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ESPaDOnS: viewing, guiding and exposure meter facilities http://webast.ast.obs-mip.fr/magnetisme/espadons_new/guiding.html
ESPaDOnS
viewing, guiding and exposure meter facilities
Viewing
The instrument aperture of ESPaDOnS consists of
two pinholes drilled within a small tilted mirror.
The central pinhole (of diameter 0.22mm or 1.6")
is used for collecting the stellar light in all three
observing modes, ensuring that 90% of the stellar
light enters the instrument in median seeing
conditions (0.7" seeing). The second pinhole
(located at a distance of 1.1mm or 7.9" to the
south of the central pinhole) is used to collect the
background light from the sky in the ’object+sky’
spectroscopic mode only. The tilted mirror (of
diameter 100") is used to reflect off the light to a
viewing camera, so that the observer can easily
focus the telescope on the central instrument
pinhole, identify the star of interest and make sure
that it fits optimally within this pinhole.
The camera we selected is model CM2-1 of the
MaxCam series, developped by Finger Lake
Instrumentation (implementing an eev ccd of type
CCD47-10 with 1kx1k 0.013mm square pixels).
Along with reimaging optics, the viewing channel
includes a filter (of schott type bg38, to select visible light only) and a density wheel (to adapt the stellar brightness to the
camera sensitivity).
The control software includes a viewing agent that can display in real time the image from this camera as observations are
carried out. When the star of interest is fitted into the central pinhole (as on the image above), the observer can see no more
than the light from the far wings of the stellar image at Cassegrain focus. When the star is properly centred into this pinhole,
this light should draw a bright ring around the central hole, as in the above example image.
Guiding
The viewing agent also include guiding facilities specifically developped for
ESPaDOnS. This tool uses the residual light from the edges of the stellar
image to evaluate any potential image decentring and remove it by
interacting with the telescope control system. If a second star is also present
in the camera field of view, the observer can also choose to offset guide on
this second star. This is obtained by simply moving the guiding zone
(depicted with a dashed circle on the above image) to the star from which
guiding must be performed, and the guider ensures that the star within the
guiding zone remains at the centre of this circular area. The sensitivity of
the camera is such that guiding can be performed with a star as faint as a V
magnitude of about 17 (when guiding on the central star), and of about 19
(when offset guiding).
The guiding algorithm used for ESPaDOnS implements a 2d gaussian
fitting (following Levenberg Marquard technique for chi square
minimisation) with two predefined null sensitivity circular area modelling
the two mirror pinholes (from which no flux is redirected to the camera).
The algorithm has proved to be rather robust when used with fake stars
(obtained by reimaging a fibre core onto the instrument aperture).
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