Four Representations of HESSI Visibilities: Which is Optimum?

We discuss four basic ways the HESSI visibilities can be stored and processed for mapping. In all of these the number and spacing of the time bins selected for the modulation profiles of each HESSI detector have direct consequences for the speed of processing and storage requirements.

The shortest time bins for HESSI will be 0.5 ms. This is small enough to obtain 8 samples per cycle for maps made near the solar limb using the finest collimator. If instead we only ask for 4 cycles per cycle, imaging is not seriously compromised, and the required array sizes are reduced by a factor of 2. Table I shows the number of bins required for each sub-collimator (N_j), given 4 or 8 bins per cycle.

Table 1

No. Bins per Half-Rotation for 8 and 4 per Cycle

Coll N_j N_j

(8/cycle) (4/cycle)

1 4000 2000

2 2309 1155

3 1333 667

4 770 385

5 444 222

6 257 128

7 148 74

8 86 43

9 49 25

total 9396 4694

In the following, we will adopt the 4 bins/cycle numbers for estimates of array sizes in different representations of the visibilities. All the estimates can be scaled up or down accordingly.