Subject: Re: JEM-X pointings for next Crab calibration

Hello Peter:

On the way back I checked the JEM-X offsets during the
March 2004 Crab calibration. (rev 170).

I used the following data as input:

                                    Crab off center positions
                                        (1.5 arcmin pixels)
           pointing                    JEM-X1        JEM-X2 
rev  pid   RA      dec      roll      X      Y      X      Y
0170 0039  84.857  +22.079  92.21   +1.4  +48.3   +1.3  +48.4
0170 0040  83.730  +23.035  92.47  -37.1  +10.0  -37.2  +10.1
0170 0081  83.778  +22.036  92.59   +1.3  +10.2   +1.3  +10.2

>From these numbers I derive that to get the Crab truly on-axis
for JEM-X1 (and for JEM-X2 as well) the nominal pointing should
be RA: 83.489, dec: +22.070  (J2000).

The precise angular separation we want between the pointings in
the 19 point dither is 0.05561 degrees (this corresponds to our
mask hexagon center-to-center separation of 3.3 mm divided by
the mask-detector separation of 3400 mm.

The roll angle will be close to 90 degrees again next time, and
this means that the reference point in the inner hexagon of the 
19 point pattern should  be:
RA: 83.490, dec +22.014 (J2000). 
This should correspond to a point shifted 0.056 degrees along 
JEM_X detector X. (This is not spacecraft X!).

The reference pointing of the outer 12 points should have the 
coordinates:
RA: 83,537, dec +21.987 (J2000).

Thanks for a good meeting!

niels