RVS Status Report RVS-CoCo-004 10 April 2002 New RVS design -------------- As part of the global accommodation of the GAIA satellite to a smaller launch vehicule, the RVS instrument has been redesigned. The global photometric + spectroscopic FoV has changed. There are now 2 photometric fields, one above and one under the spectroscopic field (instead of half a field on the left and half on the right). Therefore photometric and spectroscopic data will not be simultaneously observed, but obtained on different transits. The new spectroscopic focal plane is made of 3 star mappers (for star detection/windowing and cosmic rays rejection) and 6 side by side CCDs to collect the spectra. The dimensions of the star mappers are : 3.4 by 60 mm2 and those of the detectors 10.1 by 60 mm2. The CCDs are separated by ~1 mm dead-zones. The new pixel size is 10 by 15 microns2 (10 along and 15 across scan). The focal length has been reduced to 2.1 m, so that the angular pixel size is ~1 by 1.5 arcsec2. The dimensions of the star mappers/detectors expressed in pixels (resp. in degrees) are : 336 by 3930 / 1010 by 3930 pixels (resp 0.1 by 1.6 / 0.28 by 1.6 degrees2). Therefore the exposure times are 5.5 s per star mapper and 16.5 s per detector. The dispersion is now along scan. The resolution is still an open parameter (see the objectives section) foreseen between R = 5 000 and 20 000. The CCDs plane rotation mecanism has been removed. The precession of the satellite will therefore induce a sine transverse motion of the spectra with respect to the CCDs, of maximum intensity 0.17 arcsec/s (2.81 arcsec/CCD crossing or 1.87 pixel/CCD crossing). The spectra will, often, be 3 pixels wide (>80% of the energy). Table 1 below summarizes the new characteristics of the RVS instrument. Table 1 : Radial Velocity Spectrometer Characteristics ------------------------------------------------------ Entrance pupil 0.5 * 0.5 m2 Focal length 2.1 m Satellite spin rate 60 arcsec/s Mission duration 5 years Mean nb of transits 100 FoV ~2 * 1.6 degrees2 Fov (6 detectors) 1.65 * 1.6 degrees2 CCD (star mappers) 3 * (3.4 * 60) mm2 " 3 * (336 * 3930) pixels " 3 * (0.1 * 1.6) degrees2 Exposure time (/star map.) 5.5 s CCD (detectors) 6 * (10.1 * 60) mm2 " 6 * (1010 * 3930) pixels " 6 * (0.28 * 1.6) degrees2 Exposure time (/detector) 16.5 s Exposure time (/transit) 99 s Dispersion along scan Resolution [5000, 20000] Rotation mecanism none Radial velocity accuracies (new design) --------------------------------------- Radial velocity accuracies have been estimated by rescaling, to the new RVS characteristics, results obtained with the previous design (see RVS-CoCo-002 for more details). Table 2 below presents radial velocity accuracies (in km/s) as a function of magnitude and resolution, for single transit and over the whole mission. Those accuracies are valid for a G3V star of solar metallicity. Similar accuracies are expected for a red giant at 0.5 or 1 magnitude fainter. On the other hand, in the case of a metal poor star, the accuracies would be translated towards brighter magnitudes : ~0.5 mag ([Fe/H] = -1) and ~1 mag ([Fe/H] = -2). Table 2 : Sigma Vr (km/s) for G3V star of solar metallicity ----------------------------------------------------------- Single transit Mission mag 5000 10000 20000 5000 10000 20000 ---------------------------------------------------------- 10.0 0.87 0.50 0.24 0.08 0.04 0.03 11.0 1.29 0.85 0.53 0.21 0.13 0.08 12.0 1.60 1.07 0.72 0.29 0.18 0.12 13.0 3.18 2.14 1.42 0.34 0.21 0.14 14.0 6.92 4.93 3.65 0.86 0.66 0.49 15.0 13.98 11.55 19.59 1.45 1.06 0.75 16.0 >30.0 >30.0 >30.0 3.23 2.41 1.75 16.5 >30.0 >30.0 >30.0 5.51 4.28 3.35 17.0 >30.0 >30.0 >30.0 8.08 6.30 5.06 17.5 >30.0 >30.0 >30.0 12.73 10.87 18.46 18.0 >30.0 >30.0 >30.0 23.81 >30.0 >30.0 18.5 >30.0 >30.0 >30.0 >30.0 >30.0 >30.0 Objectives/agenda/deadlines --------------------------- The short term objectives remain similar to those define in the first "RVS management plan and status report" (RVS-CoCo-001) : 1) To optimize the RVS design : - optical and mecanical design - focal plane assembly (nb of CCDs, pixel size, ...) - spectral resolution with respect to : - RVS/GAIA science goals - crowding in dense area - on-board computation and data-handling - telemetry budget 2) Assess astrophysical diagnostics (RV, atmospheric parameters, ...) accuracy versus magnitude, source type, area density. 3) Define on-board computation and data-handling and assessed telemetry budget. The first objective, means that most of the RVS characteristics (resolution, nb of CCDs along scan, pixel size, ...) can still be modified, in order to optimize the RVS instrument, as long as the modifications do not significantly increase the cost of the spectrometer. The driver of our short term deadlines is the "Payload and Data-handling study" (hereafter PDHS), whose main objective is to evaluate/size the electronics needed to handle the acquisition, on-board treatment and transmission of the Data (observations and house-keeping). The PDHS will start in September. At that time, we will have to provide them an upper limit of the RVS data rate and on-board computation operations (derived using the highest foreseen resolution, nb of CCDs, the fainter limiting magnitude, ...). The second deadline will be december 2002. We will have to converge on the design (resolution, focal plane assembly, ...) to provide a precise estimate of the RVS data rate and on-board computation operations. After december 2002, the RVS instrument will still be in phase A for about 2 years. In this phase, the modification/optimization of the RVS instrument will continue, taking the data rate and on-board computation defined in december as new specifications. During this two years period (~2003-2004) the long term objectives (as defined in RVS-CoCo-001) will also be addressed. Table 3 summarizes the RVS agenda. Table 3 : Objectives/deadlines ------------------------------ Sep 2002 Data-rate and on-board computation upper limit Dec 2002 Choice of the RVS characteristics Data-rate and on-board computation assessment 2003-2004 Optimization of the RVS (continues) Long term objectives (see RVS-CoCo-001) RVS workshops and meeting agenda -------------------------------- Here is a short summary of the next radial velocity working group workshops and meeting. 10-11 June Ljubljana 3rd RVS Workshop 9-12 September Monte Rosa Monte Rosa Conference 13-14 September Monte Rosa 4th RVS Workshop 28-29 November Paris 5th RVS Workshop