iraf.net

Hello,I'd like some advice/insight into the tasks "sarith" and
"scombine". I have three multispectra images that I want
to combine (basically I have three 2000 seconds exposures
and would like to make a 6000 seconds exposure out of
them). Is either task better than the other? I have
tried both and the results are not quite the same. Most
significantly, "scombine" produces spikes/ditches at the
edges of each aperture, regardless of the interpolation
function used.Thank you,
Kaori Nishikida