OIS

Name

ois

Purpose (one Line Only)

Optimal image subtraction

Description

STAMP - Definition - This term refers to a localized section of the array
that contains an image of point-source. This could either be an
actual sub-array or a description of a sub-array in an larger image.

Calling Sequence

ois,image,reference,diffimage

Inputs

image - Image to be processed. If this is a string it is taken to be
a file name that will be read. The other option is to provide
the array to be processed.
reference - Reference image to be subtracted from image. Just as with
image, the input can be a string or an array.

Optional Input Parameters

Keyword Input Parameters

CONSTPHOT - Flag, if set turns on the constant photometric ratio constraint
CHISQSTAT - Threshold for excluding stamps that don't fit well.
Default=0.0 (ie., no exclusions)
GAUSSIAN - Keyword that controls the basis set used. If not provided,
a Delta-function basis set is used. To use a gaussian
function basis set either give the name of the basis set
to use or privide the description yourself. If you provide
'astier' as the name, you will get the following:
gaussian={nc:3, degree:[6,4,2], sigmas:[0.7,1.5,2.0]}
You can provide your own description by providing a
similar structure. Tag nc is the number of components.
The length of degree and sigmas must match nc.
PATH - Directory location to find the input images. Default is the
current directory.
IMGHDR - String array containing FITS header information for input image.
This is used only when the input image is provided directly,
rather than to be read from a file. This array is used as
a starting point for the saved output image (if desired).
FWHM - FWHM of the input image. Used only if image provided
directly and the header is not. This value is
otherwise read from the header from the 'SEEING'
keyword in the header.
MAXPHOTSIG- Maximum DN value for a useful signal. Any source with a peak
above this level is passed over. Default=60000.0 DN
MINFLUX - Threshold for sources to be included in the reference PSFs
to support the image subtraction. This value is relative
to the brightest source found in the image. Default=0.25
DEGREE - Degree of polynomial used to fit for the space varying kernel.
Default=0 (constant kernel)
DELP - Grid size in pixels for recalculating the space-varying kernel.
Default=1 (doesn't matter for a constant kernel)
OUTPATH - String, name of directory to save data in.
Default = current directory.
NOTE: PATH and OUTPATH must not be the same. If they are
then the SAVEIT flag will be ignored and the output will
not be generated.
SILENT - Flag that will determine whether various relevant information will
be printed. Default=0. By default, it will print the statistics
NODISPLAY - Flag that will determine whether various relevant plots will be
created. Default=0. By default, the plots will be created.
SEED - value for the seed used by the random call. Default is undefined
causing random to use the system time as the seed

Outputs

diffimage - the subtracted image

Keyword Output Parameters

CREFERENCE - the convolved reference image
FAILED - boolean indicating whether or not the subtraction failed
DIFFINFO - anonymous structure containing lots of useful information about
the difference image and what happened along the way.
This structure is undefined if the output keyword FAILED is set

Common Blocks

Side Effects

Restrictions

Note that the input to this routine is expected to already be registered
and be of the same size. The routine, dewarp, is usually used to
do this registration step.
This routine struggles as the image FWHM gets a lot larger than the
template. The execution time goes up significantly as well.

Procedure

Internal helper routines:
ois_filtpix

Modification History

Written by SwRI clinic team, 2009/11/09, optimized version derived from
the original code written by Patrick Miller. (ref)
2010/03/05, FS, added IMGSKY and REFSKY keywords
2010/03/12, FS, added SILENT, NODISPLAY, and SEED keywords
2010/04/21, FS, added CREFERENCE, FAILED keywords
2010/04/28, FS, changed shift calls in ois_coef to sshift2d with edge_zero
2010/06/10, MWB, fixed a display bug with the number of stamps changing.
Also changed default on DEGREE to 0.
2010/09/23, MWB, maxphotsig was not getting all the way to the findsrc
call that does the stamp selection. This has been
changed. Also, kernel size determination was tweaked
downward.
2010/10/13, MWB, rework of the image/reference flux ratio calculation
2010/10/21, MWB, fixed a bug where the FWHM value read from the header
was not used properly.
2010/10/22, MWB, cleaded up ois_stamps routine. Flux ratio and fwhm
determinations are much better now.
2010/11/02, MWB, added DIFFINFO output keyword
2010/11/03, MWB, minor modification to ois_convref to hold the output
image type to float (was returning a double). This
ensures the final difference image is a float.
2011/04/12, MWB, fixed a bug in stamp selection for a rare case when
the brightest good stamp is too close to the edge
of the image.
2011/12/28, MWB, Changed usage of MAXPHOTSIG. Now, this level is used
just during the stamp selection process. No change
is made during the actual differencing process.
2012/01/24, MWB, Changed FWHM filtering on stamps. It was a 2-sigma cut
and this has been relaxed to a 3-sigma cut. Also,
one case that generated a warning and tried to
run the difference anyway was changed to return with
a failure.
2012/03/10, MWB, Found and fixed a minor bug in ois_fluxcheck. It would
sometimes cause the differencing to fail if the first
stamp had already been marked bad. This routine was
changed to use the first good stamp.
2014/02/11, MWB, added more data type options for SEED
2016/05/02, MWB, integrated new ois_coeff routine from Steve Hartung that
fixes a bug related to high-order PSF variations

OIS