PHOTOSHOP YOUR WAY TO AMAZING ASTROPHOTOS

Anjal C. Sharma, Ph.D.

So you spent several hours out in the cold, hunched over your polar aligned telescope,

fiddling with your astronomical camera and captured long exposures of your favorite deepsky
objects. You’ve aligned, registered and stacked the multiple minutes long exposures using a
program like Registax to smooth out the noise and are thrilled to see a single low noise and well
exposed image on your computer screen. You sit back knowing that you’ve sweated blood to
obtain these images, and are justifiably proud of the result. However, somewhere deep in the
dark recess of your mind is a lurking suspicion that maybe, just maybe, all is not well. Maybe
just maybe, you should try to improve the images a bit so that your other astronomically inclined
buddies are zapped out of their seats when they see the images.

Well, fear not. Photoshop will come to the rescue!! Adobe Photoshop in its myriad of

versions from Photoshop 3.0 to the newest Photoshop CS3 is a powerful image enhancement
program with a multitude of functionality to help you tweak your hard earned astropix to
perfection. This tutorial is a compilation of some of the methods that I’ve learned over the years
to enhance astrophotos for viewing on the computer screen or for sharing over the internet and
for printing on your home color printer. I have an older version of Photoshop (Photoshop 5.0
Limited Edition) which is just fine for performing certain basic enhancement processes such as
levels and curves adjustment, gradient mask preparation and gradient removal, color blur and
tone adjustment and sharpening of the image.

1. The Photoshop main screen and opening your image

Double click on the Photoshop icon on your desktop to open up the main screen of the program.
It will look similar to this screenshot.

Then click on file, scroll down and click on open. A dialog box pops up which will allow you to
select the image that you want to open as shown below.

Select the image name and click open and the desired image will be displayed.

2. Levels Adjustment

As shown, the image is overwhelmed by skyfog resulting in the bright and light red brown

look of the sky background. Although both the flame and the horsehead nebulae are clearly

visible, removal of the skyglow to obtain a darker sky background would make the nebulae
stand out significantly. In addition, although difficult to see, the image has a greater contribution
from red due to the spectrum enhanced nature of my modified Digital Rebel resulting in greater
red response of the sensor. Therefore this problem has to be addressed also. The easiest way
to do this is to use the levels function in Photoshop. Click on Image, then click on adjust and
then choose levels and the histogram information for the image pops up as shown below.

In the histogram dialog box, click on the Channel (RGB) tab, and choose Red and only the
histogram for the Red channel pops up.

Now, see the small slider triangles beneath the histogram for the red channel? They define the
black (black triangle), mid-grey (grey triangle) and white (white triangle) levels within the red
histogram. We will define a new black point for the red channel by clicking on the black triangle
and sliding it to the right, until it is below where the histogram just begins to shoot up as below.

Repeat the above steps for the Green and the Blue channels and when that is done click OK on
the levels dialog box. Your result should look something like this.

Compare this image to what we started out with. Dramatic improvement, isn’t it? Not only are
the nebulae significantly more visible, the faint outer extensions of the nebulosity can be seen all
over the frame, and the details in the nebulosity really stand out. In addition, most importantly,
you’ve gotten rid of that pesky skyfog problem, and the sky looks closer to what it should. You
can stop at this stage if you wish because you’ve improved the image by 90%. However,
wouldn’t it be nice to get that additional 10% too? Well, if that strikes your fancy, read on….

3. Gradient Removal Via the Layer Mask Technique

One of the issues that always faces an astrophotographer is an illumination gradient in
astrophotos, particularly if a larger imaging sensor is used. This is a characteristic of the
telescope optics, and how large of a fully illuminated field the optical train can provide at the
prime focus plane. My Sky 90 is no FSQ or Epsilon 200; however it is acceptable as an
astrograph using the APS-C sized Digital Rebel sensor, but the images always display an
illumination gradient going from image center to image corners. A Flat Field image taken using
a white T-shirt in the daytime or of the twilight sky can be subtracted out to remove this gradient.
Dark frames taken with the same camera of the same exposure duration at the same sensor
temperature can be used to remove hot pixels and the Amp glow. What if you haven’t done this,
or like me you’re too lazy to collect darks and flats and justify your laziness by saying “I’d much
rather spend precious imaging time collecting lights….flats and darks be darned.” Well duh,
now you’ve got a problem haven’t you? Well, fear not. We have the technology to mimic a flat
field by preparing a gradient mask using the image itself and subtract it out to get a result with
extremely (though not perfectly) even illumination across the frame.

Open up the levels adjusted image in Photoshop and click on Layers and then choose

Duplicate layer. The original image will be duplicated and placed right on top of the original
image as a separate layer. Check out the layers tab on the right hand side of the main screen
and you’ll see the two layers displayed.

From the tools palette on the left of the image, click on the icon which looks like a rubber stamp.
This is the clone tool and it allows you to clone any area within the image on any other area.
Position this tool over an area of the image which is dark, and does not have any bright stars,
and press Alt and click together, to define that area as the clone result area. Now start cloning
out the bright stars and the extended nebulosity in the image.

Take your time, and be thorough as this step will define the first step to prepare a good gradient
mask. When you are finished, the result should look something like this.

As can be seen there are still moderately bright stars in the image. You’ve only changed the
upper layer, not the original image which lies below this layer. We want a “map” of the
illumination gradient across the frame, so now click on Filter, then click on Blur and choose
Gaussian Blur.

Now use the slider beneath the radius window to define the blur pixel radius, such that it
completely blurs out the stars and just barely removes any blocky patches in the image. For this
image I found a blur radius of 40.0 pixels to be perfect.

Click ok. This is the finished gradient mask layer, and needs to be subtracted out from the
original image. For this, go to the Layers box on the right hand side of the program window, and
click on the blending mode box which right now says normal.

Scroll to Difference and click. This subtracts the layer mask from the original image resulting in
extremely even illumination of the image with very little residual illumination gradient. Slide the

opacity slider to around 80% to ensure that the faint nebular regions can also be seen.

We now have to blend the layers together to result in a single layer. For this choose Layer and
scroll down to select Flatten Image. This results in a single image layer with the two merged
layers as seen below in the Layers box.

Save this image and you now have improved the image by around 97% or so. Want to improve
it even further? Well, any changes done now will only improve the aesthetic appearance of the
image, and so many of the steps below can be adjusted according to your own preferences.

4. Aesthetic Improvements – Cropping, Color Blur, Saturation, Sharpening and Curves

The first thing you can do is crop the image to show only the parts of the frame which are of
interest. In this case, the normal dithering between the individual sub-exposures have resulted

in a red line on the left hand side of the image. I’m going to crop the image to remove this
artifact completely. On the tools palette on the left of the image, click on the rectangular
marquee icon, then position the crosshairs on the bottom right corner of the image, and while
holding down the left click button, drag the crosshairs to the upper left to define the crop area as
shown above. Then click on Image and then scroll down and click on crop. This crops out the
non-selected areas, resulting in the image below.

There are several nice filter plugins available as freeware for photoshop. One of these plugins
is a series of filters called AAA Filters available for download from

www.8bf.net

. Download

these files and extract the 8bf plugin into the filters folder of photoshop. Now when you click on

Filters, you can scroll down and click on AAA Filters and choose Color Blur. This opens up the
dialog box shown. Slide the radius to around 15, set opacity to 50 and set saturation to 25.

Click okay, and the result will be a very nice and correctly color saturated image with pleasingly
softened stars as seen below.

Clicking on Filter then on Sharpen and then on unsharp mask brings up the screen below.

Set the radius to 1 pixel. Any more and you will have unnaturally sharpened stars, and you may
lose fine nebular details. Click OK. The result is a pleasingly sharpened image below.

The last thing that I like to do is a slight curves adjustment to make the nebulae stand out a bit
more without sacrificing too much fine detail. The way to do this is via a curves adjustment. At
the moment, the progression of the pixel bits from completely black to completely white is a
linear function. Clicking on Image, then on Adjust and then on Curves brings up the dialog box
shown below.

Now grab the line somewhere in the middle and drag it up a little, to obtain a curved pixel
progression function. Be gentle here, otherwise the image will look completely garish and you’ll
lose a lot of faint detail in the nebular areas. The idea here is to subtly enhance the presence of
the nebulae but not overwhelm the image brightness and contrast.

Click okay and Voila, you’re done. Save the image. Want to see a before and after? Here you
go. Enjoy.