Astron. Nachr. 318 (1997) 5, 291-304

TRAFICOS - an Echelle Zeeman spectrograph

G . HILDEBRANDT, G . SCHOLZ and J . RENDTEL, Potsdam, Germany

Astrophysikalisches Institut Potsdam

M. WOCHE, Garching, Germany *

Max-Plan&-lnstitut fiir Astrophysik

H . LEHMANN, Tautenburg, Germany

Thiiringer Landessternwarte

Received 1997 April 22; accepted 1997 May 7

We describe the TRAnsportable FIbre Coupled kchelle Spectrograph (TRAFICOS) equipped with a Zeeman analyzer and manufactured mainly for the observation of stellar magnetic fields. The spectrograph, designed at the Astrophysikalisches Institut Potsdam and constructed at the Thiiringer Landessternwarte Tautenburg, is laid out in a quasi-Littrow configura- tion. The part attached to the Nasmyth focus of the 2 m telescope contains the telescope adapter for the fibre input and output of the star and calibration light, the cases for the flatfield and the wavelength calibration, and the Zeeman analyzer. The optical scheme and the reduction software are mentioned in some details and the first results of the determination of the magnetic field and radial velocity of several stars are given showing the performance of the device in comparison with existing data.

Key words: instrumentation: spectrographs - stars: magnetic fields - chemically peculiar

A A A subject classification: 032; 116; 122

1. Introduction

Magnetic fields are observed on stars with quite different temperatures, masses, and states of evolution. In stars with masses of about one solar mass and smaller and the so-called T-Tauri variable stars the magnetic fields are an important indicator for confirmation of the dynamo theory. in stars with higher masses, especially in stars belonging to the chemically peculiar stars, CP stars, the connection between the magnetic field and the distribution of the chemical elements on the stellar surface should provide decisive arguments for the diffusion theory. In the past our investigations of stellar magnetic fields were mainly concentrated on CP stars and based on observations carried out photographically at the coudk focus of the 2 m telescope in Tautenburg. In most CP stars the temporal variations of the magnetic field observed are periodical and most frequently interpreted by the rotation of the star possessing a magnetic field, which has a lack of symmetry with regard to the axis of rotation. The widespread evidence observed of one extremum more prominent than the other requires a field geometry which differs from that of a centred or decentred dipole even with arbitrary obliquity. In order to continue the investigations on magnetic fields with an up-to-date device a fibre-fed kchelle spectrograph, equipped with a Zeeman analyzer for measurement of the circularly polarized light, was designed and constructed and is at the moment installed at the Nasmyth focus of the reflector in Tautenburg, but it can also be used at the focus of other telescopes with f/8 to f/12. For the determination of magnetic fields the observations with the new device at the Nasmyth focus should be more suitable than the previous ones at the coudC focus. The reason is that the number of light reflexions in the telescope is only two (instead of five in the coudk system), so that, on the one hand, the speed at the Nasmyth focus should be higher than at the coudC and, on the other hand, the problems of the instrumental polarization are not so difficult to handle as in the path of rays of the coudk system. Furthermore, the achromatic quarter wave plate of the Zeeman analyzer allows the magnetic field from lines of a wide spectral region to be derived, especially so for spectral lines in the red domain.

*on leave at Heraklion, Crete, Greece

292 Astron. Nachr. 318 (1997) 5

In the first phase of observations with the Cchelle spectrograph we have compared magnetic field and radial velocity measurements, especially of CP stars, with existing data, and some results are included in this paper. Future investigations are also aimed at getting answers about the relation of the magnetic field with the often very inhomogeneously distributed chemical elements on the stellar surface and their temporal behaviour, and in addit.ion about the structure and distribution of the fields observed on the stellar surface.

2. Optical and mechanical design and performance

The design of the spectrograph was driven by the requirements of transportability, of being useable for normal kchelle and Zeeman Cchelle spectra and of being modular and of expandable design. The layout of the spectrograph is a design of 4 separate modules. These are the Cchelle spectrograph, telescope adapter, fibre coupling system and Zeeman analyzer. The principal drawings of the Cchelle spectrograph and the telescope adapter, including the Zeeman analyzer, are represented in Figs. 1 and 2.

Fig. 1. Pricipal drawing of the kchelle spectrograph: 1 - Schmidt mirror as collimator and camera, 2 - fibre from telescope, 3 - fibre holder, 4 - small mirror, 5 - straylight diaphragm, 6 - CCD camera, 7 - large mirror, 8 - corrector plate, 9 - cross-disperser prism, 10 - kchelle grating

2.1. Echelle spectrograph

After the discussions of some optical design solutions for the kchelle spectrograph we decided to build the instrument in quasi-Littrow configuration. The main reasons for this decision are the compactness of this configuration and the very limited financial frame. Schroeder and Hilliard (1980) described three cases of echelle spectrographs. Our spectrograph is their case C with y = 1.5'. First descriptions of our spectrograph are given by Hildebrandt et al. (1993) and Hildebrandt et al. (1994). Similar spectrographs are in operation e.g. at Crimean Observatory Kopylov and Stechenko (1965), a t Oke Ridge Liller (1970), at the 2.1 m Struve reflector at McDonald Observatory (McCarthy et.al. 1993) and at the 5 m Hale telescope (Libbrecht and Peri 1995).

In our design a Schmidt-camera 158/250/370 serves both as collimator and camera optics. The Schmidt-camera and the cross-disperser prism are used in double pass. The prism is mounted on a rotation stage to adjust the angle of minimal deflection for different spectral ranges of the two prisms used. One prism is optimized for the normal Cchelle spectra mode, the other one for the Zeemann Cchelle spectra mode (Table 1). The wave front distortion from the prisms is less than one-half wavelength in double pass. The kchelle is an R2 grating with a blaze angle of 61.5' and 25 I/mm. The ruled area is 150 mm x 350 mm. It is mounted on a swivel around the centre of the cross-disperser prism on a radius of 300 mm and rotable around a perpendicular axis on the swivel plane to adjust different spectral ranges at minimum deviation of the prism. The angle of incidence on the grating is adjustable

G. Hildebrandt et al.: TRAFICOS 293

with a micrometer readout, allowing accurate grating tilt. The light input in the spectrograph is realized with optical step index fibres. The spectrograph is working either with a single fibre or with double fibres for Zeeman spectroscopy. All fibres can be adjusted and focused on a linear stage. The Zeeman fibres are in addition revolving arranged to adjust the optimal order separation of both Zeeman components in the Cchelle spectrum at different wavelengths. The light leaving the fibres is reflected by two plane mirrors on axis to the f/3.4 Schmidt-camera and does not go through the flat field lens. A central stray light diaphragm at the spherical mirror obscures the direct light from the fibre. The diffracted light from the grating is inclined to the incoming beam by an angle of 2 O only by grating rotation on the swivel plane and is imaged by the Schmidt-camera and a flat field lens off axis in the image plane. Fibre input and the Cchelle spectrum are located in a sophisticated configuration near to each other.

The detector is a 800 x 1200 pxl chip in MPP version, pixel size 22.5 pm from Wright Instruments Ltd. The CCD camera is adjustable on a linear stage in a large gimble-mirror mount in two axes and two angles.

Fig. 2. The telescope adapter including the Zeeman analyzer: 1 - telescope flange, 2 - focus of the telescope (on the surface of 5), 3 - collimator, 4 - objective, 5 - reflective surface of fibre holder, 6 - fibre holder, 7 - calibration light (flatfield and thorium-argon lamps), 8 - turnable mirror for the calibration light, 9 - objective for guiding, 10 - prism for guiding, 11 - ocular for the CCD guiding camera, 12 - ray to the CCD guiding camera, 13 - turnable mirror for the Zeeman analyzer, 14 - superachromatic phase plate, 15 - Glan-Taylor polarisation prism, 16 - objective for the ordinary ray, 17 - reflective surface of the fibre holder, 18 - objective for the extraordinary ray, 19 - reflective surface of the fibre holder, 20 - fibre holder, 21 - fibre holder , A - part of the guiding system, B - to the guiding CCD camera, C - parallel ray of calibration light, D - fibres to the spectrograph, E - incoming starlight from the telescope

2.2. Telescope adapter

The telescope adapter is the interface of the fibre with the telescope. The essential part of the telescope adapter is a focal reducer designed for telescopes from f/8 to f/12. The collimator is an achromatic triplet system and the cameras are achromatic lenses of different focal length, which are easily exchangeable using a lens wheel. The effective focal ratio of the telescope can be changed in this way up to €/3 €or optimal adaptation to focal ratio

294 Astron. Nachr. 318 (1997) 5

degradation and to the core diameters of the fibres used. The parallel beam in the focal reducer is by the insertion mirrors controlled by the switch centre for other instrumental functions (calibration unit, Zeeman analyzer, field viewing telescope) and helpful for visual adjustments in the whole system. The telescope adapter serves in the normal spectra mode for the coupling of star and calibration light to the fibre.

The calibration unit is a lamp housing with a thorium-argon lamp for wavelength calibration and a halogen lamp for flat field calibration as well as a housing for the power supply. These units are removable and installed at the telescope near the telescope adapter and controlled from the observing room. The calibration lamps are separately imaged on a 300 pm step index fibre, transformed into a parallel beam by an achromatic lens and reflected on axis by an insertion mirror in the parallel beam of the focal reducer. The calibration system is connected only with a 2 m long easily removable fibre with the telescope adapter. This design simulates an adjustable artificial telescope pupil i n the light of the calibration lamps. By adjustment of the calibration pupil to the telescope pupil, an accurate calibration is possible.

The stars are guided on the fibre input in the telescope adapter or in the Zeeman analyzer. The imaged starlight not going through the fibres is reflected from the reflector of the fibre holder to lens/mirror/lens optics and imaged on the chip of a CCD-camera ST-4. The guiding is controlled on a Laptop from the observing room near the kchelle spectrograph. Visual guiding directly at the telescope adapter with a special viewing telescope is also possible. The same guiding system will be used for both the normal spectra mode and for the Zeeman spectra mode.

2.3. Fibre coupling

The telescope adapter and the Zeeman analyzer are connected to the Cchelle spectrograph via three 25 m long step index fibres TQS 100/145/530. These fibres have a low focal ratio degradation for input focal ratios from f/3

The fibre inputs are fixed at the telescope with specially designed fibre holders. The front end of the fibre holder, the reflector, is a polished metallic area inclined by 25' and has a laser drilled hole in the centre. The hole diameter is 5 pn larger than the cladding diameter of the fibre. The fibre is fed through the hole and fixed inside the reflector. The inclined area reflects the light not going through the fibre to the guiding system. The quarter inch fibre holders are fixed in special mountings and adjustable in three axes and in two perpendicular angles. At the spectrograph end we are using simple quarter inch fibre holders to fix the fibres in the mountings for adjustments. All fibres are protected with plastic pipes, the fibre input ends are specially protected by metallic tubes.

to f/4.

2.4. Zeernan analyzer

Our Zeeman analyzer is a new design of a circular analyzer. The decision for a fibre coupled spectrograph required the separation of the Zeeman components before the light is fed into the fibres. The analyzer is therefore designed as a separate module mountable on the telescope adapter. The few operating Zeeman analyzers around the world are working with calcite plates or Wollaston prisms in order to get the two separated Zeeman components nearly together on the same spectrographic slit. We decided to use a Glan-Taylor polarisation prism in a parallel beam to overcome the problems of light dispersion over a large spectral range in the Cchelle spectrum and to get an f /3 to f/5 telescope focal ratio for the two Zeeman components behind the Zeeman analyzer to feed the fibres for efficient transmission to the kchelle spectrograph. The complete spatial separation of the two Zeeman components in our design is no problem for the fibre coupled spectrograph, since the two Zeeman fibre outputs are our adjustable slit. Furthermore we are using a superachromatic phase plate specially designed for a large wavelength range as possible (300 nm to 1100 nrn).

The superachromatic phase plate generates two linearly polarized components separated by 90' which are in turn separated by the Glan-Taylor prism in two spatially separated components. These two Zeeman components with the Stokes parameters I+V and I-V are imaged with achromatic lenses onto two separated fibres.

The Cchelle spectra on the detector are clearly separated according to the linear separation of the fibre outputs at the Cchelle spectrograph in the cross-disperser direction. The Zeeman analyzer was originally designed for the Nasmyth focus of the Tautenburg 2m telescope, but can be used with slight modifications on other telescopes too. The instrumental polarisation caused by the plane diagonal mirror in the Tautenburg telescope can be compensated over all wavelengths by a suitably inclined plane compensation mirror (Lehmann 1993).

We are using the pick-up mirror in the parallel beam of the focal reducer to feed the Zeeman analyzer as the compensation mirror. The telescope adapter and the Zeeman analyzer move with the telescope's declination axis. The compensation adjustment is therefore independent of the declination, but dependent on the coatings of the diagonal telescope mirror and the compensation mirror. The compensation angle has to be changed over long time scales because of ageing effects in the coatings. The Zeeman analyzer is a separate module connectable to

295 G. Hildebrandt et al.: TRAFICOS

the telescope adapter in a short time. The adjustment of the fibres, calibration and guiding operate in the same manner as in the normal spectra mode. Both the Glan-Taylor polarisation prism and the superachromatic phase plate are produced by B.Halle, Nachflg., Berlin.

2.5.

The optical design of the spectrograph is a combination of parameters of detector, camera/collimator optics, dispersing elements and fibres. We decided to use an dchelle grating with the unusual line density of 25 I/mm, in order to get a coverage of the spectrum on the chip in the main dispersion direction of up to 800 nm without gaps. The use of a prism as the cross-disperser is so as to get a more uniform order-to-order separation in the echelle orders compared with grating cross-dispersers. After running numerical experiments for different glass materials and apex angles we decided to order two different prisms, one for the normal spectra mode and one optimized for the Zeeinan spectra mode (see Table 1).

Expected per formance a n d optical design parameters

Table 1. Elements of TRAFICOS

2.6.

Elements of the spectrograph Echelle spectrograph Collimator/Camera

Echelle grating

Cross-disperser Prism 1

Prism I1

Focal reducer

CCD camera

Spectral region

Number of orders

Fibre

Fibre length Resolving power Efficiency Dimension

S t ruc tu ra l overview

Specification quasi-Littrow configuration Schmidt-camera, size of the spherical mirror = 250 x 370 mm focal length = 543 mm, diameter of the correction plate = 158 mm camera is working in image mode with flat field lens 25 lines/mm, dimension = 150 x 350 mm, blaze angle = 61.5' (fabricated by ALKOR Technologies Inc., St. Petersburg, Russia) prism I (normal mode), prism I1 (Zeeman mode) dispersion angle = 32' 56' 10" f5" height = 170 mm, basis = 194 mm, length = 120 mm glass sort = F4 (code 617 366) dispersion angle = 39' 42' 20" f 5 " height = 181 mm, basis = 201 mm, length = 146 mm glas sort = SF 10 (code 728 284) to use for f/8 to f/12 telescopes, designed for 365 nm to 850 nm collimator = achromatic triplet, camera = achromate f/3 1200 x 800 pxl, size of a pixel = 22.5 pm, maximal quantum efficiency = 40 % a t SOOOA, 10 % at 4000A and 9000A (fabricated by Wright Instruments Ltd., Enfield, England) 4000A to 86008, three steps in normal mode five steps in Zeeman mode 54 in normal mode 33 in Zeeman mode 100 pm currently in use; TQS 100/145/530 multimode planned: Polymicro fibres with 50 pm and 70 pm 25 m for the 2 m telescope in Tautenburg for 100 p m fibre 25800 about 3 to 5 % measured at a seeing of 3 to 5" 1800 x 450 x 520 mm (without case)

All optical elements of the 6chelle spectrograph are mounted in self-made mountings and fixed on the optical rail system. The thermal critical part in the Schmidt-camera is temperature-compensated with invar rods. The rail system with the 6chelle spectrograph is mounted on a 750 mm x 1800 mm standard optical table with active vibration isolation.

The instrument is enclosed in a light tight box with removable windows for adjustments and prism exchange. The whole spectrograph is very stable. The telescope adapter is a stable spectrograph housing and connected over a simple rotator for instrumental polarisation compensation with the telescope flange. This configuration is stable enough for observations in the normal spectra mode. With the Zeeman analyzer mounted the telescope flange used at the Tautenburg 2 m telescope is at its weight limit. There is slight flexure for zenith distances of more than 40'.

296 Astron. Nachr. 318 (1997) 5

3. Laboratory results, characteristics of the spectrograph

The parameters and following statements are derived from observations made with fibres of the diameter of 100 pm corresponding to 3'.'4 in the sky. The use of fibres of such diameter is necessary to fit the seeing conditions typical for the Tautenburg site. The circular fibre slit of 100 pm projected along the main dispersion direction covered 4.4 pxl of our CCD camera chip. For the determination of the resolving power the FWHMs of a sample of 175 lines of the ThAr spectrum has been used. All lines show symmetric profiles, one example being given in Fig. 3 for the emission line k6182.6 A of the thorium-argon lamp. The lines distributed over the spectral range from 5000 A to 7000 A yield a resolving power of 25800 as shown in Fig. 4.

1.00

Fi 0.80

h 0.60

0.40

a 0.20

i i 0.00

E B c- Y

al

>

X

Image: Th-Ar-llne Row: + I

I ' I ' ' I ' " " A # I I , [ l l l ' I

I' 6182 6182 6183 6183 6184

Position (WAVELENGTH)

Fig. 3. Instrumental profile. Emission line X=6182.6 A of the thorium-argon lamp

4500 5000 5500 6000 6500 7000 Wavelength

Fig. 4. Resolving power as a function of wavelength

Regarding the different dispersions of the two prisms (prism I for obtaining normal spectra and prism I1 for the Zeeman spectra) the separation of the orders is dependent on the mode used. The size (1200 x 800 pxl) and sensitivity (4000 A to 9000 A) of the CCD chip currently used generally limits the most favourable spectral region from 5000 .& t o 7000 A. In the case of normal spectra, about 54 orders, and of Zeeman spectra, 33 orders are contained in one frame, so that to cover the whole spectral region one needs three exposures for the normal and five exposures for the Zeeman mode. The minimum separation between two adjacent orders for the normal spectra is 6 pxl. The frames of the Zeeman spectra are two interleaved spectra. The separation of the orders of the two channels can be accomplished by the adjustment of the revolving two-fibre holder. With the assumption of the above-mentioned observational conditions the efficiency of the spectrograph at the wavelength 5800 A can be estimated to be 3 to 5 % observing in the normal mode, or a little smaller than the

G. Hildebrandt et al.: TRAFICOS 297

half of these values using the Zeeman mode. For a seeing of l", a star of 7th magnitude, and an integration time of one hour, a ratio S /N of about 100 in the normal mode can be achieved.

To test the wavelength stability we compared the wavelength positions of ThAr lines received over a time interval of 3 hours. The analysis of the spectra shows a difference of the positions in the dispersion direction of 50.1 pxl (M 250 m/sec). For most observations the exposure time is essentially shorter than 3 hours, so that temporal stability is guaranteed.

The most important parameters of the spectrograph, namely the central wavelength, the free spectral range, the length of the orders, the angular dispersion, the plate factor, and the resolution are plotted in Fig. 5 as a function of the order number. The essential elements of the dchelle Zeeman spectrograph are summarized in Table 1.

'oooo m

80 100 120 140 160 180 200 Order number

80 100 120 140 160 180 200 Order number

5 80 100 120 140 160 180 200

Order number

80 100 120 140 160 180 200 80 100 120 140 160 180 200 80 100 120 140 160 180 200 Order number Order number Order number

Fig. 5. Parameters of the Cchelle Zeeman spectrograph as function of the order number

4. Reduction software

For the reduction of the spectra a specially adapted package of software has been developed taking into account the possibility of using the instrument in different modes. The individual software programs are based on the general MIDAS software package with its integrated Cchelle context. Moreover, some ideas and modified parts we took over from the software package of the Cchelle spectrograph FLASH of Heidelberg, Mandel and Stahl (1991).

The reduction is divided into several steps as a semiautomatic and interactive process. In the first step, the program prepares, for the further reduction of normal spectra, a set of raw spectra of ThAr, FF, and the object and for Zeeman spectra a set of ThArl, ThAr2, FF1, FF2, and the object (two interleaved spectra of both Zeeman channels). In Fig. 6 and Fig. 7 we show an example of the spectrum of ThAr and the flatfield lamp, both obtained in one channel of the Zeeman mode.

298 Astron. Nachr. 318 (1997) 5

. : .

Fig. 6. Frame of one Zeeman channel of the ThAr spectrum in the spectral region of 5300 8, - 7000 8,

Fig. 7. Frame of the flatfield lamp. The data are the same as in Fig. 6

In the second step the program calculates the positions of the orders and those of the background using the flatfield spectra. After that the determination of the wavelength calibration X(px1) can be accomplished. In Fig. 8 is shown a typical residual plot of a wavelength calibration carried out in the reduction of the stellar spectrum of c Vir. The accuracy of the calibration amounts to f 0.018 and corresponds to that of the present wavelength tables. To reduce the Zeeman spectra this procedure has to be carried out for both spectra, of course.

Subsequently will be separated the interleaved spectra of the object into two single spectra. Fig. 9 shows an example of the spectra of the magnetic C P star y Equ. The results are stored in MIDAS-Tables for further treatment, or a break of the MIDAS-session. In the next step the program produces a one-dimensional spectrum of the object and the flatfield using the subroutines of the standard MIDAS-Cchelle context, e.g. the background correction, and calculates the quotient of object/FF. Now the possibility exists to normalize the spectrum applying to it an interactive spline-fit and to correct the measurements for the motion of the Earth. Additionally, several complementary programs are available for further investigations.

5. Astronomical observations

To get experience with the spectrograph concerning its handling, but mainly in order to test the combination of telescope/spectrograph with regard to the optical adjustment and stability, the efficiency, the correctness of the programs, etc., some representative stars have been selected for the first observations. Radial velocities have been measured in spectra of the often studied binaries p Lyr and Q Dra and values of the longitudinal magnetic field and the radial velocity have been determined for the known magnetic CP stars 53 Cam, ,f? CrB, and y Equ.

G. Hildebrandt et al.: TRAFICOS 299

- c 3

0.01

0

-0.01

Fig. 8. Residual plot of the wavelength calibration of E Vir

Fig. 9. lnterleaved spectra of y Equ of the two Zeeman channels

5.1. Radial velocity

5.1.1. p Lyr

Radial velocity values of p Lyr obtained from the measurements of the Si I1 lines 6347 and 6371 A are presented in Table 2. In Fig. 10 is shown the radial-velocity curve of star 2 (B8 component) given in the paper by Harmanec et al. (1996), Fig. 1, together with the values of Table 2, marked by crosses. All phases are from the ephemeris derived by Harmanec & Scholz (1993). As one can see, the values of Table 2 are in an excellent agreement with the expected ones following from the binary motion.

5.1.2. Q Dra

The star a Dra is also a binary with well known orbital elements, Lehmann & Scholz (1993). For the determination of the radial velocities in each spectrogram about 15 metallic lines have been used lying in the spectral region between H a and HP. The mean values for each spectrum are given in Table 3.

In order to carry out the best possible comparison of the radial velocity values of Table 3 with the existing ones we calculated once more the orbital elements of a Dra, after adding to our old data 20 recently measured values from photographic spectra. In Fig. 11 the calculated radial-velocity curve is drawn, the Cchelle values are again marked by crosses. One can see that, as in the case of P Lyr, the radial velocities of Q Dra derived from Cchelle spectra are in very good agreement with the old values. A more detailed description of the binary will be given elsewhere.

300 Astron. Nachr. 318 (1997) 5

Fig. 10. Mean radial velocities of star 2 of p Lyr vs. phase. The filled circles are measurements from red Ondiejov and DAO electronic spectra, the crosses are the values of Table. 2. The pluses around zero are the 0 -C deviations from the orbital solution.

Fig. 11. Radial velocities of a Dra VS. phase. The period is 51.4162 days, crosses correspond to the values of Table 3.

Table 2. Radial velocities of star 1 and 2 of fi Lyr Table 3. Radial velocities of a Dra HJD

50267.4612 50267.4848 50267.5160 50269.4327 50269.4695 50271.4723 50297.4693 50298.42 14 50299.4707 50300.4894 50301.4262

-2400000 phase

0.682 0.684 0.687 0.835 0.838 0.993 0.000 0.074 0.155 0.234 0.306

RV (km/s) RV (km/s) Si I1 (star 1)

+154.2 +0.5 +154.9 $0.5 +157.5 +0.5 +147.6 $1.9 $146.0

Si I1 (star 2)

-16.8 -30.5

-118.0 -171.8 -205.2 +35.4 -190.9 +42.1

Phase RV -2400000 ( W s ) 49918.3455 0.368 -43.07 f 0.45 49918.3573 0.368 -42.27 f 0.62 49919.3854 0.388 -43.94 f 0.48 50151.5783 0.904 $33.63 f 1.06 50465.7229 0.014 +46.15 f 0.43 50466.7098 0.033 $33.60 f 1.20 50470.7425 0.112 -6.68 f 0.73 50528.6094 0.237 -38.43 If 0.43

G. Hildebrandt e t al.: TRAFICOS 301

5.2. Magnetic field

5.2.1. 53 Cam

The effective magnetic field of 53 Cam varies with a period of a little more than 8 days. But the magnetic curves are quite different in their shape depending on whether photographic field measurements or observations obtained with a photoelectric Balmer-line magnetograph are used. In the latter case the observations do not show the strong anharmonic behaviour in the magnetic curve as found by photographic measurements. The discordance between the two types of magnetic curve arises probably from the individual measuring methods. In the photographic spectra preferentially the line cores and in the photoelectric observations the line wings of the right and left circularly polarized profiles are used for the measurements and the determination of the magnetic field. In the Cchelle spectra, the entire line profile of the spectral lines is included in the determination of the magnetic field. Therefore, it is reasonable to compare our results with the data found photoelectrically by Borra & Landstreet (1977). An example of a Zeeman split line is shown in Fig. 12, the different intensities in the right and left polarized spectrum are produced by not ideally illuminated fibres. In Fig. 13 the magnetic field values of Table 4 together with the values of Borra & Landstreet are presented, adopting the same ephemeris for both data sets. Even though the quality of our observations, except for one value is insufficient, indicated by the large inaccuracy, our values match quite well with those of Borra & Landstreet. Furthermore, since our measurements are based exclusively on metallic lines the good agreement with the results obtained with the hydrogen-line magnetometer shows that strong contributions from elemental patches have obviously a small influence on the measuring process.

The RVs given in the Table 4 have an amplitude as expected from the binary motion including a period of 6.66 years.

Table 4. Effective magnetic field strengths and radial velocities of 53 Cam

HJD vhase B.rr phase RV ~~

-2400000 (of B,B) (gauss) (of RV) (km/s) 50121.4626' 0.295 0.238 -7.22 f 0.21

~~

50151.4769' 0.034 -1420 0.238 -4.7 50466.5207 0.283 +4360 f 260 0.367 -6.16 f 0.44 50470.5540 0.786 -4470 f 580 0.369 -7.64 f 0.67 50494.426 1 0.760 -5090 f 610 0.379 -6.13 f 0.79 50497.4406 0.136 +1580 f 360 0.379 -7.86 f 1.21

normal spectrum only three lines measurable

Si11(8371.350)

0300.0 6308.0 0370.0 0372.0 0374.0 ' ' ' ' ' I I ' ' I I ' ' I ' ' ' '

Position (WAVELENGTH)

0.40

Fig. 12. The oppositely polarized spectra around the Si I1 line 6371.359 of 53 Cam

22 Astron. Nachr. 318 (1997) 5

302 Astron. Nachr. 318 (1997) 5

0 . 0 . 4000

1 0 0

X . X . 2000

0 0 0 0

-2000 0 . 0 .

n c3 Y

.c c P) m -4000i,, , . , . , , , ,$ , , , , ,,a$, , , , ,I

- 6000 -0.5 0.0 0.5 1.0 1.5 2.0 2.5

Phase

Fig. 13. Befi observations of 53 Cam. Filled circles correspond to the photoelectric observations of Borra & Landstreet, crosses correspond to the values of Table 5. The period is 8.0267 days

5.2.2. ,f3 CrB Concerning the magnetic data of the star ,f3 CrB the same remarks about photographic and photoelectric obser- vations can be made as for 53 Cam. In order to compare our measurements with those given in the literature we represent in Fig. 14 the photoelectric observations of Borra & Landstreet (1980) together with our two measure- ments. A sufficient agreement can be noted.

However, considering the RVs, a distinct difference occurs between the three days. The last two RVs correspond nearly exactly to the value following from the binary elements of Kamper et al. (1990), whereas the two RVs obtained one day earlier deviate remarkably. But, in Kamper’s et al. collection of RVs, some similar short-time variations of the RV exist, so that we have no reason to doubt our finding. The phases indicated correspond to a period of 10 years.

Table 5. Effective magnetic field strengths and radial velocities of p CrB

j EI J phase RV -2400000 (of &a) (gauss) (of RV) (km/s) 50268.3723 0.520 -12.05 & 0.18 50268.4299 0.228 +510 f 150 0.520 -12.26 f 0.32 50269.3549 0.520 -18.37 f 0.24 50271.3846 0.388 $500 f 100 0.520 -18.25 zk 0.25

- Y

c ).

0

Q m

1000

-500

4

- 1000 -0.5 0.0 0.5 1.0 1.5 2.0 2.5

Phase

Fig. 1.1. B,fi observations of /3 CrB. Filled circles correspond to the photoelectric observations of Borra & Landstreet, crosses correspond to the values of Table 5. The period is 18.487 days.

5.2.3. y Equ The results of a detailed investigation about the behaviour of the magnetic field and radial velocity of the long- period magnetic C P star y Equ, based also on observations with the dchelle spectrograph, have been summarized in an paper prepared for Astron. Astrophys., Scholz et al. (1997).

G. Hildebrandt et al.: TRAFICOS 303

6. Conclusions, future improvements of the spectrograph

The first observations show the suitability of the device for investigations of stellar magnetic fields, radial velocities, and measurements of detailed line profiles. In principle the modular construction of the spectrograph offers the possibility t o transfer i t , especially the Zeeman module, t o other observing sites with essentially better weather conditions, as well as t o add t o the apparatus other modules, e.g. for the measururement of the linear polarization. As we have remarked, at those rare observation times with a seeing of about 2” and after the results of some estimations about possible instrumental improvements (see below, points with a dash) an increase of the efficiency of the spectrograph of a factor three to four can be expected. But , because we must accept the moderate weather conditions occuring in Tautenburg, the modifications to increase the efficiency of the equipment have not yet been realized within the project, owing to the limited financial budget. In detail we mention: - replacement of the CCD camera by another one with an essentially higher quantum efficiency - increase of the reflective power of the mirrors inside the spectrograph - antireflective coating of the surfaces of the prisms - use of an 6chelle grating with higher reflectivity - shortening of the 25-m long fibres used (this point naturally has some restrictions in Tautenburg).

Acknowledgements. The authors want t o thank t o all the persons who have contributed to this work. We thank posthumanly especially the former director of the Thuringer Landessternwarte, Prof. S. Marx, for his unbureau- cratic help and support. The authors express thanks to Prof. J. Solf, the current director of the observatory, for his support during the continuation of the project and the allocation of observing t ime on the Nasmyth focus of the 2 m telescope. Many thanks go also to the mechanical and electronical staff of the Landessternwarte, H. Lochel, M. Pluto, and A. Kirchhof, for practical help and for many ideas during the whole phase of the construction. We are grateful t o U. Laux for optical calculations concerning the spectrograph. The realization of the TRAFICOS spectrograph could only t.ake place with the financial support of the Deutschen Forschungsgemeinschaft (DFG), code I1 C 9 Hi523/1-1, for which we say also many thanks. Finally thanks are due t o the referee Prof. J . B. Hearnshaw for his helpful comments and the careful proofreading of the manuscript.

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Addresses of the authors: G. Hildebrandt, G. Scholz, J. Rendtel M. Woche H. Lehmann Astrophysikalisches Institut Potsdam MPI fiir extraterrestrische Physik Thiiringer Landessternwarte Telegrafenberg, A27 GieDenbachstr . Sternwarte 5 D-14473 Potsdam D-85478 Garching b. Munchen D-07778 Tautenburg Germany and Germany e-mail: gscholzOaip.de Foundation for Research and Technology - Telnet: IehmOtls-tautenburg.de

Hellas GR- 71 1 10- Heraklion, Crete Greece Telne t : wocheOskinakas . phy si cs .uch. gr

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