MKFLAT

Name

mkflat

Purpose

Collect and combine CCD flat frames into a superflat frame

Description

The files are assumed to be named as 'root'NNN where 'root' is the
root of the file name that you supply and NNN is a three digit number.
If your file name has an imbedded '.' then add it to the root.
The specified range of files are all read in from FITS files. Then each
image has the overscan mean subtracted (if desired), cropped (as indicated),
input bias image is subtracted, and then the dark is multipled by each
image's exposure time and then subtracted. These images are then
normalized using the mean of the SCALE region and then averaged weighting
by the signal-to-noise ratio of each flat image. The weighted averaging
is done with AVGCLIP.PRO which does a robust average of the image
stack so that cosmic rays, stars, and other transient image defects are
eliminated. This will do a very good sky flat if enough images are provided
of different field pointings.
When done, the resulting flat image is returned to the caller and the image
is saved to a FITS file with the specified output filename.

Calling Sequence

mkflat,root,outsuf,start,nframes,bias,dark,flat

Inputs

root - Root of the file name (you must include . in the name).
outsuf - The suffix of the final output file.
start - First frame number to read (integer or long).
Start can also be a vector of explicit frame numbers to load.
In this case, nframes need not be specified and in fact will
be ignored.
nframes - Number of frames to average.
bias - Bias frame image name to subtract from each raw flat.
dark - Dark frame image name to subtract from each raw flat.

Optional Input Parameters

Keyword Input Parameters

CROP = region of original image to save, default=no cropping.
[x1,x2,y1,y2]
DDIR - Path to the raw data, default = ''
DIGITS - Optional input to indicate how many digits are in the suffix
of the file name. The default for this input is 0. In this
case it uses the ROBOCCD scheme which is three digits up
to 999. After that, it gets complicated, see numtoflist
for more information. If you were to give it a value of
three you would get the same behavior except you won't get
the ROBOCDD extension. This is really designed for when
you have four or more digits.
EXCLUDE - Optional vector of image numbers that should be excluded from
average. Default is to include all frames.
EXPKEY = String, FITS keyword to read to get exposure time, default=EXPTIME
JUSTMEDIAN - Flag, if set will stop processing after the initial median
combination of the image cube and this will be the final
answer. This should used only if the intrinsic noise
level of the result can never drop below 1 DN. Particularly
useful for a small number of flat images.
OVERSCAN = column overscan region to use for frame bias level,
default=no overscan subtraction.
RDNOISE - Read noise of CCD, [DN], default=10
SAVECHK - String with name of directory to save graphics to. The images
saved are the final flat, the original image, and the
flattened flat images. They are heavily binned if large
and are useful for figuring out if the input flats are good
or not. The default is '[[none]]' which means to not save
any of the graphics. Using this option causes the program to
use the current graphics device to open and use window 7.
SCALE - 4 element vector which, if provide, defines the region of the
array dimensions that are used to scale the mean
of the arrays before combining (. If combined in this
manner, the arrays are combined weighted by the means.
[x1,x2,y1,y2]
These coordinates apply to the pixel locations AFTER cropping.
The default is to use the center 50% of the image but not any
bigger than 200x200 subsection at the center.
MINGOOD - Minimum number of frames required to be considered a good
bias. (Default=15)
MAXSIG - Maximum sigma threshold for filtering bad frames.
Default is inherited from gradebias
default=3

Outputs

flat - Final robust averaged and normalized flat image.
good - Flag, 1 if flat is considered good, 0 if not.

Keyword Output Parameters

Common Blocks

Side Effects

Restrictions

Procedure

Modification History

95/03/10 - Initial crude version written, Marc W. Buie, Lowell Observatory
95/06/04, MWB, added EXCLUDE keyword
95/06/13, MWB, added SCALE, OVERSCAN, CROP keywords
99/06/10, MWB, added EXPTIME keyword and added documentation.
2000/02/03, MWB, rewrite to add support for multigroup FITS files.
2000/02/28, MWB, added support for frame numbers > 999.
2001/02/23, MWB, added option to provide input file list.
2001/04/28, MWB, added DDIR keyword
2004/05/05, MWB, fixed SCALE keyword default action. It was not working
right at all and the region selected was much too large.
2006/07/14, MWB, added RDNOISE keyword
2006/07/27, MWB, added JUSTMEDIAN keyword
2006/10/23, MWB, fixed output header keyword problem (remove BSCALE/BZERO)
2013/10/05, MWB, added SAVECHK keyword
2015/04/26, MWB, added use of gradeflat
2016/05/29, MWB, added DIGITS keyword