REGISTAR: Register Images Using Field Star Positions

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 "stack" 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 "on-band/off-band" 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 "reference list" 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 "reference image" 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 "photometry file") 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 "SAVE PHOT=xxx" command, and later loaded into COMMON using the "GET PHOT=xxx" 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 "fiducial" 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 "ringing" 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 "clean" 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 "REJECT=" 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 "INT" 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 "INT" 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.


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.