Worksheet 2 - Calibrating EIS data (eis_prep)#
Generally it is recommended that you calibrate your data file before you begin taking a look at it. This is because EIS data are affected by warm pixels on the CCD which make it difficult to study the images.
To calibrate your data, the standard call to eis_prep is
IDL> eis_prep,filename,/default,/save,/retain,/quiet
To see what these keywords do (and see other options), go to the wiki section on eis_prep keywords. The individual steps performed by eis_prep are described in detail in the wiki section on eis_prep.
The keyword /default makes the routine use the default processing options, and the keyword /save makes eis_prep create two files in your current working directory. These are the level-1 FITS file, and the error FITS file.
TIP: it is a good idea to compress these files with gzip and move them to a directory called /level1.
NOTE: “bad” data (referred to as missing) are flagged with a value of -100 in the error FITS file arrays (not the level-1 arrays). The missing data in the level-1 arrays are replaced with the median value of the surrounding pixels. This is useful for display purposes, but for scientific analysis you should not use any pixels flagged as missing. With the /default option, eis_prep does the following things:
- Flag any saturated data as missing (saturated data have a DN value of 16383)
- Remove the CCD pedestal and dark current
- Flag hot pixels as missing
- Flag warm pixels as missing (not implemented at present)
- Flag cosmic rays as missing
- Calibrate data, and create error arrays
Exercise#
- Using commands from Worksheet 2, how many pixels in the Fe XII 195 data array have been flagged as missing by eis_prep?
