PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM DATA_SET_ID = "RO-CAL/J/M-ALICE-2-MARS-V1.0" PRODUCT_CREATION_TIME = 2007-04-09T00:00:00 PRODUCT_ID = "ALICE_CALIBRATION_DESC" OBJECT = TEXT INTERCHANGE_FORMAT = ASCII PUBLICATION_DATE = 2007-04-09 NOTE = " This document describes the calibration procedure used for the Alice instrument aboard the Rosetta spacecraft." END_OBJECT = TEXT END The MIKE calibration routines (provided as documentation in the DOCUMENT/CODE/MIKE directory) apply various calibrations to engineering-level Alice data to convert the data from units of counts to flux units (photons s**-1 cm**-2). Four types of calibrations can be performed. They are, in order of application to the data: deadtime correction, dark correction, effective area correction, and flat field correction. These are described in more detail below and in the document headers of the calibration files themselves and the MIKE code. DEADTIME CORRECTION ------------------------------ The Alice detector electronics require a finite time to process a photoelectron pulse. As a result, if photoelectron pulses arrive too close together in time, the later pulse will not be recorded, resulting in an effective decrease in the sensitivity of the instrument that is a function of the countrate. Multiplying the data by a constant determined by the IDL procedure "MIKE_DEADTIME.PRO" corrects for this effect. The detector deadtime rate was measured in the laboratory; no in-flight measurement of this quantity is possible. At a count rate of 1 kHz, the correction factor is approximately 1.02, while at a count rate of 20 kHz, the correction factor is approximately 1.78. DARK CORRECTION ------------------------------ In-flight, the Alice detector electronics produce spurious counts at a rate of approximately 20 Hz. The spatial distribution of dark counts is approximately uniform across the detector, however there is some low-level 2-D structure to the dark counts. To measure the rate and distribution of dark counts across the detector, Alice observations are periodically made while the aperture door is closed. Since no light can enter the instrument, all counts recorded are due to the detector electronics. These dark observations are summed together to create a "superdark". To remove the dark counts from a science observation, the superdark (either a 2-D image for Histogram and Pixel List data or a scalar value for Countrate data) is scaled to the exposure time of the science observation and then subtracted from the data. EFFECTIVE AREA ------------------------------ The sensitivity of Alice is measured by the effective area of the instrument. For a point source located at infinity, effective area is defined as the area of the surface that intercepts incident photons at the same rate as is detected by the Alice instrument. Dividing the observed count rate by the effective area yields the incident flux of photons. Effective area depends on the size of the instrument aperture, reflectivities of the optical surfaces, etc., and is a strong function of wavelength. The initial estimate of the effective area of the Alice instrument was derived during laboratory testing. After launch, it was found that the in-flight effective area was a factor of 0.4 less than the ground effective area. Longward of 1450 A, the in-flight effective area of Alice has been modified such that the Alice observations of Vega match the spectrum observed by the IUE satellite. FLATFIELD -------------------------------- When uniformly illuminated by a monochromatic source, the counts detected by the Alice instrument vary from pixel to pixel at approximately the 20% level. This spatial variation in instrument sensitivity is the instrument flatfield response. During the laboratory calibration of the Alice instrument, the detector-only flatfield response was measured using a Hg "penray" lamp at 2536 A. However, THE ALICE TEAM STRONGLY ADVISES AGAINST USING THIS FLATFIELD IN THE CALIBRATION OF IN-FLIGHT DATA for the following reasons. First, the lab flatfield is not an end-to-end test of the flatfield response, i.e. any effects introduced by the optical elements of the instrument are not included. Second, the flatfield response was measured at a wavelength significantly outside the normal operating range of the Alice instrument. Significant artifacts are introduced to in-flight data by the lab flatfield. As of 2004-04-09, no suitable observations have been made from which to derive an in-flight flatfield calibration. However, such observations are planned for PC 6 and Earth flyby 2.