Image processing in Photoshop: Basics & Links, Focus on Astrophotography
Copyright © 2002-2013, Dick Locke.  All Rights Reserved.  Contact and Image Use Information 

This page discusses my overall approach to processing images in Photoshop.  (Note: I don't discuss the mechanics of Photoshop here.  If you're interested in that please check out the links below.This page describes how blend multiple "raw" frames from your camera. 

Advanced blending in Photoshop is discussed on my page here.

Andromeda Galaxy
: Believe me, they don't start out looking this good!

Before You Begin Image Processing: Start with the best quality image you can!  This gives you flexibility of future crops in case your picture framing isn't perfect every time.  Unless you're a professional photographer, I recommend using the "raw" data format on your digital camera if available.  If not, use the top quality jpeg setting.  See here for an example showing you why.


  1. I hope you have a speedy computer with plenty of disk space!  Digital images can take up a lot of space on your hard drive if your pictures are captured and stored in full resolution (as they must be for best quality).  Image editing is one of the most compute-intensive application areas.  Any 2009 or better "dual core" pc will handle normal Photoshop tasks well.  Recent versions of Photoshop us multiple processers very efficiently.
  2. Before you mess with the image, save a copy!!!

Next: Most images can be improved by taking the following steps in the order indicated. 

Note: scanners all use some sort of software that can adjust color balance, levels, sharpness, etc. Opinions vary, but must agree that if you're using Photoshop it's best that the scanning software just produce the scan & do the rest in Photoshop. The exception would be if you have surface scratches on your film, I've found that the Nikon scratch removal works well at the expense of a slight softening of the image. This is a fine tradeoff 90% of the time, as fixing a lot of scratching in Photoshop is often very time consuming. 

Most of this advice would apply regardless of the imaging program you use.  The concepts would be the same, but the tool names etc. would vary.

See the latest on my image processing approach in astronomy work here.

Finally: there are many sources of information on Photoshop & image processing on the web.

Ken Rockwell's how to use Photoshop is a good starting place

Astrophoto & photoshop
Laurie McCanna's complete list of Photoshop tutorials
Luminous Landscape Features and Instant Photoshop
Janee's Photoshop Tutorials
Excellent sharpening technique
Jerry Lodriguss Digital Techniques

Other Related:
Macro Tutorial

Read a book!  For anyone still with me, I've acquired & read a number of books on Photoshop. My processing techniques have improved both in terms of quality of the results and the time it takes to get the results. "Photoshop 6 Photo-Retouching Secrets" & "Photoshop Restoration & Retouching" are both excellent.

The same is true on the front end of the process -- the "you can't make a silk purse out of a sow's ear" saying is very relevant.  Start with a sharp, well exposed image.  I can't overemphasize the importance of taking a good picture in the first place [;-)]  I recommend  National Geographic's "Photography Field Guide" and Kodak's "Guide to 35mm Photography".  Both are excellent, and both are good for beginners.

Photoshop book discussion - 11/2004

Dan Margulis "Professional Photoshop" -  this book is not for the faint of heart, but it's really taught me a lot. The powerful techniques of blending one channel into another are covered extensively.  It is definitely not a beginner book, but I don't know that it's too technical, either. Other books I have and recommend include Photoshop Restoration & Retouching by Eismann, and two by Kelby: "Photoshop CS book for digital photographers" and "Photo-retouching secrets". None of these is specific to astrophotography, and I'm not sure any are really "beginner" books either. I've benefited from Jerry Lodgriguss' stuff specific to astrophotography.

One problem with Photoshop books is that PS can do so much that most people are only interested in a fraction of it's capabilities. I don't care about making a cool looking chrome effect, e.g., so a PS book that covers that kind of stuff wouldn't be interesting. I think the books above on "retouching" cover a lot of what you need to worry about in astrophotography.

I should also mention "Photoshop color correction" by Kieran is not bad and is more of a beginner book than Margulis. ...

Image Bit Depth:  As of 2012, some Photoshop features are only available in 8-bit mode (HDR toning).  The rest of this paragraph discusses that working in 8-bits isn't really a problem, but it was more relevant for older versions of photoshop that didn't allow layering and other features in 16 bit mode.  I previously went along with the conventional wisdom, that 16 bits was way better than 8 bits. (And Adobe wouldn't want to discourage this thinking, as it helps facilitate sales of CS. Hardware vendors are kind of in the same boat.) Dan Margulis in "Professional Photoshop" (4th ed.) has considerable discussion of 8 vs. 16 bits. Based on a good deal of experimenting, he basically concludes that "real world" differences in working with 8 vs 16 bits are negligible. (I realize that astrophotography is borderline real world, but nonetheless I think he's right.) So: I wouldn't let the 8-bit conversion in PS7 stop me from working with layers, if layers are called for. And there's a lot you can do with layers that you can't do otherwise. What I do now is capture and save in 16 bits from my camera/scanner. Allow the scanner/camera to do absolutely no image processing. I then open an image in PS, and adjust curves or levels if there's any kind of gap in the histogram (i.e. if the curve ends before the top or bottom of the scale.) I then convert to 8 bits and don't look back.  BTW, while this book is not for the faint of heart, it's really taught me a lot. The powerful techniques of blending one channel into another are covered extensively - and you need to use layers to do it

Photoshop Advanced: Dick's notes and tricks

Flat fielding techniques

(The best way to flat-field a vignetted  image is by taking 16 or so flat frames, averaging them together, and using software to remove the vignetting.  That does not address gradients, however.)

Ron Wodaski posted to following to one of the news groups describing a way to eliminate vignetting.  I used this technique above.  Jerry Lodrigus has documented another useful technique, but the disadvantage of Jerry's is that you never can get it exactly right throughout the whole frame.  The technique below allows you to do that.  From this page: Orion Area, 300mm

I've documented a simple way to created synthetic flat fields using
Photoshop in my book "The New CCD Astronomy." It works for both film and
CCD images.

The basic idea is to use an image that doesn't have extensive nebulosity in
it, and to apply Photoshop's Dust and Scratches filter with an appropriate
radius that will remove most of the stars in the image. You can use the
Clone Stamping tool to clean up any very large stars or small patches of
nebulosity that remain.

Then apply a strong Gaussian blur (usually in the range of 10-30; depends
on what's left) and you will have a very smooth and very accurate pseudo
flat-field image. The nifty part of this is that it works well for
full-color images. You can apply it in a single step by putting it in a
layer above your image, and setting the bland mode to difference. Adjust
the blending % to somewhere in the range of 80-95 (usually) to get just the
right amount of subtraction.

(DL note here: I'm wondering if the above is a mistake, as I am using more 10-15% blending than 80-95% as noted above.  Please let me know if I'm missing something :-)

This methods works well for light pollution gradients, too. For more about
my book, click on the link in my signature. I've got lots of processing
tricks that work for both CCD and digitized film images.


Processing notes for Astronomy Photos (below)

Above: M31 using Nikon D100

Combination of 4 images of about 1, 2, 3, and 4 minutes duration at ISO 1600.  Ambient temperature in the mid 30's (F).  Takahashi FS-102 with focal reducer, full frame.  Autoguided with the STV. Here's my current  DSLR Workflow.)

Processing Notes on D100 M31 (above):

  1. Start right!  I use the Camera Raw mode (no camera sharpening or jpeg conversion) which produces 9.7 MB .NEF files.  Convert to ~35 MB .tif (TIFF file format) using 16 bits/channel.  I shoot and process in the Adobe RGB color space, which holds more color data than the more common sRGB (web-ish) color space.

  2. Aligned & combined 4 images in Registar using "average" function.   Averaging images reduces noise.

  3. Import into PhotoShop CS ("version 8") maintaining 16bits/channel. (Note: as of 3/25/2005 I'm less anal about maintaining 16 bits/channel after getting the image into Photoshop.)  See discussion here: Photoshop Basics

  4. Extensive noise reduction.  Examine each RGB channel for noise, and use a noise reduction strategy on each layer separately. I use the Gaussian Blur function often, using values close to PS's (photoshop's) defaults.  I also use the "Grain Surgery" plug-in to reduce noise (works on digital noise as well as film grain), often using it with the PS blur.  Hint: Use the "Select->Color Range" function to select the dark, noisy stuff before applying the noise reduction.  This assures you don't blur the stars.

  5. Levels.  Use the black eyedropper tool (on levels or curves menu) to set black level.  (I use 15,15,15 for the black level, which is a very dark grey but not black color.  Other's use a bluer setting, and sometimes I do as well.  Using this black level helps assure some low level detail will be visible even on monitors that aren't set up properly (probably the majority).  Depending on the image I may also set the white level (245, 245, 245). 

  6. Curves.  Use the curves function to stretch the contrast appropriately.  You can hold down the right mouse button using this tool to determine the points on the curve of interest.  Then adjust the curve.  Usually on astrophotos you bring up the mid and higher (brigter) levels up, while slightly lowering only the lowest levels.  (The low levels is where most of the noise is.)

  7. Freestyle.  Do whatever else you think you need to do...  This would include any steps to alter the color balance.

  8. Re-size.  Usually 640 or 800 pixels wide.  (That was the old days.  I'm doing 800 pixels wide up to 1200 pixels or more in 2009)

  9. Sharpen.  I convert to lab mode and sharpen the lightness channel. Doing it this way assures you minimum chroma (color) noise.

  10. Convert to RGB mode and then 8 bits per channel mode.  (Have to do this in order to save as a .jpg, see below.)

  11. Convert profile to sRGB.  This assures that the conversion to the standard web colorspace is done by PS.  It matters!

  12. Save.  Save to the .jpg (JPEG) file format for web use and emailing.  I use Photoshop's adjustable quality setting to determine when the conversion to .jpg is not noticeable quality-wise on my monitor.

Skin Tone Reference Information 2017

One Answer for RGB

R: highest %
G: middle %
B: lowest %


To get a little more specific. In general,
R: Y + (15-20)
G: average of R and B
B: R – (15-20)


For example, the following values represent common Caucasian skin tones using the rules above:
* R: 80%; G: 70%; B: 60%. R is 20 points higher than B, and G is midway between R and B. That’s perfect.

*  R: 86%, G: 78%; B: 70% also reflects a nicely balanced skin tone.  Again, G is midway between R and B, and R is 16 points higher than B. This, too, illustrates a great relationship among the colors.

A CMYK Answer

Another Theory

Adult Caucasian Skin (typical CMYK ratios)
1)    Cyan = 1/5 – 1/3 Yellow
2)    Magenta + (3-10) = Yellow

Still Another

Caucasian Skin – Both Yellow (Y) and Magenta (M) should be about the same, slightly more Yellow is fine – it makes the skin look warmer, but the difference should be more than about 10-15. Cyan (C) should be about a quarter to a fifth of the Magenta value. There should be no Black (K).\


Color Wheel

CMYK Color Wheel with values

RGB Color Wheel with values


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Updated 8/28/12