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XMM-Newton Science Analysis System


echip2sky (echip2sky-0.5.1) [xmmsas_20041122_1834-6.1.0]

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Attitude variation

A complication to transforming from CHIPCOORD to sky coordinates is that the spacecraft attitude is never completely fixed, thus the exact transform varies with time. The projection from CHIPCOORD to sky coordinates over a period of time can therefore only be estimated by a sequence of transform samples, weighted by the duration of validity of the sample. The user has the facility to supply a dataset table containing a list of spacecraft attitude samples plus the boundary times between them. (The format of this list is described more fully in section 6.) This is done via parameters withatttab and atttab. Each chip image is subjected to multiple transforms, one per attitude bin; the output image consists of a weighted sum of these transformed images, the weights being proportional to the duration of each attitude bin (but see also section 3.2 below).

Notes:

• The format of the attitude sample table does NOT correspond to that of the output of atthkgen or with any other SAS product. Some time in the future a capability to produce such a table may be added to a SAS task such as attcalc.

• This scheme of weights is only accurate if the values in the input chip images are time-invariant. It may for example be unwise to make OOTE sky maps this way.

If withatttab is set to `no' (the current default), the task obtains a representative spacecraft attitude for the entire duration from RA_PNT, DEC_PNT and PA_PNT keywords in the header of each chip image dataset. NOTE in this regard that under OGIP conventions these keywords OUGHT to represent the INSTRUMENT attitude not the spacecraft. At some point this situation may be rectified.