New MESA Group Has a Systems Perspective

Jeff Valenti, valenti@stsci.edu

The Instruments Division is home to most of the instrument scientists and analysts at the Institute. Until last year, division support for the James Webb Space Telescope was drawn mainly from the Webb Instrument Team (WIT), with the remainder coming from the Telescopes group. As Institute preparations for Webb ramped up, the size of the WIT grew to more than 30 people. In response, the WIT split in May 2012 into separate groups for the four Webb instruments: Near Infrared Camera (NIRCam), Near-Infrared Spectrograph (NIRSpec), Near InfraRed Imager and Slitless Spectrograph (NIRISS), and Mid Infrared Instrument (MIRI). Support for the Webb Fine Guidance Sensor (FGS) moved to the Telescopes group. We also created the Mission Engineering and Science Analysis (MESA) group to coordinate work across instruments—and even across missions. We continue to use “WIT” as an informal term for all Webb support in the Instruments Division, even though, formally, that support is now spread across six groups.

The philosophy of MESA

A guiding principle of Webb support at the Institute is that coordinated approaches across instruments are often better than separate but equivalent approaches. The instruments have different capabilities, but from a science and operations perspective they share many requirements. MESA identifies shared requirements and convenes cross-instrument working groups to find common solutions. For example, Webb instruments measure charge as it accumulates in detector pixels during an exposure, and we need to convert charge ramps into count rates for all instruments. This conversion requires algorithms to handle reference pixels, nonlinearity, cosmic rays, pixel crosstalk, and more. Therefore, MESA supports cross-instrument working groups to define which of the many possible algorithms are actually implemented in the Webb calibration pipeline. In this way, defining common algorithms and common terminology avoids redundant development efforts and simplifies learning about multiple instruments by comparison and contrast.

Cross-instrument working groups

Currently, MESA supports the following cross-instrument working groups for Webb:

  • Exposure time calculators;
  • Proposal and planning system;
  • Operations;
  • Observatory efficiency;
  • Calibration and commissioning;
  • Pipeline; and
  • Archiving ground-test data.

This working group sequence reflects the progressive stages of an observing program: conception, implementation, execution, calibration, and archiving. Each working group has at least one representative from each Webb instrument group at the Institute. Meanwhile, external instrument teams participate in the operations and calibration working groups. Institute engineers interact frequently with the working groups, as we jointly define the capabilities of our ground system to operate Webb. Individuals from Webb instrument groups or MESA personnel lead the working groups, MESA supports the working groups by developing processes, coordinating activities, and recording results.

Operations Detector Lab

MESA is also the home of the Operations Detector Lab (ODL) at the Institute. ODL maintains a lab with a dewar for cryogenic experiments, a flight-like near-IR detector system, and, for independent testing, a controller commonly used in ground-based observatories. ODL performs detector experiments that inform operational procedures and data analysis algorithms. For example, ODL is currently investigating a practical method to characterize persistence in every pixel, at all well depths. This persistence model may become the basis for a correction in the calibration pipeline. ODL also analyzes data obtained elsewhere during ground tests. For example, ODL is currently analyzing MIRI ground-test data in order to develop an algorithm for removing low-level, periodic perturbations in the bias level.

Cross-mission synergy

Formed in the WIT restructuring, MESA initially had a strong Webb focus. However, MESA aspires to find cross-mission opportunities to pursue synergistic developments. A good example—predating MESA—is the development of a new calibration system for reference data, which both Hubble and Webb will use. Hubble systems occasionally require refreshing as technology advances. At the same time, Hubble can take advantage of developments needed for Webb. And Webb benefits by having its newly developed systems tested in the Hubble operational environment. As one example, MESA has begun investigating data-processing tools that Webb needs, but also might benefit Hubble.

The future

MESA is approaching the anniversary of a successful first year. The use of cross-instrument working groups is functioning smoothly now for Webb. We are planning to strengthen interactions between these working groups and the numerous engineering groups at the Institute. In the coming year, MESA will strive to incorporate Hubble into the cross-instrument working groups and the cross-mission efforts.