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the grist project ( ) is enabling astronomers and the public to interact with the grid projects that are being constructed worldwide , and bring to flower the promise of easy , powerful , distributed computing . our objectives are to understand the role of service - oriented architectures in astronomical research , to bring the astronomical community to the grid  particularly teragrid ,  and to work with the national virtual observatory ( nvo ) to build a library of compute - based web services .    the scientific motivation for grist derives from creation and mining of wide - area federated images , catalogs , and spectra . an astronomical image collection may include multiple pixel layers covering the same region on the sky , with each layer representing a different waveband , time , instrument , observing condition , etc . the data analysis should combine these multiple observations into a unified understanding of the physical processes in the universe . the familiar way to do this is to cross - match source lists extracted from different images . however , there is growing interest in another method of federating images that reprojects each image to a common set of pixel planes , then stacks images and detects sources therein . while this has been done for years for small pointing fields , we are using the teragrid to perform this processing over wide areas of the sky in a systematic way , using ( pq ) survey data . we expect this `` hyperatlas '' approach will enable us to identify much fainter sources than can be detected in any individual image ; to detect unusual objects such as transients ; and to deeply compare ( e.g. , using principal component analysis ) the large surveys such as sdss , 2mass , dposs , etc . ( williams et al . 2003 ) . grist is helping to build an image - federation pipeline for the palomar - quest synoptic sky survey ( djorgovski et al . 2004 ) , with the objectives of mining pq data to find high redshift quasars , to study peculiar variable objects , and to search for transients in real - time ( mahabal et al . our pq processing pipeline will use the teragrid for processing and will comply with widely - accepted data formats and protocols supported by the vo community . the grist project is building web and grid services as well as the enabling workflow fabric to tie together these distributed services in the areas of data federation , mining , source extraction , image mosaicking , coordinate transformations , data subsetting , statistics histograms , kernel density estimation , and r language utilities exposed by services ( graham et al . 2004 ) ,  and visualization . composing multiple services into a distributed workflow architecture , as illustrated in figure  [ fig : workflow ] , with domain experts in different areas deploying and exposing their own services , has a number of distinct advantages , including :    * proprietary algorithms can be made available to end users without the need to distribute the underlying software . * software updates done on the server are immediately available to all users . * a particular service can be used in different ways as a component of multiple workflows . * a service may be deployed close to the data source , for efficiency . interactive deployment and control of these distributed services will be provided from a workflow manager . we expect to use nvo services for data access  images , catalogs , and spectra  as well as the nvo registry for service discovery . as described in section  [ sec : workflow ] , much of the pipeline and mining software for grist will be built in the form of web services . one of the reasons for building services is to be able to use them from a thin client , such as a web browser . however , for such services to be able to process private data or use high - end computing , there must be strong authentication of the user . the vo and grid communities are converging around the idea of x.509 certificates as a suitable credential for such authentication . however , most astronomers do not have such a certificate , and we do nt want to make them go through the trouble of getting one unless it is truly necessary . therefore , we are building services with `` graduated security '' , meaning not only that small requests on public data are available anonymously and simply , but also that large requests on private data can be serviced through the same interface . however in the latter case , a certificate is necessary . thus the service `` proves its usefulness '' with a simple learning curve , but requires a credential to be used at full - strength ( see illustration in figure  [ fig : grad_sec ] ) . a key science - driven workflow we are constructing is illustrated in the schematic in figure  [ fig : pq - pipeline ] . the primary objectives are to search for high redshift quasars and optical transients in data from the palomar - quest sky survey . the pipeline begins by federating multiwavelength datasets , and matching objects detected with the _ z _ filter with catalogs at other frequencies . cluster analysis performed on the resulting color - color plots ( e.g. , _ i - z _ vs. _ z - j _ ) yield new quasar candidates , and outliers may indicate the presence of other objects of interest . single epoch transients are indicated by objects that are detected in one filter but not others . an object that is detected in the reddest filter is of special interest since it could be a highly obscured object or a high redshift quasar . for multi - epoch transient search , illustrated in the lower part of figure  [ fig : pq - pipeline ] , we compare new data with a database of past epochs to detect new transients or other variable objects . as described above , a primary objective of the pq survey is the fast discovery of new types of transient sources by comparing data taken at different times . such transients should be immediately re - observed to get maximum scientific impact , so we are experimenting with `` dawn processing '' on the teragrid , meaning that data is streamed from the telescope to the compute facility as it is taken ( rather than days later ) . the pipeline itself is being built with streaming protocols so that unknown transients ( e.g. , newly identified variables or asteroids ) can be examined within hours of observation with a view to broadcasting an email alert to interested parties . grist is developing a library of interoperable grid services for astronomical data mining on the teragrid , compliant with grid and vo data formats , standards , and protocols . for ease of use , grist services are built with graduated security , requiring no more formal authentication than is appropriate for a given level of usage . grist technology is part of a palomar - quest data processing pipeline , under construction , to search for high red - shift quasars and optical transients . more information on grist can be found on our project web site at .
the grist project is developing a grid - technology based system as a research environment for astronomy with massive and complex datasets . 
 this knowledge extraction system will consist of a library of distributed grid services controlled by a workflow system , compliant with standards emerging from the grid computing , web services , and virtual observatory communities . 
 this new technology is being used to find high redshift quasars , study peculiar variable objects , search for transients in real time , and fit sdss qso spectra to measure black hole masses . 
 grist services are also a component of the `` hyperatlas '' project to serve high - resolution multi - wavelength imagery over the internet . in support of these science and outreach objectives 
 , the grist framework will provide the enabling fabric to tie together distributed grid services in the areas of data access , federation , mining , subsetting , source extraction , image mosaicking , statistics , and visualization .