Editing Astrophysics Data System (section)

==Data in the system==
At first, the journal articles available via ADS were exclusively [[Image scanner|scan]]ned [[bitmap]]s created from the paper journals and the abstracts created using [[optical character recognition]] software. Some of these scanned articles up to around 1995 are available for free by agreement with the journal publishers,<ref name="data">{{cite web |date=23 June 2005 |title=NASA ADS Abstract Service Mirroring Information |url=http://ads.harvard.edu/mirror/ |access-date=2 November 2008 |publisher=Harvard-Smithsonian Center for Astrophysics}}</ref> with some dating from as far back as the early 19th century. Eventually, because of a wider spread of online editions of journal publications, abstracts would start to instead be loaded into ADS directly.

Papers are indexed within the database by their bibliographic record which contains the details of the journal they were published in, and various associated [[metadata]], such as author lists, [[Image resolution|reference]]s and [[citation]]s. Originally this data was stored in [[ASCII]] format but eventually the limitations of this encouraged the database maintainers to migrate all records to an [[XML]] (Extensible Markup Language) format in 2000. Bibliographic records are now stored as an XML element with sub-elements for the various metadata.<ref name="architecture" />

Scanned articles are stored in [[TIFF]] format at both medium and high [[Image resolution|resolution]]. The TIFF files are converted on demand into GIF files, for on-screen viewing, and [[PDF]] or [[PostScript]] files for printing. The generated files are then [[cache (computing)|cached]] to eliminate needlessly frequent regenerations for popular articles. As of 2000, ADS contained 250 [[gigabyte|GB]] of scans, which consisted of 1,128,955 article pages comprising 138,789 articles. By 2005 this had grown to 650 GB and was expected to grow further to about 900 GB by 2007.<ref name="data" /> No further information has been published (2005).

The database initially contained only astronomical references, but has now grown to incorporate three databases, covering [[astronomy]]
references (including planetary sciences and solar physics), [[physics]] references (including instrumentation and geosciences), as well as preprints of scientific papers from [[arXiv]]. The astronomy database is by far the most advanced and its use accounts for about 85% of the total ADS usage. Articles are assigned to the different databases according to the subject rather than the journal they are published in, so that articles from any one journal might appear in all three subject databases. The separation of the databases allows searching in each discipline to be tailored, so that words can automatically be given different [[weight function]]s in different database searches, depending on how common they are in the relevant field.<ref name="architecture"/>

Data in the preprint archive is updated daily from arXiv which is the dominant repository of physics and astronomy preprints. The advent of preprint servers has, like ADS, had a significant impact on the rate of astronomical research, as papers are often made available from preprint servers weeks or months before they are published in the journals. The incorporation of preprints from arXiv into ADS means that the search engine can return the most current research available, with the caveat that preprints may not have been peer-reviewed or [[proofread]] to the required standard for publication in the main journals. The database of ADS links preprints with subsequently published articles wherever possible, so that citation and reference searches will return links to the journal article where the preprint was cited.<ref>{{cite conference |url=http://meetings.aps.org/Meeting/MAR07/Event/61837 |title=myADS-arXiv: A fully customized, open access virtual journal |conference=March Meeting 2007, American Physical Society |volume=52 |issue=1 |at=U20.9 |access-date=30 October 2008}}</ref>