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.