Observing Operations | Reviews | Survey Management

Sloan Digital Sky Survey
Review of Data Processing Operations

Status of Data Processing and Distribution Systems as of July 2000
Stephen Kent
July 5, 2000

We present in this document a brief description of the major components of the date processing and distribution systems, their status as of July 2000, tasks yet to be done, and a brief description of recent progress and activities.

I. Requirements

The top level science and functional requirements are defined by two documents:

  • Science Requirements
  • Software requirements
  • In cases that the two disagree, the former has precedence.


    An overview of the data flow is shown in the following figure:

    The data flow logically breaks into 4 categories:

    1. Imaging Processing: A single night of imaging observations produce 8 DLT 4000 tapes: 6 with full frame imaging data from the 2.5m camera, 1 with log data and ancillary astrometric data from the 2.5m camera, and 1 with data from the 0.5m photometric telescope (200 GB total). The tapes are shipped via express courier to Fermilab. The data are processed through a series of pipelines: MT, SSC, Astrometric, Postage Stamp, Photometric, and Final Calibration pipelines. The outputs consist of calibrations, corrected frames, object lists, and various reduced versions of the image frames (atlas images, etc.) The object lists are written to the Operational Database (which uses Objectivity) in the form of instrumental quantities plus calibrations.

    2. Survey Planning & Target Selection: Once more plates are needed for observing at APO, the accumulated imaging data that are acceptable and cover sky available for observing in the next few months are fed through a series of programs that merge the overlapping object lists to make a unique catalog, select targets in a number of categories, lay out a set of overlapping "tiles" on the sky to cover the targets, assign targets to the tiles, and design plates (with drilling plans) for those tiles. The designs are sent to UW for actual fabrications of the plates and to APO for use in observations.

      Once a month, a plan for which areas of sky to image next is computed based on the accumulated imaging data in hand plus a strategy for minimizing the time to completion of the survey. Additionally, feedback is provided on which plates observed are "done" and which need reobserving.

    3. Spectroscopic Processing: A single night of spectroscopic observations produces 1 DLT 4000 tape. In practice, the data are also copied via network back to Fermilab for processing. The data are fed through a 2-stage spectroscopic pipeline. The outputs are 1-d spectra plus redshifts and other parameters.

    4. Data distribution: The data products are produced in the form of flat files (FITS or ascii) plus object catalogs in the operational database. The latter are calibrated and exported as another set of flat files. A subset of the files are sent via network to each of the collaborating institutions. (Tapes are written for shipment to Japan). The object catalogs and ancillary information are also written to the Science Database (SX).

    III. Status

    1. Data Acquisition System: These systems were reviewed at the APO operations meeting. They are listed here as a reminder that the same group that operated the data processing systems also maintains the DA systems.

    2. Data Processing Pipelines: With a few exceptions, all data processing pipelines meet their basic functional requirements. The major work remaining involves testing the pipelines on real data that span the full range of observing conditions encountered during the survey, enhancing the pipelines to deal with artifacts introduced in the data by the telescope or instruments, adding QA tests, fixing bugs and otherwise cleaning up the code, and documenting the code and operations procedures. Particularly troublesome areas include:

      1. The photometric calibrations have been troublesome, in particular various problems existed with detector contamination, inadequate telescope baffling, and variations in filter transmission curves that affected the accuracy of the calibration. We think the problems are solved, and some testing has been done that shows this, but addtional verification is needed. The details of the transfer of photometric calibrations from the PT to the 2.5 m will change and will change the photometric zero-points slightly. Testing has also not been thoroughly done on scans that cover the full range of observing conditions and telescope motion. The MT pipeline is known to require additional code to remove fringing in the i' and z' bands.
      2. The target selection algorithms, while close to completion, are not absolutely finished and fully verified. The galaxy selection algorithm is the most advanced, and aside from possible shifts in the photometric zero-point, it is thought to be done the remaining work to be done is verification. The quasar algorithm is 90% complete, but much testing and running of the algorithms on a variety of data remain. It will be finalized by this summer. The remaining algorithms are in various states of completion but they have less impact because they are used for categories that not intended to be complete surveys. There is an unresolved question about whether "fuzzy" boundaries are to be retained. The software at present does not adequately implement them.
      3. The criteria for acceptance or rejection of data are not fully defined, most importantly criteria on image quality. The current spec would cause rejection of virtually all data taken to date.
      4. The survey strategy process, which provides observing plans to APO, is still done by hand, and there is currently only crude feedback from the results of data processing to determine which areas of the sky are completed or in need of reobserving. Since we have so much new sky to observe, this is not a major problem at the moment, but will become so in a year.
      5. A problem with telescope tracking in certain parts of the sky complicates the astrometric calibrations and slightly affects image quality.
      6. The various camera and DA systems are producing several artifacts intermittently, including variations in the bias patterns, noise in at least one CCD, and dropped pixels. Most of these are removable in software, but require writing additional code and inserting by hand into the data processing pipelines.
      7. There are still planned enhancements to the photometric pipeline that might affect which objects are targeted.
      8. Many more global QA tests need to be implemented, e.g., comparing results of overlapping image runs, or repeat spectroscopic observations of the same object.

    3. Infrastructure: The infrastructure consists of a shared toolkit ("Dervish" and "astrotools") used by most of the pipelines, a code repository, a product versioning system, a bug tracking system, web sites, automated mailing lists, and other miscellaneous products. This infrastructure is largely in maintenance mode. There is much cleanup work that could be done but is put off as being of lower priority. The list includes:
      1. CVS (Code repository): Upgrade versions as necessary. Implement more secure operation.
      2. Astrotools: Fix numerous inconsistencies in ephemeris routines.
      3. Dervish: Clean up makefiles. Upgrade TCL to 8.0.
      4. All code: Port all products to latest Linux kernel & libraries.

    4. Data Processing: The mechanical aspects of data processing are largely in place. Data tapes are express shipped to Fermilab the critical data processing pipelines for the PT and 2.5 m imaging data are fully functional and are largely automated. Procedures exist for spooling input data, running jobs, collating and assessing the quality of the output data, writing the object catalogs to a database and bulk data to tape, and documenting the process. Similar procedures exist to handle the spectroscopic data, although they are less mature. The processing resources (CPU, disk, and tape) are in place and are adequate to handle the volume of data received to date. We are planning to upgrade the CPU and disk this fiscal year. The target selection and plate design pipelines are currently run interactively. Plate design files are distributed via a web site and are used by UW and APO. Processed data is being distributed routinely to the collaboration. To date we have processed of order 3 terabytes of raw imaging, data, designed over 240 plug plates, and processed 44 good spectroscopic plates (26,000 spectra). Troublesome areas include:
      1. The goal for turning around the imaging data through plate design is 1 month, not the 2 being achieved at present. Since spectroscopic observing is not consuming plates at the maximum expected rate, the 2 month turnaround is not yet a serious problem. Minor formatting problems with data being delivered from APO need to be resolved. Recently, correcting data for artifacts has slowed processing considerably. The planned omputing hardware upgrade will quadruple the processing power, reducing the turnaround time.
      2. Data are processed through current versions of each pipeline, and since the pipelines evolve, no single homogeneous set of processed data exists. This slows testing. It also means that target lists generated from earlier data processing runs will not match lists that would be generated from a more recent processing. However, there will be motivation to retain the spectra observed from the earlier target lists, with some process for reconciling the disparity in target lists. Our software and databases are not prepared to handle arbitrary decision trees of this sort.

    5. Data Distribution: The process by which data are pushed to the collaboration is crude. Not all data products are easily accessible at Fermilab; some are online, some are backed up to a tape robot, some are only available on tape offline, some are available but in a compressed format.

      The SX database is still being developed. New tools are needed to ease access by members of the collaboration.

      The SDSS is committed to a base level data release to the astronomical community in which the object catalogs are distributed via CDROM. The software needed to assemble the CDROMs exists as a prototype only; it was used to create a sampler CDROM in 1998. A full system will be developed at the time of the first full data release.

      An enhanced data release plan has been developed for putting additional datasets online, but this plan has not been approved or fully scoped.

    Resources and Management

    The software development, maintenance, and operations are shared amongst several institutions in the collaboration (approximately 17 FTEs total). At each institution the work is directed by one or more "PI's". The following table lists the institutions with significant responsibilities.
    1. Fermilab: Management, MT pipe, target selection, final calibration, operational database, spectro (2D), infrastructure, all data processing operations, DA maintenance, data distribution to collaboration.
    2. Princeton: Photometric pipelines, spectro (2D), photometric calibrations, mailing lists, bug database.
    3. USNO: Astrometric calibrations, operational database.
    4. U. of Chicago: Spectroscopic Pipelines.
    5. JHU: Photometric calibrations, Science Database.
    In addition, various scientists around the project contribute on an ad hoc basis.

    Decision making occurs as follows:

    1. The PIs direct the work and set priorities at their own institutions, with the proviso that support for data processing has high priority.
    2. Change requests or tasks that involve two or more pipelines or institutions are coordinates via weekely phone conferences, lead by Head of Data Processing and Distribution. Resolution is usually achieved by consensus.
    3. Decisions that impact the science of the survey or require specific test data to be obtained are elevated to the Head of Science Direction, Technical Director or the Change Control board as appropriate.
    Known problems:
    1. We are short on FTE's. We make up in part by depriving scientists of research time.
    2. Change control board is cumbersome and has not yet met.

    Submit comments and questions to the SDSS Helpdesk. Last updated 04/01/14