We present the results of a series of measurements conducted using the upgraded Fiber Optic Cassegrain Echelle Spectrograph (FOCES)1 intended to be operated at the 2.0 m Fraunhofer Telescope at the Wendelstein Observatory (Germany) in combination with a laser frequency comb as calibrator. Details about the laboratory set-up of the system integrated with FOCES are shown. Different analysis techniques are applied to investigate the calibration precision and the medium-long term stability of the system in term of changes in stellar radial velocity.
KEYWORDS: Point spread functions, Space telescopes, Signal to noise ratio, Telescopes, Image processing, Stars, Astronomy, Device simulation, Optical telescopes, Observatories
Images with trailed sources can be obtained when observing near-Earth objects, such as small astroids, space debris, major planets and their satellites, no matter the telescopes track on sidereal speed or the speed of target. The low centering accuracy of these trailed sources is one of the most important sources of the astrometric uncertainty, but how to determine the central positions of the trailed sources accurately remains a significant challenge to image processing techniques, especially in the study of faint or fast moving objects. According to the conditions of one-meter telescope at Weihai Observatory of Shandong University, moment and point-spread-function (PSF) fitting were chosen to develop the image processing pipeline for space debris. The principles and the implementations of both two methods are introduced in this paper. And some simulated images containing trailed sources are analyzed with each technique. The results show that two methods are comparable to obtain the accurate central positions of trailed sources when the signal to noise (SNR) is high. But moment tends to fail for the objects with low SNR. Compared with moment, PSF fitting seems to be more robust and versatile. However, PSF fitting is quite time-consuming. Therefore, if there are enough bright stars in the field, or the high astronometric accuracy is not necessary, moment is competent. Otherwise, the combination of moment and PSF fitting is recommended.
A modern 2-m telescope is in comissioning phase at Wendelstein Observatory since late 2013. In order to make full use
of good seeing conditions in Wendelstein, many measures were taken to reduce the image aberration to get the best image
quality. Due to its fast primary mirror, the telescope image quality depends critically on the secondary mirror alignment.
Thus a scheme of quick and accurate alignment of the secondary mirror is desired for the telescope system. We will
utilize a Shack-Hartmann wavefront sensor (SHS) to optimize the alignment for a basically well aligned telescope system.
The principle of the image aberration measurement using SHS is shortly re-introduced with this paper. Merit function
regression method can be used to align the secondary mirror of the telescope system using Zernike coefficients derived
from the reconstructed wavefront. The principle of merit function regression method is described in this paper. Optical and
mechanical layout of this telescope alignment system is also shown. A temperature stabilized box for SHS was designed
to keep the wavefront measurement precision of a commercial SHS system in the harsh conditions of an observatory site.
Mechanical design and temperature control system of the temperature stabilized box are also illustrated. The deviation of
the temperature is within 0.04 degree from the first test of the temperature stabilization experiment, which is good enough
to decrease the wavefront measurement error produced by environmental temperature variation.
One fiber-fed high resolution echelle spectrograph was built for the one meter telescope atWeihai Observatory of Shandong
University. It is used for exoplanet searching by radial velocity method and for stellar spectra analysis. One dimensional
spectra extraction from the raw echelle data is researched in this paper. Flat field images with different exposure times
were used to trace the order position accurately. The accurate background was fitted from each CCD image and it was
subtracted from the raw image to correct the background and straylight. The intensity of each order decreases towards
the order margin, and the lengths of order are different between the blue and red regions. The order tracing during the
data reduction was investigated in this work. Accurate flux can be obtained after considering the effects of bad pixels, the
curvature of each order and so on. One Interactive Data Language program for one dimensional spectra extraction was
adopted and implemented to echelle data reduction for Weihai fiber-fed high resolution echelle spectra, and the results are
illustrated here. The program is efficient and accurate for echelle data reduction. It can be adopted to reduce data taken by
other instruments even the spectrographs in other fields, and it is very convenient for astronomers.
Since 2009 the Echelle spectrograph FOCES1 is located at the laboratories of Munich University Observatories under pressure and temperature stabilized conditions. It is intended to be operated at the 2.0m Fraunhofer Telescope at the Wendelstein Observatory and it will remain under lab conditions in Munich until the telescope is fully commissioned. This has given us the unique opportunity to use FOCES as a test bed for a number of different stability issues related to high precision radial velocity spectroscopy, in particular to study spectrograph stability, illumination stability and fiber transport stability. In this paper will be presented the final optical measurement results to test temperature and pressure stabilization in the spectrograph environment with respect to simulations requirements previously published. Using measurements done by a ThAr gas discharge source, we tested the stability of our system by direct 1D spectra analysis and we verified the movement of the spot positions by changing the CCD temperature in the stabilized environment.
The Echelle spectrograph FOCES1 is currently located at the laboratories of Munich University Observatories
under pressure and temperature stabilized conditions. It is being used as a test bed for a number of different
stability issues related to high precision radial velocity spectroscopy.
We utilize FOCES to study spectrograph stability, illumination stability and fiber transport stability. With
this work we continue the series of papers that present our efforts to obtain temperature and pressure stabilization
in the spectrograph environment. In particular we present first optical measurement results and compare them
to simulations previously published. We show the movement of the image on the CCD with changes of pressure
and temperature and the stability of the spot positions in the stabilized system using measurements done by a
ThAr gas discharge source.
A commercial Shack-Hartman wavefront sensor is being used in a test setup installed at the Wendelstein 40 cm
telescope to test methods for telescope alignment based on reverse optimization. Measured low-order Zernike
wave-fronts are being used to determine the misalignment of the telescopes optical elements. Then a procedure
to optimize wave-front performance by aligning the telescope secondary mirror is applied.
The setup contains a collimating optical system, the Shack-Hartman sensor and a guiding and acquisition
camera.
The Echelle spectrograph FOCES,1 that was operated at the 2.2m Calar Alto telescope between 1995 and 2009
was moved to the laboratories of Munich University Observatories and is being as a test bed for a number of
different stability issues related to high precision radial velocity spectroscopy.
We utilize FOCES to study spectrograph stability, illumination stability and fiber transport stability.
Results from temperature and pressure stabilization are presented with this paper. We will show, that we
reach the requirements set by our model analysis approach presented in [2]. Peak to valley mid term stability of
temperature and pressure is as good as 0.002K and 0.02hPa.
The Echelle spectrograph FOCES,1 that was operated at the 2.2m Calar Alto telescope between 1995 and 2009
will be used as a test bed for a number of different stability issues related to high precision radial velocity
spectroscopy.
We utilize FOCES to study spectrograph stability, illumination stability and fiber transport stability.
The layout of this laboratory experiment will be presented in this paper together with the required and
desired spectrograph stability with respect to both pressure and temperature. We will present technical concepts
how to reach our stabilization goal as well as first results from the spectrograph thermal stabilization efforts.
Even slight changes of temperature and pressure in high resolution ´Echelle spectrographs affect the spot image
on the detector plane. At the same time astronomical applications require a stability of the measurement of up
to 1/3000 of a pixel on the CCD (with a typical pixel size being 15μm).
With this paper we present a study of the effects of thermal and pressure instabilities on ray tracing models of a
typical ´Echelle spectrograph. We conclude the required minimum stabilty in these two parameters to reach the
goal of precision spectroscopy.
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