Tips for imaging during a Full Moon

In the late 20th century, when we still captured celestial images on film, the few days around New Moon were the only times we could image deep-sky objects. The grainy, low-sensitivity films we used back then demanded the darkest skies possible for quality portraits. When First Quarter arrived, it was time to say goodbye to astrophotography for a few weeks. 

Today, with digital imaging technology, we can continue to shoot right through a Full Moon by using a few simple image-processing techniques and following half a dozen basic rules. 

Don’t worry, though — you can see your friends and family on cloudy days!

The sky during Full Moon

We all know the sky gets much brighter around Full Moon, comparable to the washed-out skies we see from the center of a large city. But there is another factor at work. The sky also gets much bluer due to atmospheric Rayleigh scattering, the process that scatters sunlight to make the sky blue during the daytime.

As a result, images taken at this time not only have a bright background, but are tinted various shades of blue depending on their angular distance from the Moon. Any process to mitigate these effects must include removing the blue sky background, subtracting out the luminance bias, and increasing the signal-to-noise ratio in our finished images.

Types of targets available

The blue color of the moonlit sky does constrain the types of objects we can successfully record. Anything with a blue tint will be greatly diminished; however, that still leaves tens of thousands of objects as targets. These include open and globular clusters, galaxies, star clouds, comet dust tails, and — with the addition of a narrowband Hydrogen-alpha (Hα) filter — emission nebulae.

As for objects you can’t shoot, avoid blue reflection and planetary nebulae, comet gas tails, dark nebulae (or those with a low surface brightness), and targets with predominantly Oxygen-III (OIII) or Hydrogen-beta (Hβ) emission.

Examining the histograms

We can start offsetting the effects of a Full Moon in an astroimage by comparing a histogram of the target under dark-sky conditions to one taken during Full Moon. A histogram displays graphically the range (from black to white) of pixel values on the horizontal x-axis, and a count of the number of pixels of a given brightness along the vertical y-axis. A dark-sky histogram from a site with no light pollution will have the red, green, and blue humps closely spaced, with the green and red a bit higher due to the colors of natural airglow. Also, the black gap before the data begin will be short, with only a small luminosity component to the sky.

A histogram of the same type of target in a Full Moon sky shows that the three colors are much farther apart, with the blue and green much brighter than the red. The blue histogram is also wider due to the contribution of the blue sky background. All three colors are much farther to the right, indicating a large luminance component to the sky. Finally, the overall noise level is higher during Full Moon.

Our goal is to correct (or at least compensate) for all these factors and make the histograms during Full Moon look just like they were shot at a dark-sky site. From there, we can use processing procedures for shots taken under dark skies.

Image processing concepts

With raw images in hand, we now can process them so the effects of the Full Moon are — for the most part — mitigated. Removing the effects of moonlight is a simple two-step process that can be done with nearly any image-processing software. 

Start by removing the excess blue and green for a neutral color balance. Next, subtract the bright luminance from the sky background. After these two steps, we can process our images in the same fashion as our dark-sky images.

Using Adobe Photoshop

After calibration of darks, flats, and bias frames, stack as you would for any raw image. Save the combined raw image as a 16-bit TIFF, and open it in Photoshop. Start by removing the blue cast. Common sense would suggest that to do this, we simply have to subtract it or tweak the color balance sliders to remove the blue. Not so! This always results in a strongly yellow subject and stars, even when the sky background is set for a neutral gray. This is because the blue histogram not only contains the blue sky, but also the target data.

But no worries, there’s a simple way to get the blue out and retain the subject’s color balance. We do this by aligning the tops of the peaks of all three color channels. Because the red is usually the lowest value, adjust the blue and green sliders on the Levels palette to bring them down to match the red channel. Don’t worry about the different widths, just align the peaks. Hit OK, and you’re almost done.

With the blue color neutralized, we next remove the strong luminance bias. Open the Levels palette again, set all three colors in the RGB mode, and move the left slider up until the background is a dark neutral gray. Try not to make it jet black — a value around 25 is best — or you’ll hide the faint details. 

You’re done with removing the Moon’s bright glow. Now you can do your standard nonlinear curves, color and saturation adjustments, and sharpening to finish the job.

Using PixInsight

It’s actually easier to use PixInsight to do these tasks than it is to use Photoshop because we can apply a linked screen stretch to the image before changing the individual histogram peaks. This makes the color balance in the raw image clearer. 

The technique is the same as above: Open the histogram palette and line up the R, G, and B peaks. Then, in a similar fashion to the process in Photoshop, move the left slider to remove the luminance bias to a value of 0.1 or so. Follow with standard processing techniques to complete the image.

Using Graxpert

What if I told you there was an even better way to process images using an incredibly powerful freeware program that not only automatically lines up the histogram peaks, but removes all the gradients and has an AI denoise function to clean up those lower signal-to-noise images to perfection? On top of all that, this application also allows various stretch levels on your image and creates a perfect dark neutral gray background.

Well, it exists: Graxpert can be downloaded from https://graxpert.com. Download the standalone version 3 and you’ll get the latest AI files and application. Graxpert will open any FITS or TIFF files and display them as a full-screen image, making it easy to spot fine details in your images. This will be important when we use the program for denoising later.

After opening your raw stacked image in Graxpert, make sure you unselect the “Channels linked” checkbox at the bottom. This will make Graxpert align the three color peaks in your image. Also, crop out the black border so as not to confuse the AI flattening algorithm later. Start with the background extraction and removal tab; for most images, the AI flattening works great. Sometimes complex nebulosity will produce dark areas in the image or remove dim nebulosity. I then choose the manual RBI method and put little sample boxes in my image in the places where the sky background appears. This works really well and your images will no longer have a Moon-induced gradient.

The next tab is Denoising, and I just leave it at the AI position. The program may ask you if it’s OK to download the latest AI. Say yes. Version 10 and later work fantastically. The slider can be set from halfway to full strength. Start with the 50-percent level and see how it looks. This denoise application is one of the most powerful available. It does take time to do its job, but you will not be disappointed with how it cleans up your images.

Finally, when saving images select the “No Stretch” option at the bottom if you want to do your own nonlinear stretch later, or one of the other choices if the stretch it suggests looks good. A level of 15 percent is usually optimal. In the Save tab, you have several options. I usually choose “Save Processed” for an image on which I will do the non-linear stretch later. If you like the suggested stretch, you can save it as “Save Stretched and Processed.”

More processing

As with any raw image that has been calibrated, gradient corrected, and denoised, you can finish the processing in your favorite program. Just remember that Graxpert by default saves images as 32-bit TIFF files, and you may want to change this to 16-bit.

Using your favorite software, start with a nonlinear stretch using the Histogram palette and move the middle slider to the left as desired. Reset the black level after each adjustment with the left slider. When the image looks right, do final color and saturation adjustments like you would for any dark-sky image. Finally, you can crop the image, rotate for a standard north-up orientation, set final background brightness, and resize for your intended use.

Narrowband imaging

Most city-dwellers have discovered that you can record some remarkable images from the bright city skies using various narrowband filters. This is generally true for moonlit skies, with one exception. While Hα and Sulfur-II filters are at the red end of the spectrum and are less affected by the blue moonlit sky, the OIII filter with its bluish-green color does not fare so well.

In fact, shooting either OIII or Hβ narrowband images is usually a losing battle. Even with incredibly long exposures, they don’t reveal much except sky fog. So go ahead and shoot Hα in the moonlight, but save the blue end of the spectrum for that next moonless night.

Combining RGB data that has had moonlight mitigated with Hα images is straightforward. I usually layer the Hα data over the RGB data in Photoshop, then colorize the Hα data with a shade of red or pink. This is easily done with the Hue/Saturation palette and selecting “Colorize.”

Then select the appropriate shade of red to tint the grayscale Hα image. Set Combine to “Lighten” in the Layers palette, and you can set the brightness, contrast, and transparency to the most suitable mix. This retains all the beautiful RGB star colors and any blue nebulosity, while bringing in the narrowband data to create the enhancement you find most pleasing.

Conclusion

So who’s afraid of that Big Bad Moon? You shouldn’t be anymore. Although you must follow some rules with a slightly limited selection of deep-sky targets, you can now take on projects you might not normally attempt. Many new targets will now be within your grasp. You have no excuse for wasting another clear night — no matter the phase of the Moon!