Beginning Astrophotography: Crescent Moonset

Computer-controlled, motorized telescope, dimly illuminated, aimed up and to the left at the distant crescent moon in the upper left, with a camera attached where the eyepiece should be at the back, using a baroque contraption.
Last night’s front-porch setup: Celestron NexStar 5 SE powered by a Celestron PowerTank Lithium and Sony α6300 camera connected using an E-mount camera adapter, aimed at the twilight moonset.

I had clear skies again last night, and I remembered to look for the Moon while it was slightly higher in the sky. I set my telescope up on the front porch shortly after sunset. The Moon presented an incandescent, imperceptibly fuller crescent facing the failing twilight.

Because it was higher, I had a better perspective, I had more time to take photos, I had more time to check my settings, and my photos had less atmosphere through which to photograph (meaning less distortion). And because the crescent was fuller, I captured more detail in my photos.

Equipment

I always remember to spell out acquisition details in my astrophotography posts, but I’ve found instead people most often ask what equipment I use. I usually don’t list this in detail, both because I’ve usually already mentioned my equipment in earlier posts and also because I find that the exact equipment I used on a given night is partially convenience and whim, not meriting any particular recommendation or endorsement. My photos are within reach of all sorts of equipment of various kinds and prices, given practice and technique, and the last thing I want to do is give someone the impression they need to spend over a thousand dollars to do what a two-hundred-dollar telescope and a smartphone can do.

However, I’m going to try to make an effort to name what equipment I use now and in the future just because it’s so commonly asked. Maybe I’ll need to reference it myself in the future, too. So last night, I used

Those are the only four pieces of hardware I used last night.

Technique

I aligned the telescope on the Moon, which let it track roughly. This meant it needed periodic corrections to keep it from drifting out of view (once every several minutes). I concentrated on keeping the extents of the arc within the viewfinder.

View of the LCD display of the camera, zoomed in on a fuzzy section of the Moon, showing bright and dark sections divided by a diagonal line.
Using the Focus Magnifier on the Sony α6300, concentrated on a section of the Moon near its terminator to fine-tune focus.

Once it was centered and roughly focused, I used a feature on my camera called the “Focus Magnifier” to fine-tune the focus. I’ve found this to be indispensable. Using this feature, I zoom in to a close up view of some section of what the camera sensor is seeing. This way, I can make fine adjustments to the telescope’s focus until I get the best possible clarity available. I can also get a good idea what kind of seeing I’ll encounter that night—whether the sky will shimmer a lot or remain still. I was lucky last night to find good focus and good seeing.

Once focus is good, it can be left alone. I ensure that the adapter is locked tightly in place so that nothing moves or settles, keeping the focal point cleanly locked on infinity.

Then I turned the ISO up—doubled it. The Moon is a bright object, so I was not keen to use something I would use for a dark site, but I settled on ISO 1600. My goal was to reach a shutter speed of 1/100 seconds, which I did, without losing the picture to noise or dimness. A higher ISO works great at a dark site, but the Moon is quite dynamic, so I felt like I had less headroom. In any case, I used 1/100 seconds’ exposure and ISO 1600 for all my photos.

I captured a short 4K video before I began so I could capture the seeing conditions that night. I recommend viewing it fullscreen, or it will look like a still photo—the sky was placid as a pond last night.

After taking the video, I realigned the telescope slightly and, using my remote controller so that I could quickly actuate it without shaking the telescope, I took 319 photos, occasionally realigning to correct for drift.

Unfortunately, Venus and Mercury had already sunk too low to get a glimpse, so I packed it up and went inside.

Processing

I moved all the photos, in RAW format, to my computer from the camera. Then I converted them all to TIFF format. These two steps took probably something like an hour and resulted in seven and a half gigabytes of data.

Screenshot of a Windows program called PIPP, using two Windows, one showing the Moon highlighted in blood red, the other with several progress bars and a list of files.
Screenshot of PIPP in use, aligning the photos of the Moon and sorting them by brightness.

Because the Moon drifted, due to the rough tracking, the photos needed to be pre-aligned. I used a piece of software called PIPP for that. Without this pre-alignment step, the tracking and alignment built into my stacking software struggled mightily with the photos and created a mess.

Its output was another series of TIFF photos. I found afterwards that two of the photos were significantly too exposed, leaving many details blown out, so I excluded them from the rest of the process, leaving me with 317 photos.

Screenshot of AutoStakkert!3, a baroque program consisting of two windows, one with a large preview of the moon, the other with lots of graphs and buttons and inputs and multiple progress bars. It is 20% through "MAP Analysis."
Screenshot of AutoStakkert!3 stacking the best 50% of the Moon photos I took into a single image.

I opened these 317 photos in AutoStakkert!3 beta. After initial quality analysis, I used the program to align and stack the best 50% of the images (by its determination). This took a bit less than ten minutes and left me with a single TIFF photo as output.

Image stacking leaves behind an intermediate product when it’s complete, which is what this TIFF photo is. It’s blurry, containing an average of all the 157 photos which were composited into it. However, the blurs in this photo can be mathematically refined more easily using special filters.1 I used a program called Astra Image to apply this further processing. In particular, I used a feature it calls “wavelet sharpening” (which can be found in other programs) to reduce the blurring. I also applied an unsharp mask and de-noising.

Finally, I used Apple Photos to flip the resulting photo vertically (to undo the inversion which the telescope causes) and tweak the contrast and colors.

The Photo

Photo of crescent moon, slender, curving down like a bowl but askew to the right. Terminator stops just short of the Mare Crisium.
Crescent moonset taken at about 8 p.m., PDT, on 20 Mar 18. Composited from 50% highest quality photos of a set of 317 taken.

Click to view the photo in fullscreen if you can. There’s a lot of detail. The terminator of the lunar surface stops just short of the Mare Crisium (the Sea of Crises), the round, smooth basalt surface right about the middle of the crescent.

I can’t help but compare this one to the photo from the night before: what a difference a day makes. I had more time to work, more photos to take, and the benefit of yesterday’s experience to help improve.

Now it’s clouded over here again—Portland weather—and I can’t practice anymore for a while.