VIR Calibration Lamps All of the material presented here has been extracted from the following reference: Melchiorri, R., G. Piccioni, A. Mazzoni, VIRTIS-M flight lamps, Rev. of Sci. Instr., 74(8), 3796-3801, doi:10.1063/1.1593784, 2003." [MELCHIORRIETAL2003] provides a comprehensive description of the calibration lamp design, testing, and spectral characteristics that will not be addressed here. The purpose of this document is to provide access to a small part of the material contained in the paper as documentation for the Dawn VIR instrument to those that do not have easy access to the journal article. Please note that the publication pertains directly to the Rosetta VIRTIS-M spectrometer and not Dawn/VIR. VIRTIS-M is a visible-infrared (VIS-IR) image spectrometer designed for the Rosetta mission; it is intended to provide detailed information on the physical, chemical, and mineralogical nature of comets and asteroids. However, VIR is a very similar instrument that uses the same lamps so the material provided here should be relevant to the VIR calibration. While the VIR lamp design is the same as VIRTIS-M, they were not built as part of the same batch that was tested and reported on in Melchiorri et al., 2003. The in-flight performance of detectors like those in VIRTIS-M and VIR may change with time and environmental factors (launch vibrations, temperature, etc.) so an in-flight calibration method is required. VIR uses lamps for internal calibrations which are characterized by a wide spectral range with a blackbody-like emission with an effective temperature of about 2400-2600 K, that covers the whole spectral range (0.2-5 micron). A precise spectral calibration is achieved by adding special filters containing many narrow absorption lines for visible and infrared ranges in front of the source. This article describes the tests and calibration of some of the flight prototypes of these lamps (VIS and IR) that were built by the Officine Galileo and calibrated by the Consiglio Nazionale delle Ricerche-Istituto di Astrofisica Spaziale e Fisica Cosmica. In order to be able to use these lamps as an in-flight calibration source, the lamp spectrum must be known (function of filament temperature). The approach taken is to use measured resistivity to approximate the filament temperature. Each lamp has a tungsten filament which tests show has stable optical emissivity, and a long lifetime. The spectrum of the radiation emitted by the tungsten filaments is featureless. In order to allow a precise frequency calibration, spectral features have been added by inserting a holmium filter for the VIS model and a polystyrene filter for the IR model (see Fig. 3, [MELCHIORRIETAL2003]). Figure 3: Holmium and polystyrene transmittance in the VIS-NIR region (modified after [MELCHIORRIETAL2003]) - provided as MELCHIORRI-FIG3.JPG Xenon gas is used to fill the lamps because this gas was found to be more stable than Argon during testing [MELCHIORRIETAL2003]. The pressure of this gas is around 110 hPa at 80 deg C. The use of Xenon may introduce problems at temperatures lower than the xenon boiling temperature (about -108 deg C). During the cold tests the lamps were cooled down to an ambient temperature of about -140 deg C where gas liquefaction may occur; however, this situation does not introduce substantial changes in the response of the lamp [MELCHIORRIETAL2003]. It has been assumed that the liquid xenon almost instantly evaporates as the filament is turned on. The final lamp spectrum is a combination of the blackbody-like emission by the filament and the absorption features due to the transmission of the filters (see Fig. 17, [MELCHIORRIETAL2003]). Once the filter features have been subtracted, it is possible to fit the lamp spectrum with a Planck curve, in order to determine the equivalent filament temperature (Fig. 10, [MELCHIORRIETAL2003]). For laboratory setup, this test gave the result T = 2600+/-100K for VIS and T=2400+/-100K for IR. FIG. 10. VIS lamp fit with an equivalent BB curve. Temperature estimated of ~2610 K (modified after [MELCHIORRIETAL2003]). see MELCHIORRI-FIG10.JPG The behavior of the filament resistance with the temperature has been monitored continuously. Any abrupt change would indicate some spurious contact among the spires. A fit to the curve (Fig. 15,[MELCHIORRIETAL2003]) provides the parameters in Table III. For temperatures higher than -50 deg C the curve tends to a straight line. FIG. 15. Dependence of the VIS and IR filament resistance on the temperature; a linear fit for the VIS type gives b = 0.0151(Ohm/degC) and a =3.94 Ohm and for the IR type: b = 0.0151(Ohm/degC) and a =4.06 Ohm (modified after [MELCHIORRIETAL2003]). see MELCHIORRI-FIG15.JPG Table III Parameters of the 3rd order fit =============================================================== term VIS IR unit --------------------------------------------------------------- a 3.9692952 4.0696665 Ohm b 0.014799736 0.014292713 Ohm/degC c 1.9703929e-005 7.7951323e-006 Ohm/(degC)^2 d -7.7914420e-009 -2.199964e-009 Ohm/(degC)^3 =============================================================== FIG. 17. Spectral variations for IR and VIS lamps. On the left side it is possible to see the holmium features. On the right side polystyrene features are not present in this band region. The maximum of the Planck curve does not change from cycle to cycle (modified after [MELCHIORRIETAL2003]). See MELCHIORRI-FIG17.JPG Resistivity and temperature are related to each other, in first approximation, by a linear equation. Assuming that geometrical factors do not change with temperature (in a first approximation), it is possible to evaluate the filament temperature by the knowledge of the resistance at ambient temperature [Eq. (1)]: R(T) 5.5 ------ + 5.0748 R(20K) Eq(1) T(K) = ----------------------- 0.0351286 In flight, there is no possibility to analyze the lamp spectrum. Equation (1) allows the retrieval of the spectral calibration. Table IV shows a comparison between the equivalent temperature measured by the electrical parameters and by the blackbody (BB) curve fit. TABLE IV. Equivalent filament temperature measured by electrical parameters and by BB curve fit (ep stands for electric parameters, and e stands for emissivity). Table IV ======================================================== Lamp model ep (K) e (K) -------------------------------------------------------- IR 2393 2436 VIS 2590 2576 ========================================================