Swansea Astronomical Society Blog

Sunday, September 15, 2013

 

The Need for Precise Image Cropping

ActroCrop Continued
On April 14, 2012 I posted here on the SAS blog, an afocal image of Saturn captured by SAS member Ken Shepherd.
Ken had used an Olympus Camedia C-480 Zoom compact camera on a homemade afocal mount with a 90mm refractor. He had captured 9 images, which due to the nature of the equipment and the technique, the subject, Saturn was not in the same place on each image.
In order to produce a stacked image with 3x the signal to noise ratio of an individual image, we used the prototype software written by Nicola Mackin, 'Precise Cropping Tool', (the forerunner to AstroCrop which has been in development since). We cropped an equal area with Saturn close to the centre from each of the images and stacked them in Registax to produce an image. The precise cropping was required because Registax requires images of equal size to be stacked. The movement of Saturn was such that Registax was unable to keep track of the 'movement' of Saturn from one image to the next.

The following animation shows the 'movement' of Saturn in each of the 9 images:


The following image is a blend of all of the images to show the relative positions of Saturn in the individual images:

It can be seen that there was considerable movement of Saturn between the images.

Registax 6 was unable to keep track of the planet in the original images, and when forced to use all of the images produced the following image for the stack:


Registax 5 did much better, but still didn't manage to produce a perfect stack:

 
It can be seen that there is a ghostly image of Saturn above the rest of the stack showed the imperfect alignment. Using a larger alignment area, Registax 5 was only able to use two of the images.

AstroCrop was applied in fully automatic mode to crop out a specified area, to save the cropped images, but also to stack the cropped images. For comparison, below is the resulting stack from AstroCrop displayed to the same brightness as the image above:


It can be seen that all of the images have been stacked correctly. The image is ready for further post processing.
The next two images show Saturn and the specified crop area, and the cropping and stacking taking place with progress bars being displayed. Click on an image to get a larger view:



The final, stacked image was post processed in the open source program, the Gimp, to produce the final image:
Stacked images, processed

 
Single image from the set of 9
 

It is to be stressed that AstroCrop is not a substitute for Registax or Autostakkert. It has different functionality and is intended for different purposes. AstroCrop can be used with images of any size, but is intended to be able to deal with very large images. Registax often finds it hard to align a number of very large images, such as those obtained from a DSLR or a bridge camera, where there is significant displacement between the images. AstroCrop works in a completely different way and within very large limits, it doesn't matter how much the object of interest is displaced between images.
AstroCrop can be used to crop precisely any image, to precisely defined dimensions, or can crop a batch of similar images to the same dimensions and register the images during the process. The final stack is a bonus that allows one to move on to post process the resulting image in other software.
AstroCrop is a sophisticated, precise cropping tool.
The release version will be freeware and will be able to work with JPG image files.
The full version will be available for a small charge and will be able to deal with JPG, BMP, TIFF (8 and 16 bit) files. The program will be able to read and write all of these formats and will be able to save out combinations of formats simultaneously, or a single format selected by the user.
The software is in advanced beta testing and the freeware release will occur in the near future.
Watch this space
The software is developed by SAS member Nicola Mackin, an independent IT consultant and software developer; at Alternative Systems.

Steve Wainwright
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