4 Fiber locked auto guider (FLAG)

The goal of FLAG is to cancel at fiber output all effects arising from stellar-image photocenter XYZ motions. Admittedly nothing is done about instantaneous image blurring, which should then become responsible for any remaining perturbations.

4.1 Principle

The fiber input itself plays the role of position detector. One introduces two small oscillations of stellar image in two perpendicular directions X and Y, with frequencies F_x and F_y. A small translation of a lens on its axis produces a Z-translation of the image, hence a focusing change with frequency F_z. At the output of the fiber, any type of single-pixel intensity detector generates a modulated signal; it uses the NIR through a dichroïc beamsplitter. Three synchronous demodulators at frequencies F_x, F_y and F_z reconstruct three DC error signals SX, SY and SZ. The first two are applied to some fast tip-tilt device, while SZ may go either to the Z-lens, or to the telescope secondary-mirror motor.

  

Figure 6: FLAG basic trade-off; numerical simulation for a single channel. Growth of error-signal (arbitrary units) versus X or Y oscillation amplitude (unit: fiber radius), and decrease of efficiency. Three cases shown, for FWHM of Gaussian seeing pattern equal to 0.5, 1, or 2 fiber diameters. Usual compromise in our tests: amplitude 0.25, efficiency 0.98. Similar curves are found for Z. For all three channels active, efficiency then drops to 0.94

The image oscillations widen the seeing pattern, hence induce some loss at fiber input; numerical simulation for a single X or Y parameter is shown by Fig. 6; for Z the curves are similar. When all 3 channels are active together, and at least for amplitudes $\ll$ fiber radius (as used in practice), the error signals remain the same, with little cross-coupling, while the overall efficiency is the product of the individual ones. Oscillation amplitude is adjustable at will, and the optimum trade-off depends on stellar color and magnitude. In practice, we have kept constant amplitude, obtained nearly constant performance, up to M_v = 8, and measured a 6% loss by switching all three oscillations on and off. The dichroïc loses another 15% of the visible beam.

There are some possible variants, all untried so far: With minor changes, a photon-counter could be substitued. A single frequency might be used for X and Y, with $90\hbox{$^\circ$}$ phase difference; image trajectory would then become a circle rather than a Lissajous pattern. Instead of a dichroïc, a weak achromatic beamsplitter might bleed-off a small fraction of the visible beam. Finally, a double scrambler might be incorporated without any change of the system.

4.2 Optical system

Our design was intended for the Coudé focus of the 152-cm OHP telescope (Fig. 7); furthermore, it made use of available or inexpensive elements only. The f/27 beam is converted to f/2.55, and the star image is formed on the 50 $\mu$m-fiber entrance which accepts 2.7 arcsec on the sky. The two plane-parallel "scanning'' plates (6), close to the Coudé focus, introduce two small oscillations of the image ($\sim 0.2$ arcsec) in two perpendicular directions X and Y, with frequencies F_x (1 190 Hz) and F_y (955 Hz). The larger "guiding'' plates (5) can introduce a tilt up to $\pm 4$ arcsec. The field lens (7) (focal length 70 mm) re-images the telescope pupil on the achromat (8) which is mounted on a small loudspeaker (drilled through its axis) and oscillates in Z at frequency F_z (215 Hz). The very short 6-mm focal length makes chromatic aberration negligible, which is important as guiding and focusing are checked in the NIR.

  

Figure 7: Autoguider optics: 1) tungsten lamp, 2) circular Gaussian-pattern screen, 3) simulated telescope-pupil stop, 4) commutation mirror, 5) guiding plates ($10\times 10\times 20$ mm), 6) scanning plates ($5\times 5\times 5$ mm), 7) field lens, 8) translating achromat, 9) dichroïc, 10) guiding detector, 11) atmospheric dispersion compensator, 12) beamsplitter or removable mirror, 13) finder video. Commutation mirror is removed for star beam

At the fiber output, the beam is split by a 600-nm cut-off dichroïc plate (9). The NIR part falls on a Si avalanche diode, Peltier-cooled (10). In order to make direct measurements of beam shape and position, the visible part produces images of the near- and far-field as in Fig. 2. Later on, this same beam will be sent to the EMILIE spectrograph (Bouchy et al. 1999). An atmospheric-dispersion compensator (11) incorporates a pair of counter-rotating low-dispersion prisms piloted by a PC; it is independent of the guider. A finder (12-13) is used to center the star image on the fiber ($5\times 7$ mm CCD video camera with a $2\times 3$ arcmin field of view).

  

Figure 8: Autoguider electronics. GD: guiding detector, PR: preamplifier, HP: high pass filter, LP: low pass filter, DIV: analog divider, BP: band pass filter, DEM: demodulator, PID: proportional-integrator-derivator, S/H: sample/hold, PA: power amplifier, OSC: oscillator, V/F: voltage-to-frequency converter, MONO: monostable multivibrator, INV: inverter, DA: differential amplifier, REL: relays

The commutation mirror (4) interchanges stellar and reference beams. A set of Gaussian screens (2), illuminated by a tungsten lamp (1), simulates stars under different condition of seeing. An aperture stop (3) with a central obstruction, is located in a plane conjugate of the telescope pupil; hence the beam shape matches that from the telescope.

4.3 Electronic control system

The system is controlled by purely analog electronics (Fig. 8); it was built without outside help, entirely on solderless test boards. Let us follow the X-guiding channel. An harmonic oscillator signal at frequency F_x is applied to the X-actuator. If the image mean position is not centered on the fiber input, the guiding detector signal GDS is modulated at F_x. For exact centering, the F_x fundamental must be nulled, and only harmonic 2 remains; hence, a demodulator at F_x provides a DC error signal, and a proportional-integrator-derivator (PID) optimizes the servo response. The oscillation actuators have 1.5 kHz resonant frequencies, the guiding ones about 200 Hz. A first version of the system used only 2 plates, both for scanning and guiding, with the slower actuators; it operated correctly, but was slower.

For the Z-channel, the oscillator signal F_z goes to the Z-lens loudspeaker. The DC correction can be applied either to the loudspeaker or to the telescope secondary-mirror driving motor. We tried both schemes; with the loudspeaker, Z-correction was fast but perturbations appeared on X- and Y-channels, probably due to poor optical alignment. Since telescope defocusing is very slow, the second scheme (shown in Fig. 8) gave adequate results. Two V/F converters provide slow pulse trains (one pulse for every few seconds); two monostable multivibrators adjust the length of the motor-on intervals, hence correction speed. The system is roughly equivalent to the Integrate channel of the PID.

The rest-point of all three servos is independent of the mean GDS intensity, but the response time is not, and they would become sluggish when the beam passes through weak clouds. Hence an automatic gain-corrector is added at preamplifier output. An analog divider provides the ratio of AC to DC components; the output is a stabilized-amplitude AC signal, and is fed to the demodulators. The system works well within a 10/1 range of GDS intensity, greater than needed for recording spectra in practice.

During short but total cloud-induced breaks, recording has to stop, and it does not matter that the error signal is no longer available. However, when the star reappears, one wants the autoguider at least to pick-up the image automatically. A level detector triggers at 1/10 th of the maximum GDS intensity, and commands a sample/hold amplifier to preserve the power amplifier input signal during the break. Thus, the autoguider is able to find the image at the point of the field where it was last seen.

Analog X-Y displays of either the error signals or the guiding plate tilts are shown on an oscilloscope screen. This allows the observer to monitor the behavior of the system and to correct telescope pointing when the image drift has exceeded the guider-correction field. Later on, we may automatize the process by sending the very low-frequency guiding errors directly to the fine-guiding telescope motors.