This value is used only when constructing the Mapping between two different SkyFrames (for instance, when calling astConvert or astFindFrame). If the transformation between the two SkyFrames causes positions to shift by less than SkyTol arc-seconds, then the transformation is replaced by a UnitMap. This could in certain circumatances allow major simplifications to be made to the transformation between any pixel grids associated with the two SkyFrames (for instance, if each SkyFrame is part of the WCS FrameSet associated with an image).
A common case is when two SkyFrames use the FK5 system, but have slightly different
Epoch values. If the AlignSystem attribute has its default value of
" , then the
transformation between the two SkyFrames will include a very small rotation (FK5
rotates with respect to ICRS as a rate of about 0.0005 arc-seconds per year). In most
circumstances such a small rotation is insignificant. Setting SkyTol to some suitably
small non-zero value will cause this rotation to be ignored, allowing much simpler
transformations to be used.
The test to determine the shift introduced by transforming between the two SkyFrames is performed by transforming a set of 14 position spread evenly over the whole sky. The largest shift produced at any of these 14 positions is compared to the value of SkyTol.
The SkyTol value is in units of arc-seconds, and the default value is 0.001.