Mosaicing Large Fields:
I will be mosaicing ACS/WFC fields together.  What is
the offset in the U2 direction that can be taken before a gap appears in
the coverage of the sky between the two pointings?  I know the ACS field
of view is roughly 202" by 202", but with the distortions, etc., is
there a more precise number?  Same question for a U3 offset.
I could probably figure this out from the distortion equations but maybe
this is a relatively common question so someone has already got an
answer.

Reply

See the following report (ACS ISR 01-07) for some carefully-designed 
mosaic patterns that take this into account:

http://hst.stsci.edu/acs/documents/isrs/isr0107.pdf


Below is a set of corner coordinates for the two WFC chips (courtesy
of Colin Cox). This should allow you to accurately address the issue 
of optimal offsets in the mosaic. 
Pixels are numbered so that the center of the bottom left pixel in (x,y)
is (1,1)
We take the detector edges as running from 0.5 to 4096.5 in x and 0.5
to 2048.5 in y. This excludes the physical overscan regions

WFC1
(x,y)                   V2      V3              Amp corner
(0.5,0.5)               154.45  258.23          
(4096.5, 0.5)           356.82  250.28
(4096.5, 2048.5)        359.46  148.99          B
(0.5, 2048.5)           160.05  159.42          A


WFC2
(x,y)                   V2      V3                      
(0.5,0.5)               149.98  362.98          C
(4096.5, 0.5)           354.44  357.74          D
(4096.5, 2048.5)        356.86  252.94
(0.5, 2048.5)           154.54  260.76

These results are in metrology V2V3. To get the best estimate for FGS
measured positions, add (2.24, -6.79) to each V2,V3 position. This may
not matter for offset calculations. This is discussed further in a recent
ACS ISR No 02-02 by Cox etal.