Deep Sky Astrophotography in Moonlight
March 23, 2014
The Moon is full about every four weeks. That’s a given, like snow in winter where I live. Within about a week either side of full, the Moon floods the night sky with reflected sunlight. This can be, among other things, beautiful, romantic, eerie, tranquil or even photogenic in its own right. But it creates real challenges for deep sky imaging.
Much like man-made light pollution, the Moon’s light washes out faint objects. Since I’m not willing to surrender two weeks of deep sky imaging to the Moon every month, I’ve developed a few strategies to keep the shutter open even when the Moon is big and bright.
Location, Location, Location
When the Moon is 3-7 days on either side of full, I can usually find some pretty dark areas of sky in the north, especially on very transparent nights when there are fewer particles in the air to scatter moonlight. The moon follows a lower altitude path in the summer than winter, so is further away from my northern target area in summer than in winter (but the nights are so short!).
Choose Star Clusters
Open clusters and globular clusters are my targets of choice during this time, especially if I want to acquire a whole colour image data set in one session (I use a mono camera with filters). Since they are often overlooked by imagers, compared to galaxies and nebulae, this gives me a chance to give open and globular clusters their due in my image collection.
Take Short Exposures
I have found that taking many relatively short exposures gives better results than fewer long exposures. I have found this to be generally true, but especially when the skies are bright from moonlight or light pollution. One reason is that shorter images have fewer (or no) saturated pixels in the bright areas. Another reason is that stacking more images does a better job of removing artifacts in individual frames, like hot or cold pixels. Finally, a lower background brightness in the image gives more dynamic range between the darkest and brightest pixels in the image.
Plan to Return Later
My colour pictures are made up of four or five channels, each taken through a different filter (red, green, blue, clear and sometimes Hydrogen-alpha). The R, G and B channels don’t provide much brightness information in the final image (that comes from the clear filter), so I often can collect colour data when the moon is bright, even for galaxies and nebulae. I can also get good H-alpha data in moonlight — even on the night of full Moon – because of the narrow range of light wavelengths that passes through my H-alpha filter. But I usually wait for a moonless night to shoot through the clear (luminance) filter, which provides nearly all the brightness information in my final image. A few days after full Moon, I can shoot Luminance data between the end of twilight and when the moon rises above the trees that shield my eastern horizon.
Remove Light Gradients During Processing
The reality is that even on moonless nights most of us will capture unwanted light pollution in our images along with the precious photons we’re really after. Most image processing software comes with tools that can reduce the impact of unwanted light gradients in images. My personal favourite is the Dynamic Background Extraction tool in PixInsight. Gradient Exterminator is an easy-to-use plug-in for Photoshop.
Telescopes, cameras and mounts need a lot of regular TLC to keep working at their best. The period around full Moon each month is when I tend to do these things. Even though I have a “permanent” setup, I change optics, camera orientation, and guiding parameters regularly. My scope’s collimation and my mount’s polar alignment and pointing models need to be periodically updated. New software configurations for imaging and mount control need testing. When the moon is out I sometimes take test shots to see how to best frame objects in future imaging sessions.
Most amateur astroimagers know that there’s a sort of “Drake equation” for the probability of getting a good night’s imaging. And guess what: the news isn’t good. For most of us imaging time is very limited due to clouds, work, family commitments, other hobbies (really??), equipment malfunctions and even short summer nights. With a few carefully chosen strategies, you can stop the Moon from robbing half of your imaging time every month, and make sure you’re equipment is ship-shape for those moonless nights deep sky imagers crave.
And don’t forget to look at the moon with any size telescope or the naked eye. It is beautiful. It’s also a good first target for imaging with any kind of camera.