Interactive Data Language
(IDL)
Margit Haberreiter
(haberreiter@lasp.colorado.edu)
LASP, room 135
Acknowledgement: Marty Snow
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Outline
What is IDL?
Basic Syntax
Functions, Procedures, and Library
Reading & Writing
Plotting & Printing
Image Processing
And Stuff….
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Pro test
Save it as test.pro
.compile filename
filename
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Interactive Data Language
• IDL was born here at LASP
• Very popular among scientists:
– Handles arrays of data easily (spectra, time series,
images, etc.)
– Easy to learn
– Portable
– Large existing user library
• Online Help – type “?” at prompt
• Type help at prompt for contents of
variables
• Interactive or Compiled, your choice!
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Data Types
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Integer (and Long Integer)
Floating Point (and Double Precision)
String
Structure
Pointer
Also, complex, unsigned integers, etc.
IDL is “dynamically typed” which means a variable can be defined as one
type, and then be changed to another within the same program unit.
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Scalars, Vectors, Arrays
Problem 1
• Scalar
– 42
– ‘help!’
a=5 & b=0. & c=0.d
• Vector
– [1,2,3,5]
– [‘red’,’blue’,’happy’,’Burma Shave’]
• Array
– Up to 8 dimensions
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Definition of arrays
arr1=intarr(3)
arr2=fltarr(2,a)
arr3=dblarr(a,b,c,d…)
arr4=strarr(11)
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Structures - Problem 2
• Can hold mixed data types
– S={day:0,time:0d0,label:’ ‘}
– S.day=5
– S.label=‘happy time’
“tag A, B, C”
• Can make arrays of structures
– S2=replicate({time:0d0,label:’’,values:fltarr(100)},n_obs)
– S2[4].values=findgen(100)
• Names or Numbers
– S2.time is equivalent to S2.(0)
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Rules of the Road
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Always start counting at zero.
For help at any time, type help.
Case Insensitive (EXC: filenames).
Odd is true, Even or Zero is false.
An array of length 1 is not the same as a
scalar. But you can reform it.
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Squiggly or Square?
• Subscripting variables -- [] square
• Parameters of a function – () round
• Definition of a structure – {} curly
IDL is lax about enforcing these rules, but it is to your benefit to follow them.
Example:
a=exp[5] ;6th element of variable “exp”
a=exp(5) ;e^5
You might have named a variable the same name as a library function!
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Subscripting - Problem 3
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z=a[5]
z=a[0:2]
z=a[3:*]
mask=[5,6,7,8,9] & z=a[mask]
Range=where(mask gt 2 and mask le 4)
a=indgen(10)
Mask2=mask(range) z=a[5]
z=a[0:2]
z=a[3:*]
mask=[5,6,7,8,9] & z=a[mask]
Range=where((mask gt 6) and (mask
le 8))
Mask2=mask(range)
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The Size of Arrays
• Size(x)
• n_elements returns the number of elements
in an array (vector, scalar).
• An undefined variable has zero elements.
• Often takes the place of size().
• Used to check if user has supplied
parameters
• Used to index loops.
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Basic IDL Syntax
Problem 4
a=‘Hello World’
print,a
;This is a comment
a=5;dynamic data types
b=4.5*findgen(100)
z=a+$ ;continuation
sqrt(b)
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Operators
• Arithmetic
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• Boolean Logic
+
*
/
^
mod
• Min/Max
– #
– ##
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and
not
or
xor
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eq (a=b)
ge (ab)
gt (ab)
le (ab)
lt (ab)
ne (ab)
• Relational Operators
– < (a<b is the lesser)
– > (a>b is the greater)
• Matrix Multiply
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–
–
–
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Note that you can’t use any of
these letter combinations as
variable names!
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FOR loop
for variable=init,limit[,increment] do statement
for i=0,100 do print,i,vector[i]
for variable=init,limit[,increment] do begin
statements
endfor
Loops in IDL execute slowly (relatively
speaking), so try to use something
else, like where.
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As IF…. Problem 5
if expression then statement
if expression then statement else statement2
if expression then begin
statements
endif else begin
statements
endelse
Good programming style 
avoid hardcoded numbers in
procedures
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for x=0,5 do begin
if x gt 5 then begin
z=dblarr(10,10)
z=z+.1
z[3,2]=1.
endif else begin
z=dblarr(10)
z[x]=!pi
endelse
endfor
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meanWHILE
while expression do statement
while expression do begin
statements
endwhile
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just in CASE
case expression of
expression1: statement
expression2: begin
statements
end
expression3: statement
else: statement
endcase
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Example:
x=3
CASE x OF
1: PRINT,
2: PRINT,
3: PRINT,
4: PRINT,
'one'
'two'
'three'
'four'
ENDCASE
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Other Flow Control Statements
• STOP – excellent way to debug (.con or .c to continue)
IDL Help: This example illustrates how, unlike CASE,
• GOTO
SWITCH executes the first matching statement and
– goto,label
– label:
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•
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any following statements in the SWITCH block:
x=2
SWITCH x OF
1: PRINT,
2: PRINT,
3: PRINT,
4: PRINT,
ENDSWITCH
'one'
SWITCH
'two'
'three'
CONTINUE
'four'
BREAK
MESSAGE – stops program and prints message
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Procedures and Functions
• A list of IDL statements that could have been
entered at the command prompt.
• Contained in separate file or files.
– Compiled automatically if files are properly named.
Filename matches procedure with a .pro extension.
USE LOWER CASE!
– All procedures compiled from start of file but
stops when procedure name matches file name.
– Environmental variables define directories that
IDL will automatically search. USER LIBRARY
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Compile and Run
• Type procedure name at command prompt to
run it. IDL will search for an appropriately
named file and automatically compile and run
the procedure.
• .r or .compile will compile the procedure, but
not run it.
• IDL will compile all procedures in file until it
hits the one matching the file name, then it
will stop. Put the named procedure last in the
file.
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Simple Procedure
pro simpleprocedure,a,b,c
c=a+b
end
To run it, type:
simpleprocedure,var1,var2,result
print,result
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Simple Function
function eve,number
if number mod 2 eq 0 then return,0
return,1
end
To run it, type:
result=eve(var)
print,eve(var)
print, ‘The result is ‘, result
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What’s the difference?
Not much.
Use a function to get a value that you’ll use
right away (like an expression).
EXAMPLE:
simpleprocedure,eve(0),eve(5),result
Use a procedure when you return multiple
results and don’t want to just wrap them
together in a structure or array.
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Parameters and Keywords
Problem 7
pro proc2,input1,input2,result,doplot=doplot
if n_elements(doplot) eq 0 then doplot=0
result=input1*sin(input2/!pi)
if doplot gt 0 then begin
set_plot,'win'
!P.CHARSIZE=3.
; system setting
!P.MULTI=[0,1,3]
; system setting
plot,input2,result,psym=-4
plot,input1
Order of Parameters is critical.
plot,sin(input2/!pi)
Order of Keywords is irrelevant.
endif
end
Keywords can normally be abbreviated
xtit instead of xtitle
xs instead of xstyle
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Variables: Global or Local?
Problem 8
Variables are local….sort of (Attention!!)
A procedure can modify any of its
parameters, which will change the
variable in the calling procedure.
pro proc1
v=0
print,v
proc3,v
print,v
end
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pro proc3,var2
var2=var2+1
end
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Error and stop
• If the IDL interpreter hits a STOP or
encounters an error, flow halts at the local
level. You can type ‘help’ to view contents of
variables and which line of code you’re at.
• To return to the top level – retall (return
all the way)
• To clear everything, type .f (full reset) or
.reset -> Attention: all variables deleted
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Reading & Writing & Saving
• openr,openw,openu (Read, Write,
Update)
• readf,readu (Formatted, Unformatted)
• close or free_lun
• IDL save files:
– save,file=‘data.idl’,var1,var2
openr,lu,’file.txt’,/get_lun
– restore,’data.idl’
while not eof(lu) do begin
readf,lu,data
endwhile
free_lun,lu
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Slight Gotcha…
pro wont_work
openr,lu,’file.txt’,/get_lun
data=fltarr(100)
for i=0,n_elements(data)-1 do begin
readf,lu,data[i]
endfor
free_lun,lu
end
IDL passes by value, not by
reference, so this won’t
work.
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Slight Gotcha…
pro will_work
openr,lu,’file.txt’,/get_lun
data=fltarr(100)
i=0
while not eof(lu) do begin
readf,lu,temporary
data[i]=temporary
i=i+1; counter
endwhile
free_lun,lu
data=data[0:i-1]
end
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IDL passes by value, so this
will make IDL happy.
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Task: Read data from file
Problem 9
pro willwork,filename,data
openr,lu,filename,/get_lun
; get_lun: sets file unit number
data=fltarr(2,1000)
i=0
while not eof(lu) do begin
readf,lu,temporary1,temporary2
data[0,i]=temporary1
data[1,i]=temporary2
i=i+1
; counter
endwhile
free_lun,lu
data=data[*,0:i-1]
set_plot,'win'
!P.MULTI=0
plot,data(0,*),data(1,*),title='Data from file'
end
Run the procedure
Add a plot using a parameter (see slide 22)
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Homework 1—LISIRD Data
• Go to http://lasp.colorado.edu/lisird and
retrieve solar Lyman alpha (121.5 nm)
data for at least two missions.
• Save data as text file.
• Write an IDL procedure to read data.
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More on plotting…
Basic IDL Plotting Procedures
• Line plots
– plot
– oplot
– histogram
• Contour plots
– contour
• Surface plots
– surface
– shade_surf
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Line ‘em up Problem 10
• X=FINDGEN(360)
• Y=SIN(X*!DTOR)
• PLOT, X, Y, XRANGE=[0,360], /XSTYLE, XTIT=‘X’, $
YTIT=‘Y’, TIT=‘Sample Line Plot
• Z=COS(X*!DTOR)
• OPLOT, X, Z, LINESTYLE=2
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Adding text to plot
• X=FINDGEN(360)
• Y=SIN(X*!DTOR)
• PLOT, X, Y, XRANGE=[0,360], /XSTYLE, XTIT='X', $
YTIT='Y', TIT='Sample Line Plot'
• Z=COS(X*!DTOR)
• OPLOT, X, Z, LINES=2
• XYOUTS, 100, 0, 'cos(x)'
• XYOUTS, 190, 0, 'sin(x)'
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Histograms
• Y=RANDOMN(SEED, 100, /NORMAL)
• H=HISTOGRAM(Y, BINSIZE=0.2, LOCATIONS=L)
• PLOT, L, H, PSYM=10 ; histogram mode
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Graphics Keywords
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change the style of the data
add a title
manipulate an axis/change tick marks
change the format of text
change coordinate systems
etc.
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Styles and Symbols Keywords
• Symbols
• Lines
– LINESTYLE={0,1,2,3,4,5}
– THICK: change the
thickness of the line
(default is 1.0)
Value
Meaning
0
Solid
1
Dotted
2
Dash
3
Dash Dot
4
Dash Dot Dot
5
Long Dashes
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Value
Meaning
1
Plus sign (+)
2
Asterisk (*)
3
Period (.)
4
Diamond
5
Triangle
6
Square
7
X
8
User defined
9
Undefined
10
Histogram mode
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– PSYM={1,2,3,…10}
– SYMSIZE: change the size
of the symbol
– USERSYM: procedure to
define PSYM=8
– PSYM= -{1,2,3,…8}: solid line
connecting the symbols
– N.B.: PSYM=3 (period) does
NOT show up well in
postscript plots and does not
scale nicely with symsize.
Better to use PSYM=1,
SYMSIZE=.3 (or similar)

Negative value of “psym”
creates line+symbols
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Titletown USA Keywords
• TITLE: place a string of text at the top of a plot
– TITLE=‘This is my title’
– TITLE_STRING=‘This is my title’
TITLE=TITLE_STRING
• SUBTITLE: place a title below the x-axis
• [XYZ]title places title on x, y, or z axis
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Axes & Tick Mark Keywords
• To manipulate individual axes:
– Set the range:
• [XYZ]RANGE=[min, max]
– Change the axis style
• [XYZ]STYLE=number
• Multiple effects can be achieved by adding values together
– Label an axis:
• [XYZ]TITLE=string
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Value
Meaning
1
Force exact axis range
2
Extend axis range
4
Suppress entire axis
8
Suppress box style axis
16
Inhibit setting y-axis minimum to
zero
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Axes & Ticks Keywords 2
• To manipulate individual axes (cont.):
– Change the axis thickness
• [XYZ]THICK=number (default is 1.0)
• Use axis procedure to draw axes with different scales
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–
–
–
X=FINDGEN(360)
Y=SIN(X*!DTOR)
; system variable π/180
PLOT, X, Y, XRANGE=[0,360], XSTYLE=9, XTIT='X (Degrees)'
AXIS, XAXIS=1, XRANGE=[0,2*!PI], XSTYLE=1,
XTITLE='(Radians)'
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Clipped title
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Fixed clipping…
plot,x,y,xstyle=9,xtit='X (degrees)',ymargin=[4,4]
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Text Formatting Keywords
• To change the size of text
– CHARSIZE=number (default is 1.0)
• To change the thickness of the text
– CHARTHICK=number (default is 1.0)
– Or set system variable:
• !P.Charsize=3
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Coordinate systems
• IDL has 3 coordinate systems
– DATA (default)
• Uses the range of values of the data
– DEVICE
• Uses the device (X window or Postscript device)
coordinates (i.e. pixels)
– NORMAL
• Normalized coordinates from 0 to 1 over the 3
axes
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Adding color
• Color tables
– LOADCT[, number]
– XLOADCT (widget)
• Graphics keywords:
– BACKGROUND=number
– COLOR=number
– Number from 0 to 255, corresponding to the color
table
– color=fsc_color(‘red’) ;using library routine
• On Mac, Windows: DEVICE, DECOMPOSED=0
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Contour
• X=FINDGEN(360)
• Z=SIN(X*!DTOR)#COS(2*X*!DTOR)
• CONTOUR, Z, NLEVELS=6, XRANGE=[0, 360], $
YRANGE=[0, 360], /XSTYLE, /YSTYLE, XTIT='X', YTIT='Y', $
TIT=‘Sample Contour Plot’
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Surface plots (shaded surf)
• SHADE_SURF, Z, XRANGE=[0, 360], $
YRANGE=[0, 360], /XSTYLE, /YSTYLE, XTIT='X', $
YTIT='Y', TIT='Sample Surface Plot 2'
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Colored surface
loadct,3
shade_surf,z,tit='Sample Surface Plot 3',shades=bytscl(z+1)
Bytscl translates the values of the array into a byte array from 0 to 255
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Surface plots 2 (wire mesh)
• SURFACE, Z, XRANGE=[0, 360], $
YRANGE=[0, 360], /XSTYLE, /YSTYLE, XTIT='X', $
YTIT='Y', TIT='Sample Surface Plot 2'
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Changing appearance of mesh
z2=rebin(z,180,180)
surface,z2
z3=rebin(z,90,90)
surface,z3
The possibilities are endless!
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Multiple plots on a page
• !p.multi=[a,b,c]
– a: number of plots left on page
– b: number of columns
– c: number of rows
• Example:
– 6 plots per page (3x2), begin plotting in the top
left
– !p.multi=[0,3,2]
• To return to a single plot per window,
!p.multi=0
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Making postscript files
• SET_PLOT, ‘PS’
• DEVICE, FILENAME=‘example.ps’
– , /LANDSCAPE
– , /PORTRAIT, YOFFSET=1, YSIZE=9, XSIZE=6,
/INCHES
– , /COLOR, BITS_PER_PIXEL=8
• Plotting commands
• DEVICE, /CLOSE
• SET_PLOT, ‘X’
• For Windows, SET_PLOT, ‘WIN’
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Detailed plotting...
set_plot,'ps'
tek_color
!p.font=1
!p.color=0
!p.background=1
;!p.MULTI=[0,2,4]
!P.MULTI=0
ang = string("305B)
device,filename=fileout+'*.ps',/color,ysize=12, xsize=17,xoffset=1,yoffset=10
;*****************************************************************
plot,x1,y1,linestyle=1,/ylog,thick=6,$
tit='',CHARSIZE=1.5, xtit='!Ml!N (nm)',XCHARSIZE=1.5,$
ytit='intensity (erg s!E-1!Ncm!E-2!NHz!E-1!N)',YCHARSIZE=1.5,$
xrange=[x0,x1],xs=1, $
yrange=[y0,y1],ys=1
;*****************************************************************
; overplot
;*****************************************************************
oplot,x2,y2,linestyle=0,thick=3
legend_mh01,textarr,charsize=charsize,textcolor=colors,colors=colors,$
box=0,position=[.5,.55],/normal
xyouts,0.85,0.85,‘a‘,charsize=1.5
print,'Have a look at the plot :)'
end
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; Angstrom symbol
; /nodata does not plot the data
; !E ->exponent; !I-> index; !N ->normal font
; !M -> mathfont, e.g. !Ml ->lambda
; label of figure panel, i.e. a, b, c, d....
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Making other types of graphics
files
• Use tvread function from David
Fanning’s library www.dfanning.com
• plot,x,y
• im=tvread(/png,file=‘filename’,/nodialog)
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Putting it all together
Homework assignment 2
• Using LISIRD, create a composite Lyman- time
series using SORCE, TIMED SEE, UARS SOLSTICE,
and SME data
– Plot each instrument as a different color
– Label each instrument
– X-axis: year
Y-axis: W/m2/nm
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Sample Solution
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Imaging basics
• Images in IDL are just arrays. All standard
mathematical operations are available.
• Images are usually saved in FITS format
– Header and array of integers (image)
– Use Goddard IDL Astronomy Library procedures to read
both the header and image
• Raw images must be processed before they can be
used for science. Example:
– Remove the background counts (dark)
– Correct for variations across the CCD (flatfield)
Corrected image 
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Rawimage  darkimage
Flatimage  dark image
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Displaying images
• Basic IDL commands to display an array/image
– TV, image
– TVSCL, image
• Resize an image
– REBIN(image, xsize, ysize)
– xsize and ysize must be integer factors of the
original size of the image
• Add color by loading a color table
– E.g. SOHO EIT has its own color table for each
wavelength
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and Stuff
•
•
•
•
•
•
•
•
•
where
size
histogram
string manipulation
file_search
map projections
n_elements
reading file formats (CDF, FITS, PNG, etc.)
pointers
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Where
Use WHERE to find data values meeting
some criterion.
– good=where(data gt 0.)
– better=where(data gt 1.5 and $
time gt julday(1,1,2005))
data2=data[good] ;new variable data2
data=data[better] ;redefine variable data
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Size
• The size function returns lots of good
information about your variable.
(dimensions, n_elements, data type, etc.)
s=size(data)
An undefined variable is a valid type, so
check for size or n_elements.
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Histogram
• There are billions and billions of uses
for histogram.
h=histogram(data,binsize=2.5,omin=omin)
bins=findgen(n_elements(h))+omin
http://www.dfanning.com/tips/histogram_tutorial.html
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Strings
• IDL understands regular expressions
• Concatenate strings with +
myname=‘yourname’
result=strpos(myname,‘o',0)
result=strupcase(myname)
result=strlen(myname)
strsplit, strmid, ….
June 11, 2009
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file_search
• Retrieves a list of filenames into a
string array.
list=file_search(path+’*.idl’)
for i=0,n_elements(list)-1 do begin
restore,list[i]
plot,time,irradiance,psym=-4
endfor
All your string manipulation skills can be used to
create the list of files, of course.
dialog_pickfile is an interactive way of selecting files.
June 11, 2009
IDL for REU students
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Plotting Data on a Map
• set up map projection
– map_set,/continents,/mercator
(mercator: cylindrical map projection)
• plot positional data on it
MAP_SET, /MERCATOR, 0, -75, 90, CENTRAL_AZIMUTH=90, $
/ISOTROPIC, LIMIT= [32,-130, 70,-86, -5,-34, -58, -67], $
/GRID, LATDEL=10, LONDEL=10, /CONTINENTS, $
TITLE = 'Transverse Mercator‘
Other options: /ROBINSON, /ORTHOGRAPHIC, … ?map_set
June 11, 2009
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Standard Data Formats
• IDL save files: restore, filename
• FITS files: d=mrdfits(file,exten,head)
• netCDF: read_netcdf,file,data,status
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Pointers
• Variable contains a reference to data in
memory, instead of containing the data.
• Needs to be de-referenced to get the
data.
get_sorce_telemetry,data,info,jd1,jd2,$
externalelement=‘solstice_a’,$
itemname=[‘grat_pos’,’detector_a’,’int_time’]
gp=(*data.(0)).science
June 11, 2009
IDL for REU students
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User Library Routines
•
•
•
•
•
•
legend
pause
psymdot
which or file_which
fsc_color
rainbow
June 11, 2009
IDL for REU students
Margit Haberreiter
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Coyote’s Guide to IDL
• www.dfanning.com The real source for
tips and tricks
June 11, 2009
IDL for REU students
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Colors and Plots
• xpalette to show color table
• tvrd (“TV read”)to grab graphics window
and write to file
• add color definitions to startup file
• remember to make plots color-blind
friendly if possible
(linestyles --- and symbols xxx)
June 11, 2009
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File Tips
• help IDL’s automatic compiler by always
naming your files with lower case
• IDL stops compiling when it hits the
routine that matches the filename
June 11, 2009
IDL for REU students
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IDL 7 Workbench
• New version of IDL uses Eclipse
• Backwards compatible as always
June 11, 2009
IDL for REU students
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Tips & Tricks for Workbench
• Hover help (and open declaration)
• Redefine key bindings to whatever you’re used
to: ctrl-shift-L
• double click on history entries
• cntrl-i shifts focus to command line
• cntrl-space (or alt-slash) for command
completion
• .edit filename
• search works like grep—search for content in
files
June 11, 2009
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Just the beginning….
• Remember, there’s a lot of IDL
knowledge around here, so feel free to
ask questions.
• Many user library routines might
already do whatever you had in mind.
• Online help manual has lots of examples.
• SolarSoft, Widget programming, Object
Oriented...
June 11, 2009
IDL for REU students
Margit Haberreiter
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