REGISTAR will register (spatially align in 2-D) an image with respect to a
common reference image by determining the mean row and column shifts
between stars common to the target and reference images. The positions of
the reference stars must be pre-selected using the MARKSTAR command.
REGISTAR is designed to allow simple interactive and batch-mode alignment
of images. For example, if you are going to ;SPMquot;stack;SPMquot; a number of images of
an object together into a single cumulative image, REGISTAR may be used to
correct the images for small frame-to-frame shifts (usually due to guiding
errors) before adding them together. Other common uses are alignment of
image pairs used for ;SPMquot;on-band/off-band;SPMquot; emission-line imaging or
photometric color maps, and aligning elements of an image mosaic before
merging them into a single large image with the MERGE command.
Before using REGISTAR, it is necessary to use the MARKSTAR command to
create a ;SPMquot;reference list;SPMquot; containing the positions of stars to be used as
the common reference frame for the image registration. To do this, one
first chooses a single image from the set to act as the ;SPMquot;reference image;SPMquot;
and then uses MARKSTAR to mark all stars found in common between the frames
to be registered. The best strategy is to select stars which are neither
too faint, too bright (especially not saturated stars), or too crowded to
give a good centroid. The list of reference star positions determined with
the MARKSTAR command (also known in VISTA parlance as a ;SPMquot;photometry file;SPMquot;)
is stored in an internal COMMON block. This reference list will be used by
all subsequent calls to the REGISTAR command. However, if MARKSTAR is run
again, or if another photometry file is loaded using the GET command, the
reference list will be lost, and REGISTAR may produce spurious results.
Once the reference list has been created with the MARKSTAR program, it may
be stored on disk using the ;SPMquot;SAVE PHOT=xxx;SPMquot; command, and later loaded into
COMMON using the ;SPMquot;GET PHOT=xxx;SPMquot; command. This allows the user to
pre-select stars interactively, then run REGISTAR on a large number of
images in batch mode by reading in a prepared reference star list, or to
repeat the registration procedure on other images sharing the same
reference stars without having to repeat the selection process with
MARKSTAR. The user may also use the VISTA utilities for editing or
updating a photometry file to change a predefined reference star list, and
then repeat the registration procedure. REGISTAR will not work without a
photometry file loaded, either by MARKSTAR or GET.
Image registration in previous versions of VISTA was accomplished using a
combination of the interactive MARKSTAR command and the SHIFT command.
However, this had the distinct disadvantage of precluding batch
registration, and also made the procedure more cumbersome as MARKSTAR
resets the coordinates of the stars in the photometry file each time it is
run, causing the ;SPMquot;fiducial;SPMquot; to change. In addition, the user had to
determine the sign of the relative offset, and feed it by hand to the SHIFT
command. With REGISTAR, the reference star list is fixed, and the shift is
applied automatically.
The positions of the individual reference stars are found by multiple
centroiding. Once the relative offset in rows and columns between the
target image and the reference frame has been determined, the target image
is then shifted and re-binned, using either 4th-order Lagrangian
interpolation (default), or a somewhat slower but arguably more precise
SINC interpolation scheme. SINC interpolation should only be used on
images which are well-sampled (or even oversampled), as it has a tendency
to produce undesirable ;SPMquot;ringing;SPMquot; artifacts in the presence of rapid changes
in pixel-to-pixel intensity. SINC interpolation should never be used with
under-sampled star images.
REGISTAR searches in the neighborhood of the reference star positions for
those stars on the target image. The maximum distance that REGISTAR will
search in rows and columns is +/-3 pixels. The search limits may be
changed using the RSHIFT= and CSHIFT= keywords. Setting the search size
too large poses the danger of REGISTAR getting lost and grabbing the wrong
stars, hot pixels, or cosmic ray events. (It is a good idea to ;SPMquot;clean;SPMquot;
images of cosmic rays and hot pixels before attempting a registration; see
ZAP and TVZAP for details). Setting RSHIFT or CSHIFT too small runs the
risk of missing the stars if the relative shift puts them outside the
search box.
If it is already known that the target image is shifted relative to the
reference image by a large amount (e.g., you know that the target is
shifted by at least 5 pixels in rows and -10 pixels in columns), this may
be specified using the DR= and DC= keywords. DR and DC need only be given
to the nearest pixel or two. Without a suitable offset, REGISTAR could get
lost and either give up or give a spurious answer.
When 5 or more reference stars are used, REGISTAR iterates to calculate the
best mean shift between images by rejecting stars which give discrepant
shifts. The default rejection threshold is 4-sigma from the mean shift,
but this may be changed using the ;SPMquot;REJECT=;SPMquot; keyword. Too small of a
rejection threshold could eliminate valid stars, and too large could accept
spurious centroids. Once the mean shift calculation has converged,
REGISTAR will print the shifts for each of the stars on the image, and the
mean shift with an uncertainty (sigma).
REGISTAR provides for interactive control of the shift calculation via the
;SPMquot;INT;SPMquot; keyword. This is especially useful in cases where the image quality
or depth of the target image is not comparable to that of the reference
image. The ;SPMquot;INT;SPMquot; keyword stops after each iteration and provides an
opportunity for the user to change the weights of the stars used. REGISTAR
uses a normalized weighting scheme. A weight of 0.0 (zero) is used to
reject a star. Similarly, a weight of 1.0 accepts a star. With INT, the
user determines when the registration solution has converged. After
accepting a shift, the image is then shifted to the common reference frame.
After using REGISTAR, the target image should line up with the reference
image to within the accuracy allowed by the image quality. Most often this
is limited by the frame-to-frame repeatability of the stellar PSF.
REGISTAR includes a record keeping option available via the LOG keyword.
This is useful for identifying problems after a long batch registration run
of many images. In addition, it has also proven useful as providing data
to help track autoguider performance on a sequence of images.
<#5090#>Examples:<#5090#>
NOTE: At this time, a maximum of 100 stars are allowed to be used To change
this, the parameter MAXSTAR must be changed in the REGISTAR and GETMEAN
subroutines, both of which live in the same source code file.