Processing High Resolution

Images for Publication, Internet or Printing

Uploaded 10/11/04

 There are several ways to look at getting images ready for publication. Are we trying to show the elements of a dense star cloud, or rich Milkyway field, or perhaps a bright crimson red nebulosity to an advantage? The answer I have come to the conclusion on is you can use one of two methods to highlight your subjects.

For nebula, especially faint tendrils in a rich star field, it is well known that applying the "erosion filter" in Photoshop can enhance the visibility of dim nebula by removing all the dim stars and reducing the bright ones, but not affecting the nebula.

I am demonstrating the opposite condition here, using the new software Pix Insight to implement some dramatic changes in an image, to make it more compatible with your intended output device, or for magazine or book publication.

The Problem.

Images taken with fast focal ratio instruments have very small stars. The smallest size of the stars on film or CCD are directly proportional to the f/ number, not the aperture or focal length (Assuming good seeing) . A fast 50mm lens for example can produce stars about the size of a living blood cell for example, about 5 microns. A full size enlargement of this image up to 16x20 for example will yield a tremendous number of faint stars, and of course a highly detailed image. But if we reduce the size of the print by the use of a now standard digital printer (like Walmarts new digital photo printers) or resize for the internet to 640x480, or even try to print the image at a paltry 5x7 size, we start loosing all the faint stars. Why? If the original had stars that when scanned or were originally digital that were only one or two pixels across, they will resize to sub pixel' and effectively disappear at any smaller sizes. Down below are two images Ill explain a bit later to demonstrate this resampling problem.

One Solution.

Mathematically there is one way to keep all those hard earned faint stars, and print, or display on a web page everything you recorded. The latest version of Pix Insight has a new dilation filter that you can adjust the parameters on, unlike Photoshops one slider does all approach. Here's the theory, If you increase the size of the stars not by defocusing (which would blur them out and remove them) by a round kernel dilation process, you effectively expand the disk of the star, but don't change its brightness. So if we are to make a image half sized in which the stars are about one pixel to start with, you would double the star size by dilation, then resize the picture to its final size. Then the stars would be once again one pixel. Further reductions in size will require more dilation. I used a setting of about .4 pixels to get the images below. Remember - use only JUST enough dilation to make the faintest stars you want in the final picture become visible, and NO more. You will love what it rescues from your original hard earned images! Here are the settings used for this process:

Two Examples.

Look carefully at the two thumbnails below. The top one is a shot of Cassiopeia with a fast lens for 5 minutes at ASA 3200 from my backyard. Its more vague glows and certainly one or two stars. The second image has had dilation and SGBNR applied with Pix Insight. I over exaggerated the dilation to make this thumbnail to prove a point - the faint stars are there in the original, and you can make them more visible in smaller sizes to any extent you wish with this process. Now look at the 1200x800 images, and see faint stars in the second image that are not even present in the first. Its hard to believe that its not the same image!


Here is yet another tool for the modern astrophotographer to enhance their images to their full potential. The key to most astrophotography is to show off the final images in either print, internet, or magazine publication (the ultimate). I hope you found this technique as useful as it has been to me in getting the most out of my schmidt camera and wide field film, and digital images !

Select an image size for a larger view: 1200 x 800

Select an image size for a larger view: 1200 x 800
Lens: 28 - 135mm f/3.5 Canon EF/ IS Platform: Robotic Barn Door tracker Camera: Canon 10D ASA: 3200 Exposure: 5m Filters: None Location: Payson, Arizona Elevation: 5150 ft. Sky: Seeing 6/10, Transparency 6/10 Outside Temperature: 15 C Processing Tools: Photoshop CS, Pix Insight HOME GALAXIES EMISSION NEBS REFLECTION NEBS COMETS GLOBULARS OPEN CLUST PLANETARIES LINKS


FastCounter by bCentral