Swansea Astronomical Society Blog
Friday, September 20, 2013
Harvest Moon with a mobile phone
A Harvest Moon, is the September full Moon that is closest to the equinox.
Colin Elphic
The full Moon with a QHY 5-II CMOS camera
Steve Wainwright
Sunday, September 15, 2013
The Need for Precise Image Cropping
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:
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:
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:
.JPG)
.JPG)
The final, stacked image was post processed in the open source program, the Gimp, to produce the final image:
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
Cropping with precision
This software allows the user to crop an image to specific dimensions rather than using an arbitrary cropping tool such as is found in all image processing programs. So, if I wish, for example, to crop out an area of exactly 1500 x 1500 pixels from a large image, I can do this precisely. If I have a number of images of the same object and I wish to crop them all, the software will crop them either semi-automatically, or fully automatically to exactly the dimensions required. Moreover, the cropped images will have been registered. The end of the automatic cropping process for a set of images is a set of registered images of exactly the same, prescribed dimensions, and a stacked image ready for further processing.
This is a screenshot of the program with the "about" screen displayed:
.png)
A Pentax K-x DSLR was directly connected to an f/5, 150mm Newtonian and 23 images of the Moon were captured. They were submitted to AstroCrop and automatically cropped as shown above. This is the resulting average of the cropped images, post processed in Photoshop:
Similarly, a Fujifilm Bridge camera with 40x optical zoom was attached to a CG5 equatorial, driven mount and 28 images of the Moon were captured. They were cropped and stacked in AstroCrop to produce the image below:
I shall be writing and talking about this software in the near future when it is in its release version.
It is fundamentally different from software such as Registax and Autostakkert and is not intended to be a competitor. It is a precise cropping tool that will give cropped images of exact, precise dimensions, something that is not easy to achieve in most image processing software. If a set of replicate images are cropped and registered, then the stacked image produced at the end will have a higher signal to noise ratio than an individual image in proportion to the square root of the number of images stacked. This image is ready for further processing.
Nicola will soon be releasing this software as a freeware program that for a small price can be upgraded to a program with more functionality. It is good to see yet more astronomical software coming out of Wales.
Steve Wainwright
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