Image Processing

Processing of astronomical images is a careful teasing out of details from images, which on the surface look like nothing is there to be seen. What I want to do here is give a brief introduction to how I process my images and how that has developed over the years showing how I tease out of a seemingly flat and featureless image the photos you see on these pages.

When I started out I didn't bother with stacking images mainly because I had not heard of Deep Sky Stacker and the only previous experience I had with astrophotography was with a film camera, and stacking negatives just doesn't work! This changed by searching the internet on taking astrophotographs and I learned of Deep Sky Stacker and other software some of it free, and some not.

One astrophotographer who used a DSLR was Carole Pope and her website has a useful page on imaging with a DSLR and I based my early workflow on the advice contained in this rather lengthy but excellent page. This worked for a while and many of my early photos are as a result of this advice.

Like everything we move on and develop our techniques and one influential source of information was Nik Szymanek's book Shooting Stars now in its second edition. I would recommend this book for the information and images it contains. It can be obtained from the Astronomy Now webstore.

I also invested in a copy of Making Every Photon Count which is a wide ranging book by Steve Richards, covering equipment, setting up, finding, focusing, framing, and tracking the object then the basic techniques of processing. The book concludes with advice on drift alignment and collimation and a look at his own kit.

I struggled with a number of problems apart from the electronics of getting the information from camera to computer. One of which was the vignetting in the images and noise which still seemed to be present despite stacking the images. That was when I came across Tanja Schmitz and her photograph of M42 taken in the city of Johannesburg on her Photographing Space website. When I looked at her gear list and recognised stuff that I was using I sat up and paid attention. What I was impressed by, was the number of files the photograph was comprised of.

With that in mind, this year I set out to improve my astrophotography and my 2020 page is the result. I now have a full set of darks, flats and bias files for the camera and the two main ISO settings I use. I now pay more attention to things like ISO which I used to change at will to try to get more information in the image. I have dropped the Canon image acquisition software I used to use, unless I am using my EQ5 mount, in favour of image acquisition with the Stellarmate device.

During the year I came across a book on astrophotographic processing with Affinity Photo and Nik Szymanek had also done a series in Astronomy Now using the same software. For those of you who don't know, Affinity Photo is a contender for the Photoshop crown as many photographers I know are moving to using what is a much cheaper but still very capable piece of software. The book I found was published by Dave Eagle and so I purchased a copy. It was money well spent not just for the techniques Dave used but for the tips on Plugins for Affinity Photo he used. So my workflow is now as follows.

After taking a series of exposures around 40 or more I use Deep Sky Stacker to produce a tif file of the stacked images. This is then opened in Affinity photo without any modification in DSS and the first thing I have been doing recently is using the very useful plugin, AstroFlat pro from ProDigital software this evens out the illumination and tidies the image up, often considerably. I then, gently stretch the image with curves and levels to bring out the details of the object I have photographed, this is repeated as necessary. Another plugin I have installed is HLVG which removes a green cast which can sometimes affect astroimages.

To give an idea of the effect the astroflat plugin has had compare the following two images of M10.

The left hand image has been produced using the plugin whilst the right hand one has been produced using more conventional methods employing calibration files. Despite using darks, flats and bias files I have still got a problem with vignetting due to the amount of image stretching I have employed fortunately cropping can remove most of this. Using the plugins has almost eliminated the need for the additional calibration files.

The effect you see is not solely due to the plugins but the additional processing I have been able to do to get a darker sky and more contrast in the image.

To all those who have inspired me I am thankful.