ADAHELI (ADvanced Astronomy for HELIophysics) is a small-class (500 kg) low-budget (50 MEuro) satellite mission for the study of the solar photosphere and the chromosphere and for monitoring solar flare emission. ADAHELI’s design has completed its Phase-A feasibility study in December 2008, in the framework of ASI’s (Agenzia Spaziale Italiana) 2007 “Small Missions” Program (calling for two missions at 50 MEeuros each, plus the launch budget).
ADAHELI’s main purpose is to explore Sun’s lower atmosphere in the near-infrared, a region so far unexplored by solar observations from space. ADAHELI will carry out observations of the solar photosphere and of the chromosphere at high-temporal rate and high spatial and spectral resolutions.
Spacecraft configuration and internal layout. From top left: ADAHELI in the VEGA fairing; ADAHELI’s overall layout; detailed view of the ADAHELI scientific payload’s layout.
ADAHELI will contribute to the understanding of Space Weather through the study of particle acceleration during flares. A radiometer operating in the millimeter radio band will continuously monitor the solar disk, throughout the spacecraft’s life time.
ADAHELI’s baseline instruments are a 50-cm high-resolution telescope operating in the visible and the near-infrared, and a lightweight full-disk radiometer operating at millimeter wavelengths (90 GHz).
The core of the telescope’s focal plane suite is the spectral imager based on two Fabry-Perot interferometers, flying for the first time on a solar mission. The instrument will return fast-cadence, full bi-dimensional spectral images at high-resolution, thus improving on current slit-scan, mono-dimensional architectures. Moreover, the possibility of working in polarized light will enable full 3D magnetic field reconstruction on the photosphere and the chromosphere. An optional instrumental package is also being proposed to further extend ADAHELI’s scope: a full-disk telescope for helioseismology based on a double Magneto-Optical Filter, a Neutral Particle Analyzer for magnetospheric research, an Extreme Ultraviolet imaging and spectro-radiometry instrument. These options fall outside the prescribed budget.
ADAHELI, flying a Sun-Synchronous orbit at 800 km, will perform continuous, long-duration (4-h), daily acquisitions, with the possibility of extending them up to 24 h.
ISODY: the visible and infrared solar telescope
ISODY (Instrument for solar dynamics), is ADAHELI’s main instrument, a 50-cm solar telescope operating in the visible and the near-infrared spectral bands. ISODY is equipped with a focal plane suite composed of a spectral imager, based upon two Fabry-Perot interferometers operating in the near-infrared region around 850 nm, and a broad-band channel for high-resolution imaging in the visible. ISODY aims at obtaining high spatial, spectral and temporal photometric and polarimetric resolution images of the solar photosphere and chromosphere.
OTU and the focal plane suite
The focal plane suite, designed at CNR/INOA, Florence, under Thales Alenia contract, is optically connected to the telescope through the Optical Transfer Unit (OTU).
The key element of the OTU is the Collimation Lens Unit, CLU, see (Suematsu et al., 2008). CLU has very strict requirements, requiring radiation-hardened optics and structure for delivering high throughput, temperature-insensitiveness of focus position and the ability to compensate for the Gregorian’s large field curvature. It also requires to be free of instrumental polarization and introduce negligibly small chromatic aberration in the observation wavelength range.
The CLU is a camera lens composed of 6 lenses with an IR/UV-rejection filter at its entrance. The first two lenses, of synthetic fused-silica, are placed Sunward, so that they work as another shield to the inner lenses, more susceptible to radiations.
The Focal Plane Assembly (SPF) comprises two optical/near-infrared channels: the narrow band (NB) and the broad band (BB) channels, as well as the Correlation Tracker (CRTR) channel.
The optical instrumentation inside SPF comprises 25 items or assemblies. A dichroic mirror (DM) transmits part of the telescope beam towards the BB channel in the wavelength range 530–670 nm and reflects part of the beam to the NB channel in the range 850–860 nm.
The principal optical path of the NB channel is formed by 3 folding mirrors (M4–M6) and 3 converging lenses (L1–L3), which successively collimate the solar and the pupil images. After L2 are the two Fabry-Perot interferometers (FP1, FP2) used in axial-mode and in classic mount and, between them, a filter wheel (FW) carrying a hole, a dark slide and various interference filters. The principal optical path of the BB channel is formed by 5 small folding mirrors (M7, …, M11), one re-imaging lens (L4 + L5), one variable neutral density filter and one filter wheel (FW), carrying a hole, a dark slide and 4 interference filters. A variable neutral density filter may be used to calibrate the radiation flux for different wavelengths and different exposure times.
Moretti, P.F. et al., Future instrumentation for solar physics: a double channel MOF imager on board ASI Space Mission ADAHELI, Astrophysics and Space Science, Volume 328, Issue 1-2, pp. 313-318, 2010
Greco V., Cavallini F., Berrilli F., The telescope and the double Fabry-Pérot interferometer for the ADAHELI solar space mission, Space Telescopes and Instrumentation 2010: Optical, Infrared, and Millimeter Wave. Edited by Oschmann, Jacobus M., Jr.; Clampin, Mark C.; MacEwen, Howard A. Proceedings of the SPIE, Volume 7731, article id. 773142, 2010
Berrilli F et al., The Fabry-Perot interferometer prototype for the ADAHELI solar small mission, Proceedings of the SPIE, Volume 8148, id. 814807, 2011
Stangalini et al, DIMMI-2h a MOF-based instrument for Solar Satellite ADAHELI, Proceedings of the SPIE, Volume 8148, id. 81480U, 2011