% @(#)descri.alq	19.1 (ESO-IPG) 02/25/03 14:03:16 

              Standard descriptors for images + tables           951031  KB

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Images
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Integer descriptor NAXIS :
Contains the number of dimensions (or axes) of the image data.
The maximum number for NAXIS is 3, but most applications work only for
NAXIS = 1 or 2, a few applications (like EXTRACT) also for NAXIS = 3.
  
Integer descriptor NPIX :
Array of NAXIS (i.e. up to 3) values containing the no. of pixels in each axis.
  
Double precision descriptor START :
Array of NAXIS values containing the world coordinate of the first pixel of
each axis.
  
Double precision descriptor STEP :
Array of NAXIS values containing the distance between pixels on each axis 
expressed in units of the world coordinate system corresponding to that axis.
  
Character descriptor CUNIT :
String of (NAXIS+1)*16 characters containing the unit of the data and the units
of the world coordinates of each axis.
Here is specified, what the data and world coordinates represent, e.g.
'INTENSITY', 'MICRON', 'ANGSTROM', 'FLUX' or 'ARC_SEC'.
  
Character descriptor IDENT :
String of up to 72 characters, to contain the source of the data, object and 
date of acquisition, e.g.:
      IDS HD6034  06/09/1980
      ESO  CCD  10 June 1981  flat field
  
Real descriptor LHCUTS of 4 values :
First 2 values contain user defined low + high cuts for scaling when the image 
data is displayed.
Last 2 values contain real minimum + maximum of image data and are maintained
by the MIDAS system.
  
Integer descriptor DISPLAY_DATA of 9 values :
This descriptor holds information about the way the frame was loaded into 
the display. 
First 2 elements contain scaling values, negative values indicate expansion,
positive values indicate shrinking of the image (note, that for historic
reasons the sign is the opposite of what you use in the LOAD/IMAGE command).
The next two values are the center pixels in x,y. 
DISPLAY_DATA(5) = 1 or 0 depending upon if the frame was averaged or not when 
loaded with scaling factors < 0.
The values of DISPLAY_DATA(6,7,8,9) are the frame and screen pixels (x,y) for
a given fixpoint.
  
Character descriptor HISTORY:
String to hold the reduction history of a frame. All relevant MIDAS commands 
add a line of 80 chars. to this descriptor.
Since the maximum length of any descriptor is 32767 elements, there is
only space for history_lines from 400 operations. So if you are doing a lot
of operations on the same frame, it may be better to turn off the automatic
addition of history_lines to descriptor HISTORY.
Adding an integer descriptor (1 element) named HISTORY_UPDA to the frame 
and setting it to 0, inhibits the history addition.
  
Integer descriptor HISTORY_UPDA (1 element):
Serves as update flag for descriptor HISTORY of the same frame. 
If set to 1, descriptor HISTORY is updated. 
If set to 0, descriptor HISTORY is not updated.


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Tables        
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Integer descriptor TBLCONTR:
Contains 10 elements with information about the table contents.
1 - no. of allocated columns (no. of words per row)
2 - no. of allocated rows
3 - no. of defined columns (NDC)
4 - no. of defined rows
5 - reference column
6 - sorted column
7 - physical table format on disk
8 - table version
9 - items reserved for column descriptors
10 - number of selected rows

Character descriptor TLABLxxx of 41 chars.:
Holds for each column the label (element 1 -16) and the format (33 - 41),
xxx is the column no. in the range 001, ..., NDC.

Integer descriptor TBLENGTH:
Has NDC elements (in encoded form) defining for each column its type 
and no. of items.

Integer descriptor TBLOFFST:
Has NDC elements defining for each column its offset in bytes from the
beginning of the table.

Character descriptor TSELTABL of 64 chars.:
Holds the selection information. Contains "-" if whole table is selected.
  
