IBIS: A New Instrument
for Solar 2-D
Spectroscopy
Gianna Cauzzi
K. Janssen, F. Cavallini & K. Reardon
INAF-Osservatorio Astrofisico di Arcetri
Firenze (Italy)
1. High Resolution Constraints
• Solar observations at high spatial and temporal
resolution are needed for understanding the nature
of convection, properties of solar magnetic fields,
coronal heating, etc.
• Observations of small scale structures require high
performances from both telescopes (large aperture,
seeing “correction”) AND focal plane instruments:
• Instrumental constraints include: - spectral resolution
>200,000; - spatial resolution <0.15” (100km on the
Sun); - temporal resolution < 10 sec (for any line
profile). Finally, an extended FOV is desirable.
• High throughput is hence a necessity.
2. Multiple Etalon Imaging System
• A system employing multiple etalons is in principle
capable of satisfying the above requirements:
• Etalons are highly transparent devices, affording
spectral selection with ease and rapidity of tuning. The
use of multiple etalons “in series” allows a very high
spectral purity.
• Moreover, etalons provide spectral information over
an extended FOV.
• The obvious drawback is the need to sequentially tune
the device to different wavelengths in order to
retrieve the spectral profile of any chosen line.
3. IBIS: Interferometric
BIdimensional Spectrometer
• IBIS is a double etalon system, developed at the INAFOsservatorio Astrofisico di Arcetri (OAA). The project is led
by Fabio Cavallini (OAA), with contributions from the Univ.
of Florence and Rome-Tor Vergata.
• The system employs two air-spaced Fabry-Perot
interferometers of high finesse (40) of 50 mm diameter. It is
optimized for the red part of the visible spectrum (5800-8600
Å). The etalons are mounted in classical configuration.
• IBIS has been installed at the DST/NSO since June 2003 (fed
by AO corrected beam). Figures and Tables in the following
provide details on instrumental characteristics.
Fig. 1 IBIS Layout. Red lines define the main optical path (thin line identifies white
light and monitor cameras), while gray and yellow define calibration paths utilizing,
respectively, a He-Ne laser (for aligning purposes) and a continuum lamp (for
wavelength calibration) as sources. The 50 mm diameter, air-spaced Fabry Perot's are
mounted in a collimated beam.
Table 1. IBIS “assigned” characteristics
• Optimized for 5800-8600 Å range
• Currently 5 ranges of operations (prefilters):
• 5896 - Na D1
• 6301 - FeI (magnetic + telluric lines)
• 7090 - FeI (g=0) +TiO
• 7224 - FeII (g=0) + TiO
• 8542 - CaII
• Prefilters width: 3 – 5 Å
• Spatial scale = 0.08”/pix
• FOV = 80” (diameter)
Table 2. IBIS “measured” characteristics
•
•
•
•
•
•
Transparency = 16%
Typical texp= 20–100 ms
Tuning time= 20 ms
Frame rate = 2.5–4 frames/s (CCD limited)
Spectral Resolution = 212,000-274,000 (20-40
mÅ passbands)
Drift of transmission profile = <10 m/s in 10 h
Fig. 2. Portion of the solar spectrum observed with IBIS (blue) compared with the
expected values (atlas convolved with instrumental profile, orange). The lines at 6302
and 6302.7 Å are of terrestrial origin.
Fig. 3. IBIS raw images of Sun center acquired on June 02, 2004.
Left: continuum. Right: blue wing of photospheric, non-magnetic FeI 7090.4 Å. FOV is about 80” in
diameter, exposure time was 25 ms. The continuum image clearly shows abnormal granulation in
areas where small dark structures are visible – these corresponds to areas of weak magnetic fields
(see next Figures). The large scale intensity gradient within the image on the right is due to a blueshift of the transmission profile moving from the center to the edge of the FOV. Max. blueshift is
about 70 mÅ (wavelength dependent). Small scale structures are obviously related to “cross-talk”
between intensity and velocity in the (inter)granular field.
Fig. 4. As in Fig. 3, for the CaII 8542 Å line.
Left: about -1.2 Å from line core. The intensity gradient due to the blueshift of transmission profile
(max = 90 mÅ) is not obvious, due to the large width of this spectral line. Inverted granulation is
clearly visible in the “quiet” portion of the image, while patches of enhanced brightness
correspond to areas of abnormal granulation and weak (network) magnetic fields.
Right: line core intensity map, showing chromospheric structures related to (low-lying) magnetic
loops connecting the network points with surrounding opposite polarities.
4. First Applications
• Structure and dynamics of low photosphere is
currently being addressed (see talk by Janssen et al.,
these proceedings).
• The use of CaII 8542 Å as a chromospheric diagnostic
at high resolution is being investigated. In particular,
we are studying a rather complete data set that
includes co-spatial, co-temporal observations of
“quiet” Sun in CaII 8542 (IBIS); CaII K (Halle filter,
tuned at K2v wavelengths); photospheric longitudinal
magnetic field (MDI high cadence), for about 1 hr.
• Next figures show first results.
Fig. 5. Background: images of CaII 8542 at about -1 Å from line core. Data were acquired on
June 02, 2004 at 15:23 UT. Spatial resolution of single images is about 0.3”.
Right: black contours outline bright patches, corresponding, at least for the larger
conglomerations, to “chromospheric network”.
Left: contours from MDI simultaneous magnetic map indicate fields of positive (red) and negative
(blue) polarity. Levels are at 50 and 150 G. MDI pixel size is 0.6”.
While correspondence is very good, not all the CaII brightenings correspond to a magnetic
structure. This could be due to a variety of factors, such as: - flux being below the MDI threshold
for a single map; -intensity brightness due to the equivalent of K2v bright points; -influence of
oscillations in CaII 8542.
Fig. 6. Scatterplot of CaII 8542 – 1 Å intensity vs magnetic values for the single maps displayed in
Fig. 5. The general trend is very similar to what is observed for CaII H and K, with a best fit given
by a power law of exponent ~ 0.6, and a saturation at about 400 Gauss. The scatter can probably
be reduced once temporal evolution is taken into account (the analysis of individual line profiles
might offer further insights). CaII 8542 could hence be used for “proxy magnetometry” within any
IBIS observational sequence.
5. Further Remarks
• IBIS is a highly performant and versatile instrument. Its
narrow passband, and high stability, make it
particularly suitable for spectroscopic investigations at
high spatial resolution.
• Instrumental characteristics have been precisely
determined from laboratory tests and observational
campaigns. This is particularly important for high
resolution observations. A set of programs for data
reduction and analysis is under construction.
• IBIS is available to the community via the normal TACreviewed proposal process of NSO.
• Future plans include tests with polarization analyzer,
and the upgrade to a faster camera.
Acknowledgements
• This research was partially supported by the European
Commission through the European Solar Magnetism
Network (contract HPRN-CT-2002-00313)