
   CONTENTS

        A.  HOW TO SET UP A SIMULATION
        B.  THE SIMULATION PARAMETERS
        C.  CRYSTAL SHAPES AND ORIENTATIONS



   A.  HOW TO SET UP A NEW SIMULATION

   First try running some of the pre-stored simulations.    
   Change some of the controls settings to see their effects.

   Before going further in this section, click 'Reset' on 
   the control panel.  This restores the default settings 
   and clears internal variables.  
  
   The controls will now be set to run the 'Startup' 
   simulation.   

   1) On the control panel in the 'Crystals' section, the 
      left hand up-down control changes the 'Crystal type 
      number'.  Click the controls until this shows '1'.

   2) Select a crystal shape by clicking the 'Shape' button.  
      A file open dialogue box show the crystal shape files 
      available.  The shape files should normally be
      stored in the home directory of the HALO program.  

      Select "Hexagonal Ice Prism.xsh".   

   3) Now select the orientations that the ice crystal can 
      have.   Click the 'Orientation' button.  The file open 
      box will open again and this time will show the 
      orientation files available.

      Select "Random.xan"

      The Halo website describes the effects of different
      Crystal orientations.

   4) Check that the '%tot' box shows 100.  This box sets 
      the percentage population for each channel.   

   5) Unless the start-up simulation has been changed, you 
      will find that the 'Xtal Report' will now be displayed. 
      This automatically displays whenever the total crystal
      population does not add up to 100%

      On the Control Panel click the up-down buttons beneath
      'Crystals' to select channel 2.  Clear this by 
      clicking the button beneath captioned 'Clr 2'.

      Repeat for channel 3.  The Xtal Report should now show
      only one channel with a non zero percentage.  

   5) Now select the display type.

      From the menu under 'Projection and Plot Style' select
      'Zenith centered Fisheye' if it is not already selected.

   6) Select the plot style

      From the box immediately below, select 'Black dots 
      on white'

   7) Select the number of rays

      In the 'Rays '000' box type 10.  i.e. calculate 10,000 
      ray paths through the crystals.

   8) Click 'Start'.   

      HALO simulates the 22 and 46 degree circular halos 
      from randomly orientated hexagonal prism ice 
      crystals.

      Each ray that emerges from a crystal is plotted as 
      a black dot.

      The Status panel at lower left shows when the 
      simulation is complete and the time taken.   A 
      Pentium 500 will take 2-3 seconds.

   9) Changing the ice crystal proportions

      In the 'c/a ratio' box, enter 4.   The hexagonal 
      prisms are now four times as long as their 
      cross-section.  The 'c/a ratio' is the ratio of the
      length of the prism to its breadth as measured 
      between opposite edges.

      Run again.  Notice that the 46 degree halo is fainter 
      because a smaller proportion of the total rays now 
      pass though the end faces to form the 46 degree bow.


   10)Another halo.

      Click the 'Orientation' button again and select 
      "Horiz column 1deg dispersion.

      In the 'c/a ratio' box, enter 2.  The hexagonal 
      prisms are now twice as long as their cross-section.  
      The prisms are aligned with their long axis almost 
      horizontal.  The '1deg dispersion' indicates that 
      HALO varies the axial direction so that the tilts 
      overall have a Gaussian distribution with a standard 
      deviation of 1 degree.

      Click 'Start'.  The main halos are now the upper 
      and lower tangent arcs to the 22 degree halo.   A 
      parhelic circle is also visible.  HALO simultaneously 
      generates ALL the halos produced by the selected 
      crystal shape and orientation.

      If the Control Panel is in the way, hide it by 
      clicking the 'X' button.  Show it at any time using 
      the Menu bar.   Click 'Controls' 'Control panel'.  
      This toggles the visibility of the controls.

      The simulation can also be better seen if it is
      shifted to the right of the screen.   If it is central,
      go to 'Appearance' on the main menu and click 'Offset
      Simulation'.   The next simulation will be offset 
      to the right.




     
      B.  THE SIMULATION PARAMETERS

      [Note:  Some controls might not be visible.  
              Visibility can be customised by going to 
              the main menu, clicking 'Tools' then 
              'Options'. ]

      SOLAR ALTITUDE
      Adjust the solar altitude by changing the value in the
      box in the yellow 'sun' section at top left of the
      control panel.  The units are degrees.   
      
      This and many other controls 'freeze' when a 
      simulation is being run.

      SOLAR OR LIGHT SOURCE DIAMETER
      A value of 0.5 (degrees) reproduces halos as they 
      appear in Earth's skies.   However, a value of zero 
      (point source) is sometimes more convenient for 
      preliminary studies.

      To enter other values, for example halos on Mars or
      on a moon of Jupiter or Saturn, simply enter the apparent
      diameter of the sun in degrees. 

      VIEW
      Try changing the view to "Sun centered fisheye" by 
      clicking the arrow in the box that currently displays 
      "Zenith centered Fisheye".   Click 'Start' again to 
      display the new view.

      BRIGHTNESS LEVELS
      Better definition and range of halo brightness is 
      displayed using more brightness levels.  Instead of 
      the "Black dots on white" choose "Grey shades on white".   
      You will need more rays to run this option - change 
      the number to 80,000 (enter 80).  Keep the orientation 
      file as "Horiz column 1deg disp".  
      
      Click 'Start' to run again.  The intensities of the 
      halos are now represented by the graduated grey shades.

      Improve the finesse by increasing the number of LEVELS 
      to the maximum of 256.  

          This assumes that your display is set to 'True
          Color (32 bit)'.   If not, levels greater than 
          8 should not be selected - better still, switch
          your PC display to 'True Color (32 bit).

      The number of rays traced will need to be increased, 
      perhaps to over one million.   Be patient and watch 
      a smoothly gradated simulation appear.

      The number of rays can be set to a large number, say 
      50,000,000 and the simulation stopped when the 
      intensities are as required.

          The ADD button.
          Sometimes a large simulation is run but the final
          result needs more rays.   Once the simulation is 
          complete, select the number of additional rays 
          needed.  Then press the ADD button rather than START.

          Rays are added to the existing simulation.

      COLOURED BACKGROUND
      Change the display method to "Grey shades on color".  
      This displays halos against a coloured background of 
      choice.   The default background is dark blue, but 
      any color can be selected by opening the 'Sky Tuner'
      from 'Appearance' on the menu bar.  A separate help 
      item describes the Sky Tuner.
      
      OTHER COLOURS
      The worktop, horizon, sun and edge colours can all be 
      changed from the menu.  'Appearance', 'Colors'.  
      Any colours chosen are stored with the other simulation 
      parameters (see later).  New colours do not show until 
      a simulation is started.  The default colours can be 
      restored at any time.  

      SYMMETRY BUTTONS
      The extended panel shows two 'Symmetry' buttons 'LR' 
      and 'Ang'.   Click them to toggle them 'On' or 'Off'

      LR
      Most halo displays are symmetrical 'left' and 'right' 
      about a meridian passing through the sun.  When the
      button in 'On',  HALOSIM uses this symmetry to nearly 
      double its simulation speed.   Best results are with 
      a large number of rays and grey scales.  Disable it 
      by choosing "off".   

      ANG
      Halos from random oriented crystals are symmetrical 
      about the sun.   If the angular symmetry button is
      set to "On" the time to plot these halos is very 
      considerably shortened.   When this option is chosen 
      there is no advantage in also setting the meridian
      symmetry option to "Yes" - indeed, HALOSIM will turn 
      it off when the simulation starts to run.
      

      Both button settings will sometimes change when a 
      simulation is started.   HALOSIM checks whether the 
      symmetry options are physically realistic and changes 
      them if not.   For this reason, there is no need to 
      click the angular symmetry button to 'Off' unless 
      you really want to simulate a circular halo at low
      speed.

      There is no loss of accuracy when the symmetry options 
      are enabled.


      USER SELECTED VIEW DIRECTIONS
      Choose "User center Camera View".  This view is that 
      obtained by a camera.   Select "Black dots on 
      white" - a fast simulation is better for initial choice 
      of views and angles.  Keep the "Horiz column 1deg 
      dispersion" orientation file.
  
      With the solar altitude set to 40 degrees,  choose a 
      camera view direction of 20 degrees altitude and 180 
      degrees azimuth.  The 'camera' now points away from 
      the sun.

      The parhelic circle, anthelion, Tricker and diffuse 
      arcs are revealed.

      The Field of View setting can be used to zoom in on 
      displays.   The ZOOM controls from the menu are more
      powerful but unlike the camera view, the zoom settings
      are currently not stored with simulations.

      DISPERSION MODEL
      The orientation file names contain a value for the 
      amount that the crystal deviates from the exact 
      preferred orientation.   HALO follows the file
      instruction and applies one of three dispersion 
      models

         1)  GAUSSIAN
         2)  TOP HAT or HEAVYSIDE
         3)  SIMPLE HARMONIC OSCILLATOR

      GAUSSIAN is normally used and the file dispersion 
      value is the standard deviation of the distribution.

      The other models can be used to examine their effects
      on the appearance of halos.   

      Select the distribution type from the box in the 
      'extended' part of the Control Box.
  
      The file dispersion value is always the distribution 
      standard deviation but the Top Hat and SHO models 
      produce distributions of finite width.  This is 
      computed within HALO and shown in the 'Full width' 
      box.  Use 'Tools''Options' to reveal the width box.


      REFRACTIVE INDEX AND WAVELENGTH
      In the 'extended' part of the Control Box.  

      A default value of about 1.31 normally appears in the
      refractive index box, the value for water-ice and 
      yellow light of ~580 nm wavelength.
 
      The refractive index and wavelength boxes are linked.
      If the value in either is changed, the other will 
      alter to show the corresponding value for the material
      selected.   The circle between them shows the colour
      corresponding to the wavelength displayed.   

      HaloSim uses only the refractive index so it is 
      possible to experiment with different values and 
      disregard the colour and wavelength.

      These controls are not shown when full-color displays
      are simulated.   This is because these displays need
      refractive index data for all visible wavelengths and
      HALOSIM gets this directly from the crystal "Material"
      that is selected for each channel.

    
      MATERIAL BUTTON

      [use 'Tools''Options' to reveal]

      The default crystal material is ordinary ice.

      This button allows a different material to be selected
      for a particular crystal channels.   It is possible to 
      compute displays for mixtures of crystals of several
      materials.

      A file must be created for each material to describe 
      its refractive index.   The text inside the water-ice 
      file "Water-Ice.xmt" explains how to do this, or ask
      the authors!

      Monochrome displays do not need a material file except
      to properly show the wavelength for a particular
      refractive index.   Instaed, the refractive index could
      be entered directly - the wavelength box will usually
      go blank.   This does not matter.





      C.  SELECTING OTHER CRYSTAL SHAPES 
          AND ORIENTATIONS

      All common hexagonal prism ice crystals can be 
      simulated with the "Hexagonal Ice Prism.xsh" 
      crystal shape file.   The c/a ratio (or length to 
      breadth ratio) is set within the program from 
      the 'Control Panel'.   It's default value is 1.

      SIMULATING A PYRAMIDAL CRYSTAL DISPLAY

      Some less common halos are formed by pyramidal 
      ice crystals.  These can occur in different "habits" 
      that have facets of different sizes but the same 
      interfacial angles.   A different crystal shape 
      file is needed to describe each habit.  One is 
      included,  'Pyr_.30_.30_.... .xsh', and others can 
      be made using 'Pyramid Maker' described elsewhere 
      in Help.

      Press 'Reset' then 'Shape'.

      In the File Open dialogue box select 
      'Pyr_.30_.30_... .xsh'

      Click 'Orientation' and select 'Random.xan'.

      Choose a solar altitude of 22 degrees (the arc 
      radius produced by ordinary hexagonal ice crystals) 
      and several million rays.

      Choose a "Sun centered fisheye" view and the "Black 
      dots on white" display mode.

      Press Start to simulate odd radius circulare arcs. 


      'PYRAMIDAL' PARHELIA

      Change the crystal orientation by selecting "Plate 
      1deg disp.xan".   These crystals are aligned with 
      their large hexagonal faces nearly horizontal.   
      Hexagonal crystals aligned this way produce the 
      familiar sun dogs and circumzenithal arcs.

      With pyramidal crystals selected, the rare parhelia 
      to the odd radius circular halos are simulated.


      OTHER CRYSTAL ORIENTATIONS

      HALOSIM comes with file sets for all the common 
      crystal orientations:

             Random
             Singly oriented columns
                      (called 'Horiz column ... ' )
             Plates
             Parry oriented columns
             Lowitz oriented columns

      Each type has a range of angular dispersions and if the
      required dispersion(s) are not there, custom files can 
      be made as described in Help under 'Making Orientation 
      Files'.






      



