ADONIS Prefocal Optics

Fabry-Perot Etalons:
Atmospheric Dispersion Corrector :

Note: The prefocal optics are installed on the optical bench by the La Silla 3.6-m Telescope Team and then controlled by the astronomer using the ADOCAM software.

Also note: The Fabry-Perot etalons and the coronograph should be not be installed on the bench during the night. Removal of all prefocal optics out of the bench can be done during the night. Access to the prefocal optics is permitted only when the telescope is at zenith.

Fabry-Perot Etalons:

Before applying for time please contact the Adonis instrument scientist for a Fabry-Perot feasibility discussion.

To obtain high resolution spectroscopy (R = 1000, 2200) in the K-band, two Fabry-Perot etalons can be mounted in front of the camera. Their finesse and resolution for different wavelengths are tabulated in Tables 1 and 2. The Fabry-Perot optics were optimized for use with the 50 mas/pixel scale. In this case the shift in passband across the field of view is less than 0.4 nm. Transmission in the wavelength range 1.9 to 2.4 µm] is: 85 % for the low resolution etalon; and 65 % for the high resolution etalon.

Both etalons need to be calibrated each day by the 3.6-m Team. A scan of spectral lines as close as possible to the science line are recorded before each observing night using a spectral lamp. For a list of the available spectral lines see the Sharp II manual.

Wavelength [µm]FinesseSpectral
Resolving Power
Table 1: Low resolution Fabry-Perot

Wavelength [µm]FinesseSpectral
Resolving Power
Table 2: High resolution Fabry-Perot


The combination of adaptive optics and coronographic techniques providing at the same time high angular resolution and high dynamic range is a powerful tool to study the close environment of bright objects in different fields of astrophysics.

Observatoire de Grenoble, in collaboration and under contract with ESO, has designed and manufactured a new coronograph to be used as a prefocal optics on ADONIS. The optical sketch of the coronograph is shown in the next figure:

It is composed of an occulting mask introduced at the output F/45 focus of ADONIS and a Lyot stop located on a pupil image inside the SHARP IR camera.
Please note that there is no reason to use the coronograph with COMIC, the COMIC detector already being an antiblooming device.
The occulting mask is optically conjugated with the detector and the Lyot stop with the telescope pupil. The Lyot stop, also called apodizing mask, is used to block the diffracted light from the edges of the telescope primary and secondary mirrors and from the secondary mirror support (including the spider arms).

The coronograph focal plane unit allows to insert different occulting masks at the ADONIS F/45 output focal plane. These masks will be mounted on dedicated barrels, each of them supporting up to 3 masks. The observer will thus be able to insert anyone of those 3 masks in the IR camera field of view by remotely moving a motorized linear stage from the ADOCAM control software. Typically 30 seconds will be necessary to switch from one mask to another one. If needed, there is also a way to manually move the mask along the perpendicular axis (limited excursion range).
Please note that it is necessary to dismount the coronograph to exchange one barrel for another (20mn). This delicate manipulation should be avoided during the night.
We strongly recommmend the observer to keep the same barrel for a given night.

The actual diameters (within an accuracy of 5 %) of the masks are as follows:

Mask size in arcsec (")
Barrel 10.420.630.72
Barrel 20.841.001.40
Barrel 32.003.925.50
Table 3: Availables masks for Adonis coronograph

The detection limit at 2" from the central object in K band for a total integration time of 6 mn with an occulting mask of 0.8" in diameter is typically 100 000 fainter (12.5 magnitude per pixel) than the peak intensity of the star observed without the mask, with the same angular resolution.

User's Guide

The Coronograph User's Guide (2000) is available as a gzipped postscript file (460 kB).


A wire grid linear polarizer can be inserted into the beam in front of the camera. Its specifications are summarized in Table 3. Note that the image position on the detector slightly moves according to the polarizer position angle. Note also that 50% of the incoming light is lost.

Any polarizer position angle can be set by the user via the ADOCAM software. A nice feature of ADOCAM is that a sequence of different polarizer angles can be executed automatically for which images are taken at each angle.

SubstrateCalcium Fluoride
Spectral range 1-9 µm
Grid period0.25 µm
Transmission for polarization
perpendicular to the wire grid at 1.5 µm
83 %
Transmission for polarization
parallel to the wire grid at 1.5 µm
3 %
Degree of polarization at 1.5 µm 93 %
Table 3: Specifications of the wire grid linear polarizer

Atmospheric dispersion corrector

The atmospheric dispersion prevents diffraction limited observation at higher zenith distances in J- and H-band. To correct this wavelength dependent aberration, an atmospheric dispersion corrector can be placed in front of the camera.
This has the drawback of introducing further ghosts problems. Figure 5 illustrates the image elongation for a zenith distance of 60° without correction and using an ADC.

This ADC is built of two pairs of prisms made of ZnSe and ZnS-Multispectral. Each pair of prisms is cemented. All the surfaces are coated for highest transmission. The average transmission of the complete ADC in the wavelength range from 1.1 to 2.45 µm is 95 %.

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