AstroCrop - Image Registration and Batch Cropping
AstroCrop is an application which automatically registers and precisely crops a series of images based on a single reference frame. It can be used to automatically crop a series of images or be used as an image stabilisation tool whose data can be easily processed by other stacking applications.
AstroCrop is currently available for Microsoft Windows and 64-bit Linux distributions. A native macOS version will be released in due course.
A static tripod can be used to hold a DSLR with a long lens or a Bridge camera with high optical Zoom, such as the 60x optical Zoom of a Panasonic Lumix DMC-FZ72 bridge camera for capturing images of the Moon or Sun (suitably solar filtered.) This allows for very quick setup for imaging.
The camera can be set to capture images in bursts of several images. However, to obtain the highest quality results it is necessary to capture large numbers of images to be stacked. It is possible to capture more than 100 images in rapid bursts before problems of image rotation creep in. The problem is that whilst the camera can be held quite still enough to capture images without movement blur, there is inevitable, and usually substantial movement of the object being imaged between bursts of images, not withstanding the steady movement of the Sun or Moon across the field of view due to the Earth’s rotation.
The result is the capture of a large number of images where the object of interest is in very different places in a relatively large image. This makes it virtually impossible for stacking software such as Registax to keep up with the movement of the object from image to image. What is required is to precisely crop out the object in the centre of a user-specified area and to save out the cropped images ready for stacking. Moreover, because the cropped images are smaller than the original images in which there was lots of wasted space, the stacking process if facilitated and speeded up. AstroCrop does this automatically, but there is also a manual mode for precise cropping of individual images.
Similar problems arise when afocal images are captured of planets etc often on manually tracked mounts or Dobsonian telescopes. By using AstroCrop to precisely crop the images, the problem of object movement between images is solved.
The following is a quick tutorial on how to use AstroCrop to process a series of images which have extreme movement between frames. It would be very difficult to stack images like these, particularly if there was a lot of them. AstroCrop is able to register each image and align them ready for processing in other stacking applications.
Unregistered stack of the example images to show the degree of movement between images.
Reference Frame Selected
Crop Area Selected
Crop area placed on image
Ready to crop.
Starting the cropping procedure.
Automatic cropping in progress.
All of the images are now registered and ready for cropping in software such as Registax, AutoStakkert! or lxnstack.
Future Development of AstroCrop
AstroCrop was written some time ago and so only supports reading and writing 8-bit image files (TIFF, Bitmap and JPEG files). I intend on writing a new version which will support 16-bit TIFF, FITS and DSLR Camera RAW files as well as outputting 8 or 16-bit SER files.