iraf.net

Hi
I've successfully flatfeilded my 8k 36"kpno data.
I would now like to combine various pointings into
1 image.
I've tried using mscstack, but it doesn't correctly
compute the offset between images.Do you have any other suggestions ???
thanks,
-pete challis

Hi Pete,What do you mean MSCSTACK doesn't correctly compute the offset between
images? Do you mean the result is not very good or that the mode
values are just plain wrong? It may be that the mode values used for
a zero offset are not adequate to make the best final image. It is
tricky to get this right.The task MSCIMATCH is available to compute the scaling and offset
values based on object photometry. These puts the results in the
keywords MSCSCALE and MSCZERO in the image headers and MSCSTACK can
be set to use these values with the syntax "!mscscale" and "!msczero"
in the scale and zero parameters. You can use the mode method in
MSCSTACK, MSCIMATCH, or some other way to determine the scaling you
need. Getting the best match between CCDs and then between exposures
is challenging and there is no special trick except experimentation.We are exploring some options including an improved MSCIMATCH and
a technique of fitting and subtracting a low order sky fit to each
CCD (using IMSURFIT) before MSCIMAGE. Then no offset scaling should
be required in MSCSTACK. This seems very promising and you could
try it. The only gotchas are to be sure the threshold limits in
MSCIMATCH and MSCSTACK are changed from the defaults which reject
data less than 1. Also in MSCIMAGE setting the blank value for
flagging the gaps to a significantly negative value rather than zero.I hope this helps.Cheers,
Frank