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\paperID{P10-14}
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\title{The ATST Virtual Instrument Model}

\author{Steve\ Wampler and Bret\ Goodrich}
\affil{National Solar Observatory, 950 N. Cherry Ave, Tucson, AZ USA 85750}
\contact{Steve Wampler}
\email{swampler@noao.edu}
\paindex{Wampler, S.}
\aindex{Goodrich, B.}
\authormark{Wampler \& Goodrich}

\keywords{ATST, software: architecture}

\begin{abstract}          % Leave intact
The Advanced Technology Solar Telescope (ATST) is intended to be the
premier solar observatory for experimental physics.  Unlike its night-time
counterparts that operate with relatively fixed instrument sets, ATST's
science goals and requirements are best met by a laboratory style
instrument configuration, where scientific requirements often mean that
instrumentation must be assembled by scientists to meet the unique demands of
each experiment.

In order to maximize observing efficiency the ATST software and control
systems must be designed to operate smoothly in this environment.
To meet the requirement of providing flexibility in a laboratory style
operations environment, the control system uses a Virtual Instrument Model.
This report introduces this model and briefly outlines its salient
characteristics.  The aim is to provide some insight into the approach
being proposed as part of the overall software and controls design and to
provide a foundation for discussions on the advantages and disadvantages
of using a virtual instrument model.
\end{abstract}

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\section{Introduction}

The \htmladdnormallinkfoot{Advanced Technology Solar Telescope (ATST)}{http://atst.nso.edu/} is a new 4-meter telescope optimized for solar astronomy.
It has a number of novel mechanical features
as part of that optimization: an off-axis design, heat stop, integrated
high-order AO, and a hybrid dome are several of these features.  From a
software perspective, none of the above pose any particular problem and
for that reason the majority of the ATST software systems follow design
models commonly used in modern night-time and radio astronomy systems.

One of the key science requirements does have a significant impact on the
system design both in mechanical systems and in software.  The ATST is
required to provide the flexibility inherent in a laboratory
environment\footnote{``ATST Science Requirements Document", SPEC-0001, p 58}.
The existing
\htmladdnormallinkfoot{Dunn Solar Telescope (DST)}{http://www.nso.edu/nsosp/dst} at Sunspot, NM, is specifically mentioned as a model that well illustrates
the desired flexibility.  On the
DST a series of optical benches on a protected rotating platform provide the
principal support for observing.  Scientists can construct instruments
specific to their experimental needs from existing components.  While a few
instruments are 'facility' and consist of a fixed set of components, even
these instruments may be combined with other components using dichroics, beam
splitters and slit reflections.  The ATST mechanical systems provide a
similar flexibility through eight optical benches: on a two level rotating
coudé platform in the telescope pier.

This approach differs considerably from current night-time observatory design.
Because most observing stations on night-time telescopes are exposed and often
subject to rotating gravity vectors, instruments developed for nighttime
observing are more fixed and considerable effort is spent to ensure that each
instrument supports a wide range of science experiments. However, an inherent
lack of flexibility results.  Often the first task required of an astronomer
wishing to perform an experiment it to locate a telescope and instrument
combination that is capable of performing the experiment.  The flexibility
provided by the DST and required of the ATST allows the scientist to construct
the instrument to meet the needs of the experiment.

Unfortunately, the DST is almost 40 years old and the software and controls
system has evolved over this time in a more or less ad hoc fashion.  The ATST
challenge for software has been to develop a more formal model that supports
this flexibility.  This formal model is then fit within a general control
system patterned after existing modern telescope control system design. This
document outlines the observing model used at ATST and concentrates on a
description of the \emph{Virtual Instrument Model}: that portion of the
observing model supporting flexible, laboratory style operation.

\section{The Experiment}

Observers at ATST are interested in performing experiments.  A central tenant
of the ATST control system model is that the system should be adapted to the
requirements of the experiment.  A laboratory environment provides flexible
support to carry out experiments that are likely not understood or defined at
the time the laboratory is designed.  Consequently, experiments are a formal
concept within the model.

In ATST, an
Experiment\footnote{A description of the terminology used for ATST observations may be found in the ATST documentation, "Operational Concepts Definition Document", SPEC-0013}
includes a \emph{Science Program} of \emph{Observations} and a
\emph{Virtual Instrument} capable of performing those observations.
(The Experiment also ultimately includes Results but that aspect is not
relevant to this discussion.)
Observations contain sequences of operational steps describing
the behavior of the instrument.  Each operational step consists of a set of
configuration parameters and a simple command describing a state change within
the instrument, collectively referred to as a Configuration. This use of
science programs is typical of modern observatory operations and matches
similar functionality provided at SOLIS, Gemini, VLT, ALMA, and other
observatories.

What differentiates the ATST approach from these other observatories is that,
instead of adapting experiments to fit within the bounds imposed by
instruments consisting of fixed components, the ATST observer can construct a
virtual instrument from available components to meet the needs of the
particular experiment.  This provides a great deal of flexibility in the
nature of experiments that can be performed at ATST.

\section{The Virtual Instrument}

Instruments consist of one or more \emph{Components}.  Some components may be
purely mechanical with no associated software (e.g., a dichroic).  Others
may be purely software (a sequencer).  Most, however, include both mechanical
and software aspects (cameras, scanners, etc.).  These last components are
called Devices.

In a conventional instrument the set of components that comprise the
instrument are fixed and permanently associated with each other. Nevertheless,
there is some software that understands these associations.  Thus the primary
difference between a virtual instrument and a conventional instrument is
merely that the associations within a virtual instrument are not fixed but
rather managed by software. A subtle difference that is implemented in the
ATST virtual instrument model is that telescope components can also be
associated as part of a virtual instrument.  An experiment that needs to
perform drift scanning across the solar disk can include the telescope mount
as a component.  An experiment performing near-limb observations is likely to
include the occulter as a component.

Virtual instruments are composable from a set of available components.
Scientists, as part of the design of their experiments, construct the
instrument they need for the experiment by assembling the requisite components
and associating them with a virtual instrument.  Virtual instruments are
uniquely identified and the component associations are preserved.
This allows virtual instruments to be readily reconstructed for future
experiments, if needed. Some virtual instruments are used so often that the
physical associations are also maintained.  These become the \emph{facility
instruments} for ATST. Facility instruments include a visible high resolution
spectrograph, a near-IR polarimeter, a broad-band imager, various tunable
filters, and others.  Virtual instrument identifications are also associated
with Experiments.

From a control perspective, once the component associations have been made
the control of a virtual instrument is identical to that of a conventional
instrument.  This simplifies the integration of instrument operation within
the otherwise conventional ATST control system.  

\begin{figure}
\epsscale{0.5}
\plotone{P10-14_2.eps}
\caption{The Balatron Mark IV Virtual Instrument}\label{P10-14:figure4}
\end{figure}

Components are hierarchical and may be composed from other components.  In
particular, instruments themselves are components.  Composition of instruments
is a common feature of operation of the DST and is expected to be a key
operational characteristic of ATST as well.  With a few additional components
several facility instruments may be associated into a new virtual instrument.
Figure~\ref{P10-14:figure4} shows one of several instrument layouts proposed by
K.S. Balasubramaniam and G.A. Gary at the May 2003
\htmladdnormallinkfoot{ATST Boulder Workshop}{http://atst.nso.edu/meetings/may03}.
           
Some experiments require cooperation between ATST and off-site observatories
(including off-planet).  A virtual instrument may include special proxy
components for coordination with off site facilities.

Elements of a virtual instrument do not need to be under computer control;
dichroics, beamsplitters and other static optical components need no
active control.
However, for bookkeeping purposes these \emph{passive components} are
included in the definition of a virtual instrument.

\section{Issues}

The virtual instrument approach does little to address a fundamental issue
associated with laboratory style operations.  In conventional systems with
fixed instruments great care can be, and is, taken to ensure that components
are properly aligned and movements are coordinated.  In a laboratory
environment where instrument components may be assembled, placed, and replaced
by observers it is much more difficult to ensure proper alignment
automatically.  The result is that instrument setup is often more time
consuming than with conventional instrumentation.
The availability of multiple optical benches provides ATST with the
ability to overlap
instrument setup and alignment with active observing, however, reducing the
effect of this issue on observing efficiency.

\section{Summary}

The virtual instrument model is a powerful concept for supporting the
flexibility inherent in a laboratory environment.  It formalizes the
approach already used in such environments enabling integration of the
laboratory environment into the conventional modern telescope control system.

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