viewing, guiding and exposure meter facilities
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
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")
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
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
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.
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
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).
As a by-product, the guiding algorithm also produces in real time the width and flux of the
stellar image (and thus the average seeing and magnitude), as well
as an estimate of the fraction of the total flux that was fitted into the central pinhole.
The guiding agent displays this information in the status server and can (on users' request) plot
it as a function of time on a graphical window, an example of which
is shown on the right (recorded in arbitrary guiding conditions).
To check that the flux entring the spectrograph is maximum and corresponds to the
expectations, the observer can use the exposure meter implemented
within the spectrograph, picking off a small fraction of the beam (of order 0.1% of the
total flux) on its way from the main collimator to the grating. The detected count rate
(in the range of about 10 counts/s to 2 million counts/s) corresponds to stellar V
magnitudes of about 4 to 18 (depending on the color of the star of interest).
The detected count rate, as well as the number of counts accumulated during an exposure,
are displayed in real time in the status server. The observer can also activate, on
request, a graphical window (see example panel on the right)
displaying the count rate information, both on a linear scale (top graph) and on a
logarithmic magnitude scale (bottom graph). Both graphs include both the instantaneous
meaurements (light blue curve) and values averaged over the last 30 measurements (green
curve). The standard deviation on the same sample of 30 measurements is also indicated
in the appropriate box and updated in real time.
This tool is very useful to check how much the observing conditions are varying with
time, and to potentially correct the situation (eg by refocussing the telescope or fine tuning
the guiding zone) if necessary.
© Jean-François Donati, last update May 18 2004