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 "" 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 21:38:35 19-Jul-2024 BST

Hi, Jian

Thanks for doing this! Really useful not to have to read a whole FITS file -- especially the huge files we recently discussed!

On a point of refinement -- and this is something we should consider in the team meeting in October -- I'm not sure that we can really speak of a single EIS pointing. The reason I say this is the North-South offset of about 20" between the two wavelength channels, and also the 1" East-West offset between them. In a sense, each wavelength channel has its own pointing. But perhaps this is factored into your routine already?

Does EIS_PREP correct for these offsets?

--David R Williams, 26-Sep-2007

Hi David,

Your comments are valuable. Thanks.

I don't think EIS_PREP corrects these offsets, people have to do it by themself manually.

There is a preliminary report from Deb (MSSL) to show the shifts between SW & LW CCDs and XRT. The fits file has 22 lines: 8 lines in SW CCD and 14 lines in LW CCD:

Y shifts (arcsec):

[20,20,20,20,20,19,19,19] (SW)  [4,2,2,2,3,0,0,1,1,1,0,0,0.5,3] (LW)
X shifts (arcsec):
[0,0,0,0,0,0,0,0.5] (SW) [3,3,1,1,1,3,0,1,1,2,3,3,2,1] (LW)

Here’s a summary:

1. The pointing information in eis fits header is for LW CCD only.

Note that the heliocentric coordinates stored in the fits headers and returned 
by the EIS software apply to the long wavelength band, not the short wavelength band. 
This is because the He II 256 line was used to co-align with the SOT data.

2. Images on SW CCD are 16-20" (or pixels) higher (ie. in Y-direction towards north pole) than LW CCD.

This may vary with wavelength

3. Images on SW CCD are 2" (or pixels) righter (ie. in X-direction towards solar-west limb) than LW CCD.

The fact is that the two CCD images arise from different halves of the primary mirror. 
The focal points of the two mirror halves could thus be different.

4. Slit-tilt effection

--JianSun 21:38:35 19-Jul-2024 BST