Step 1- The Raw Image:
Please note- I wrote this simple tutorial several years ago, when I was
interested in DSLR astrophotography with the Canon 10D. Although
I exclusively do CCD astrophotography now, many of the principles
involved in image processing have remained the same, so I've decided to
keep this tutorial on my website in case anyone might benefit.
of my Canon 10D work
acquiring and processing Raw Format files. Much
of what I show is based upon the use of Photoshop, although the basic
principles could be followed using alternative image processing
programs such as ImagesPlus. For
this tutorial, I will use one of my previous images of
M45 (details provided in the image below). This image was
acquired, raw converted, dark frame calibrated, aligned, and stacked
using standard techniques. Please refer to Mike Unsold's
excellent video tutorials for ImagesPlus (or check with other websites)
for details regarding these initial steps in image processing.
Below is a typical stacked result that will serve as a starting
point for our image processing tutorial. You may wish to download this
file for practice after you have finished with the tutorial,
although the quality is compromised due to the need for size reduction
and jpg compression:
click on image for a higher resolution view)
Notice that the image is dark, which
typical for raw converted Canon files (since they are derived from a
relationship between channel number and channel intensity). In
to transform the image into something that we are familiar with, the
function must be made non-linear (i.e., stretched, shown in step 2) as
first step in processing. With a keen eye, however, you will
that this photo will turn out very well- the stars are pinpoint, and
there is a fair amount of nebulosity already visible (click on this and
all other images for a higher resolution view).
Astrophotography Page Step 2-
Stretching the Raw Image:
The use of the Curves function is an indispensible part of processing
astrophotos, and it will be used to stretch the original "Raw" format
Canon 10D file in this step. This step is not necessary if you
are starting with jpg format files that have already been stretched by
internal digital camera software. However, the use of Canon "Raw"
format with the 10D or 300D is highly recommended over the jpg format,
since it permits capturing the data in 12 bit mode, without internal
image processing by the camera (like stretching or noise reduction)
that could lead to artifacts or loss of resolution. (Note: If you
are using Photoshop Elements, you will notice that it unfortunately
does not contain a Curves function. However, there is third party
software available called "Hidden Elements" that permits the use of
Curves as well as other functions with Photoshop Elements).
(Please click on image for a higher
1. Open the Curves window- Go to "Layers, New Adjustment Layer,
Curves". I am using Photoshop CS, but this sequence
should be similar for recent versions of Photoshop.
2. Adjust the curves similar to what is shown.
You will notice that the background brightens, and that a
significant amount of nebulosity can be seen. This faint
is contained within the left hand portion of the x-axis of the Curves
graph, or the left hand portion of the histogram. This region
must not be clipped. Also notice that the histogram has shifted to the
right as a result of stretching the pixel intensity distribution.
will be corrected in step 4.
Step 3- Correction of Vignetting (next page):
Please note: Graphics on this website may not be reproduced
without author permission.