calculating gain and readnoise of coadded frames

[AndrewS]

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?

[kwrsema]

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