Narrow band imaging surveys are a powerful tool to detect and study the evolution of line emitter objects. Since the selection procedure is specifically suited for this kind of objects, selection effects present in other techniques (broad band imaging or spectroscopy) are avoided. However, deep surveys require the ability to observe different emission lines (Hα, Hβ, Oxygen, Ly-α) at different redshifts. This requires using a large number of filters that, given the sizes required for most instruments for 8-10m class telescopes, is a very expensive proposition.
Tunable Filters (TFs) provide a neatly way to overcome this problem. The tunable filters are conventional Fabry-Perot etalons in that the cavity thickness, defined by high reflectance plates separation, can be adjusted in a wide thickness range with high accuracy. By using gaps at least an order of magnitude narrower thant those employed by conventional Fabry-Perot devices, allow to observe an almost monochromatic fiel-of-view within the Jaquinot spot.
The OSIRIS (OSIRIS Tunable Emission Line Object Survey) instrument TFs are equivalent to have available several thousands standard filters in the range 365--1000 nm, with the advantage that a TF passband can be narrow than the typical narrow band filters used. Tunable imaging allows obtaining line fluxes simultaneously for all objects within OSIRIS field-of-view, avoiding at the same time sky emission lines, and at an arbitrary selected wavelength compatible with the etalon configuration and with an arbitrary (within the restrictions of the etalon parameters) passband width. A great advantage is the reduced contribution of sky and target continuum photon noises when narrower filters are used, and a very good continuum subtraction since off--band images are obtained very near the emission line and with the same spectral response than the on-band image. Moreover, the implementation of charge shuffling techniques, that allows a very good sky subtraction since sky variations are averaged on line, will increase even more this difference.
Surveys using tunable filters, as CADIS (Calar Alto Deep Imaging Survey), and the TTF--FGS (Taurus Tunable Filter Faint Galaxy Survey) detect one order of magnitude more objects (when normalizing for telescope size and exposure time) than conventional narrow band surveys as the Suprime-Cam of Subaru and the New UCM Survey.
Narrow band surveys with Tunable Filters in large telescopes will constitute a deep sky probe with unprecedented sensitivity. Whereas there are other instruments using TFs in operation (the Kyoto 3D spectrograph) or in project (the Prime Focus Imaging Spectrograph of the SALT Telescope) OSIRIS will be an unique instrument, thanks to its larger FOV (8.5'x 8.5'), range (365--1000 nm) and telescope size (10.4m). No other wide field TF in common user instruments for 8--10m class telescopes will be available at or near GTC Day One, OSIRIS will provide GTC with unique capabilities compared with similar telescopes, and OTELO will supply a unique database in survey area, sensitivity and target discrimination.
OTELO
Home PageLast update December 1, 2005, by Héctor Castañeda