PhotoShop'ing Your Astronomical Images - The Quick Basics

Photoshop is an amazing and amazingly deep program. There's massive tomes filling entire walls of bookshops on using Photoshop. There's series of courses devoted entirely to this one program. No way can we expect you to have already mastered Photoshop, or even to learn a tiny fraction of it's capabilities. I certainly haven't either. However, you can go 70% of the way towards turning your picture into it's full potential with just a few commands. So I'll walk you through the basics you'll need for doing your Astro 8 or Astro 9 images.

First, as described in the preliminary procedures, you use CCDOPS to dark subtract (and perhaps flat field - ask Rick), and convert to color, saving as a TIFF file. You stack together your individual frames into a composite in Registax and save as a 16bit TIFF file. Then, and only then, are you ready for Photoshop to make your final adjustments and artistic flourishes.

First, Adjusting the Composition

Once your photo is taken, the main tool you have for adjusting the composition is cropping out unwanted border areas.Click on the dotted rectangle on the left side icon array. Bring the cursor over to your picture and click it once, move the cursor again, and click again. You'll get a dotted border. Then click on Image | Crop and your border will be trimmed off. If you don't like your new composition, you can go back. Important - this is true for any change you make! - on the right side of your screen should be the history of all changes made (if not, go to Window | History is checked.). Right-click on the command you made and click on the pop-up menu delete and you're back to the version before the command you just deleted.

Next, Adjusting Brightness and Contrast

Most astronomical subjects are faint, and your raw TIFF file image will probably not show off your faint nebula or galaxy to optimum effect. In the toolbar, go to Image | Adjustments. Under this menu, you can either use Levels, or better, use Curves.

Levels: Click Levels and you'll see a histogram of how many pixels are at what brightness level, and you'll see two arrows underneath. The left one sets which level will be 0 (black) and the right one sets the maximum (white) brightness of the Level'ed image. Everything in between is intermediate gray, in a linear way. As a rule of thumb, bring the left arrow to the base of the "mountain" and bring the right arrow leftward to the base of the other side of the "mountain". Play with it until it looks best. Your goal is not to make the night sky pure black, but very dark gray, and to minimize the areas which are saturated white.

Curves: This is a fancier version of Levels, setting what's called a "transfer function" which is not necessarily linear, but can be about anything you want. Click on the curve to set an anchor point and then shifting around the anchor point to see what it does to your image. For deep sky objects you'll pobably want to bring the right side of the slanting line down and he left side up. But play with it until you like it. If you want a "solarized" look, make your curve go up and down and back up again.

Next, Adjusting Color Saturation

Star clusters and galaxies are dominated by stars, which are thermal radiators and have subtle colors. You can pump up the colors by Image | Adjustments | Hue and Saturation and move the Saturation slider to the right. Overdo it and you'll pay big time, with bad color speckling and lurid, phoney looks. For nebulae, you probably can't get away with any saturation adjustment, as emission nebulae are already fully saturated. Maybe just a tad, to help out the stars in the image.

Next, Adjusting Sharpness

Often the focus for our images is a little "soft". You can sharpen up the stars with Filter | Sharpen. Under this menu my favorite is Smart Sharp, but Unsharp Mask is also a good choice. Play with the amount. Don't sharpen too much or you'll make your images really grainy. Go easy. You'll notice that you can't sharpen without also increasing the noise in the dark sky areas. Color pictures will show enhanced color speckling. Not to worry (yet)...

Next, Reducing Noise

Photographing faint nebulae, especially from a light-polluted site like Cabrillo Observatory, and your pixels will be undersampled. Pixel-to-pixel shot noise will cause speckling. Photoshop CS2 can reduce this to some extent with median combining noisy areas neighboring pixels. Click on Filter | Noise | Despeckle. or alternatively Filter | Noise | Reduce Noise.

That's the basics. If you want to get deeper into it, you can use the Layers command, or astronomy-specific actions. Actions are pre-packaged collections of commands done in order, automatically when invoked. Actions allow the Photoshop community to devise their own processing algorithms and share them. Here's a text file explaining the Astronomy Actions which I've recently bought. And here's a website to get you introduced to actions. Here's another good page on actions.

Finally, Adjusting the Size

For printing, you'll want to make sure your picture is sized right. (I require an 8x10" print for Astro 8 and 9). I'll suggest 300 dots per inch (300 dpi) for printing. (if for your own purposes you're only publishing to the web, then 72 to 100 dpi is what you want). Click Image | Size and in the resulting menu, set the longer dimension to 10 inches. Make sure the boxes at the bottom Scale Styles and Constrain Proportions and Resample are all checked. Use Bi-cubic as the resampling mathematical algorithm.

When you're done, give me the digital version of your picture and I'll upload into our Astro 8 or Astro 9 gallery pages so the whole world can enjoy your creation!

Special Situations - Moving Targets

One of the coolest things to photograph are comets. Every one is unique and most are pretty beautiful. But, the closer and brighter they are, the faster they generally move across the sky, making it pretty tough to get a sharp image of both the comet and the background stars. But it can be done! It takes more work, and a deeper knowledge of Photoshop. There's a nice article in the June '07 Sky and Telescope on digital comet photography, including a brief mention of how to combine images to show both sharp stars and a sharp comet. Here are two websites which will help. Richard Bennion's and Bernard Hubl's. The key to the technique is to use a stacking program which includes a median filter, so that if most images have dark sky instead of a star in the stack, you get dark sky. The resulting stack will the show a deep image of the comet with no stars. Then, a combine is done without the median feature, stacking on stars and not the comet. The comet will then be a blur and the stars will be sharp. Then in Photoshop you use layers to combine the two images. We have been using Registax for stacking, but CCDStack or Sigma have the median feature. I've just learned about this and have yet to experiment with it. Note first that CCDstack is a $200 piece of software.

A Worked Example

Most good astrophotographers will tell you that 90% of a great image is in the post-processing. You can do an amazing amount of improvement with software these days. Follow along on this link to see a worked example I've posted on a separate page.