ROSETTA PTOLEMY DATA CALIBRATION DESCRIPTION =========================================================================== Revisions --------- 2010-06-10 first issue, SONC 2015-06-18 sniff mode conversion factors added, A. Morse Ptolemy calibration =================== Sensors calibration (thermocouples and pressure) ------------------------------------------------ The process of converting the sensor voltages measured by Ptolemy into calibrated sensor readings is described in the document RO-LPT-OU-TN-3146 (Ptolemy Sensors Calibration). The document is located in the DOCUMENT directory. The data tables of thermocouple emfs (electro-motive force) against temperature for type K and N thermocouples can be found in the file THERMOCOUPLE_CALIB_DATA.TXT located in the CALIB directory. Conversion of counts into densities ----------------------------------- The Ptolemy electron multiplier detector is operated in an ion counting mode. An ion impacting the electron multiplier creates a pulse of electrons about 50 ns duration. The detector electronics counts the number of pulses i.e. 1 ion count = 1 ion. The sensitivity of the ion trap depends upon the following factors: 1. Number of electrons emitted by the nano-tip ionisation source. This was found to be highly variable, varying by factor of at least 5 even when control voltages are held constant 2. Number of electrons that enter the ion trap which is affected by the ion trap RF voltage during the ionisation phase of the scan function. 3. Number of ions created in the ion trap during the ionisation phase of the scan. This depends on the ionisation time, the ionisation cross section and the ion density. 4. Proportion of ions that are stored in the ion trap which depends on ion mass and RF voltage during the ionisation phase. 5. Proportion of stored ions that are ejected into the electron multiplier detector during the ramp phase of the scan function 6. Proportion of ions impacting the detector that generate an electron pulse detected by the ion counting electronics. The mass spectra calibration ---------------------------- There are two types of files needed for the spectra calibration. One contains the WGA2 (Waveform Generator ASIC) scan function and the other gives the relation between bin number and mass. The information from the file containing the scan function is used to calibrate the spectra and each spectrum should point to a scan function. The files are located in the CALIB directory. The file, "WGA2_SCAN_FUNCTION_20010101.TXT" describes the data stored in Ptolemy that was used to control the mass spectrometer. It describes how the mass spectrometer was operated for WGA2. Details are described in documents "RO-BER-RAL-TN-3401.PDF" (WGA and RICA specification) and "RO-LPT-OU-MA-3102.PDF" (On Board Software User Manual". The file, "WGA2_CALIBRATION_20010523.TXT" is the calibration file. It contains a table with the following columns. - Bin number is the raw data bin number - an integer from 0 to 1023. - DAC value is the RF DAC at the start and end of each bin. An integer value from 0 to 4095. - Mass (amu) is the calibrated atomic mass at the start and end of each bin. A positive number between 0.00 and 200.00 fixed to two decimal places. - Description describes how the mass spectrometer is operating - Valid MS indicates whether the ions detected form part of the mass spectrum. All the spectra aquired during the thermal vacuum tests and PC#4 used WGA2 scan function and this calibration file. The mass spectra calibration - WGA3, WGA4 as part of Sniff Mode ---------------------------- There is a two-step process for Sniff Mode WGA3 and WGA4 calibration. The first step is to convert from raw data bin format to a calculated RF DAC value. RF DAC is a measure of the theoretical voltage applied to the ring electrode. For WGA3 the conversion is: RF DAC = 358 + 2x, where x is the bin number. For WGA4 the conversion is: RF DAC = 670 + 3x, where x is the bin number. The six mass spectra of Sniff Mode are interleaved WGA3-WGA4-WGA3-WGA4-WGA3-WGA4 RF DAC is related to mass (strictly m/z), by operation-specific conversion factors which must be applied in the format: m/z = ax^2 + bx + c, where 'a', 'b' and 'c' are operation-specific conversion factors and 'x' is RF DAC. Following conversion to RF DAC, WGA3 and WGA4 data can be interchanged since RF DAC is non scan function-specific. The conversion factors are derived empirically by a process of iterative peak-matching. The conversion factors are supplied for each operation of Sniff Mode in attached file: SNIFF_MODE_CONVERSION_FACTORS.TXT in the CALIB directory. The conversion factors are repeated below. PC12 refers to the Sniff Mode undertaken during PC12. Lutetia 1-5 refer to the five Sniff Modes undertaken during the July 10th 2010 flyby of asteroid 21 Lutetia PHC refers to the Sniff Mode of Post Hibernation Commissioning PDCSXX refer to the Sniff Mode of the relevant PDCS block Agilkia refers to the 'Post Bounce' Sniff Mode shortly after comet arrival. Abydos 1-6 refer to the six Sniff Modes undertaken at the final Abydos landing site during FSS. Operation a= b= c= PC12 -4.0000E-07 3.9158E-02 -1.6961E+00 Lutetia 1 -4.0000E-07 3.8037E-02 -9.9389E-01 Lutetia 2 -4.0000E-07 3.9279E-02 -1.5408E+00 Lutetia 3 -4.0000E-07 3.9429E-02 -1.7052E+00 Lutetia 4 -4.0000E-07 3.9429E-02 -1.7052E+00 Lutetia 5 -4.0000E-07 3.9429E-02 -1.7052E+00 PHC -4.0000E-07 3.9565E-02 -1.9714E+00 PDCS1A -4.0000E-07 3.9506E-02 -1.9011E+00 PDCS1B -4.0000E-07 3.9506E-02 -1.9011E+00 PDCS2A -4.0000E-07 3.9245E-02 -1.5226E+00 PDCS2B -4.0000E-07 3.9427E-02 -1.8325E+00 PDCS3A -4.0000E-07 3.9506E-02 -1.9011E+00 PDCS3B -4.0000E-07 3.9506E-02 -1.9011E+00 PDCS4A -4.0000E-07 3.9506E-02 -1.9011E+00 PDCS4B -4.0000E-07 3.9506E-02 -1.9011E+00 Agilkia -3.95E-07 3.9646E-02 -2.0514E+00 Abydos 1 -4.0000E-07 4.0065E-02 -2.3854E+00 Abydos 2 -4.0000E-07 4.0065E-02 -2.3854E+00 Abydos 3 -4.0000E-07 4.0117E-02 -2.1955E+00 Abydos 4 -4.0000E-07 3.9681E-02 -1.9757E+00 Abydos 5 -4.0000E-07 3.9418E-02 -1.6869E+00 Abydos 6 -4.0000E-07 3.8986E-02 -1.1612E+00 Please refer to the SNIFF_MODE_CONVERSION_FACTORS.TXT file for calibration sources.