Cute-SCIDAR: An Automatically Controlled SCIDAR
Instrument for the Jacobus Kapteyn Telescope
J. J. Fuensalida
1, B. García-Lorenzo
1, *, J. M.
Delgado
1, C. Hoegemann
1, M. Verde
1, M. Reyes
1
and J. Vernin
2
1: Instituto de Astrofísica de Canarias; 2: Laboratoire Universitaire
d'Astrophysique de Nice
In February 2004
the Cute-SCIDAR instrument was installed at the 1m Jacobus Kapteyn telescope
(JKT) for a systematic monitoring of the atmospheric turbulence at the Observatorio
del Roque de los Muchachos (ORM). The proper knowledge of the atmospheric
turbulence structure is crucial for optimising the efficiency of adaptive
optics systems. SCIDAR has proved to be the most contrasted and efficient
technique from ground level to obtain the optical vertical structure of the
atmospheric turbulence. The classical (
Vernin
& Roddier, 1973;
Rocca,
Roddier & Vernin, 1974) and generalised SCIDAR (see e.g.
Klueckers
et al., 1998;
Ávila,
Vernin & Masciadri, 1997;
Fuchs,
Tallon & Vernin, 1994) techniques analyse the scintillation patterns
produced at the telescope pupil by the light coming from the two stars of
a binary system. Turbulence profiles as a function of height, C
N2
(h), are derived through the inversion of the autocorrelation of scintillation
patterns. Wind vertical profiles, V(h), are derived from the cross-correlation
of a series of scintillation patterns relative to a reference pattern.
Figure 1 sketches the SCIDAR technique.
|
Figure 1. SCIDAR technique layout. From the observation
of binary stars, we get a stack of consecutive images of scintillation. The
average auto-correlation function of them gives information of the CN2
profiles, and average cross-correlation functions give the wind of the layers.
[ JPEG | TIFF ]
|
The main drawback of SCIDAR observational campaigns is the tedious setting
up of the instrumentation and the computational effort needed to infer the
nightly turbulence and wind profiles. Therefore, systematic recording of
turbulence and wind structure requires a huge number of highly qualified
human resources. Consequently, the development of a fully automated SCIDAR
device is of increasing importance to characterise the atmospheric turbulence
and fix the input requirements and limits of the future multi-conjugate adaptive
optic systems to be installed at the ORM. The Instituto de Astrofísica
de Canarias (IAC) has developed a SCIDAR instrument providing high performance
in automatic control and data reduction, the Cute-SCIDAR. It has been designed
for the 1m JKT, with the goal of monitoring the vertical turbulence with
a high temporal coverage. This device is not only restricted to the JKT but
can also be used on other telescopes.
Technical Description
Figure 2 presents the optical scheme of the SCIDAR
instrument. From the observational point of view, the SCIDAR technique requires
that the detector is able to move along the optical axis to allow selection
of the different conjugated planes. Moreover, the rotation around the optical
axis is most than desirable for a SCIDAR instrument: because the star beams
should be properly orientated on the detector (with its rows) in order to
simplify the data reduction.
|
Figure 2. SCIDAR instrument optical scheme.
[ JPEG | TIFF ]
|
The Cute-SCIDAR allows the automatic control of any of the SCIDAR instrument
components. The detector is lodged in two devices permitting the motion in
the XY plane (perpendicular to the optical axis) to correct the small flexure
displacements in the observational plane. The maximum range in the XY plane
is 25mm. A long electronically controlled rail to place the detector in the
adequate conjugated plane provides the movement along the optical axis, Z
direction. This motion also facilitates the instrument focusing procedure,
since it permits to easily verify (using a single star) the state of collimation
of the beam. The maximum displacement in the Z direction is 300mm. The current
detector is a commercial sensitive CCD camera of PCO. The instrument can
rotate up to 270° with respect to the telescope through a crown wheel.
Another complementary mechanism is a diaphragm, placed in the focal plane
of the telescope (see the scheme of
Figure 2), and
also electronically controlled. The diaphragm mechanism permits the proper
alignment of the observing binary star with the instrument optical axis.
After a short successful commissioning at the Carlos Sánchez telescope
at the Observatorio del Teide in Tenerife, the Cute-SCIDAR was installed
at the JKT in February 2004.
Figure 3 (left) shows
the Cute-SCIDAR already installed at the JKT.
Figure 3
(right) shows the essential instrument components. In this figure, we can
see the diaphragm and collimator (1) within the instrument cover. The detector
can be seen at the bottom opened door (2) and the crown wheel is the golden
ring (3) connecting the instrument and the telescope. The label (4) indicates
the electronic box controlling the mechanical elements of Cute-SCIDAR.
|
Figure 3. Views of the Cute-SCIDAR instrument installed
at the Jacobus Kapteyn Telescope. [ JPEG |
TIFF ]
|
Control Software
A specific software package for the control of the different mechanical components
and a pre-processing on-line data evaluator has been developed. A user-friendly
interface based on MS-WINDOWS XP allows handling the different instrument
components from the telescope control room.
Figure 4
shows an example of the quick-look data interface: the left upper image corresponds
to the pupil image of a binary star (the data recorded at the detector),
and the right upper plot is the 2D normalised auto-correlation of this image;
the bottom plots are cross-correlations of the left upper image and a reference
image. The bottom right plot is a X-cut along the 2D autocorrelation function
showing the presence of at least two turbulence layers (the two peaks to
the left and right of the central brightest peak).
|
Figure 4. User interface window showing an example
of the on-line data evaluator. [ JPEG | TIFF ]
|
Observational Campaigns at the JKT
After a successful commissioning of the Cute-SCIDAR at the JKT last February
2004, we have started a monitoring program of the atmospheric turbulence.
We are carrying out monthly one-week observing SCIDAR campaigns and we already
have data corresponding to 30 nights of observations. On each night we can
record more than 1500 different atmospheric turbulence vertical profiles
above the Observatorio del Roque de los Muchachos. Preliminary results obtained
from these data have been recently presented to the astronomical community
(
Fuensalida
et al., 2004;
Hoegemann
et al., 2004;
Fuensalida
et al., 2003).
Figure 5 shows the temporal evolution
of the atmospheric turbulence profiles along an observing night at the JKT.
In this figure, the X-axis corresponds to the time (in UT) along the night
referred to midnight. The Y-axis is the altitude above sea level, and the
colour bar on the right side indicates the values of CN2(h). The dome seeing
contribution has been rejected.
|
Figure 5. Evolution of turbulence vertical profiles
at the Observatorio del Roque de los Muchachos during the night of 23-24
April 2004. The mean seeing derived from these profiles was 0.84 arcsec being
in good agreement with the DIMM measurements. [ JPEG | TIFF ]
|
In semester 2004B we will continue the monitoring of the turbulence structure
at the JKT after the extension of the agreement with the Isaac Newton Group
of Telescopes. The Cute-SCIDAR team thanks the staff of the Isaac Newton
Group of Telescopes for their usual high standard of service.
¤
References:
- Ávila, R., Vernin, J., & Masciadri, E., 1997, Appl. Opt.,
36, 7898. [ First citation in text |
ADS
]
- Fuensalida, J. J., Chueca, S., Delgado, J. M., García-Lorenzo,
B., González-Rodríguez, J. M., Hoegemann, C., Mendizábal,
E. M., Reyes, M., Verde, M., & Vernin, J., “Vertical structure of the
turbulence in the observatorios of the Canary Islands: parameters and statistics
for adaptive optics”, Astronomical Telescopes and Instrumentation: The industrial
revolution in Astronomy, SPIE Proc., 21-25 June 2004, Glasgow, Scotland
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| ADS
]
- Fuensalida, J. J., Delgado, J. M., García-Lorenzo, B., Hoegemann,
C., Reyes, M., Verde, M., & Vernin, J., “An automatically controlled
SCIDAR instrument for the Roque de los Muchachos Observatory”, Second Workshop
on Extremely Large Telescopes, SPIE Proc., September 7-12 2003, Backaskog,
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]
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]
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B., Mendizábal, E. M., Reyes, M., Verde, M., Vernin, J., “Cute SCIDAR:
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]
*Email contact: Begoña García (
begona.garcia@iac.es)