10 STEPS TO PROCESSING:
My recommended workflow for great images
Here I present my workflow for processing long-exposure deep-sky images taken with a DSLR. This workflow assumes you’ve turned on Long Exposure Noise Reduction, to have the camera take dark frames at the same time as the images, so all images come out of the camera with dark frames already subtracted and noise-reduced. Yes, it is certainly possible to subtract separate dark frames during processing. Indeed, most other tutorials will advise that. However, popular programs for doing so (such as Deep Sky Stacker) must by-pass any opportunity for working with the original Raw files using the power of Adobe Camera Raw, a step I feel is essential for the finest images from DLSRs. And, since I’ve found that subtracting dark frames later is never as effective as having the camera do it, I feel this workflow yields DSLR images with the lowest noise and maximum detail.
#1 Import and Select
While much of the processing that nightscape and time-lapse images need can be accomplished from within Lightroom, for deep-sky images Photoshop becomes essential. So I use Bridge’s Photo Downloader, not Lightroom, to import images. Once on your drive in an organized folder, use Bridge to inspect your images for flaws. Trailed stars can happen, even when auto-guiding, for reasons that are often elusive. Images with severe trailing will likely be unusable. Check for unwanted intrusions such as aircraft or satellites. Many satellite trails can be subtracted from a stack (see Step 4). Check for frames with clouds and haze drifting through. Some hazy frames can be used to good effect to add photogenic glows around stars. I’m assuming you’ve shot a set of at least four “sub-frames” for each deep-sky object. Stacking four doubles signal-to-noise ratio. To double it again you have to stack 16! Open up your stacking set in Adobe Camera Raw (ACR).
IC443 Supernova Remnant and M35 Open Cluster, in Gemini
Raw single image as it came out of the camera. Click on any screenshot to download a full-sized JPG for closer inspection.
IC443 Supernova Remnant and M35 Open Cluster, in Gemini
Final result: a stack of 10 x 6 minute exposures at f/4.5 and ISO 800 with a TMB 92mm apo refractor and filter-modified Canon 5D MkII.
Processed entirely in Photoshop and Adobe Camera Raw.
Diffraction spikes added with Noel Carboni's Astronomy Tools actions.
#2 Develop Raw: Add Curves and Contrast
Once you are in ACR use the Basics and Detail panels to apply contrast enhancement and noise reduction to each image, as I outlined under 10 Steps to Processing Nightscapes. However, for many deep-sky images, Contrast and Clarity often need to go to 100%! For even more contrast boost turn to the Curves panel. Bring down the blacks and boost the mid-tones, while holding back the highlights. As you make adjustments, watch the histogram. Don’t slam it to the left – the sky should not be pitch black – but don’t clip highlights to the right. However, this is the point in the workflow where have access to the full dynamic range of original raw data to recover detail. Use the Whites and Highlights sliders bring out details in bright areas, such as the cores of clusters and galaxies. Use the White Balance Tool (the eyedropper) to correct colour by clicking on what should be a neutral black area. Due to changing sky conditions during the shoot, you might need to tweak the white balance of images individually.
#3 Develop Raw: Correct Optical Flaws
If you are shooting with camera lenses, use the Lens Correction panel to fix the darkening of the corners – vignetting – as well as lens distortions and chromatic aberration. Use Profile>Enable Lens Corrections to fix most lens flaws automatically, based on ACR’s lens database. Under the Color tab turn on “Remove Chromatic Aberration.” For manual lenses and for all telescopes, go to the Manual tab and dial up the Amount of vignetting correction and shift the Midpoint (moving the slider right shifts the correction out to the corners) until the field looks uniform. For all but the fastest and vignetting-plagued telescopes, this should “flatten” the field, brightening the corners without any need to shoot and apply separate “flat field” frames. At this point you can also use the Adjustment Brush (keyboard shortcut K) to paint in “local adjustments,” tweaking selected areas of an image. The Spot Removal tool (B) can remove dust blotches, though perhaps not without cloning in fake stars. Use it sparingly
#4 Process: Stack for Smoothing
The goal is to develop your set of “sub-frames” so each looks as good as possible, though having frames that look a little light and overexposed at this stage is just fine. Indeed, each should be well-exposed, as per the edict to “expose to the right.” Once you have a set of images developed to your satisfaction, you need to stack them so the noise averages out, smoothing the image still further. The more sub-frames you have the better. You don’t need a specialized program to stack. Open all the images in Photoshop (version CS6 or higher), then go to File>Scripts>Statistics, and in the dialogue box click “Add Open Files” and “Attempt to Automatically Align.” Under the Choose Stack Mode menu, select either Mean or Median combine. Mean produces the smoothest result, but Median works well for averaging out satellite trails and other oddities present on single frames. In moments you have a stacked, aligned, and averaged master image. It's a powerful Photoshop function, yet few are aware of it.
#5 Process: Apply Smart Filters
The stacking process automatically creates a “smart object.” By going to Layer>Smart Objects>Stack Mode you can change the stacking method, should you decide to use Median instead of Mean. Double clicking on the smart object “unpacks” it to reveal the sub-frames that compose it, to edit them if you wish. Any filters get added as “smart filters” that can be adjusted at any time. Reduce Noise is great – click on Advanced and you can smooth just an individual colour channel. The mask that accompanies smart filters allows you to apply those filters to just selected areas. If you wish to apply smart filters each with a different mask, apply one, then go to Layer>Smart Objects>Convert to Smart Object to create a smart object nested within another. Now add another smart filter with a different mask. I rarely need to do that, but it’s a trick worth knowing. Smart objects can be nested many times and unpacked for re-editing.
#6 Process; More Curves and Contrast
The secret to bringing out the faintest wisps of nebulosity in a deep-sky image is Curves. We applied them in Adobe Camera Raw but we need to apply them again here. Use the Adjustments panel to add successive Curves adjustment layers. It’s best to apply curves in a series of small, iterative layers, not in one fell swoop. With each adjustment pull down the black end and boost the mid-tones in a classic S-curve. In the Curves panel select the Targeted Adjustment Tool (the little arrowed hand at top) to click on an area of the image you want to punch up. Bring up an area of nebulosity, and bring down an area of dark sky. A final application of a Brightness & Contrast adjustment layer can snap up an image still more. How much contrast stretch you apply depends on your taste. As always, watch the histogram to avoid slamming it to the left or right. Ensure an evenly distributed range of pixel values. Hit the circular “Refresh” button above and right of the histogram to render the plot as it should appear.
#7 Process: Correct Colour
As you stretch the contrast you’ll also see the colour shift and intensify. Any off-colour imbalance that was leftover from the initial processing in ACR will now get exaggerated. There are many ways to correct colour at this point. You can use Levels to adjust individual colour channels. Or use Color Balance. I prefer to use Selective Color. Choose Neutrals to make overall colour balance shifts. Or Blacks to affect the dark sky. Choose individual colours for punching up (or down as the case may be) the reds and magentas of nebulas, or the blues of galaxy arms. For example, under Reds decrease Cyan to punch up the reds, or boost the Magenta and Yellow sliders. A Vibrance adjustment layer can further boost colours, or pull them back. Don’t create fluorescent images! But also don’t be afraid to enhance what’s there. There is no "correct" way these objects should look, but use the histogram to check for overall off-colour tints.
#8 Process: Make Use of Masks
Much of the power of Photoshop is in masking. For every adjustment layer you add you can use the mask (the white box at right) to apply the adjustment to just a selected area. To do this, double-click the mask to bring up the Mask Properties panel. Invert the mask to turn it black. Now use a white brush to paint over areas where you want the adjustment to apply. Use the [ and ] keys to adjust the size of the brush. You can also change its hardness and opacity. Paint with a low-opacity brush to build up gradual changes. The entire mask can also be feathered or turned down in opacity. Choose the Gradient Tool (keyboard shortcut G, grouped with the Paint Bucket) to draw out a gradient across a mask, useful for applying a colour correction or brightness shift across an image. In this example I applied Brightness & Contrast tweaks to just the top and bottom edges to counteract the shadowing from the DSLR's mirror box.
#9 Process: Sharpen for More Detail
Deep-sky objects often benefit from a final clever technique of sharpening. While you can apply a smart filter such as Smart Sharpen early on, most sharpening is best left to a final step. Here's the trick:
Once you have an image looking great for colour and contrast, hit this poorly documented but useful four-fingered shortcut: Command+Option+Shift+E (Control+Alt+Shift+E on Windows). This function, which surprisingly has no menu command, creates a “stamped” flattened version of your entire image duplicated onto a new layer at the top of the layer stack. Select it and turn it into a smart object (Filter>Convert for Smart Filters). Now go to Filter>Other>High Pass and apply a 5- to 10-pixel radius high-pass filter to the layer. It’ll turn ugly grey! Until you …
… Change that layer’s Blend Mode (the pull-down menu at the top of the Layers panel) from Normal to Overlay. (Soft Light and Hard Light also work for less or more effect.) The whole image magically sharpens. Add a mask to the layer (click the little black circle icon at the bottom of the Layers panel), then Invert the mask, and paint it with a soft-edged low-opacity white brush to apply the sharpening to selected areas, such as a galaxy or nebula. Because you applied the high pass as a smart filter to its own masked sharpening layer, you can adjust the strength of the sharpening and the area it affects at any time.
All very powerful, yet non-destructive. That’s the power of Photoshop. Indeed, everything we've done to the image has remained within a non-destructive workflow. No pixels were ever harmed in the making of this image! You can re-work your image at any time – the next day, week, or year. But only if you do this ...
#10 Process: Document and Save!
At this point, you’re done. Save! And backup! Always save the layered master as a Photoshop file you can return to later to readjust settings. And you will, when you view the image with fresh eyes.
Once you’re happy, flatten the image and save that version under a different file name or format. (I save it as a 16-bit TIFF.) But first, please do this: Go into File>File Info to add keywords, a description of the image –what it consists of, how you took it – and personal data such as copyright, contact info, etc. While the file will preserve camera data such as the date, the camera model, its ISO speed and exposure time, it’s up to you to type in what gear you used and any other pertinent information. By recording all that information into the image’s “metadata” it will carry through into any versions of the image you export (such as 8-bit, down-sized JPGs in the sRGB colour space), to post on the web or send to friends and publications.