OHANA - an interferometric Network at Mauna Kea Telescopes

A joint informal meeting was held on March 16/17th 2000 in Waimea between the following participants: The purpose of the meeting was to discuss the concept of OHANA (Optical Hawaiian Array for Nanoradian Astronomy), proposed in 1996 by Mariotti et al. (A&ASS, 116, 381-393. SPIE 3350, 785-792.) for the coherent coupling of existing large telescopes on the Mauna Kea site by means of optical fibers. The attendees look forward to expanding participation in OHANA discussions, including other potential partners from the site.

1. Overview

The site with its telescopes, as it is right now, has a unique interferometric potential, that is only achievable with optical fibers. The long baselines and the large apertures extend the scientific capability of existing ground interferometers (VLTI, KI) and complement future space missions (TPF, Darwin/IRSI).

Such an implementation will contribute to longer term issues regarding the role of hectometric or kilometric arrays of large telescopes.

A cooperative program between institutes and telescopes represented on the summit of Mauna Kea generated enthusiasm amongst the participants of the meeting. A step by step approach was considered including:

  1. Simple demonstration of feasability,
  2. Highly focused science with pairs of telescopes,
  3. Long term evolution towards an imaging array.

2. Rationale

Two technological breakthroughs in the last years make this project entirely feasible: There is no other site on earth which provides the combination of superb seeing, largest collecting areas with AO, near kilometric baselines and easy potential cooperation between partners. The severe competition for observing time must be weighted against the scientific capabilities offered by these factors.

The resolution at 2 microns of an 800 meter baseline such as that defined by Keck and CFHT or Gemini is 0.5 milliarcseconds. Conservative estimates of the limiting magnitudes for visibility determination (see below) lead to a value of K=12 in snapshot observing.

With these values, a highly focused science program can soon be directed towards AGNs and ULIRGs. 0.2~0.5 milli-arcsecond resolution at K gives access to their very inner part, otherwise inaccessible. Even a few accurate visibility values departing from unity would significantly constrain models. Spectral resolution is not initially required. With the limiting magnitudes, tens of objects are readily accessible. In many cases, a significant fraction of the flux comes from the core as demonstrated with AO at the 100 mas. These data will extend by a factor of 9 in resolution the KI and VLTI results.

A longer term development would give access to fainter galactic and extragalactic sources and considerably extend the scientific capabilities in terms of sensitivity, u-v plane coverage and imaging potential.

3. Instrument overview & sub-systems

The initial instrument concept is based on the following:

3.1 AO operation of each telescope,

This is standard operation with no special requirement

3.2 beam extraction from each focal plane with a single mode fiber,

Due to the AO capability of beam steering, a simple optical module ensures the feeding into the fiber

3.3 beam transport through the fiber from each telescope to a recombination laboratory,

This step can be split in two: a) from telescope to its base, where routing must account for bending and twisting (possible polarization impact). b) From telescope base to laboratory through existing underground ducts with no site impact whatsoever.

3.4 controlled optical delay,

Long delays are necessary (up to 300 meters or more). Delays are achieved with slew-and-clamp and continuous lines properly folded to reduce volume and to be compatible with existing space in the telescopes. Vacuum and/or air operation can be implemented in successive project phases. Use of single mode components requires small size optics in the line (5~10cm).

3.5 beam recombination and signal detection,

This subsystem is entirely identical to the IOTA/FLUOR recombiner, as fiber transport erases memory of the telescope size. A proven solution is therefore readily available. A classical high sensitivity array detector is needed with only a few pixels being used.

3.6 instrument control.

Thanks to fibers, each item is a self-contained independent package. Connectivity between fibers with small losses (a few percent) ensures modularity. Items 3.2 and 3.3 are specific to the project and require prototyping and/or testing. Item 3.2 has already successfully been tested on a large telescope (ESO 3.6 meter). Item 3.3 raises the issue of polarization and this has already been addressed and solved in FLUOR (or polarization maintaining fibers could be considered if necessary). Item 3.4 is based on well established principles. Item 3.5 and 3.6 are standard.

Item 3.2 can be subject to further development to accomodate dual beam extraction (already planned for Keck and VLTI without fibers).

4. Plan

The plan is organized in three phases.

Phase I. The objective is to demonstrate 3.2 and 3.3.a, and to elaborate a funding plan for phase II. Phase I includes:

Phase II. aims at interferometric operation between pairs of telescopes and highly focused science on specific targets, proving the astronomical value of the whole project.

Possible choices for telescope pairs are: a) K1 and K2; b) K1 or K2 and IRTF or Subaru; c) CFHT and Gemini. a) is readily available but combines interferometry designed telescopes, b) involves more partners, c) combines non-interferometric telescopes and therefore models the project goals. East-West and North-South baselines provide specific pros and cons.

Phase II includes:

Phase III. aims at full exploitation and pending on previous results, cooperation prospects and funding capabilities, may include:

5. Implementation of Phase I

A student from the Ecole doctorale of Paris (Graduate school in astronomy) will most likely be selected to get started for his PhD with Phase I, beginning by a short (ca. 2 months) visit to Hawaii starting around end August (tbd). Costs of the visit will be shared between Meudon (travel) and local institutions (local expenses). This visit will allow familiarization with the AO systems and the local partners, and conceptual design of the extractor.

Meudon will most likely be able to fund the construction of the extraction set-up and to provide short (ca. 2m) fibers for it. Local partners will handle the interfaces of the extractor with the telescopes focal planes. The procurement and possibly cost of fibers for the routing tests (ca. 50 m) will be evaluated.

It appears essential to involve the fiber company (located in Brittany and sole possible provider) through a visit of its Director, Mr G. Mazé, to the site. This will be proposed to him and rediscussed for costs sharing Further steps of phase I will be discussed at the end of this first round, to be completed by end 2000 for the initial study.

A joint supervision of the PhD thesis between Meudon and some local partner may seriously be considered.

6. Management

The OHANA committee is hereby created. The committee's objective is to coordinate and organize its member institutions in the planning and operations of OHANA until a more formal project structure is in place. The OHANA committee will also communicate plans and activities to the astronomy community, and serve as a contact point for potential additional partners who may wish to participate in OHANA.

The initial institutional participation in the OHANA committee consists of:

Canada-France-Hawaii Telescope Corportation
Department Spatial, Observatoire de Paris-Meudon
Gemini Observatory
Institute for Astronomy, University of Hawaii
W.M. Keck Observatoty
National Optical Astronomy Observatories
The initial committee memebers are: The OHANA committee contact for information is:
Olivier Lai,
CFHT, P.O. Box 1597,
Kamuela, HI 96743, USA.
tel (808) 885 7944
fax (808) 885 7288
email: lai@cfht.hawaii.edu

Additional participating Mauna Kea observatories are anticipated and welcome.

7. Funding

The OHANA committee believes that the initial membership will successfully obtain funding for phase I, described above.

A P.I.-Co.I structure will be established to develop the detailed concept and budget, to seek funding and to carry out phase II.

Back to Main page
 
 
Page mainained by Olivier Lai

Last modified: Mon Apr 17 15:09:14 HST