diff --git a/Framework/Algorithms/src/CalculateSlits.cpp b/Framework/Algorithms/src/CalculateSlits.cpp
index e8905f9ca467aa9c4478ea566aa3b79598d9d604..1014251fc3ca599756530d3ee06c457b5dbdcef1 100644
--- a/Framework/Algorithms/src/CalculateSlits.cpp
+++ b/Framework/Algorithms/src/CalculateSlits.cpp
@@ -39,7 +39,7 @@ const std::string CalculateSlits::category() const {
 /// Algorithm's summary for use in the GUI and help. @see Algorithm::summary
 const std::string CalculateSlits::summary() const {
   return "Calculates appropriate slit widths for reflectometry instruments "
-         "based on the instrument setup and desired resolution and footprint of the experiment.";
+         "based on the instrument setup, desired resolution, and desired footprint of the experiment.";
 }
 
 //----------------------------------------------------------------------------------------------
diff --git a/MantidQt/CustomInterfaces/inc/MantidQtCustomInterfaces/QtReflMainView.h b/MantidQt/CustomInterfaces/inc/MantidQtCustomInterfaces/QtReflMainView.h
index 1306417ed5c84ffd9139390a259a08e08a250d6d..ab2f48dfa2db8daac520f43ab82517c3a5325e67 100644
--- a/MantidQt/CustomInterfaces/inc/MantidQtCustomInterfaces/QtReflMainView.h
+++ b/MantidQt/CustomInterfaces/inc/MantidQtCustomInterfaces/QtReflMainView.h
@@ -12,136 +12,138 @@
 #include <QSignalMapper>
 #include "ui_ReflMainWidget.h"
 
-namespace MantidQt
-{
-  namespace CustomInterfaces
-  {
-
-    /** QtReflMainView : Provides an interface for processing reflectometry data.
-
-    Copyright &copy; 2014 ISIS Rutherford Appleton Laboratory, NScD Oak Ridge National Laboratory & European Spallation Source
-
-    This file is part of Mantid.
-
-    Mantid is free software; you can redistribute it and/or modify
-    it under the terms of the GNU General Public License as published by
-    the Free Software Foundation; either version 3 of the License, or
-    (at your option) any later version.
-
-    Mantid is distributed in the hope that it will be useful,
-    but WITHOUT ANY WARRANTY; without even the implied warranty of
-    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-    GNU General Public License for more details.
-
-    You should have received a copy of the GNU General Public License
-    along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
-    File change history is stored at: <https://github.com/mantidproject/mantid>
-    Code Documentation is available at: <http://doxygen.mantidproject.org>
-    */
-    class DLLExport QtReflMainView : public MantidQt::API::UserSubWindow, public ReflMainView
-    {
-      Q_OBJECT
-    public:
-      QtReflMainView(QWidget *parent = 0);
-      virtual ~QtReflMainView();
-
-      /// Name of the interface
-      static std::string name() { return "ISIS Reflectometry (Polref)"; }
-      // This interface's categories.
-      static QString categoryInfo() { return "Reflectometry"; }
-
-      //Connect the model
-      virtual void showTable(QReflTableModel_sptr model);
-      virtual void showSearch(ReflSearchModel_sptr model);
-
-      //Dialog/Prompt methods
-      virtual std::string askUserString(const std::string& prompt, const std::string& title, const std::string& defaultValue);
-      virtual bool askUserYesNo(std::string prompt, std::string title);
-      virtual void giveUserInfo(std::string prompt, std::string title);
-      virtual void giveUserWarning(std::string prompt, std::string title);
-      virtual void giveUserCritical(std::string prompt, std::string title);
-      virtual void showAlgorithmDialog(const std::string& algorithm);
-      virtual std::string requestNotebookPath();
-
-      //Plotting
-      virtual void plotWorkspaces(const std::set<std::string>& workspaces);
-
-      //Set the status of the progress bar
-      virtual void setProgressRange(int min, int max);
-      virtual void setProgress(int progress);
-
-      //Get status of the checkbox which dictates whether an ipython notebook is produced
-      virtual bool getEnableNotebook();
-
-      //Settor methods
-      virtual void setSelection(const std::set<int>& rows);
-      virtual void setTableList(const std::set<std::string>& tables);
-      virtual void setInstrumentList(const std::vector<std::string>& instruments, const std::string& defaultInstrument);
-      virtual void setOptionsHintStrategy(MantidQt::MantidWidgets::HintStrategy* hintStrategy);
-      virtual void setClipboard(const std::string& text);
-
-      //Accessor methods
-      virtual std::set<int> getSelectedRows() const;
-      virtual std::set<int> getSelectedSearchRows() const;
-      virtual std::string getSearchInstrument() const;
-      virtual std::string getProcessInstrument() const;
-      virtual std::string getWorkspaceToOpen() const;
-      virtual std::string getClipboard() const;
-      virtual std::string getSearchString() const;
-
-      virtual boost::shared_ptr<IReflPresenter> getPresenter() const;
-
-    private:
-      //initialise the interface
-      virtual void initLayout();
-      //the presenter
-      boost::shared_ptr<IReflPresenter> m_presenter;
-      //the models
-      QReflTableModel_sptr m_model;
-      ReflSearchModel_sptr m_searchModel;
-      //the interface
-      Ui::reflMainWidget ui;
-      //the workspace the user selected to open
-      std::string m_toOpen;
-      QSignalMapper* m_openMap;
-      MantidWidgets::SlitCalculator* m_calculator;
-
-    private slots:
-      void on_actionNewTable_triggered();
-      void on_actionSaveTable_triggered();
-      void on_actionSaveTableAs_triggered();
-      void on_actionAppendRow_triggered();
-      void on_actionPrependRow_triggered();
-      void on_actionDeleteRow_triggered();
-      void on_actionProcess_triggered();
-      void on_actionGroupRows_triggered();
-      void on_actionClearSelected_triggered();
-      void on_actionCopySelected_triggered();
-      void on_actionCutSelected_triggered();
-      void on_actionPasteSelected_triggered();
-      void on_actionExpandSelection_triggered();
-      void on_actionOptionsDialog_triggered();
-      void on_actionSearch_triggered();
-      void on_actionTransfer_triggered();
-      void on_actionImportTable_triggered();
-      void on_actionExportTable_triggered();
-      void on_actionHelp_triggered();
-      void on_actionPlotRow_triggered();
-      void on_actionPlotGroup_triggered();
-      void on_actionSlitCalculator_triggered();
-
-      void on_comboSearchInstrument_currentIndexChanged(int index);
-      void on_comboProcessInstrument_currentIndexChanged(int index);
-
-      void setModel(QString name);
-      void tableUpdated(const QModelIndex& topLeft, const QModelIndex& bottomRight);
-      void showContextMenu(const QPoint& pos);
-      void showSearchContextMenu(const QPoint& pos);
-    };
-
-
-  } // namespace CustomInterfaces
+namespace MantidQt {
+namespace CustomInterfaces {
+
+/** QtReflMainView : Provides an interface for processing reflectometry data.
+
+Copyright &copy; 2014 ISIS Rutherford Appleton Laboratory, NScD Oak Ridge
+National Laboratory & European Spallation Source
+
+This file is part of Mantid.
+
+Mantid is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 3 of the License, or
+(at your option) any later version.
+
+Mantid is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+File change history is stored at: <https://github.com/mantidproject/mantid>
+Code Documentation is available at: <http://doxygen.mantidproject.org>
+*/
+class DLLExport QtReflMainView : public MantidQt::API::UserSubWindow,
+                                 public ReflMainView {
+  Q_OBJECT
+public:
+  QtReflMainView(QWidget *parent = 0);
+  virtual ~QtReflMainView();
+
+  /// Name of the interface
+  static std::string name() { return "ISIS Reflectometry (Polref)"; }
+  // This interface's categories.
+  static QString categoryInfo() { return "Reflectometry"; }
+
+  // Connect the model
+  virtual void showTable(QReflTableModel_sptr model);
+  virtual void showSearch(ReflSearchModel_sptr model);
+
+  // Dialog/Prompt methods
+  virtual std::string askUserString(const std::string &prompt,
+                                    const std::string &title,
+                                    const std::string &defaultValue);
+  virtual bool askUserYesNo(std::string prompt, std::string title);
+  virtual void giveUserInfo(std::string prompt, std::string title);
+  virtual void giveUserWarning(std::string prompt, std::string title);
+  virtual void giveUserCritical(std::string prompt, std::string title);
+  virtual void showAlgorithmDialog(const std::string &algorithm);
+  virtual std::string requestNotebookPath();
+
+  // Plotting
+  virtual void plotWorkspaces(const std::set<std::string> &workspaces);
+
+  // Set the status of the progress bar
+  virtual void setProgressRange(int min, int max);
+  virtual void setProgress(int progress);
+
+  // Get status of the checkbox which dictates whether an ipython notebook is
+  // produced
+  virtual bool getEnableNotebook();
+
+  // Settor methods
+  virtual void setSelection(const std::set<int> &rows);
+  virtual void setTableList(const std::set<std::string> &tables);
+  virtual void setInstrumentList(const std::vector<std::string> &instruments,
+                                 const std::string &defaultInstrument);
+  virtual void
+  setOptionsHintStrategy(MantidQt::MantidWidgets::HintStrategy *hintStrategy);
+  virtual void setClipboard(const std::string &text);
+
+  // Accessor methods
+  virtual std::set<int> getSelectedRows() const;
+  virtual std::set<int> getSelectedSearchRows() const;
+  virtual std::string getSearchInstrument() const;
+  virtual std::string getProcessInstrument() const;
+  virtual std::string getWorkspaceToOpen() const;
+  virtual std::string getClipboard() const;
+  virtual std::string getSearchString() const;
+
+  virtual boost::shared_ptr<IReflPresenter> getPresenter() const;
+private:
+  // initialise the interface
+  virtual void initLayout();
+  // the presenter
+  boost::shared_ptr<IReflPresenter> m_presenter;
+  // the models
+  QReflTableModel_sptr m_model;
+  ReflSearchModel_sptr m_searchModel;
+  // the interface
+  Ui::reflMainWidget ui;
+  // the workspace the user selected to open
+  std::string m_toOpen;
+  QSignalMapper *m_openMap;
+  MantidWidgets::SlitCalculator *m_calculator;
+
+private slots:
+  void on_actionNewTable_triggered();
+  void on_actionSaveTable_triggered();
+  void on_actionSaveTableAs_triggered();
+  void on_actionAppendRow_triggered();
+  void on_actionPrependRow_triggered();
+  void on_actionDeleteRow_triggered();
+  void on_actionProcess_triggered();
+  void on_actionGroupRows_triggered();
+  void on_actionClearSelected_triggered();
+  void on_actionCopySelected_triggered();
+  void on_actionCutSelected_triggered();
+  void on_actionPasteSelected_triggered();
+  void on_actionExpandSelection_triggered();
+  void on_actionOptionsDialog_triggered();
+  void on_actionSearch_triggered();
+  void on_actionTransfer_triggered();
+  void on_actionImportTable_triggered();
+  void on_actionExportTable_triggered();
+  void on_actionHelp_triggered();
+  void on_actionPlotRow_triggered();
+  void on_actionPlotGroup_triggered();
+  void on_actionSlitCalculator_triggered();
+
+  void on_comboSearchInstrument_currentIndexChanged(int index);
+  void on_comboProcessInstrument_currentIndexChanged(int index);
+
+  void setModel(QString name);
+  void tableUpdated(const QModelIndex &topLeft, const QModelIndex &bottomRight);
+  void showContextMenu(const QPoint &pos);
+  void showSearchContextMenu(const QPoint &pos);
+};
+
+} // namespace CustomInterfaces
 } // namespace Mantid
 
-#endif  /* MANTID_CUSTOMINTERFACES_QTREFLMAINVIEW_H_ */
+#endif /* MANTID_CUSTOMINTERFACES_QTREFLMAINVIEW_H_ */
diff --git a/MantidQt/CustomInterfaces/src/QtReflMainView.cpp b/MantidQt/CustomInterfaces/src/QtReflMainView.cpp
index 6f599771e4f4ffc5b63452900b01485518c834b5..f3632eaf125e82c4d98e20af56f2a998ad70bcea 100644
--- a/MantidQt/CustomInterfaces/src/QtReflMainView.cpp
+++ b/MantidQt/CustomInterfaces/src/QtReflMainView.cpp
@@ -262,12 +262,16 @@ namespace MantidQt
     void QtReflMainView::on_comboProcessInstrument_currentIndexChanged(int index)
     {
       ui.comboSearchInstrument->setCurrentIndex(index);
+      m_calculator->setCurrentInstrumentName(ui.comboProcessInstrument->currentText().toStdString());
+      m_calculator->emitInstrumentChangedSignal();
     }
 
     /** This slot is used to syncrhonise the two instrument selection widgets */
     void QtReflMainView::on_comboSearchInstrument_currentIndexChanged(int index)
     {
       ui.comboProcessInstrument->setCurrentIndex(index);
+      m_calculator->setCurrentInstrumentName(ui.comboProcessInstrument->currentText().toStdString());
+      m_calculator->emitInstrumentChangedSignal();
     }
 
     /**
@@ -299,6 +303,7 @@ namespace MantidQt
     */
     void QtReflMainView::on_actionSlitCalculator_triggered()
     {
+      m_calculator->setCurrentInstrumentName(ui.comboProcessInstrument->currentText().toStdString());
       m_calculator->show();
     }
 
diff --git a/MantidQt/MantidWidgets/inc/MantidQtMantidWidgets/SlitCalculator.h b/MantidQt/MantidWidgets/inc/MantidQtMantidWidgets/SlitCalculator.h
index 199550efb76f7309a1956f1c946d82bbbd561e29..f88271b88c5cc957ba55b5add0fc6073772d8bf0 100644
--- a/MantidQt/MantidWidgets/inc/MantidQtMantidWidgets/SlitCalculator.h
+++ b/MantidQt/MantidWidgets/inc/MantidQtMantidWidgets/SlitCalculator.h
@@ -4,49 +4,62 @@
 //----------------------------------
 // Includes
 //----------------------------------
+#include "MantidAPI/InstrumentDataService.h"
+
 #include "WidgetDllOption.h"
 #include "ui_SlitCalculator.h"
-
 #include <QDialog>
 
-namespace MantidQt
-{
-  namespace MantidWidgets
-  {
-    /** SlitCalculator : A calculator for Reflectometry instrument slits
-
-    Copyright &copy; 2015 ISIS Rutherford Appleton Laboratory, NScD Oak Ridge National Laboratory & European Spallation Source
-
-    This file is part of Mantid.
-
-    Mantid is free software; you can redistribute it and/or modify
-    it under the terms of the GNU General Public License as published by
-    the Free Software Foundation; either version 3 of the License, or
-    (at your option) any later version.
-
-    Mantid is distributed in the hope that it will be useful,
-    but WITHOUT ANY WARRANTY; without even the implied warranty of
-    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-    GNU General Public License for more details.
-
-    You should have received a copy of the GNU General Public License
-    along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
-    File change history is stored at: <https://github.com/mantidproject/mantid>
-    Code Documentation is available at: <http://doxygen.mantidproject.org>
-    */
-    class EXPORT_OPT_MANTIDQT_MANTIDWIDGETS SlitCalculator : public QDialog
-    {
-      Q_OBJECT
-    public:
-        SlitCalculator(QWidget* parent);
-        virtual ~SlitCalculator();
-    protected:
-        Ui::SlitCalculator ui;
-    private slots:
-        void on_recalculate_triggered();
-    };
-  }
+namespace MantidQt {
+namespace MantidWidgets {
+/** SlitCalculator : A calculator for Reflectometry instrument slits
+
+Copyright &copy; 2015 ISIS Rutherford Appleton Laboratory, NScD Oak Ridge
+National Laboratory & European Spallation Source
+
+This file is part of Mantid.
+
+Mantid is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 3 of the License, or
+(at your option) any later version.
+
+Mantid is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+File change history is stored at: <https://github.com/mantidproject/mantid>
+Code Documentation is available at: <http://doxygen.mantidproject.org>
+*/
+class EXPORT_OPT_MANTIDQT_MANTIDWIDGETS SlitCalculator : public QDialog {
+  Q_OBJECT
+public:
+  SlitCalculator(QWidget *parent);
+  virtual ~SlitCalculator();
+  void setInstrument(std::string instrumentName);
+  Mantid::Geometry::Instrument_sptr getInstrument();
+  void setCurrentInstrumentName(std::string instrumentName);
+  std::string getCurrentInstrumentName();
+  void emitInstrumentChangedSignal();
+
+protected:
+  Ui::SlitCalculator ui;
+
+private:
+  Mantid::Geometry::Instrument_sptr instrument;
+  std::string currentInstrumentName;
+  void setupSlitCalculatorWithInstrumentValues(
+      Mantid::Geometry::Instrument_const_sptr);
+signals:
+  void instrumentChanged();
+private slots:
+  void on_recalculate_triggered();
+};
+}
 }
 
 #endif /* MANTID_MANTIDWIDGETS_SLITCALCULATOR_H */
diff --git a/MantidQt/MantidWidgets/src/SlitCalculator.cpp b/MantidQt/MantidWidgets/src/SlitCalculator.cpp
index 08255d1a7e14fbf4cb8e17e50e13a3c4ff2ce317..7a456a5ec653b37ba6886ab59128920a81182a6d 100644
--- a/MantidQt/MantidWidgets/src/SlitCalculator.cpp
+++ b/MantidQt/MantidWidgets/src/SlitCalculator.cpp
@@ -1,48 +1,106 @@
 #include "MantidQtMantidWidgets/SlitCalculator.h"
 #include "MantidAPI/AlgorithmManager.h"
-#include <math.h>
+#include "MantidAPI/Progress.h"
+#include "MantidGeometry/Instrument/InstrumentDefinitionParser.h"
+
 
-namespace MantidQt
-{
-  namespace MantidWidgets
-  {
-    SlitCalculator::SlitCalculator(QWidget *parent)
-    {
-      Q_UNUSED(parent);
-      ui.setupUi(this);
-      on_recalculate_triggered();
-    }
+#include <math.h>
 
-    SlitCalculator::~SlitCalculator()
-    {
-    }
+namespace MantidQt {
+namespace MantidWidgets {
+SlitCalculator::SlitCalculator(QWidget *parent) {
+  Q_UNUSED(parent);
+  ui.setupUi(this);
+  if (currentInstrumentName == ""){
+      currentInstrumentName = "INTER";
+      setInstrument(currentInstrumentName);
+  }
+  on_recalculate_triggered();
+}
+void SlitCalculator::emitInstrumentChangedSignal(){
+    on_recalculate_triggered();
+}
+SlitCalculator::~SlitCalculator() {}
+void SlitCalculator::setInstrument(std::string instrumentName) {
+  // we want to get the most up-to-date definition, so we use the current
+  // date/time
+  auto date = Mantid::Kernel::DateAndTime::getCurrentTime().toISO8601String();
+  // find the full path to the definition file
+  auto filename =
+      Mantid::API::ExperimentInfo::getInstrumentFilename(instrumentName, date);
+  // parse the XML that we have found for the definition
+  Mantid::Geometry::InstrumentDefinitionParser parser =
+      Mantid::Geometry::InstrumentDefinitionParser(
+          filename, instrumentName,
+          Mantid::Kernel::Strings::loadFile(filename));
+  // retrieving the mangled name of the instrument
+  std::string instrumentNameMangled = parser.getMangledName();
+  // See if we have a definition already in the InstrumentDataService
+  if (Mantid::API::InstrumentDataService::Instance().doesExist(
+          instrumentNameMangled)) {
+    // If it does, set the associated instrument to the one we have found.
+    this->instrument = Mantid::API::InstrumentDataService::Instance().retrieve(
+        instrumentNameMangled);
+  } else {
+    // We set the XML that we have found for the instrument.
+    Mantid::API::Progress *prog = new Mantid::API::Progress();
+    this->instrument = parser.parseXML(prog);
+    delete prog;
+  }
+  setupSlitCalculatorWithInstrumentValues(instrument);
+}
 
-    void SlitCalculator::on_recalculate_triggered()
-    {
-      //Gather input
-      const double s1s2 = ui.spinSlit1Slit2->value();
-      const double s2sa = ui.spinSlit2Sample->value();
-      const double res = ui.spinResolution->value();
-      const double footprint = ui.spinFootprint->value();
-      const double angle = ui.spinAngle->value();
+void SlitCalculator::setupSlitCalculatorWithInstrumentValues(
+    Mantid::Geometry::Instrument_const_sptr instrument) {
+  auto slit1Component = instrument->getComponentByName("slit1");
+  auto slit2Component = instrument->getComponentByName("slit2");
+  auto sampleComponent = instrument->getComponentByName("some-surface-holder");
+    // convert between metres and millimetres
+  const double s1s2 = 1e3 * slit1Component->getDistance(*slit2Component);
+  ui.spinSlit1Slit2->setValue(s1s2);
+  const double s2sa = 1e3 * slit2Component->getDistance(*sampleComponent);
+  ui.spinSlit2Sample->setValue(s2sa);
+  
+}
+Mantid::Geometry::Instrument_sptr SlitCalculator::getInstrument() {
+  return this->instrument;
+}
+void SlitCalculator::setCurrentInstrumentName(std::string instrumentName){
+    this->currentInstrumentName = instrumentName;
+}
+std::string SlitCalculator::getCurrentInstrumentName(){
+    return this->currentInstrumentName;
+}
+void SlitCalculator::on_recalculate_triggered() {
+  const auto currentInstrument = getInstrument();
+  if (currentInstrument->getName() != currentInstrumentName){
+    setInstrument(currentInstrumentName);
+  }
+  // Gather input
+  const double s1s2 = ui.spinSlit1Slit2->value();
+  const double s2sa = ui.spinSlit2Sample->value();
+  const double res = ui.spinResolution->value();
+  const double footprint = ui.spinFootprint->value();
+  const double angle = ui.spinAngle->value();
 
-      //Calculate values
-      Mantid::API::IAlgorithm_sptr algSlit = Mantid::API::AlgorithmManager::Instance().create("CalculateSlits");
-      algSlit->initialize();
-      algSlit->setChild(true);
-      algSlit->setProperty("Slit1Slit2", s1s2);
-      algSlit->setProperty("Slit2SA", s2sa);
-      algSlit->setProperty("Resolution", res);
-      algSlit->setProperty("Footprint", footprint);
-      algSlit->setProperty("Angle", angle);
-      algSlit->execute();
+  // Calculate values
+  Mantid::API::IAlgorithm_sptr algSlit =
+      Mantid::API::AlgorithmManager::Instance().create("CalculateSlits");
+  algSlit->initialize();
+  algSlit->setChild(true);
+  algSlit->setProperty("Slit1Slit2", s1s2);
+  algSlit->setProperty("Slit2SA", s2sa);
+  algSlit->setProperty("Resolution", res);
+  algSlit->setProperty("Footprint", footprint);
+  algSlit->setProperty("Angle", angle);
+  algSlit->execute();
 
-      const double s1 = algSlit->getProperty("Slit1");
-      const double s2 = algSlit->getProperty("Slit2");
+  const double s1 = algSlit->getProperty("Slit1");
+  const double s2 = algSlit->getProperty("Slit2");
 
-      //Update output
-      ui.slit1Text->setText(QString::number(s1,'f',3));
-      ui.slit2Text->setText(QString::number(s2,'f',3));
-    }
-  }
+  // Update output
+  ui.slit1Text->setText(QString::number(s1, 'f', 3));
+  ui.slit2Text->setText(QString::number(s2, 'f', 3));
+}
+}
 }