IBIS is a new instrument for solar bidimensional spectroscopy, now under construction at the Arcetri Astrophysical Observatory. It essentially consists of two Fabry-Perot interferometers, piezo-scanned and capacity servo-controlled, which are used in classic mount and in axial-mode, in series with a narrow-band interference filter. This instrument will operate on a large field of view (80����) and on a large wavelength range (5800–8600 ˚A), with high spectral (λ/Δλ � 250000), spatial (�0.2) and temporal (� 5 frames s−1) resolution. When completed in

2002, it will be one of the leading instruments for solar research, well suited for a new generation telescopes such as THEMIS

Berrilli, F.

Cantarano, S.

Cavallini, F.

Egidi, A.

English

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IL NUOVO CIMENTO Vol. 25 C, N. 5-6 Settembre-Dicembre 2002IBIS (Interferometric BIdimensional Spectrometer)(∗)F. Cavallini(1), F. Berrilli(2), S. Cantarano(2) and A. Egidi(2)(1) Arcetri Astrophysical Observatory - Firenze, Italy(2) Departimento di Fisica, Universita` di Roma “Tor Vergata” - Roma, Italy(ricevuto il 10 Giugno 2002; approvato il 7 Agosto 2002)Summary. — IBIS is a new instrument for solar bidimensional spectroscopy, nowunder construction at the Arcetri Astrophysical Observatory. It essentially consistsof two Fabry-Perot interferometers, piezo-scanned and capacity servo-controlled,which are used in classic mount and in axial-mode, in series with a narrow-bandinterference filter. This instrument will operate on a large field of view (80′′) andon a large wavelength range (5800–8600 A˚), with high spectral (λ/∆λ  250000),spatial ( 0.2′′) and temporal ( 5 frames s−1) resolution. When completed in2002, it will be one of the leading instruments for solar research, well suited for anew generation telescopes such as THEMIS.PACS 96.60.-j – Solar physics.PACS 96.60.Ev – Solar instruments.PACS 95.55.Qf – Photometric, polarimetric, and spectroscopic instrumentation.PACS 01.30.Cc – Conference proceedings.1. – IntroductionAt the end of 1999 the construction of a new instrument for solar bidimensional spec-troscopy started in Arcetri, which has been called IBIS, an acronym for InterferometricBIdimensional Spectrometer. This instrument has been built with the contribution ofthe Arcetri Astrophysical Observatory, the Department of Astronomy and Space Scienceof the Florence University, and the Department of Physics of the Rome “Tor Vergata”University.When completed, at the end of 2002, IBIS will replace the Italian PanoramicMonochromator (IPM) [1], installed at THEMIS, the French-Italian solar telescope op-erative at Tenerife (Canary Islands).Notwithstanding their different name, the two instruments are very similar, at leastin principle, since both have been designed to take monochromatic images of the solar(∗) 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 673674 F. CAVALLINI ETC.surface. The question is: why a new instrument, so similar to another one, alreadyoperative and producing scientific results?2. – The ideal bidimensional spectrometerTo answer the previous question, let us consider firstly the more relevant instrumentalcharacteristics required to an ideal bidimensional spectrometer for solar physics.1) High spectral resolving power: λ/∆λ ≥ 200000, to analyse narrow photosphericlines.2) High wavelength stability: a maximum drift of the instrumental profile ≤ 10 m s−1on some hours, to ensure the repeatibility of the wavelength selection.3) Large useful spectral range: a useful spectral range of  1000 A˚ or more, to allowa large selection of spectral lines of high diagnostic value.4) Large field of view: a field of view of  1.5′ or more, to allow the study of activeregions, and the interaction between adjacent structures.5) High spatial resolution: a spatial resolution of  0.2′′, to study single, elementarystructures, with sizes of  100 km on the solar surface.6) High temporal resolution: an acquisition rate of several frames s−1, to study theevolution of the observed phenomena on short time scale.As already noted [2], IPM only partly satisfies these requirements. As a matter offact, it has a high spectral resolution and a high wavelength stability, but also is affectedby some limitations. So, the useful spectral range ( 200 A˚) and the field of view(5′′) are too small. Moreover, mainly due to the Universal Birefringent Filter (UBF),used as order sorter for the interferometer, the overall transparence is low, imposing anexposure time of about 200–300 ms. This implies that, while IPM has been designed tocompletely exploit the spatial resolution allowed by the telescope ( 0.2′′ for THEMIS),due to the long exposure time, the effective spatial resolution is lowered by the seeingeffects. Finally, due to the long exposure time and to the long wavelength setting timeof UBF ( 1 s), the acquisition rate cannot be higher than  0.7 frames s−1.Among the requirements detailed above, the high spatial resolution certainly is themost demanding, not often obtainable with ground-based observations.On the other hand, no space-based instrument of this kind yet exists, as it requires alarge telescope and a very large bandwidth to transmit the enormous data flux.Special considerations must therefore be given to these characteristics when planninga new instrument.There are two primary methods that are currently used in solar physics to correctthe seeing effects that limit the spatial resolution obtainable from the ground: adaptiveoptics, and the so-called phase diversity technique.As is well known, adaptive optics is a hardware technique that allows the correction, inreal time, of the incoming distorted wave-front. Tests, recently carried out at the NationalSolar Observatory (Sacramento Peak), have shown the possibility of successfully usingthis method on a small solar field of view ( 10′′×10′′), in order to reach the diffraction-limited resolution of the telescope.Phase diversity, instead, consists in the simultaneous acquisition of two images of thesame solar region, one in focus and another out of focus, by a known amount. The post-facto comparison of the two images allows the determination, and subsequent correction,IBIS (INTERFEROMETRIC BIDIMENSIONAL SPECTROMETER) 675of the atmospheric wave-front over the whole field of view. The phase diversity methodrequires very short exposure times (less than about 40 ms), in order to freeze the seeingconditions in each single exposure. In turn, this requires a very high throughput of thetelescope and instrument combination and, for this reason, recent results obtained withthis method are limited to data acquired with broad-band filters, devoid of any spectralinformation.Starting from these considerations, IBIS has been designed to allow very short expo-sure times (≤ 20 ms), so that the spatial resolution of 1 m class telescopes can be fullyexploited by using the phase diversity technique.In any case, if adaptive optics will become available in the future, IBIS can takeadvantage of it, using longer exposure times for a more accurate photometry, but stillallowing very short total acquisition times.3. – IBISThe instrumental layout has been already described in detail [2] and only some generalcharacteristics will be reported here.IBIS essentially is formed by two Fabry-Perot interferometers, piezo-scanned andcapacity servo-controlled (see table I), which are used in classic mount and in axial-mode, in series with a narrow-band interference filter (FWHM = 3 A˚).At the end of the principal optical path there are one or two CCD cameras (seetable II), depending on the use of the phase diversity technique. Near the entrance ofthe instrument, a beam-splitter picks off a small amount of light ( 0.25%), which thenpasses through a second set of broad-band interference filters (FWHM = 50 A˚), centeredat the same peak wavelength of the narrower ones. A suitable optical system then formsa “white light” image of the same solar region on a further CCD camera, identical to theother ones. Thanks to the use of only one electronic shutter, placed near a pupil image,the two CCD cameras simultaneously take a monochromatic and a “white light” imageof the same solar area.Additional secondary optical paths are provided to continuously monitor the selectedsolar area, to find the initial tuning conditions, to verify and to adjust the parallelism ofthe interferometer plates and their orthogonality to the optical axis.In table III the more relevant instrumental characteristics of IBIS are compared tothose of IPM. As may be seen, IBIS has a high spectral resolving power and a highwavelength stability, as IPM, but also a larger wavelength range (2800 A˚ vs. 200 A˚ ofIPM), a larger field of view (80′′ vs. 51′′ of IPM), a shorter exposure time (5–20 ms vs.200–300 ms of IPM) , and a higher acquisition rate (5 frames s−1 vs. 0.7 frames s−1 ofIPM).4. – The choice of the optical mounting4.1. A single interferometer . – As said before, the two interferometers of IBIS areused in classic mount and in axial mode, and this choice is relevant to assure the bestinstrumental performances. Let us discuss, therefore, in more detail advantages anddisadvantages of different optical mountings.As well known, a Fabry-Perot can be used for imaging in classic or in telecentricmount.In classic mount the interferometer is placed in a pupil space where the image iscollimated: each image point is therefore formed by a cone of rays, normally incident676 F. CAVALLINI ETC.Table I. – Fabry-Perot interferometers characteristics.Manufacturer Queensgate Instruments Ltd.Type Mod. ET50Clear aperture 50 mmPlate separation 2.300 mm, 0.637 mmWedge angle 20′Coating Multilayer broad-bandReflectivity 92–94%Wavelength range 5600–8600 A˚Defects of each plate λ/150 maximumat 6328 A˚ after coatingon the interferometer, containing all the directions allowed by the optics, and covering asmall area of the plates.In telecentric mount, instead, the interferometer is placed in an image space wherethe pupil is collimated: each image point is therefore formed by a beam of rays incidenton the interferometer at an angle less or equal to the maximum one allowed by the optics,and covering a large area of the plates.Each mounting shows advantages and disadvantages.Let us consider firstly only one perfect interferometer (an interferometer the plates ofwhich are perfectly parallel and without defects) used in axial mode.In classic mount, in this case, each interference order (hereafter assumed as the in-strumental profile) is generally narrower than in telecentric mount, but it shows a sys-tematical blue-shift when moving from the optical axis towards the edge of the field ofview.On the other hand, in telecentric mount the wavelength of the instrumental profileis the same on all the points of the image plane, which can be therefore considered asspectrally homogeneous, and this is the reason why this mount is often preferred forimaging.However, if we consider now a real interferometer, the effect of plate defects (generallymore relevant than the parallelism errors) is different on different mounts.In telecentric mount, due to the plate defects, the spacing on the small area coveredby each cone of rays generally differs from the mean interferometer spacing. As a con-Table II. – CCD and camera characteristics.Camera manufacturer Princeton Instruments Inc.Camera type PentamaxMaximum acquisition rate 5 MHzDynamic range 12 bitsCCD manufacturer KodakCCD type KAF-1400CCD format 1317× 1317 pixelsPixel size 6.8× 6.8 µmActive area 8.98× 7.04 mmFull well capacity 45 ke−Readout noise 20 e− at 5 MHzIBIS (INTERFEROMETRIC BIDIMENSIONAL SPECTROMETER) 677Table III. – Comparison between IPM and IBIS characteristics.IPM IBISWavelength range 4600–6800 A˚ 5800–8600 A˚Calibrated ranges 5184 A˚ (Mg b1), No calibration5380 A˚ (C I), is necessary5576 A˚ (Fe I),5890 A˚ (Na D2),6438 A˚ (Cd red line),6563 A˚ (Hα)Full Width at Half 17–26 mA˚ 21–42 mA˚Maximum (FWHM)Spectral resolving power 260000–270000 205000–320000(λ/∆λ)Wavelength drift ≤ 10 ms−1 on 10 h ≤ 10 ms−1 on 10 hField of view 51′′ 80′′(circular)Transparency 0.7–2.0% 10.0–16.2%(Hβ–Hα)Exposure time 200–300 ms ≤ 20 ms(S/N ≥ 100)Wavelength setting time 1 s  1.5 msAcquisition rate  0.7 frames s−1  5 frames s−1sequence, the wavelength position of the instrumental profile ramdomly changes whenmoving from a point to another point of the image plane, which therefore cannot beconsidered as strictly spectrally homogeneous.In classic mount all the collimated beams associated to the image points cover a largearea of the plates. Consequently, the defects are equally averaged, and they produce awidening of the instrumental function which is the same in all the points of the finalimage plane.In table IV is shown, as an example, the instrumental profile (λ = 7200 A˚, f-number= 110) of one of the interferometers used for IBIS (spacing = 2.300 mm, see table I),when used in classic and in telecentric mount.We may conclude that, for a single interferometer used in axial mode (as for IPM),the choice between classic and telecentric mount is only a compromise between spectralresolution and spectral homogeneity of the image plane.678 F. CAVALLINI ETC.Table IV. – Instrumental profile of a single Fabry-Perot (axial-mode).Classic TelecentricPeak transparency (%) 81.3 43.2FWHM (mA˚) 32.3 79.1Equivalent width (mA˚) 40.9 40.9Shift (mA˚) −74 ±134.2. A multi-interferometer . – The situation is completely different when two or moreFabry-Perot in series are used for imaging. Let us consider, for example, a double-interferometer in telecentric mount and in axial mode. In this case, the random wave-length fluctuations of the instrumental profile produced by the first Fabry-Perot randomlyadd to those produced by the second one. The result is that on each point of the finalimage plane the instrumental profile not only changes in wavelength, as for a single Fabry-Perot, but also in shape. In table V is shown, as an example, the instrumental profile(λ = 7200 A˚, f -number = 110) of IBIS, when used in classic and in telecentric mount.It may be seen that, as said before, the instrumental profile changes in wavelength, butalso in transparency, in width, and in equivalent width. Moreover, all the instrumentalprofiles are asymmetric, except those corresponding to equal shifts produced by bothinterferometers.We have to note that the strong fluctuations of the equivalent width (±34%) willproduce equally strong fluctuations of the radiation flux on the final image plane. So,a field uniformly lighted by a continuous source will be reproduced at the exit of theinstrument as a dishomogeneous field, with dark and light patterns. The flat field canremove this effect from images obtained on the continuum, but cannot obviously removethe effect produced by the shape fluctuations of the instrumental function from imagesobtained on a line profile.In classic mount, instead, the radial blue-shift only depends on the wavelength and onthe incidence angle, which is the same for each image point and for both interferometers,securing the tuning over all the field of view. The resulting instrumental profile, simplyobtained as a product between the single instrumental functions of the two interferome-ters, has therefore the same shape over all the final image plane.We may conclude that, while for a single interferometer both classic and telecentricmounts can be used, for two or more interferometers, telecentric mount so heavily affectsthe instrumental profile, that classic mount seems to be the only possible solution.Table V. – IBIS instrumental profile (axial mode).Classic TelecentricPeak transparency (%) 66.1 17.7–39.7FWHM (mA˚) 30.4 76.6–80.5Equivalent width (mA˚) 26.1 15.8–32.1Shift (mA˚) −74 ±28IBIS (INTERFEROMETRIC BIDIMENSIONAL SPECTROMETER) 6795. – The ghost imagesThe proposed solution of a double-interferometer used in classic mount and in axialmode does not consider a problem, typical of any multi-Fabry-Perot: the ghost imagesproduced by inter-reflections between different interferometers.This problem generally is solved by tilting one or more Fabry-Perot of a small amount,sufficient to allow ghost images to clear the field [3, 4]. However, this solution alsoproduces some serious, unwanted effects. In telecentric mount the tilt gives rise to astrong asymmetry of the instrumental function; in classic mount it produces differentinstrumental profiles in different points of the image plane.Moreover, both in classic and in telecentric mounts, the tilt also yelds an equal, strongloss of the overall transparence ( 46% for IBIS). In particular, while in telecentric mountthe tilt produces a dishomogeneous darkening of the pupil and a homogeneous darkeningof the image, in classic mount it produces opposite effects.In any case, the final conclusion is depressing: the result of these considerations, infact, is that, both in classic and in telecentric mounts, a multi-interferometer is alwayscharacterized by a bad instrumental profile and a heavy loss of transparency.This impasse has been solved for IBIS by using the two interferometers in classicmount and in axial mode, and solving the problem of ghost images by placing the inter-ference filter between the two Fabry-Perot. A filter with a so narrow passband, in fact,also has a small peak transparency of  30%, and reduces therefore the ghost imagesfrom an intensity of  10% to  1% of the principal one.In this way we obtain a double interferometer with a good instrumental profile andwithout any loss of light. The only remaining problem, that is the radial blue-shift ofthe instrumental function, can be easily solved by extending the spectral scanning of thesame amount towards the red.This solution, obviously, cannot be also used for three or more Fabry-Perot, and this isthe reason why we preferred for IBIS two interferometers and a narrow-band interferencefilter, which limits the useful wavelength range to  2 A˚, to more interferometers inseries, whith a wider interference filter [5].REFERENCES[1] Cavallini F., Astron. Astrophys. Suppl. Series, 128 (1998) 589.[2] Cavallini F., Berrilli F., Cantarano S. and Egidi A., Proceedings of the 1st Solar andSpace Weather Euroconference on The Solar Cycle and Terrestrial Climate, Santa Cruz deTenerife, ESA SP-463 December 2000, pp. 607-610.[3] Loughhead R. E., Bray R. J. and Brown N., Appl. Opt., 17 (1978) 415.[4] Kentischer T. J., Schmidt W., Sigwarth M. and v. Uexku¨ll M., Astron. Astrophys.,340 (1998) 569.[5] von der Lu¨he O. and Kentischer T. J., Astron. Astrophys. Suppl. Ser., 146 (2000) 499.

IBIS (Interferometric BIdimensional spectrometer)

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IBIS (Interferometric BIdimensional spectrometer)

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