TORUS1:[GLL_RAW.INFO]HOW_TO_LOOK_AT_EUV_DATA.DOC 4-june-1996 jjg

This document is intended to give you a basic overview
of how the EUV works so that you have enough information
to do basic plots and data processing on the data.  

Basics:

   The EUV instrument covers the 549 to 1280 Angstrom range of the 
spectrum using 126 photon detectors(2 additional detectors are 
no longer used).  Each detector covers a small portion of this 
range.  The detectors are read out sequentially from the shortest
 wavelength to the longest. This process is called a scan and 
takes approximately 21.4 milliseconds to execute.  Because the 
instrument is mounted on the spinning portion of the spacecraft,
 a series of scans will cover an arc of the sky as the spacecraft 
spins.  Spin information from AACS is used to allow the same portion
of the sky to be scanned on each spin. Data taking does not occur
when the AACS information is not available.  Information on when
AACS data is not available is hard to get from the ground, so the
EUV keeps track of how many times it can take data.  This is 
done by incrementing a counter, called the integration counter,
every time that data-taking begins.  This value is contained
in the housekeeping values, along with information about the
instrument command, spin rate, and time values.

   The instrument stores data as a two dimensional array, one
dimension representing sky position and the other representing
wavelength. The sky position dimension is divided in to sectors.
Each sector represents all the counts taken in an angle defined
by the number of scans in the sector times the spin rate times
21.4 milliseconds. Sectors are scanned consecutively from the
starting angle until the commanded number of sectors have been
scanned.

   The wavelength dimension is stored as superpixels. A superpixel
is essentially a memory location where the counts from 2 or 4 
pixels are stored.  The pattern for storing the pixels into 
superpixels is defined by loading a fixed pattern noise (FPN) table 
into the instrument's memory.  This table gives each pixel a superpixel 
where its counts are to be stored. So when pixel n is scanned, the 
counts are stored in the superpixel number stored in location n of 
the FPN for the appropriate sector. Because different FPNs can be 
loaded, the EUV can scan give higher wavelength resolution in different
ranges. There are 3 FPN's currently in use: specialized  FPNs for 
both the Io torus and Jupiter's aurora, and a general purpose  FPN that 
is a compromise between the other two. It is extremely important to 
know which FPN was used to take a data set so that can be interpreted 
correctly. (see attached picture)

More Information:
625-270 EUV functional requirements(look on notebook shelves)-- this
document contains official description of the instrument.

TORUS1:[GLL_RAW.INFO]EUV_FIDUCIAL.DOC --this file contains information
about EUV housekeeping values and what they mean.

Space Science Reviews  Vol 60 p 518 to 529 -- paper on UVS/EUV
instruments.

TORUS1:[GLL_RAW.INFO.MICRO_LOADS]EUV_4P1.SRC--last resort! Source code for 
the EUV microprocessor.  The documentation in here explains most of the 
details very well, but you probably shouldn't bother trying to understand 
the code.

DATA FILES:

EUV data is stored in in lonarr(1132) files, designed to be used with IDL.
Detailed information about the structure of these records can be found
in TORUS1:[GLL_RAW.INFO]EUV_P2_RTS_HDR.DOC.

PROGRAMS:

I have already written a large number of functions for processing EUV data in
the file TORUS1:[GLL_RAW.SOFTWARE]EUV_PLOT_PHASE2.PRO. The main function
is called EUV_PLOT and it allows you to make several different plots and to
extract the data as a (sector,pixel) dimensioned array. There are also lots
of very useful subroutines for things like making wavelength scales, 
converting superpixels to pixels, and extracting data from data files in an 
easy manner.  See EUV_PLOT_PHASE2.DOC in the same directory for more 
information.