ROB digitization facility

This website provides a description of the digitization and archiving facility at the Royal Observatory of Belgium (ROB). The ROB digitizer creates a computer readable digital copy, with unprecedented geometric accuracy, of the analogue sky images archived on photographic plates. As part of the current ESPaCE project we propose to digitize photographic plates of the natural satellites of the planets Mars, Jupiter and Saturn. Using the new technology ROB digitizer we create a high precision digital data set for the extraction of the geometric positions of the natural satellites and stellar images visible on the plates. After the astrometric reduction, these extracted plate positions will yield high precision astrometric positions in the sky as a function of time and serve as input data in combination with radio science space data for the calculation of new improved ephemerides of these natural satellites.

Introduction

Recently, a digitization and archiving facility has been created at the ROB under the Belgian D4A project (Digital Access to Aerial- and Astro-photographic Archives) that aims to digitise the astro-photographic collections of ROB, of the aerial-photographic collections of the National Geographic Institute of Belgium, of the Belgian Royal Museum of Central Africa and those of the DI07 project, also including the photographic collections of the Royal Institute of the Art Patrimonium (De Cuyper et al. 2009 and 2012). The new technology ROB digitizer uses specific optics, a diffuse illumination and nanotechnology mechanics to create a computer readable digital copy, with an unprecedented geometric accuracy, of the analogue sky images archived on the photographic plates. These digital images allow for an automated extraction of the positions of the celestial objects archived on the plates.

The value of photographic plates

Photographic plates contain valuable temporal information about the position and the brightness of celestial objects that cannot be reproduced by modern instruments. Hence, old photographic observations (up to more than one hundred years old for natural satellites) are the most important data archives for reconstructing the orbital evolution of the studied bodies as a function of time over a long time span.

Reduction of the data

The astrometric reduction of the photographic plate images consists of extracting the positions of the heavenly objects visible on the plates and transforming these plate positions into astrometric positions on the celestial sphere. The use of a computer readable digital copy of the photographic images automates the extraction process and improves the accuracy of the extracted plate positions. Unfortunately, the digitization process introduces errors due to the illumination, the electronics and the optical distortion of the objective/detector unit. As the optical distortion caused by the objective/detector unit of the digitizer is stable in time, the induced systematic error can be corrected. In addition, the telescope optics and the Earth's atmosphere deform the positions, shape and brightness of the images of the objects on plates. These are usually corrected, using a distortion model determined by measuring the plate positions of the stars of a well-known open star cluster (like the Pleiades or Praesepe). Astrometric reduction is needed to determine the center of the figure of the observed natural satellites and of the reference stars visible on the same photographic exposure. In particular, as the partly resolved images of many natural satellites are far from spherical due to an inhomogeneous albedo and to their often only partial illumination by the Sun (similar to the well known phase figures of our Moon), the determination of the center of the figure is often hard to fix.

Necessary means to undertake the astrometric reduction

Digitising these plates in an optimal way is crucial to the accuracy of the disclosed photographic data. The level of detail in astro-photographic plates requires a very high optical resolution to produce their high geometric and radiometric accuracy in the digital copy, and precludes the use of image sharpening filters (filters to make edges look sharper) in the scanning process. This excludes the use of the common commercial (flat bed)scanners. Commercial colour scanners normally use one dimensional CCD (Charge Coupled Device) rows moving at constant speed in one direction during the digitization. Hence, part of the image that is captured by a pixel also falls on it during the next integration step. Moreover, the time that the light coming from a particular point of the original image is projected on a single pixel also varies. This means that the details of the image are smeared out over neighbouring pixels in the direction of the scan. This creates a "soft" looking image. Furthermore, commercial scanners, used at a simple fraction of their optical resolution, will only read out that fraction of their pixels. Hence the digital image will only contain that fraction of the detail contained in the original image. When used at a non-simple (complex) fraction of their optical resolution, pixel rebining will take place. Lastly, most commercial scanners also apply an image "sharpening" filtering and have a local geometric accuracy between 2µm and 10µm. Other purpose build astronomical plate scanners such as the StarScan (US Naval Observatory, Washington DC), are more accurate than commercial scanners, with a geometric accuracy of between 2µm and 0.2µm.

The ROB digitizer

The ROB digitizer DAMIAN (Digital Access to Metric Images Archives Network) is a 2D-digitizer facility of high geometric and radiometric resolution and was precision built between 2002 and 2007. This machine can digitize photographic images up to 342mm wide on glass plates, film sheets and film rolls. A cast aluminium counter pressure plate and plate trays with a central opening corresponding to the actual image size are needed (De Cuyper et al. 2004, 2005 and 2006). The XY-table is an adapted Aerotech ABL3600 air bearing system with nanometer positioning technology. A high frequency servo-loop system uses the position of the XY-table read by Zerodur Heidenhain encoders with a precision of 0.001 µm for steering the linear motors of the X and Y axes. The mechanical subsystem designed and build according to the ROB specifications by Aerotech, Pittsburgh, includes an automatic plate holder assembly and a plate exchange robot with plate tray magazines and a turntable for the photographic glass plates and film sheets and an automatic film roll transport system. These custom made devices allow for a rapid exchange of the photographs to be digitized without manual intervention. The heart of the current optical system consists of a BCi4 CMOS camera from C-Cam Vector International and a Schneider Xenoplan telecentric 1:1 objective, mounted on a Z-axis on a granite bridge above a central opening in the granite table. The back light illumination system consists of very bright LED's (lifetime min. 50000h) controlled by a high precision power supply. The capacity the reproduce the positioning of the XY table is better than 0.01µm and the positioning accuracy is greater than 0.05µm.

The digitization process

The digitisation process introduces errors due to the illumination, the electronics and the optical distortion of the objective/detector unit. To remove the first two, the raw images need a correction for flatness and darkness. The current optical system consists of a BCi4 CMOS camera from C-Cam Vector International, with a 9mm by 7mm non-cooled Ibis 4 digital detector of 1280 by 1024 pixels of 7µm x 7µm, mounted on a Schneider Xenoplan telecentric 1:1 objective. This objective/detector unit creates a digital copy of the photographic plate of about 3620dpi. The digitization process is done in a step and stare mode. Because the digital distortion of the current objective/detector unit rises to 3µm at a distance of 3.5mm from the optical axis, the digitization process uses increments of 5mm in both directions of the XY-table axes. As this optical distortion caused by the objective/detector unit of the digitiser is stable in time, the extracted plate positions can be corrected for this systematic error. Software was developed at the ROB for the automation of the step-and-stare digitization process, for the processing of the raw images, for the darkness and flatness of field corrections, for creating digital images in TIFF and FITS formats, and for the determination of the parameters of the correction model for the optical distortion caused by the objective/detector unit of the digitizer. Thanks to a partnership with the United States Naval Observatory (USNO), the ESPaCE project benefits from a large collection of high quality photographic plates of the planetary system that have been obtained between 1926 and 1998. In particular, it contains hundreds of observations of natural satellites of Mars, Jupiter and Saturn, the major targets of the today's space programs. The ROB digitisation facility will digitize all these photographic plates. The data will be used to provide new ephemeris kernels for these natural satellites. Thereafter, all data will be published in refereed journals and made accessible worldwide through a proper astrometric database.

References

De Cuyper J.-P., Winter L., de Decker G., Zacharias N., Pascu D., Arlot J.-E., Robert V., Lainey V., 2009, New Astrometric Reduction of the USNO Photographic Plates of Planetary Satellites, Astronomical Data Analysis Software and Systems XVIII ASP Conference Series, Vol. 411, proceedings of the conference held 2-5 November 2008 at Hotel Loews Le Concorde, Québec City, QC, Canada. Edited by David A. Bohlender, Daniel Durand, and Patrick Dowler. San Francisco: Astronomical Society of the Pacific, 275.