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diff --git a/noao/obsutil/src/doc/findgain.hlp b/noao/obsutil/src/doc/findgain.hlp new file mode 100644 index 00000000..91bf7eaf --- /dev/null +++ b/noao/obsutil/src/doc/findgain.hlp @@ -0,0 +1,168 @@ +.help findgain Nov01 noao.obsutil +.ih +NAME +findgain -- calculate the gain and readout noise of a CCD +.ih +SYNOPSIS +FINDGAIN uses Janesick's method for determining the gain and read noise +in a CCD from a pair of dome flat exposures and a pair of zero frame +exposures (zero length dark exposures). +.ih +USAGE +findgain flat1 flat2 zero1 zero2 +.ih +PARAMETERS +.ls flat1, flat2 +First and second dome flats. +.le +.ls zero1, zero2 +First and second zero frames (zero length dark exposures). +.le +.ls section = "" +The selected image section for the statistics. This should be chosen +to exclude bad columns or rows, cosmic rays and other blemishes, and +the overscan region. The flat field iillumination should be constant +over this section. +.le +.ls center = "mean" +The statistical measure of central tendency that is used to estimate +the data level of each image. This can have the values: \fBmean\fR, +\fBmidpt\fR, or \fBmode\fR. These are calculated using the same +algorithm as the IMSTATISTICS task. +.le +.ls nclip = 3 +Number of sigma clipping iterations. If the value is zero then no clipping +is performed. +.le +.ls lsigma = 4, usigma = 4 +Lower and upper sigma clipping factors used with the mean value and +standard deviation to eliminate cosmic rays. +Since \fBfindgain\fR is sensitive to the statistics of the data the +clipping factors should be symmetric (the same both above and below the +mean) and should not bias the standard deviation. Thus the values should +not be made smaller than around 4 sigma otherwise the gain and readnoise +estimates will be affected. +.le +.ls binwidth = 0.1 +The bin width of the histogram (in sigma) that is used to estimate the +\fBmidpt\fR or \fBmode\fR of the data section in each image. +The default case of center=\fBmean\fR does not use this parameter. +.le +.ls verbose = yes +Verbose output? +.le +.ih +DESCRIPTION +FINDGAIN uses Janesick's method for determining the gain and read noise +in a CCD from a pair of dome flat exposures and a pair of zero frame +exposures (zero length dark exposures). +The task requires that the flats and zeros be unprocessed and uncoadded so +that the noise characteristics of the data are preserved. Note, however, +that the frames may be bias subtracted if the average of many zero frames +is used, and that the overscan region may be removed prior to using this +task. + +Bad pixels should be eliminated to avoid affecting the statistics. +This can be done with sigma clipping and/or an image section. +The sigma clipping should not significantly affect the assumed gaussian +distribution while eliminating outlyers due to cosmic rays and +unmasked bad pixels. This means that clipping factors should be +symmetric and should have values four or more sigma from the mean. +.ih +ALGORITHM +The formulae used by the task are: + +.nf + flatdif = flat1 - flat2 + + zerodif = zero1 - zero2 + + gain = ((mean(flat1) + mean(flat2)) - (mean(zero1) + mean(zero2))) / + ((sigma(flatdif))**2 - (sigma(zerodif))**2 ) + + readnoise = gain * sigma(zerodif) / sqrt(2) +.fi + +where the gain is given in electrons per ADU and the readnoise in +electrons. Pairs of each type of comparison frame are used to reduce +the effects of gain variations from pixel to pixel. The derivation +follows from the definition of the gain (N(e) = gain * N(ADU)) and from +simple error propagation. Also note that the measured variance +(sigma**2) is related to the exposure level and read-noise variance +(sigma(readout)**2) as follows: + +.nf + variance(e) = N(e) + variance(readout) +.fi + +Where N(e) is the number of electrons (above the zero level) in a +given duration exposure. + +In our implementation, the \fBmean\fR used in the formula for the gain +may actually be any of the \fBmean\fR, \fBmidpt\fR (an estimate of the +median), or \fBmode\fR as determined by the \fBcenter\fR parameter. +For the \fBmidpt\fR or \fBmode\fR choices only, the value of the +\fBbinwidth\fR parameter determines the bin width (in sigma) of the +histogram that is used in the calculation. \fBFindgain\fR uses the +\fBimstatistics\fR task to compute the statistics. +.ih +EXAMPLES +To calculate the gain and readnoise within a 100x100 section: + +.nf + ms> findgain flat1 flat2 zero1 zero2 section="[271:370,361:460]" +.fi + +To calculate the gain and readnoise using the mode to estimate the data +level for each image section: + +.nf + ms> findgain.section="[271:370,361:460]" + ms> findgain flat1 flat2 zero1 zero2 center=mode +.fi + +The effects of cosmic rays can be seen in the following example using +artificial noise created with the \fBartdata.mknoise\fR package. The +images have a gain of 5 and a readnoise of 10 with 100 cosmic rays added +over the 512x512 images. The zero level images have means of zero and the +flat field images have means of 1000. The first execution uses the default +clipping and the second turns off the clipping. + +.nf + cl> findgain flat1 flat2 zero1 zero2 + FINDGAIN: + center = mean, binwidth = 0.1 + nclip = 3, lclip = 4., uclip = 4. + + Flats = flat1 & flat2 + Zeros = zero1 & zero2 + Gain = 5.01 electrons per ADU + Read noise = 10.00 electrons + cl> findgain flat1 flat2 zero1 zero2 nclip=0 + FINDGAIN: + center = mean, binwidth = 0.1 + nclip = 0, lclip = 4., uclip = 4. + + Flats = flat1 & flat2 + Zeros = zero1 & zero2 + Gain = 2.86 electrons per ADU + Read noise = 189.5 electrons +.fi + +.ih +BUGS +The image headers are not checked to see if the frames have been +processed. + +There is no provision for finding the "best" values and their errors +from several flats and zeros. +.ih +REVISIONS +.ls FINDGAIN - V2.12 +New task derived from MSCFINDGAIN. This makes use of the new clipping +feature in IMSTATISTICS. +.le +.ih +SEE ALSO +imstatistics +.endhelp |