BACHES (BAsic eCHElle Spectrograph)

News: The ESO-Messenger issue of September 2007, includes an articule about BACHES in page 62. The complete issue of this ESO-Messenger can be downloaded in full as a PDF file here.

BACHES is our smallest echelle spectrograph; with a resolution of R=20000 and a total weight of 2Kg including the CCD camera (SBIG ST-1603ME) is probably the smallest echelle spectrograph ever built. The instrument has been designed to work with a focal F10 ratio (typical in amateur's telescopes) and it can be attached to the telescope using the standard 2" eyepiece holder or with an standard T-2 thread.

BACHES can be used with cameras with the KAF-1603ME CCD (1530x1020 square pixels at 9x9µ) like the SBIG ST-8XME, or preferebly its cheaper and lighter sister the SBIG ST-1603ME.

A web-cam (here a Philips ToUCam Pro-II PCVC-840K) with a 1.25" adapter, is used to visualize the field of view and the slit of ~100x25 microns.

In total 29 orders (from 32-61) can be imaged in the CCD, covering from 350nm till 780nm..

Typical exposure times were of 300s for stars of magitud 3 with the Celestron CGE 11" at Paranal (see picture below), however we believe there is still some place for improvement in the slit (~10%), reducing the comma distortion (~20%), better polar alignment (~5%), but in particular a wind shield to reduced the vibrations produced by medium-strong wind (~%40). Introducing automatic guiding in the slit is our next goal. Seeing was of 1.7 arcsec the best day.

Data reduction for BACHES is available with MIDAS and its package ECHELLE: Each picture is bias and dark current substracted and bad pixel eliminated. The image is wavelength calibrated with a thorium-argon lamp, extracted and rebined. MIDAS scripts are used to automatize the identification of the thorium-argon lines for wavelength calibration and for executing a kind of pipeline. Spectra is calibrated inmediately after exposure by running the MIDAS pipeline in the same machine where the data is generated.

The MIDAS procedures (including demo data) to calibrate and reduced BACHES spectra is available under the compressed-tar file baches-1_4.tar.gz. A Shockwave Flash demo of the BACHES calibration and data reduction can be downloaded here.

In fact only a laptop with WindowsXP is needed to control the webcam, the control of the CCD camera and the data reduction pipeline with MIDAS, which runs in the Linux emulator Cygwin/CygwinX.

The following table shows the dispersion and resolution obtained around 400, 500, 600 and 700nm:

	BACHES attached to a Celestron CGE 11" Casegrain telescope with a Losmandy Equatorial G-11 mount. Observation period: 25.03-31.03.2007 Site: Next to ESO Residencia in Paranal (Chile), Cerro Paranal with VLT in the background. Seeing: 1.7 arcsec. Strong wind at night.
	Close up to BACHES, SBIG ST-1603ME CCD camera and Philips ToUCam Pro-II. Total weigth including CCD camera, webcam and cables: 2Kg
	Spectrum of Aldebaran overimposed with the Thorium-Argon calibration lamp. First light with BACHES taken in Garching the 12.03.07, with AGAPE Celestron CGE1400 telescope and 60s exposure time.
	Spectra from 4 Be stars centered in H-alpha taken at BaseCamp Paranal the 31.03.07. delta-cen (Mag. 2.5) zeta-tau (Mag 3.5) kappa-cma(Mag.3.5) eta-cen (Mag 2.2) (Note: in the plot labels zeta-tau and kappa-cma are interchanged)
	Orders 45 to 55 (up-down) of tau-bootis, a star of spectral type F6IV with magnitud 4.5 and a know extra-solar planet 4 Jupiter mass and a period of 3.3 days.
	Orders 32 to 44 of tau-bootis
	Evolution of H-alpha in zeta-tau taken at Paranal in 4 different days.
	Evolution of H-alpha in zeta tau in 44 days. The structure of H_beta is roughly consistent with the one of H-alpha as far as the three local minima (or just inflection points) are concerned.
	Evolution of H-beta in zeta tau in 44 days. But the overall line emission is quite different between H-alpha and H-beta. Moreover, there are very strong differences between the 2 H-beta profiles while the variability in H-alpha is small.
	Spectrum from the thorium-argon calibration lamp with a 5s exposure. 1500 lines are detected and 80% are tipically identified (calibrated) with the MIDAS/ECHELLE calibration procedure. The resolution measured after calibration and rebin was of: 19600@660nm and 21600@560nm. thorium.fit raw spectrum of the lamp thorium_wr.fit extracted spectrum in orders thorium_wrm.fit merged mono-dimensional spectrum
	Plot of the resolving power R ((Lambda/DeltaLambda) for 1000 thorium lines along the complete spectrum. This plot is the product of executing the MIDAS procedure: Midas 001> @@ baches_resolving_power demo_thar_wrm.fit from BACHES data reduction package available under the compressed-tar file baches-1_4.tar.gz