The digitized archive

A preliminary study devoted to define the characteristics of the digitization work was carried out by simultaneously testing three digitization devices on a sample of 80 plates extracted from the archive. The used devices were two commercial scanners (Epson Perfection 1200, Epson Expression 1680 pro) and a system endowed with a 1kx1k CCD camera, the latter developed at the Tor Vergata Rome University.

The selected plates were extracted from the CaII K time-series; we choose two plates for each available year, one acquired in Winter, the other one in Summer. Test images of the selected plates, as well as of a linear target, were acquired under different settings of the three devices. After analyzing the obtained images we decided to perform the digitization work by using the commercial scanner Epson Expression 1680 pro, with the setting 1200 dpi and 16 bit significant data. In fact, the scanner setting of 1200 dpi corresponds to a scale of 0.62 arcsec/dot in most of the produced digital images.

Due to the limitations of the typical local seeing, which during best conditions was worse than 2 arcsec, with higher scanner settings (1600, 2400 dpi), no space information content would be added in comparison with the acquired image. On the other hand, lower resolution settings (600, 800 dpi), would not allow to resolve some details that could be present occasionally on the solar disk observation.

Moreover, even though most analyzed plates seemed to be under-exposed, we decided to store the digital images as 16 bit grey-scale data. We considered that a reduction of dynamic range acquisition to 8 bit would determine occasionally loss of photometric information stored on the original observation. In fact, the effects of such reduction depend on exposition and potential defects of individual plates and thus have to be evaluated with care for each image. However, more digital products were obtained with the increase of the information content per pixel to 16 bit. In particular, with the 1200 dpi setting, the image size increases from 13 to 46 Mb assuming 8 and 16 bit data significance, respectively. The instrument and settings so defined allowed the scanning of plates in groups of four for most of the archive; each scanning produced a tiff format 8435x11153 pixel image (179 Mb) and took about 5 minutes. The digitization of the whole archive lasted about one year. All the obtained images were stored on DVDs.

The digital images were assembled in a database which contains for each observing day all the data available plus additional information including the characteristic curve for the photographic calibration of the image, the pertaining information written on observation log-books, and the ccc. The database containing all these data is presently stored on a dedicated PC at the Osservatorio di Roma and can be accessed through a web interface. The user interface has a menu type structure. Several links give general information on the Arcetri solar photographic archive, the main characteristics of stored observations, and the main results obtained from them. At present, users may down-load images in JPG and FITS formats and data obtained by the image analysis as ASCII files.