In this paper we consider the possibility of modulationless measuring the magnetic-field strength and the differential line-of-sight velocity, based on

using a diffraction spectrograph and multichannel CCD photodetectors. The optical system for implementing the method is described. Results of trial observations

are presented. When measuring the differential velocity, the spectrograph’s inherent

noise is reduced by factors of 35–40. It becomes possible to measure the longitudinal

magnetic-field strength in a single exposure even if the CCD array is used. This can

be particularly attractive when reducing the influence of atmospheric instabilities

on the result of magnetic-field measurement using CCD arrays

Kobanov, N.I.

Makarchik, D.V.

English

Scientific Open-access Literature Archive and Repository

IL NUOVO CIMENTO Vol. 25 C, N. 5-6 Settembre-Dicembre 2002Developing modulationless measuring of magnetic fields anddifferential velocities at Sayan observatory(∗)N. I. Kobanov and D. V. MakarchikInstitute of Solar-Terrestrial Physics - P.O. Box 4026, Irkutsk, Russia(ricevuto il 10 Giugno 2002; approvato il 24 Settembre 2002)Summary. — In this paper we consider the possibility of modulationless measur-ing the magnetic-field strength and the differential line-of-sight velocity, based onusing a diffraction spectrograph and multichannel CCD photodetectors. The opti-cal system for implementing the method is described. Results of trial observationsare presented. When measuring the differential velocity, the spectrograph’s inherentnoise is reduced by factors of 35–40. It becomes possible to measure the longitudinalmagnetic-field strength in a single exposure even if the CCD array is used. This canbe particularly attractive when reducing the influence of atmospheric instabilitieson the result of magnetic-field measurement using CCD arrays.PACS 95.85.Sz – Gravitational radiation, magnetic fields, and other observations.PACS 01.30.Cc – Conference proceedings.1. – IntroductionAt the same time, in relation to the observer, the object under investigation canparticipate in a variety of other motions such as large-scale flows, solar rotation, diurnalrotation of the Earth, and its ecliptic motion. Thus the absolute value of the line-of-sightvelocity of the object under investigation is made up of many components which, what ismore, are time variable. These problems are grossly dramatized when wave motions areinvestigated. The element observed can be affected simultaneously by tens and hundredsof oscillatory modes with different periods, propagation directions and spatial scales.Based on the resulting noise-like mixture obtained by conventional methods, it is difficultand sometimes impossible to understand the character of motions of the object underinvestigation. The differential method makes it possible to extract from this mixture thewave motions with the directions and spatial scales specified. By its nature, this methodis best suited to the investigation of wave motions since the observation of the wave(∗) Paper presented at the International Meeting on THEMIS and the New Frontiers of SolarAtmosphere Dynamics, Rome, Italy, March 19-21, 2001.c© Societa` Italiana di Fisica 695696 N. I. KOBANOV and D. V. MAKARCHIKa bCCDSλ /4λ /4Dλ∆LI III IICCDSλ/4Dλ∆0 0Fig. 1. – Simple optical layout: a) for measurements of differential velocity; b) for measurementsof longitudinal magnetic field.itself intrinsically implies a comparison of the motion at several (two at least) points ofspace [1, 2].The differential method can be used most efficiently in the case of a sharply defineddirectedness of the wave motions along, for example, the longitudinal axis in a prominencefilament or in the case of a radial symmetry of the motions in a sunspot. Furthermore,the spectrograph instrumental noise is totally suppressed, and the influence of large-scaleatmospheric irregularities is reduced. It should be noted that in differential measurementsthe spectral line asymmetry effect on the signal is reduced. The total sensitive gain ofvelocity measurements is by factors from 10 to 100, depending on the contribution tothe signal from the diffraction spectrograph noise which is effectively suppressed by thismethod.Fig. 2. – Spectral components position as the response to magnetic field or differential velocity.DEVELOPING MODULATIONLESS MEASURING OF MAGNETIC FIELDS ETC. 6972. – Method and instrumentThis paper gives an account of a new method for measuring the differential line-of-sight velocity and the longitudinal magnetic-field strength in the modulation-free modeusing CCD devices. Figure 1 presents a simplified optical design for performing suchmeasurements.Orthogonal polarized beams will pass from elements I and II through the spectrographentrance slit, thanks to polarization prism L. Polarization prism D in fig. 1 is used toaccomplish a displacement of these beams in inverse direction along the spectrographdispersion to a distance equivalent to the half-width (the width at the half-intensitylevel) of the working spectral line. In our case the directions of polarization of the beamsmake up 45◦ with the direction of the grooves of the diffraction grating.In their simplest version, polarization prisms L and D represent calcite plates. Intheir more sophisticated version, these plates consist of the combination of two elementspermitting the linear separation of the polarized beams to be varied over the requiredrange. When investigating the propagation direction of waves, it is frequently necessaryto change the direction of image splitting, which leads to a change in the position ofthe plane of polarization. In order for the polarization of the beams to be matched tothe angular position of prism D, irrespective of the angular position of prism L, twoachromatic quarter-wave phase plates are added to the optical system. One of themis oriented with its axes at 45◦ to the polarization direction of the beams within theprism, and is rigidly attached to prism L. This quarter-wave plate converts the linearpolarization of the beams to the circular polarization where the mutual orientation ofpolarization prism L and deflector D may be arbitrary. The axis of the second quarter-wave plate, that is placed behind the spectrograph entrance slit, is oriented parallel tothe dispersion direction of the spectrograph. At the spectrograph output, in this case,two identical images of the spectrum are produced, which are shifted with respect to oneanother along the dispersion by a small amount ∆λ0 which is determined by prism D.Spectral line images will be superimposed in the plane of the light-sensitive surface of theCCD array. By separating the spectral components by the line half-width, it becomespossible to use in measurements the steepest and linear portions of the line profile, which,on the one hand, ensures a maximum sensitivity to changes in the distance between thecomponents, and, on the other, an independence from the signal from common shifts ofthe components within the linear part of the profile.It is remarkable that if all polarization optics placed in front of the spectrographentrance slit is removed from the system (fig. 1b), then we get an instrument suitable formodulation-free measurements of the longitudinal component (projection onto the lineof sight) of the magnetic-field strength H‖.The principle of V and H measurements is explained in fig. 2, showing the positionof the spectral components relative to the center of the conventional “electronic slits” ofthe CCD detector. Identical groups of pixels produce “electronic slits”, in each of whichmeasurements are made of the integral light flux from a portion of the line wing “cutout” by a corresponding “electronic slit”. It is evident from fig. 2 that, depending on thesign of the measured magnetic field or on the differential line-of-sight velocity, the initialdistance between the spectral components ∆λ0 either decreases or increases thus causingcorresponding changes of the light flux intensity in the “electronic slits”. Furthermore,an increase in intensity in the central “slit” (between the components) is accompaniedby its decrease in the lateral ”slits” (wings), and vice versa. Besides, an additional,“electronic slit” is placed for recording the intensity of the continuous spectrum. This698 N. I. KOBANOV and D. V. MAKARCHIKFig. 3. – The interface window (Russian version) that shows a part of real spectrum near Ni I,4857.makes it possible to perform a calculation of signals simultaneously by several formulas,and to compare results. A comparison of the integral fluxes at the center and in theblue and red wings can be accomplished with a very high accuracy. With a 12-digitanalog-to-digital converter, for example, the sensitivity of measurements reaches 10−4 ofthe measured light flux. The values of the longitudinal magnetic-field strength and ofthe differential and line-of-sight velocity are calculated by simplified formulas:H‖ =Φrbl − (Φbl +Φrl )Φrbl + (Φbl +Φrl )kH ,(1a)Vd =Φrbl − (Φbl +Φrl )Φrbl + (Φbl +Φrl )kV ,(1b)V =Φbl − ΦrlΦbl +ΦrlkV ,(1c)where Φrl , Φbl , and Φrbl are the light fluxes integral by the respective electronic slits;and kH and kV are the calibration coefficients.For comparison purposes, use was also made of the formulaH‖(Vd) =(ΦrblΦc− b0)kH(kV),(2)where Φc is the intensity of the continuous spectrum, and b0 is the coefficient of zerosetting of the scale which is determined experimentally or calculated.It should be noted that by using “electronic slits”, it is possible to easily simulate theoperating mode of the Doppler compensator [3]. In this mode, the “electronic slits” thatare placed in the blue and red wings, are shifted in such a way as to maintain the zeroDEVELOPING MODULATIONLESS MEASURING OF MAGNETIC FIELDS ETC. 699Fig. 4. – Time series of the line-of-sight velocity in disk center (Fe I, λ = 525 nm): a) Dopplercompensator mode; b), c) differential method.value of the difference signal. Results of measurements in this case are independent of theprofile slope, and the calibration procedure is maximally simplified. The sensitivity toline-of-sight velocity changes in the last case is slightly reduced since the “electronic slit”changes its position, and different sets of elements always participate in the measurement.Internal errors are added as a consequence of the different sensitivity of pixels.Interestingly, the proposed optical design (fig. 1) is also suitable for measuring H‖in the mode of a so-called λ-meter [4]. For this purpose, it will suffice (using two pairsof “electronic slits”) to keep track of the position of the center of gravity of each ofthe spectral components. The difference of the measured shifts is proportional to themagnetic-field strength, and the semi-sum is proportional to the line-of-sight velocity.The initial splitting of the spectrum, that is ensured by deflector D, can in this case beincreased for the sake of convenience. In doing so, it should be remembered that theimmediate vicinities of the spectral line must not contain any fine-structured features(which is especially hard to do in a sunspot). Otherwise one will have to increase thewidth of the “electronic slits”, which will result in a certain decrease in the sensitivity ofmeasurements as the process of measurements will involve portions of the line profile witha smaller slope. The result of measurements with such a method is not affected by changesin the profile brightness and slope from pixel to pixel, which is an obvious advantage.The calibration procedure for different spectral lines is substantially simplified since inthis case, it is sufficient to know the Lande factor of a given line. In the IR spectralrange, such measurements are easier to realize when compared with the visible.The interface that has been developed in the form of dialog windows eases also theon-line control process of observations, and calculations in the on-line mode leads to asubstantial reduction of the memory used on a hard disk to record results of measure-ments.To realize observations with the line-array (firm Toshiba, TCD 1301 D, array length29 mm: 3700 pixels, pixel height 200µm, and width 8µm), a program was developedto operate in the Delphi programming environment for the Windows 95/98 operational700 N. I. KOBANOV and D. V. MAKARCHIKFig. 5. – Telluric line (λ = 687.4 nm): a) line-of-sight velocity measured by Doppler compensatormode; b) differential velocity measured by differential method; c) the same on b (other scale).system. All the control parameters for the operation with the array described beloware specified in the program interface, which simplifies the conduct of observations bythe user, and makes it possible to promptly change the observing modes. The dataaccumulation time and the write-in file name for calculated signals are specified in oneof the windows. A “live” spectrum is displayed in the other window, which is renewed30 times per second; the position and width of the “electronic slits” are also displayedthere. Figure 3 shows this program interface window where the display shows the workingportion of the spectrum near the line of Ni 485.7 nm (in the area of overlapping of thespectral components), and the arrangement of three “electronic slits” in the line: centralin the core, and lateral (“red” and “blue”), respectively, in the red and blue wings ofthe line. Amongst several modes of viewing the “working” spectrum, there is a zoomingcapability, which improves the accuracy of accommodation of the “electronic slits”. Inthe other dialog window, it is possible to determine the formulas, by which the signal isto be calculated in the “on-line” mode, and results of a calculation rather than raw datawill be written in to the file.Fig. 6. – A slice through the spot of active area NOAA 9169 (21.09.2000), in a line λ = 5250, 02nm: a) magnetic-field strength; b) line-of-sight velocity; signals calculated with formula (2)—thick and (1)—thin.DEVELOPING MODULATIONLESS MEASURING OF MAGNETIC FIELDS ETC. 7013. – ObservationsThe first observations according to the technique described above were performed inSeptember-October 1999 by using a linear CCD array on a horizontal solar telescope ofthe Sayan observatory and were continued in 2000 (with a CCD matrix as well). Theseobservations were of testing character and were aimed at studying the main character-istics and features of this method. Observations by using the conventional technique(imitation of the Doppler compensator mode) were performed with the same linear CCDarray for comparison. In observations performed in 2000, an RTE/CCD-256-H (Prince-ton Instruments) CCD matrix was used. The cooler of the matrix operating on thePeltier effect allows for the use of a wide range of operating temperatures. The linearsize of the photosensitive window is 0.5 × 2 cm (256 × 1024 square pixels), one pixelis  20µm. The Fe I line (λ = 525.02 nm) was observed in spectra measured withboth a linear CCD array and a CCD matrix. This is a purely photospheric line with anequivalent width of 12 pm and the Lande factor equal to 3. Figures 4b, c present timeseries of the differential line-of-sight velocity measured at the center of the disk in theundisturbed photosphere in a line of 525.02 nm (obtained with a guide and tracking).The noises in b), c) are real signals of velocity caused by atmospheric motions (about3–4′). Oscillations with a 5-min period (3 mHz), which are characteristic of the solarphotosphere, are clearly pronounced in the record. A similar time series obtained withthe same CCD array in the mode of Doppler compensator imitation is shown in fig. 4a.A higher noise level in the record of fig. 4a as compared to that in fig. 4b,c is obvious.This is due to the effect of the internal instabilities of an astronomic grating spectrograph(thermal inhomogeneities of the air mass filling the spectrograph’s volume, local pressurevariations, residual strains of movable mechanical parts, etc.).In order to determine more precisely the gain achieved upon noise suppression inthe differential method, a similar observation was performed in the oxygen line of theEarth atmosphere (λ = 687.4 nm). As is known, telluric lines can be employed asnatural references in measurements of Doppler velocities with an accuracy of up to 5–10 m/s [5, 6]. Figure 5a shows the measurements results of the spectrograph’s intrinsicnoise recorded by a linear CCD array in the mode of a Doppler compensator following upthe position of a telluric line. As we see, the instability amplitudes reach 200–300 m/s ata recording time constant of 10 s over time intervals of tens to hundreds of seconds andmay substantially distort the velocities measured. The data in fig. 5 allow us to estimatethe degree of spectrograph noise suppression in measurements by the differential method:the contribution of the spectrograph noise decreases by a factor of 30–40. This meanstheoretically that the sensitivity in differential measurements with a CCD array mayexceed the sensitivity in conventional measurements of the Doppler velocity by the samefactor.Figure 6 shows the results of processing a frame obtained by using the CCD ma-trix. Figure 6b is the line-of-sight velocity distribution calculated with two formulas:thick curve is the ratio of the difference of the lateral components to the continuum (for-mula (2)), and thin curve is the ratio of the difference of the side components to theirsum (formula (1)). The difference between curves in fig. 6b is most likely determinedby the fact that, in the second case (thin line), the calculated signal is more exposed tothe influence of line profile changes. Figure 6a shows the distribution of the longitudinalmagnetic-field strength along the slit. The sensitivity of measurements H‖ is roughlyestimated at no worse than 10–20 Gauss. A detailed investigation of the errors inherentin this method will be made elsewhere.702 N. I. KOBANOV and D. V. MAKARCHIK4. – ConclusionsThe chief merit of the method considered here is that the measured quantities (Vd, H‖or H‖ simultaneously with V ) can be obtained in no longer than one exposure with theCCD array. But this means in particular that the method can be highly attractive in in-vestigating the fine structure of the magnetic field or of the velocity field. In observationswith the CCD matrix, it is possible to reduce the exposure time to 1–5 ms, with suchvalues of exposure, the atmosphere looks like “frozen”, and does not introduce any ad-ditional errors into measurements. An improvement on the sensitivity of measurementsis favored by a significant decrease of the contribution to the signal from the spectro-graph internal noise which are inherent in this method. Note that in investigating waveprocesses, a highly valuable property of the method is the possibility of implementing itusing spatial filtering in the direction and wavelength.∗ ∗ ∗The work is supported in part by the Russian Foundation for Fundamental Re-search, grant No. 00-15-96659 and Federal program “Astronomy”. We thank Mr. V.G. Mikhalkovsky for his assistance in preparing English version of the manuscriptand V. V. Zhigalov for his assistance in first work phase.REFERENCES[1] Kobanov N. I., Solar Phys., 82 (1983) 237.[2] Kobanov N. I., Astron. Astrophys., 143 (1985) 99.[3] Kobanov N. I. and Makarchik D. V., Instrum. Exp. Tech., 44 (2001) 530.[4] Semel M., Astron. Astrophys., 5 (1970) 330.[5] Balthasar H. and Thietle U. and Wohl H., Astron. Astrophys., 114 (1982) 357.[6] Kuhn J., Astrophys. J., 264 (1983) 689.

Development modulationless measuring magnetic fields and differential velocities at Sayan observatory

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Development modulationless measuring magnetic fields and differential velocities at Sayan observatory

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