Hi,

It's not always safe to get "XCEN", "YCEN" information directly from EIS fits (main) header.

Here are some suggestions (according to emails between Ignacio U., Ken D., Viggo H. and John M.):

1. use the fits bte header (ie the data_object methods)

IDL> fname='eis_l0_20070320_030501.fits.gz'
IDL> obj=obj_new('eis_data',fname)
IDL> xpos=obj->getxpos()
IDL> ypos=obj->getypos()
IDL> xcen=total([max(xpos),min(xpos)])/2.
IDL> ycen=total([max(ypos),min(ypos)])/2.
IDL> print,xcen,ycen
      21.4457     -976.781

2. use methods provided by the hdr_object

IDL> fname='eis_l0_20070320_030501.fits.gz'
IDL> hdr=obj_new('fits_hdr',fname)
IDL> xcen=hdr->getxcen_ti1(/offset)
IDL> ycen=hdr->getycen_ti1(/offset)
IDL> print,xcen,ycen
       21.410865      -976.24575

3. I wrote a simple wrapped idl program called "eis_getpinfo.pro" to extract pointing information from EIS fits file (ie no need to read full fits file)

IDL> fname='eis_l0_20070320_030501.fits.gz'
IDL> eispointing=eis_getpinfo(fname)
IDL> print,eispointing
       21.410865      -976.24575       583.06600       512.00000
The values mean: [XCEN, YCEN, FOVX, FOVY] (unit: arcsec)

Hope this helps. :)

Any comments?

JianSun 13-Mar-2026 06:41