Subject: JEM-X input for Crab calibration (and dither pattern)

Hello Peter:

Regarding the Crab calibration:

We prefer to have many short duration pointings with adequate
telemetry rather than a few longer pointings. The purpose is to 
illuminate the full detector surface while minimizing the
obscuration by the collimator. Therefore we prefer a 19 point
hexagonal pattern aligned with our mask pattern (which is
parallel to that of SPI) but with only 3.34 arcmin between
neighboring pointings. Given adequate telemetry (15+15 packets)
we can do with about 650 s per pointing. If absolutely necessary
we would rather see a reduction of the time per pointing than
a reduction of the 19 pointings. If telemetry is a major obstacle
then we should concentrate on JEM-X1 this time rather than doing 
a poor job on both units.

We would still like to have JEM-X2 activated and taking data
during the periods where IBIS or SPI are the prime instruments.
To get good data for JEM-X2 the DFEE should be powered up
one orbit prior to the start of the calibration (to achieve
temperature stabilization) and the JEM-X spcific observation
should be placed in the second half of the calibration sequence
to assure that JEM-X2 has stabilized - even after being dormant
for half a year.

If the detailed planning of such an observation is complicated
by the uncertainty of the precise date where it will be performed
we can relax our requirement on the alignment of the movements
with the mask pattern. The movements off axis are small, and 
alignment errors up to 10 degrees will have an insignificant
effect on the end result. We will in any case make full use of
the post-facto knowledge of the attitude data.

I attach my first notes to the ISWT on this issue. 


with best regards


niels


***************************************************************

> 
> Next Crab calibration
> 
> We request for the coming Crab calibration a "mini-dither" in a 
> hexagonal pattern matching the JEM-X mask pattern - i.e. the 
> separation between the dither points should be 3.34 arcminutes.
>  
> The purpose of this calibration is to map with good accuracy the 
> uniformity at the pixel level of the sensitivity of the JEM-X 
> detectors. These uniformity data are badly needed to improve our 
> image deconvolution and spectral extraction software.
> 
> We request a pattern of 19 pointings close packed around the
> JEM-X 'on-axis' pointing. With an adequate telemetry allocation 
> (15+15 packets for the two JEM-X units) we need a total time of 
> 12.5 ksec or 650 s per pointing.
> 
> The orientation of the hexagonal pattern relative to the spacecraft 
> axes must be the same as the orientation of the normal hexagonal 
> dither pattern. (The hexagonal patterns of the JEM-X masks are 
> oriented parallel to the hexagonal pattern of the SPI mask).
> 
> 
> ****************************************************
> 
> 
> Shift of the centerpoint for observations with multiple hexagonal 
> dither patterns.
> 
> 
> To reduce the artefacts in images obtained with standard hexagonal 
> dithering it is proposed to shift the center for each new 7-point 
> cycle similarly to the shifts now introduced for repeated 25-point 
> patterns.
> 
> I propose here a sequence of 13 different center positions allowing 
> observations of up to 325 ksec with hexagonal dither without 
> repetition of any pointing.
> 
> Six of new center points are displaced by 40 arcminutes from the 
> primary target in a hexagonal pattern parallel to the nominal 
> observation pattern. Six more center points are obtained by 
> displacement by 72 arcminutes in a hexagonal pattern turned by 
> 30 degrees with respect to the nominal observation pattern.
> Together with the original pattern centered on the primary target 
> we will then obtain 13 sequences of 7 pointings (91 different 
> pointings in total). 
> 
> The end result, when all 13 patterns are completed will be a 
> hexagonal pattern with 91 pointings separated by 40 arcminutes.
> 
> The pattern is crudely sketched below. O marks the position of
> the primary target. The small o's are six positions of the
> original hexagonal dither pattern, 2 degrees (120 arcminutes)
> off the primary target. The x's marks six shifted positions of 
> the center for subsequent dither patterns, they are 40 arcminutes 
> from O. The y's are six further center points, 72 arcminutes off
> center.
>   
>             o              o
> 
> 
> 
>                    y
> 
>                 x     x
>             y             y
> 
>  o            x    O    x             o
> 
>             y             y
>                 x     x
> 
>                    y
> 
> 
> 
>            o               o
> 
> 
> ****************************************************
>