Echelle spectrograph itself is described in detail in documents that can be found on the NOT web pages.
The FIES spectrograph report to STC (Lindberg & Frandsen, Sept 1998)
FIES technical report (Lindberg, Lens-Tech, Skellefteaa, Sept 1998)
FIES: a high resolution FIber fed Echelle Spectrograph for NOT (Frandsen & Lindberg, in Astrophysics with the NOT, 1999)
FIES: The high-resolution Fiber-fed Echelle Spectrograph at the Nordic Optical Telescope (Telting et al. 2014, AN335, 41)

The Echelle orders run vertically on the CCD, and in the blue region they are sufficiently far apart to allow for an interlaced calibration spectrum (ThAr or sky) to be recorded simultaneously (see below for details).

The spectrograph allows a fixed wavelength setting only. Except for the focus drive and spectrograph shutter there are no motorised moving parts in the spectrograph.

The spectrograph can be focused with the computer-operated focus drive, which moves some of the optics in the spectrograph camera towards or away from the CCD. The spectrograph shutter is operated by the CCD controller, and sits at the entrance of the spectrograph just behind the fibres.

Focussing the spectrograph:
Using up/down Hartmann masks we found a top/bottom tilt of the detector of half a pixel Hartmann shift. If optimally focussed, this tilt does not seriously compromise the spectral resolution.

Fibre unit

New bundle: bundle C
In the fall of 2009 the new bundle C was installed. This bundle has all 4 science fibers and the simultaneous-ThAr fiber fully working, i.e. the new bundle can obtain a simultaneous sky spectrum using Fib2 (med-res R~46000) in conjunction with Fib3 (med-res).
For the other fibers (Fib1, Fib3, Fib4, Fib5), the description of bundle C follows that of bundle B below.
Bundle C is actually the refurbished bundle A.

Design of the the Fiber bundles B and C (Lindberg, 2011)

Spare bundle B
In June 2007 a fibre unit B was installed. This unit has a total of 3 fibres: a 200 micron (Fib1, low-res R~25000 ), a 100-micron (Fib3, med-res R~46000 ), a 100-micron with 50 micron exit slit (Fib4, high-res R~67000 ). The sky apertures are 2.5 arcsec diameter for the low-res fibre and 1.3 arcsec for the med- and high-res fibres.

Additionally there is a separate high-res calibration fibre for the simultaneous ThAr mode. This fibre can be used in combination with target fibres Fib3 (med-res) or Fib4 (high-res).

Below the first bundle is described; most things apply to the newer bundles as well. However, with bundle B one cannot obtain a simultaneous sky spectrum as Fib2 does not exist.

Old bundle A (decommissioned)
The telescope and spectrograph are coupled through the science fibres that are about 40 meters long, Polymicro FBP type. There are 4 fibres of which at most 2 can be used at the same time. The 200-micron diameter fibre provides a low spectral-resolution mode (R~25000), with relatively low losses due to seeing. One of the 100-micron fibres provides high spectral-resolution (R~67000) and the other two 100-micron fibres provide medium spectral-resolution (R~46000). The redundant med-res fibres allow for a simultaneous interlaced sky spectrum to be obtained.

At the telescope end, the starlight is imaged onto the target acquisition mirrors, which have circular apertures of about 3.3 arcsec and 1.5 arcsec diameter for the 200 and 100-micron fibres respectively. The telescope pupil is imaged onto the fibre entrance by means of a spherical microlense which sits directly behind the aperture. Fibres are selected using a sliding mask which opens access to either one fibre or two adjacent fibres.

At the spectrograph the 4 fibres end in a line, with a fifth fibre that comes from a calibration unit located in the spectrograph room. This fifth fibre allows for simultaneous ThAr spectra to be recorded. The echellograms produced by each of the fibres are slightly shifted in the cross-dispersion direction.

At the fibre exit, the high-res and the calibration fibre are equipped with a slit of 50 micron width, which is needed to reach a spectral resolution of R~67000. The slit losses are approximately 40%. The light of all 5 fibres passes a focal extender that narrows the beam into the spectrograph to limit the grating-overfill losses due to focal-ratio degradation in the fibres. The lens multiplies the Fratio by a factor 1.76, resulting in a beam of approximately F/7.5 going towards the collimator.

The science fibres as seen from the telescope end, using StanCam.
Left: hole 1 (low-res, 200 micron) and hole 2 (med-res) are open, the calibration arm is at hole 4.
Right: hole 3 and hole 4 are open, the arm is at hole 1.
The small dark holes are the apertures to the fibres, and are 1.5 arcsec and 3.3 arcsec diameter for the 100 and 200-micron fibres respectively. These images were taken with the domelights on, such that it is possoble to see the calibration arm.

Calibration units

The fifth fibre connects to a calibration unit housing 4 or 5 lamps, of which usually only a halogen and a ThoriumArgon are used. The latter is used for the 'simultaneous ThAr mode', in which an interlaced ThAr spectrum can be recorded while exposing on-sky. The halogen lamp is used to define and trace the orders of the Echellogram of the fifth fibre.

A second calibration unit is located in the telescope adapter, which calibrates the science fibres. A rotating arm, that transmits light from either a halogen or a ThAr lamp, can be located infront of one of the science fibres at the time. When the arm is infront of a fibre, the light from the telescope is blocked.

Spectral resolution and wavelength range

The CCD is not big enough to sample the full echellogram. As a result there is no order overlap in the red ( > 7200 Angstrom), and only a limited number of Echelle orders can be sampled. We have currently chosen a fairly blue setting that samples the Balmer jump and runs redwards from it.

Due to the way the fibres are mounted at the spectrograph entrance the Echellograms of each of the fibres are a bit shifted with respect to eachother in the cross-dispersion direction (X direction of the CCD). At the red end the CCD cuts off the most orders for the low-res fibre and the least for the high-res fibre: see the wavelength range in the table below. The spectral resolution is taken from the FWHM of arc lines at the blaze, and can be slightly lower at the extremes of the orders. Note that the lamp light does not have a central obscuration as the telescope has; in case of incomplete radial scrambling in the 40 meter long fibres, the spectral resolution for starlight might be somewhat lower than for lamp light.

Fibre bundle A (decommissioned)
fibreFWHMresolutionwavelength range
#1 low-res 6.3 pix250003630 - 7265 (orders 157 - 79)
#4 med-res 3.4 pix470003640 - 7360 (orders 156 - 78)
#2 med-res 3.4 pix470003640 - 7360 (orders 156 - 78)
#3 high-res 2.3 pix680003640 - 7455 (orders 156 - 77)
#5 high-res calib

Fibre bundle B (spare bundle)
fibreFWHMresolutionwavelength range
#1 low-res 6.4 pix 25000 3630 - 7170 (orders 157 - 80)

#3 med-res 3.4 pix 46000 3630 - 7260 (orders 157 - 79)
#5 high-res calib 2.4 pix 65000 3640 - 7360 (orders 157 - 78)
#4 high-res 2.4 pix 67000 3640 - 7360 (orders 156 - 78)

For bundle C, we find similar values as in the above tables.

Fibre bundle D with CCD15
fibreFWHMresolutionwavelength range
#1 low-res 5.9 pix 25000 3620 - 8700
#4 high-res 2.2 pix 67000 3640 - 9110

A list of order numbers and wavelength ranges per order and dispersions per pixel can be found here . Note that the CCD is too small to sample all the listed orders together. Order overlap is achieved below 7200 Angstrom.

System efficiency

The plot below shows two measurements made at different nights, for a very blue and a very red wavelength setting. These settings were chosen to check the system efficiency throughout the optical domain. The 200-micron low-res fibre was used, as this fibre suffers the least from seeing losses and losses due to atmospheric dispersion.

The system efficiency includes the full light path: atmosphere, telescope, fibres, spectrograph and the detector. For the low-res fibre the system efficiency at the blaze peaks at about 9% in the R band. In regions where there is order overlap, the combined efficiency peaks at about 9.5%.

The 100-micron fibres suffer from seeing and atmospheric dispersion losses. If not using the ADC, it is important to center the star on the fibres using a colour filter in the beam to the target acuisition camera (StanCam) to minimise the atmospheric dispersion losses in the wavelength region of interest.

Fibre bundle A (decommissioned)
fibre system efficiency
#1 low-res 9%
#2 med-res 5%
#3 high-res 2.5%
#4 med-res 8%

Fibre bundle B (spare)
fibre system efficiency
#1 low-res 9%
#3 med-res 9%
#4 high-res 5%

Fibre bundle C (default)
fibre system efficiency
#1 low-res ~9%
#2 med-res ~8%
#3 med-res ~9%
#4 high-res ~5%

Inter-order distance and PSF

For all fibres the inter-order distance (measured between adjacent order centers) decreases from 33.2 pixels at 4000 Angstrom to 16.3 pixels at 7000 Angstrom. For the old bundle A the profile FWHM of the orders is 7.3 and 4.3 pixels for the 200 and 100-micron fibres respectively. For fibre bundle B the profile FWHM of the orders is 7.0 and about 4.0 pixels for the 200 and 100-micron fibres respectively.

wavelength inter-order distance
4000 33.2
4500 27.8
5000 23.4
5500 20.8
6000 18.7
6500 17.3
7000 16.3

For the different fibres the echellogram is shifted in X position. For each fibre the X-position at two wavelengths is indicated in the table below, which allows to estimate the shifts between the echellogram of two particular fibres.

Fibre bundle A (decommissioned)
fibre X-position at 4000 Â X-position at 7000 Â
#1 low-res 626.7 2040.8
#4 med-res 611.8 2028.4
#2 med-res 602.7 2019.8
#3 high-res 593.9 2011.2
#5 calibration 585.3 2002.7

Fibre bundle B (spare)
fibre X-position at 4000 Â X-position at 7000 Âcentral order
#1 low-res 615.6 2065.1 128
#3 med-res 587.2 2037.6 127
#5 calibration 578.1 2028.9 126
#4 high-res 569.0 2020.1 126

Spare bundle B: for simultaneous ThAr observing mode, the calibration fibre #5 can be used in combination with either the high-res fibre #4 or the med-res fibre #3. As the red light of the simultaneous ThAr is filtered away, both cases will give sufficient distance between the interlaced orders over the full wavelength domain.
There is no possibility of doing a simultaneous sky spectrum with this bundle.

Default bundle C: this bundle allows simultaneous-ThAr mode for Fib3 and Fib4, and simultaneous-sky mode for Fib3 (target) + Fib2 (sky). Inter-order distances are very similar to those of the previous bundles. At approximately 6300Å the sky spectrum starts to get close to the target spectrum of the neighboring order.

On-blaze detail of an image using the high-res fibre (halogen light) with the simultaneous ThAr calibration fibre.


The current E2V CCD42-40 detector fringes quite substantially. In flatfields the fringes become noticable from 6100 Angstrom, and reach 15% peak-to-peak amplitude at 7200 Angstrom (for all fibres).

From test setups we know that the fringe amplitudes increase dramatically further towards the IR, but the current wavelength setting does not feature such very large fringe amplitudes.

Wavelength zero-point accuracy

Scientific analyses of stellar spectra obtained during a single night or during a few-night run, point at the following achievable wavelength/velocity zero-point accuracy:

  • stellar spectrum followed by a single separate ThAr exposure using the same fibre: accuracy better than 150 m/s
  • stellar spectrum with simultaneous ThAr light in calibration fibre: accuracy better than 15 m/s
  • short stellar spectrum with separate ThAr spectrum before and after in same fibre: accuracy better than 15 m/s .

    Results show that for a series of blue-sky spectra (i.e. the solar spectrum) with 10000 e- peak flux in simultaneous-ThAr mode, with exposure time 15 seconds, a wavelength zero-point accuracy of better than 4 m/s can be achieved within a single day. We expect that the same holds for well-illuminated stellar spectra.

    From a sequence of ThAr spectra taken during a continuous 28 hours (6/10/2008), we have been able to measure the Echellogram shift due to atmospheric air-pressure variations. The ThAr spectrum shifts by aproximately 120 m/s per millibar (HPa). Note that the diurnal atmospheric pressure 'tide' amounts to about 1 Hpa peak-to-peak amplitude. When using simultaneous-ThAr mode this air-pressure effect is largely calibrated out.
    Note that normally the drifts due to temperature changes should be (much) lower than those due to pressure effects, as the temperature is kept constant (within approximately 0.02 degree) in the FIES room and spectrograph. It is expected that the effect of 1 millibar pressure change is similar to that of 0.1 degree temperature change.

    Another recent result from scientific analysis: the RMS/sqrt(#lines) of the wavelength solution decreases by almost 40% when going from a 5s to 20s ThAr (med-res fiber, bundle B). Apparently the penalty from saturation and diffraction of the strong lines is less than the what is gained from making the forest of weaker lines stronger.

    Latest results

    Results reported from planet-confirmation programs point at a problem with the observations made with Bundle C as of 2010. From on-sky tests it has become clear that the radial-velocity accuracy of the medium-resolution fiber F3 in this bundle depends on the atmospheric conditions. In case of stable seeing, intra-night radial-velocity accuracy of better than 10m/s can be achieved. However, in case of variable seeing the radial-velocity accuracy can be three times worse than that: 25-30 m/s.

    For the high-res fiber F4 in Bundle C the RV accuracy does not differ that much from Bundle B. Intra-night radial-velocity accuracy of 5-10 m/s has been observed regularly, regardless of the atmospheric conditions.

    Tests with the fiber shaker show that in cases of unstable seeing the poor radial-velocity accuracy of the medium-resolution fiber F3 in bundle C can be minimised. When shaking the fiber, the radial-velocity accuracy achieved is close to that in stable seeing conditions, and better than 10m/s.
    The shaking has no positive effect in normal seeing conditions.

    Shutter limitations

    The new main shutter (1-inch diameter, located at spectrograph entrance) has a minimum opening time of 0.03 seconds.

    The exposure meter

    Since fall 2008, FIES is equiped with an exposure meter that can be used to monitor the count rates during an ongoing exposure. The exposure meter picks off light inside the spectrograph that otherwise would have fallen outside the bottom of the CCD. As the red orders are longer than the blue orders, and hence more red light is falling off the CCD, the exposure meter is more sensitive for red stars than for blue stars.

    The exposure meter is placed under a small pickoff prism that intercepts part of the beam at the intermediate focal plane inside the spectrograph, after the light has been dispersed by the echelle grating, but before cross-dispersion. The placement of the pickoff prism is such that no photons are lost from the part of the echellogram that is sampled by the 2kx2k CCD.

    Below the pickoff mirror is a shutter that can be closed to protect the photomultiplier. The shutter will open when doing a non-dark exposure on the exposure meter.

    The Hamamatsu H8259-01 photomultiplier is continuously providing counts to an external counter that is placed in the front room. The photomultiplier is sensitive over the full FIES range, peaking between 300-600nm. The external counter can be undestructively read out every 3-4 seconds. The accumulated counts are written to the FIES status database by the FIES computer.

    The status program FIESTA is providing a graphical history of the exposure meter counts based on the accumulated count in the FIES status database.

    The dark current of the exposure meter is on the order of 150 counts per second, and has not been accounted for in the below table.

    We have calibrated the exposure meter for fibre bundle B:
    Exposure meter counts corresponding to CCD overexposure (65000 ADU, gain=0.7e-/ADU)
    Stellar spectral type ExpMeter counts for Fib#1 ExpMeter counts for Fib#3 ExpMeter counts for Fib#4
    O9 star 300,000,000 250,000,000 320,000,000
    F2 star 190,000,000 185,000,000 240,000,000
    M0 star 80,000,000 100,000,000 90,000,000

    Additionally we have to calibrate the absolute count rates for stars of different brightness.

    Atmospheric Dispersion Corrector (ADC)

    In 2010 the first on-sky tests with the ADC were performed. Although the ADC is primarily developed for use with FIES, it can for some cases be valuable for other instrument as well. The ADC is located under the 'roof' of the adapter, and swings into the telescope beam above the FIES and STANCAM pickoff mirrors.

    ADC transmission as observed by ALFOSC and NOTCAM
    U B V R I J H K
    88% 88% 90% 91% 89% 84% 85% 67%
    Note: in the K band significant emission from the ADC prisms should be expected, leading to about 0.5 mag higher background level.

    The ADC consist of a set of rotating prisms, that when positioned adequately can fully correct for up to approximately AM=3 . The ADC provides an unvignetted FOV of 3.1 arcmin diameter, and hence can be used for ALFOSC/NOTCAM on-axis spectroscopy as well.

    When the ADC is in the beam, the telescope focus needs an offset of about 560 units.

    When in the beam, the ADC support arm will obscure some of the FOV of the autoguider. This needs to be sorted out. It is expected that the ADC obscures the top-lft part of the U-shaped guide field.

    When using the ADC, note that the guide star is not corrected by the ADC. This means that gradually the target will move away from its initial position, if the ADC is used in 'continuous mode'. This may result in object shifts of about 1 arcsec in half an hour of guiding. Hence caution is required when using the 'continuous mode', and for many applications the mode 'automatic-at-preset' should be considered instead.

    Below some first results of using FIES with the med-res fiber F3, with and without the ADC, for different airmasses. Red-drawn spectra are taken with the ADC. All spectra are normalised to the (black) AM=1.1 spectrum taken without ADC. The spectra were obtained with the B-filter infront of the fiber-viewing camera. For AM>1.5 (or so) factors of 3-4 can be won at the extremes of the FIES wavelength coverage, when using the ADC.

    It was also found that the ADC is beneficial for accurate RV measurements at high airmass. Without ADC the RV varies parabolically as a function of echelle order, whereas this effect is much smaller when using the ADC.

    Simultaneous sky mode

    In 2010 the possibility of making simultaneous observations of a target star and the sky background was investigated using fiber 2 and 3 (both medium resolution) in fiber bundle C. Tests were made using blue-sky spectra.

    A throughput comparsion between fiber 2 and fiber 3 has been made and shows the science fiber (fiber 3) to be 41% more efficient than fiber 2, in mean. A polynomial fit to the difference is shown below.

    When using the instrument in simultaneous sky mode the inter-order distance between the sky spectrum and the science spectrum decreases towards the red, causing problems with scattered light substraction when FIEStool is used for the reduction. Below is shown the difference between a spectrum taken with fiber 3 and a spectrum taken in simultaneous sky mode. At a wavelenght of 6700AA the difference between the two spectra is larger than 2.5% with increasing discrepancy towards the red.

    Calibration frames can be obtained individually for the two fibers (F2 and F3) so two sets of flats and wavelength-definition frames should be made (in the afternoon). Also a set of two wavelength-calibration frames (one for each fiber F2 and F3) should be made everytime an update of the wavelength solution is required, to get accurate wavelengths for both the sky spectrum and the science spectrum. To calibrate the throughput difference of the two fibers a set of blue-sky exposures should be made before sunset in an ABBA sequence (in this case F2 --> F3 --> F3 --> F2) to average out the effect of the changing light level, when merging the frames. Exposure times of 20 seconds are appropriate for this. Make sure the telescope is pointing AWAY from the Sun before opening the dome. Note that the dome-lights used for dome flats doesn't provide sufficient illumination for this calibration.
    Also note that a full reduction of simultaneous sky observations is not yet implemented in FIEStool, so the reduction of simultaneous sky frames requires reducing the science frames twice, using the corresponding order-definitions and wavelength definitions for each of the fibers respectively.

    Modal Noise

    Prompted by the RV-stability problems seen with Bundle C, we have looked into the possibility of detecting modal noise in FIES. Modal noise may appear as a consequence of the fiber-output speckle pattern being different at different wavelength, and different as well when the fibers are moved. When a variable or moving speckle pattern is inserted into the spectrograph, the amount of light vignetted by the exit slit (high-res fiber) or the grating (all fibers) or other items that may vignette the beam will be variable as a function of time due to the inclusion/exclusion of speckle points close to vignetting edges, and hence as a function of wavelength and time 'noise' will be recorded in the spectra. See e.g. Baudrand & Walker 2001.

    Such modal noise will not flat-field out, as due to the fiber movements invoked by the telescope, the flatfield will change as a function of time as well.

    Modal noise is a symptom of non-perfect scrambling of the light in the fibers, and it may be possible that the RV-stability problem of bundle C is another symptom of this. A possible solution to cure the modal noise is a fiber shaker, which may also improve the general scrambling characteristics of the fibers in bundle C.
    Tests with a fiber shaker will be conducted shortly.
    Below the results that reveal the existence of modal noise with fiber bundle C.

    Modal Noise for fiber bundle C

    The following experiment was carried out in December 2010, with fiber bundle C. For both fiber 3 (med-res) and 4 (high-res) two series of 21 flats (top calibration unit) were obtained. For each fiber, between the 2 sets of 21 flats, the fiber hose on the dome floor was repositioned manually onto a chair. During these tests the telescope power was switched off.

    For fiber 4 each of the 2 sets of 21 flats were combined into a bias-subtracted masterflat. The masterflats corresponding to before and after fiber repositioning were then divided into each other, and the S/N was derived from these quotient flats, in 11 orders covering the wavelength region of 500-560nm. The 11 orders of the quotient flats are displayed at the bottom half of the below figure.
    As a check we performed the same steps of analysis for the first 10 and last 10 of the set of 21 flats taken before repositioning the fiber. The corresponding 11 orders are displayed at the top half of the figure below, and are not expected to suffer from modal noise as the fiber was not repositioned between these two 10-flat sets.

    As can be judged from the figure, the bottom set of orders shows much more irregularity as a function of wavelength than the top set, whereas based on photon statistics the opposite would be expected. In fact the irregularities seem to spread over broader regions than that oridinary photon noise does.
    Clearly the repositioning of the fiber onto a chair did change the flatfield significantly. Expressed as S/N, the results are listed in the following table.

    S/N in flats that are and flats that are not affected by modal noise

    Expected S/N for photon statisticsMeasured S/N
    Fiber 3, 2x 10 flats control 754 718
    Fiber 3, 2x 21 flats repositioned 1094 654
    Fiber 4, 2x 10 flats control 834 682
    Fiber 4, 2x 21 flats repositioned 1206 447

    As can be seen from the difference between expected and measured S/N, the repositioning of the fiber has a major influence on the results. This, especially in the case of Fiber 4 which has an exit slit to enhance the spectral resolution. The large effect for this fiber may be because of the exit slit, which provides an extra plane of vignetting, or may be by chance because of the way the repositioning of the fiber on the dome-floor was done.

    It is clear that modal noise induced by fiber movements is severely limiting the maximum S/N that can be reached in FIES spectra. It remains to be checked if the other fiber bundle is affected similarly by modal noise.

    The fiber shaker

    The new fibre bundle C for FIES gave improved throughput and a working sky fibre. Unexpectedly, however, the accuracy of radial-velocity measurements with the medium-resolution fibre degraded considerably when observing in unstable seeing conditions. Several remedies were tested, but without decisive success. One possibility was that a variable nonuniform illumination might propagate from the fibre entrance to the exit, but might be averaged out by moving a section of the fibre slightly, but frequently during the integration.

    As a test, an interim fibre shaker or scrambler to move the fibre cable a few cm every couple of seconds was built from LEGO building blocks, motors and controllers, courtesy of Felix Telting. As the test was very promising, a more permanent prototype was acquired for further testing.

    The new fiber shaker is installed after the part of the fibers that are moved by the telescope, about 10 meters away from the spectrograph. The shaker translates the fiber bundle transversely in the horizontal plane, with a stroke of several cm and a frequency of a few Hz. The frequency can be chosen. Photographs of the shaker can be found here.

    Tests with the fiber shaker show that in cases of unstable seeing the poor radial-velocity accuracy of the medium-resolution fiber F3 in bundle C can be minimised. When shaking the fiber, the radial-velocity accuracy achieved is close to that in stable seeing conditions, and better than 10m/s.
    The shaking has no positive effect in normal seeing conditions.

    Tests with the shaker aimed at improving the modal noise (see above) did not show any positive effect.

    The fiber-shaker is currently not offered to the community.

    John Telting