Turning your Raw Digital Pictures into Beautiful Wall-Worthy Masterpieces!


Let's take an example of a typical digital imaging project, using our favorite equipment set-up. Lets see how each stage changes the your image...

Step 1. Under the Heavens - Getting your Raw Images

I will set up the equipment for you. The 8" f/4 LXD75 or Megrez 80mm f/6 refractor, and ST2000XCM color CCD camera is our favored set up.Example, at the Astro 27 - "Field Astronomy at Pinnacles National Monument" weekend field class Sept 23, '06, I stayed up afterhours and took a series of images of the AE Auriga nebulae region. AE Aurigae is an eclipsing binary and is in a star formation region which includes both red Hydrogen-alpha emission and also blue dust reflection nebulosity, making it one of the most beautifully colorful subjects in the sky. It's notoriously difficult to photograph however since it's very faint. It requires very dark skies and lots of exposure time. Our standard procedure is to take a seris of 5 minute long exposures, and then stack them together. Why 5 minutes? Because Windows XP, in it's infinitesimal wisdom, will bonk and give you it's infuriating "Windows must close this application. Would like to send a message to Microsoft"? pop-up and it does it AFTER you've invested your full exposure time. If this were to happen after an hour-long exposure, you would be VERY upset! 5 minutes, I can resign myself to once a night or so. Also, you never know what other tragedies will befall your exposure - an errant airplane with flashing lights, a piece of floating space junk, an on-coming car with headlights, a careless student flashlight, a gust of wind... So it's good practice - 5 minutes per shot.

The first stages of reduction you can do in the lab, or if you want, you can do them at home. The software needed is CCDOPS which is available for free download from the Santa Barbara Instrument Group website, and Registax, also available for free.

...It'll look like this when first presented at the conclusion of your 5 minute picture.


In CCDOPS, I subtract the relevant dark frame and apply a flat field, then click on single shot color process and use the sRGB + gamma option. This best preserves the wide dynamic range in a nebula picture. My image now shows color. The ST2000XCM is a 1600x1200 pixel camera which uses a filter mask so each 2x2 pixel array has 2 green+red+blue filters over the pixel array. This color image thus has half the original resolution. CCDOPS then does an interpolation on the colors of the original pixels to get a 1600x1200 color image. CCDOPS outputs as an 8-bit image (wish it would output 16 bits but it won't), which I save as a .TIF file

For this subject, I took 14 5-minute images one after the other. Dark subtracting, flat fielding, and single-shot color processsing each. Then in Registax I stacked all frames on top of each other. This gives you the equivalent of a single 70 minute exposure. It may look no brighter, but that's because of the automatic brightness scaling here. We'll fix that. The point is, this 16bit image now has only 1/sqrt(14) or about 27% as much pixel to pixel noise as the 5-minute frames. So called "shot noise" is like this. Each quadrupling of the exposure length cuts the noise in half. You'll get to a point of diminishing returns after an hour or so, unless you have the patience of Ghandi that is.


Now, you're ready to start your artistic work - polishing up your nugget to produce a masterpiece! This I do in Photoshop CS2. Photoshop is, of course, a vast program which requires large investments in time to master. You can take multiple college courses and still not tap the potential of this software. We have it loaded onto the computers in the astronomy lab room. We'll use just some of the most obvious features, and for more sophisticated touches we'll use a neat set of macros (called "actions" in Photoshop lingo) developed by Noel Carboni specially for astronomical images (highlighted recently in Sky and Telescope Magazine) - Astronomy Tools v. 1.4.

First, I'll adjust "levels" to get a more workable image. "levels" sets what will be considered black and what will be considered max'ed out white on your image. Image | Adjustments | Levels, and you'll see a histogram which looks more or less like a mountain or perhaps a series of mountains. Move the left triangle rightward and the rightmost triangle leftward until your nebula brightens enough to work with. You don't want to have your image lying near the bottom of your range of values so you're essentially confining your output brightness range to a similar range on the input image. Now our nebula has brightened up and we can see what we're working with.

At this point, raise the magnification to around 200x (lower left corner) so you can really see what's happening. Stars are always too bloated in my opinion. Maybe because our camera isn't a $10,000 model with a bazillion megapixels, as seems to be the standard for those who get published? Maybe because CCDOPS outputs only an 8-bit color image to begin with and so we don't get the dynamic range to accomodate the stellar profiles? For this shot, I apply an AstroTools action. Right side menus, click on "actions" tab and find "make stars smaller", 4 times in succession is about all you can get away with before artifacts get too obtrusive.

The previous step introduced small scale blotchiness in the dark sky and the nebula. I'll deal with this by applying the Astro Tools action "space noise reduction". This reduces noise in the intermediate brightness sky. (There's also "deep space noise reduction" which didn't seem to help on this particular image).

Now that we've fixed up the sky, let's do one more sharpening pass. Filter | Sharpen | Unsharp Mask (68%).

Now we're done with the sky, the stars, and the nebulosity detailing. Let's bring up the nebula. Image | Adjustments | Curves. Curves is a more versatile way (compared to "Levels") of defining the transfer function between the input levels and the output levels. Here I used just two set points on the curve, making an "s" to raise the contrast and lower the sky brightness.

One more adjustment - let's pump up the color a bit. It helps bring out the always subtle coloring of stars, and with a red + blue nebula like this, adds some drama to the contrast. You can easily overdo this step if you have an H-alpha emission nebula alone. Don't make it garish! Image | Adjustments | Hue & Saturation and adjust saturation only - I did +5 here. It's immoral to adjust the hue of a nebula!

We're almost done. Now we just have to fiddle with the artistic composition. You should ideally do this when you're taking the shot, but sometimes it's too time consuming to twist and refocus and twist and refocus... I feel this shot works better tilted over. Image | Rotate Canvas | 90CW. Then, as a right-hander, I prefer to flip the image left-to-right so Image | Rotate Canvas | Flip Canvas Horizontal

Now lets crop it to the proper aspect ratio and composition. On the left palette, select the rectangular marquee tool at the extreme upper left, frame your final image, then Image | Crop.. You'll probably have to go back and forth several times until you're happy. Remember, you can click on any step in the "history" tab and right-click "delete" to undo steps.

Be sure to click on the final image to see it in all it's glory! As a final step for each of these web-published versions you see above, I click Image | Size and set the long dimension to 777 pixels (about right for 1024x768 viewer's computer monitors), then File | Save for Web and make sure the resolution is 72 per inch and adjust the quality until the size is a little under 200K, to not take up too much download bandwidth. If instead you want to get a large print, you'll want to preserve the full resolution .TIF file and bring it to e.g. Bay Photo Lab down the street.

One more Photoshop CS2 option to experiment with; CS2 can take several images of different exposure length and stack them to get a single photo which shows the full dynamic range of the array of original images. It's called the HDR (high dynamic range) option. For many faint astronomical images this will not come into play, but for some - I'm thinking in particular of the Orion Nebula - it's an essential step. Here's is a web page I found which describes the steps.


It's possible to over-process an image and turn noise into apparently genuine detail. That's why you should only apply the Astronomy Tools Photoshop actions once or twice at the most, and go easy on sharpening! Here's a good article on the problem.