iraf.net

below is the parameters I used for imcombine for coadding frames of m13 cluster:imcombine ("@m13-5.txt//bfreg",
"m13-5.fits", headers="", bpmasks="", rejmasks="", nrejmasks="", expmasks="",
sigmas="", imcmb="$I", logfile="STDOUT", combine="average", reject="crreject",
project=no, outtype="real", outlimits="", offsets="none", masktype="goodvalue",
maskvalue="0", blank=0., scale="none", zero="none", weight="none", statsec="",
expname="", lthreshold=0.001, hthreshold=INDEF, nlow=1, nhigh=1, nkeep=1,
mclip=yes, lsigma=3., hsigma=3., rdnoise="1.73", gain="2.13", snoise="0.",
sigscale=0.1, pclip=-0.5, grow=0.)the rdnoise value of 1.73 was for each individual frame that was coadded.
the gain value of 2.13 was for each individual frame that was coadded.I think to find the gain of the new image is simple 2.13N where N is the number of images that went into the composition.What is the formula for finding the new readnoise?

When averaging, the readnoise goes down by sqrt(N), with N the number of frames you're averaging, so new readnoise = readnoise/sqrt(N).