Contents

Preparation

Important notes

Spectral light source

To align the Fabry-Perot plates a flat mono-spectral (in terms of the spectral resolution of the Fabry-Perot) illumination of the ADONIS object plane is necessary. This illumination is produced by a spectral lamp in combination with an integrating sphere.

For the spectral calibration of the etalons several emission lines at different wavelengths can be used:

More detailed information on the line emission of the spectral lamps are listed in the documentation, which is already in La Silla.

Adonis optical bench

The surfaces of the Fabry-Perot etalons need to be perpendicular to the optical axis of the ADONIS system. One can achieve this by tilting the last ADONIS mirror M9. The reflex of the white light fiber with the Fabry-Perot surface should follow back the same way as the incoming light. This can easily be checked by moving a piece of paper right behind mirror M9. Please bear in mind, that tilting mirror M9 requires a new alignment of the SHARP II+ camera. Therefore one should run through this procedure in the very beginning of an observing run.

Sharp II+ camera

For the calibration procedures the image scale of tex2html_wrap_inline175 should be used, because in this scale the flux per pixel is maximized. The illumination of the pupil can be checked with the pupil imaging lens. To get enough signal per pixel a integration time of 3 seconds is recommended. Before working on the Fabry-Perot etalons themselves, one should check several topics without the etalons:

Initializing the Fabry Perot Etalons

Before controlling the Fabry-Perot etalon by the COMIC computer, the capacitance bridges of the CS100 control unit have to be balanced. For both etalons the setup conditions are listed in table 1.

  table37
Table 1:   CS100 setup conditions

The RESPONSE TIME should be set to tex2html_wrap_inline181 (blue scale), the MODE has to be switched back to BALANCE and the METER DISPLAY to OFFSET.

Now the CS100 control unit can be switched on. When changing the MODE to OPERATE the yellow BALANCE indicator should go out and the green OPERATE indicator should come on after a delay of about two seconds. The X,Y and Z meter will now give an indication of how much correction is being applied in the three axes to achieve parallelism. When switching the METER DISPLAY to QUADRATURE ERROR, the meters will read zero.

If the OUT OF RANGE indicator illuminates and the system reverts to the BALANCE mode, the system was not able to close the control loop, because the QUADRATURE ERROR exceeds more than 5V. This could happen because of wrong PARALLELISM or QUADRATURE BALANCE settings or because of wrong or missing connection to the Fabry-Perot etalons.

Before handing over the control to the COMIC computer, the MODE has to be switched to BALANCE again.

Controlling the Fabry-Perot etalons by the COMIC computer

In the present configuration of the COMIC Acquisition Software it is not yet possible to switch between the configuration of the two Fabry-Perot etalons within the menu system (the implementation will follow). Therefore, before starting the ``bench'' software on the Comic computer, the ``cam_bench.init'' file has to be modified. Within this file there are two lines starting, with ``FP1'' and ``FP2'', telling the system the parameters of the tuning formula of the Fabry-Perot etalons. One of these two lines has to be commented out, so that the system only recognizes one etalon. The tuning parameters A, B, E and EBZ are describing the relation between the transmitted wavelength tex2html_wrap_inline191 at a specific order m and the decoder values z, which are transmitted to the CS100 controller.

equation44

The values for the tuning parameters A,B and E for both Fabry-Perot etalons are listed in table 2.

  table47
Table 2:   Tuning parameter for the Fabry-Perot etalons

In principle the parameter EBZ is not necessary, because it can be combined with B to a single parameter. The reason for implementing two parameters is, that the spacing between the two Fabry-Perot plates -- equivalent to the transmitted wavelengths -- for a decoder value of zero changes slightly when switching off and on the CS100 controller. Therefore the parameter B represents an average zero spacing and EBZ is the present correction. This parameter EBZ should be reconfirmed each time a Fabry-Perot etalon is prepared for observations. This procedure is described later in a separate section.

If the configuration in the ``cam_bench.init'' file is set to the right Fabry-Perot etalon, one can start the Comic bench software by entering ``xtart bench'' (if not already done during the system startup). On the CS100 controller the MODE indicator should switch from OPERATE to BALANCE and the manual control will be DISABLED. Now it is possible to control the etalon by the routines implemented in the COMIC software.

If nothing special is specified, the system automatically choices the best interference order m for a given wavelength and also tunes the CVF to the wavelength.

Both options can be suppressed:

Use the BLOCKED mode, if you want to scan a line, being sure that all the wavelengths are observed at the same spectral resolution (the spectral power of a Fabry-Perot is given by the product of its finesse and the observed order). The NONSEPARATED mode can be useful, if you are scanning a line only in a very narrow wavelength range ( much smaller than the spectral resolution of the CVF), because you are saving the time necessary for moving the filter wheel. Especially for alignment and calibration of the Fabry-Perot etalons this mode should be used.

Confirming parallelism of the etalon plates

Aligning the Fabry-Perot etalons

The general procedure for aligning the etalons is described on page 6 and 7 of the CS100 OPERATING INSTRUCTION ( you can find a copy in La Silla). Here are some additional remarks for the configuration used in SHARP II+.

Calibration the Fabry-Perot etalon plate spacing

Because the z spacing of the Fabry-Perot etalons changes slightly when switching off and on the CS100 controller, the tuning parameter EBZ in the ``cam_bench.init'' files has to be reconfirmed or modified each time.

Wavelength calibration

The correlation between the transmitted wavelength and the decoder values for the z-spacing was allready discussed in the section ``Preperation''. Beside the zero distance of the two Fabry-Perot etalon plates, none of the tuning parameters should change with time. Nevertheless the tuning formula should be reconfirmed at the beginning of each observing period by scanning several emission lines. The lines suitable for this purpose are listed in the beginning of this operation manual. The deviation from the true wavelength should be less than tex2html_wrap_inline225 of the FWHM of the passband. If there is a significant difference, the tuning parameter should be recalculated by a least square fit, starting with the old values.

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