The `Ohana concept considers interferometric coupling of 3 to 10 meter class telescopes of Mauna Kea taking advantage of two technological breakthroughs: adaptive optics and coherent transport of light with single mode optical fibers. This concept, proposed in 1996 (Mariotti et al, AASS, 116, 381-393, SPIE 3350, 785-792), was informally discussed at a first meeting in March 2000. It was considered worth pursuing by the participants and a number of preliminary actions was undertaken.
A plan was drafted comprising three phases:
A follow up meeting was held on December 18th/19th 2000 in Waimea at CFHT headquarters, and was attended by the following participants:Phase I: demonstration of beam extraction from AO foci with fibers and routing to ground (year 2001). Phase II: first coupling of a pair of telescopes leading to fringes acquisition, visibility measurements and scientific impact (2002- mid 2003). Phase III: progressive implementation and completion of the array capability with full scientific program (from 2003 onwards).
Hiroyasu Ando (Subaru) Chris Dainty (Imperial College) Greg Fahlman (CFHT) Wolfgang Gaessler (Subaru) Don Hall (Ifa - UH) Rolf Kudritzki (Ifa - UH) Antoine Labeyrie (OHP) Fabien Malbet (Obs. de Grenoble) David St Jacques (Subaru) Francois Reynaud (IRCOM) Steve Ridgway (NOAO) Francois Rigaut (Gemini) Jean Rene Roy (Gemini) Hideki Takami (Subaru)
The goals set forth for this second meeting were to:get Ph.D. student (Julien Woillez) and funds in France, begin implementation study, inform and invite new potential partners, inform the community (web site, SPIE), think on science cases and prepare a second meeting. All of these actions have been closed at the start of the meeting.
Obtain instrument status and progress reports of different groups, Establish a schedule for the upcoming year 2001 (Phase I) and define technical interfaces, Expand the `Ohana committee with new partners, Establish science cases, Define phase II.
Members of IRTF, UKIRT and Subaru were invited by the `Ohana
committee and largely contributed to the progress of the meeting. They
have been invited to join the `Ohana committee.
The meeting was divided into two parts. The first day addressed
technical issues and the second day was dedicated to science cases and
to definition of action items.
The use of single mode fibers for beam transport over hectometric
distances in the visible and near infrared has been demonstrated in laboratory
experiments in Japan (MIRA) as well as in France (IRCOM), and will be pursued
at Observatoire de Paris for K band fibers.
It is believed that the existence of an empty 4 inch duct linking
telescopes could be available for a permanent link, but requires confirmation
from UH. It appears that for a temporary connection of close observatories
on the Eastern ridge, an overground solution is feasible and indeed preferable.
However, for the western side, the wikaiu bug habitat may prevent overground
routing of the fibers in which case an underground solution readily exists.
Several solutions have been discussed. For a baseline involving
Keck telscopes the fringe tracker camera may be available, as long as the
dedicated optics are provided for fiber focusing. The Meudon group is considering
building a Nicmos array. The availability for an innovative detector at
University of Hawaii should be investigated.
Integrated optics beam combination (IONIC) is now demonstrated on the sky and would simplify `OHANA's beam combination. Pupil densification could provide a direct imaging capability when a number of telescopes are available (Labeyrie, Guyon). In the case of imaging of complex sources, combining `Ohana data with KeckI and VLTI data will allow to fill out the UV plane more evenly and cover more spatial frequencies.
Four illustrative science cases were presented and they already
proved to be strong science drivers for phase II of the instrument.
In AGNs, the topics of interest are the size of the Broad Line
Region and the strucrure of the base of jets in the infrared. This will
require baseline on the order of or greater than 200 meters. However, the
sensitivity required will imply the need for the larger apertures. Significant
results can be obtained by obtaining visibilities in H band and Pa{beta};
a moderate spectral resolution is needed for the determination of the size
of the BLR. For the study of jets, calibrated fluxes and polarimetric measurements
will be needed. A deeper study is necessary in order to examine the significance
of a few visibility measurements for constraining models.
`Ohana will be able to probe the inner central AUs of the circumstellar
environment, including the inner accretion disk, and star/disk interface
region, which hold important clues on the accretion/ejection mechanism.
Thermal emission from dust heated by the viscous accretion process dominates
the near-infrared continuum. It will be possible to constrain the
size of the disk truncation radius (expected to lie close to the corotation
radius - 0.07 au) and discriminate between different disk models (magnetized
versus purely hydro). Brgamma observations with moderate spectral resolution
(a few 100) would constrain the geometry of the star/disk interface region
and the wind formation regions. Baseline greater than 200 meters will be
needed to answer these questions. However, the sensitivity is not an issue,
and the smaller telescopes can be used for this program. Even a very limited
number of visibilities constrain models. About 100 sources are accessible.
K band is the most suited for this program.
The distance ladder can be improved in two ways. First by indirect
measurements: angular diameter measurement coupled with linear diameter
derived from models. Second, by direct measurement: angular diameter variation
coupled with spectroscopic photosphere velocity determination. The sample
of sources for `Ohana comprises at least 100 targets for the indirect measurement
and from 60 up to more than 100 for the direct measurement. Besides, the
resolving power of `Ohana will allow to study the atmosphere of these stars
and compare with current models which are a source of bias in the final
distance ladder calibration. A large fraction of this program can be achieved
with moderate baselines (~200 meters) and pupils (4 meters).
The modeled diameter is on the order of 100 micro-as. However
a measure of this diameter will strongly constrain the equation of state
for cooling models. This is unique science as the full resolution of `Ohana
is required as well as the largest pupils: the resolution of `Ohana is
425 micro-as in H band and so a diameter measurement translates into a
95% visibility measurement. The required accuracy on the visibility is
1% and there are half a dozen candidates within 10 pc.
A modular injection device, the interfaces of which will have
to be adapted to the f-ratio of each input beam, will be designed for Keck,
CFHT and Gemini telescopes and possibly Subaru pending steric compatibility.
It will verify the injection, measure the efficiency of AO to fiber coupling,
investigate polarisation and fiber stability during the transport from
focus to ground and give clues to the final sensitivity of the array. The
tests, to begin on the telescopes in the fall of 2001, are expected to
be completed by the end of that year. A large fraction of the tests will
be done during daytime, while technical and/or discretionnary time could
be considered for the sky validation.
Both silica and fluoride glass fibers should be used for these tests and the injection modules should preserve compatibility for both types of fibers.
Cost for phase I includes the construction of 2 injection modules (provision for phase II), a length of 50 meters of infrared fibers (and also silica fibers), laboratory measurement of beam transport in it and the assembly of an available IR detector. These costs can be beared by Meudon. New visits of Woillez to Hawaii will be necessary and it is wished to continue the arrangement which supported Woillez visit in November to December 2000.
Phase I also includes the design and cost estimate of phase II for
which the following goals and requirements were established. A requirement
is the minimum achievement at which a task is said to be completed, while
a goal is the ideal objective one would like to reach.
Demonstrate feasability of Phase III Obtain fringes with 2 different telescopes equipped with AO on Mauna Kea the diameter of the telescopes should be greater than 3 meters and the minimum baseline should be 150 meters. Demonstrate coupling of telescopes of unequal diameter The demonstrator source should be fainter than K = 8 Meaningful visibilities should be obtained with 5% accuracy. Meaningful results should be published as a refereed paper. J or H or K bands transit interferometer + "slew and clamp" delay line delay line solution for phase III.
same as requirements but with 2 pairs (1 North/South, 1 East/West) Baseline > 400 meters, Limiting magnitude K > 12, Unique science, J, H and K, sideral motion tracking, fringe tracking.
All this has to be achieved within the following constraints:
Phase II should not only demonstrate the technical feasability
of telescopes' coupling on Mauna Kea summit, but also lead to some unique
science results beyond the VLTI and Keck Interferometer capabilities. Main
parameters for the choice of telescopes are: telescope aperture, baseline
length, baseline orientation (North/South orientation simplifies delay
line problem).
The science discussion led to the following potential objectives,
ordered by probable increasing difficulty:
Extending the VLTI and KI resolutions, any of those programs can bring unique science.Cepheids require long (>200~400m) baselines and moderate apertures. Even one visibility at J, H, or K brings important information. Young Stellar Objects require at least 150~200m baselines and moderate apertures. A few visibilities, using two different baselines (e.g. 200 and 400m) at J, H or K and possibly moderate spectral resolution (Br{gamma}) bring important information. AGNs require at least 200 meters baselines with large apertures at H or K, possibly with moderate spectral resolution. Proper interpretation of a few visibilities in a complex object require more study. Brown dwarves may require both largest baselines and apertures. Even one well calibrated visibility in H or K is important.
Three baseline configurations satisfy the phase II requirements, offer the technical demonstration capability and contribute to these science goals. Among all possible configurations, this choice appears to select the simplest ones, which can be ordered by increasing difficulty:
For any of these baselines, the same pair of matched infrared and/or silica fibers ca. 450 meters in length can be used.CFHT-Gemini (~150m ~NS) with simple delay lines (slew and clamp) at Gemini or CFHT. CFHT-UKIRT (~350m ~NS) with simple delay lines at Gemini. UKIRT having no AO for the moment, it will be equivalent to a 1.5 meter aperture if tip-tilt is corrected. IRTF-Subaru (~405m ~EW) with Keck delay lines. IRTF should be equipped with AO by early 2003.
Recombining equipment can be made available from existing systems for both infrared and silica fibers.
Cost estimates for phase II will be consolidated in 2001 and discussed among partners. Yet, a significant fraction of the cost (beam combiner and possibly fibers) may be covered as the french contribution to the project.
Phase II is likely to extend over 2002 and 2003. Its precise definition
requires additional studies and will be the subject of the third `Ohana
meeting to be held in the fall of 2001.