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Macduff: An astronomical example

Published onJul 14, 2020
Macduff: An astronomical example
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Summary: This is a thought-experiment about how to unify and upgrade the flow of observations from astronomical observatories into an accessible stream of observations, events, and collections of various sizes.

Status: Thought experiment for discussion, glossary of terms, and cross-field glossary of related terms in the world of astro event brokers.

Background

An observatory network might have a few million observations a night. Most telescopes capture much more data than they preserve; they make their best effort to save what might be useful to their own future work or to others who could use something they’ve seen in the context of othe robservations.

As a result, astronomical ‘event brokers’ listen to the observation streams of a network and filter for potential events that different audiences [usually: other observatories] might be interested in, in order to co-associate further observations or annotations.

What does the network need to do to turn a subset of observations into structured collections?

What does a broker need to do to maintain its own registry (possibly large, largely unused, speculative datasets — costs of storage covered by its brokerage service) for original + filtered collections?

Terminology

Telescopes are real-world data sources with

  • hardware type

  • configuration

  • calibration functions tied to the {type, config}

  • calibration data updated at least nightly, tuning those functions

    • weather data, if terrestrial, informing this

  • location

An observation from a scope has

  • target location

  • angular magnification

  • field angle (field of view)

  • time

  • duration (time over which light is integrated)
    this allows things to be somewhat fuzzy: you could compile many observations of different durations from a raw video- or image-stream.

  • implicit noise: mechanical, electrical, astronomical, cosmic rays.

An event is the appearance of something new in the sky. It has

  • time of first observation (a lower bound)

  • duration (if transient)

  • classification

  • weight / likelihood of being a real event, observed from most locations that happened to observe the same target (concept not standardized)

An event stream from a set of sources is a feed of timestamped events that cross some threshhold of likelihood to be interesting.

In addition to communication protocols for managing a shared distributed feed, aligning events across different scopes requires cross-calibration.

  • Where the scopes use the same config, just a comparison of parameters.

  • Where setups are different, a mapping function that projects both into comparable space+time coordinates.

An event broker is a service that subscribes to event streams from many observatories, lets researchers and observatories register interest in certain kinds of events, and submits requests to participating observatories to follow up on a possible event by renalyzing their own observations from that location, or making new ones.
For instance, traditional scopes may subscribe to the event feed of LIGO to look at points in the sky where there was a possible gravitational event. Or a broker with access to many feeds could track where each scope is looking and give it feedback, if anyone else recorded an observation in that region, to reanalyze its raw data before discarding it for possible additional observations of the same event.

Examples

Past examples:

VOEventNet (2007),
SkyAlert (2009),
CRTS (2009),
automated classification of transients (2011)

Current examples:

  • the Large Synoptic Survey Telescope (’15) : a raw feed of 1M potential transient events a night; a simple ‘event broker’ to help flag events of interest to common users; and access to the full stream (for a fee) granted to external brokers who flag different subsets for different audiences.

  • the Antares (+)and GROWTH projects — both developing machine-learning tools for automating filtering + classifying events, allowing for automated telescopes to choose where to look based on the results

  • Maximizing Science in the LSST Era” (pp.138-40, 145-9) has many detailed examples, including data sharing tools, community brokers w/ local data filters, cross-matching of alerts, and the need for common protocols of communication b/t brokers and other data services (e.g., SIMBAD and NED)


Macduff:In

Inputs. Say we have 6 telescopes in our network, Ta - Tg,
run by Anubis, Balor, Charon, Dante, Eurydice, and Frank.

Ta and Tb are the same hardware in different locations.
Tc is different hardware with roughly the same settings.
Td is a scope capturing a continuous imagestream, converted later into chunked observations.
Te is an orbital scope.
Tf is a virtual-aperture scope (combined data from multiple scopes on different sides of the planet)

Macduff:Out

Some of the outputs desired from this network: raw data, existence, interest, alignment, combination, and replication.

Raw data - The observational data synthesized into inferred events. Much of this is not stored for long, but feedback from others may lead to reanalysis of observations that would otherwise be ignored. References to observations and their bulk properties are important.

Existence - What events have been observed in this region / within this time-range? With what level of confidence that this was a real event and not noise?

Interest - What demand is there for events of different shapes? What observatories have registered monitoring scripts with event brokers to help strengthen coverage of certain events?

Alignment - Did anyone else observe something close to this {time/place/classification}? Did anyone observe something sub-threshhold? How have various brokers prioritized events for others to look at or replicate?

Combination - An aggregate feed from many sources. Combined events: events where the combined observations from multiple sources/spectra pass a combined threshold. A combined feed of only combined-event data.

Real-time replication - A prioritized feed of recent events that merit further observation, often time-sensitive [transient events]. A streamlined feed of high-priority events and their followups, perhaps segmented or filtered by the types of scopes that could respond [which spectra are most interesting to observe, what features are needed].

Current concepts of event brokers often address replication: observatories have limited scope time, it takes time to move a large scope’s focus across the sky to a new target, and for interesting events we want to capture them in as many bands as possible. So if you have an infrared scope and have just finished an observation and want to know where to look next, you may want to know if any other scopes near your current field are looking for IR confirmation of a potential event.

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