Lunar Imaging Beyond Basics - Part 2
In the two 'Basics' articles, I mentioned the essential elements for good lunar imaging: Simple cameras, inexpensive software, lots of frames and good post-processing. In Part I of 'Beyond Basics' I wrote about getting plenty of daylight practice. Now let's look at those final important aspects of lunar imaging: exposure time and the right time to shoot.
In general, you don't want exposures (frames) of a lunar image to be too long. If they're too long, the normal atmospheric quivering will blur the image. In addition, you don't want to saturate the brightest spots on the image because you'll lose detail. So shorter is better. But if you make the image length too short - freezing the quivering and avoiding saturation - you can end up with as much CCD noise as there is signal (photons) from the moon. Stacking can help reduce the impact of the noise, but getting more signal in each frame is still best.
Once you finally get out under the moon, center your target and adjust the shutter speed (and gain if it's controllable) to maintain the brightest bright area near the peak of your camera's range and keep the lowest dark space near 0 as possible. (Most capture software can give you constant feedback on the brightest and darkest spot on the image.) If the gain is controllable, you might need to seesaw the shutter and gain values. High gain will make things brighter (allowing shorter shutter speeds) but it will add more noise. Reducing the gain makes the image darker and can force you to decrease the shutter speed (potentially introducing more blur). As I mentioned, try to use statistical measure of the image data rather than estimating brightness and darkness from the laptop screen.
The goal is to make the sky background as close to 0 as possible without touching 0 and the brightest spots as close to the camera's maximum as possible without touching the maximum. This makes sure that your images will reproduce the brightness range that the moon is showing in your particular image.
The bad news is that the values you used on one night can be totally useless on the same night if you pan to a much brighter or darker spot on the moon. A shot with the terminator running down the middle will be relatively dark, even at the brightest spot. A shot of the fully illuminated part of the moon will be hugely bright (with very few naturally dark spots). These will require various shutter/gain settings for the best results. So even the simple act of shooting different parts of the moon or using different fields of view can require adjustments so that you keep the proper dynamic range.
Adjusting for proper dynamic range is second in importance only to focusing. Like focusing, mistakes in dynamic range are often irreparable. If you saturate an area, all the detail in that area cannot be recovered. If your low point is too high, you will likewise throw away dim detail. If the values are too "squeezed" together then you effectively reduce the ability to show variations in brightness. Getting the dynamic range properly set for one image can only be handled by practice and experimentation with your camera's settings. (If you have software that can adjust shutter speed and gain for optimal imaging, congratulations! I'm not aware of anything that can do that.)
Switching gears completely, I want to talk about another kind of "timing". When is the best time to take lunar pictures??
Part of the answer concerns the atmosphere. Of course a dark sky with perfect seeing and transparency is great. But since that never happens here, what are our choices? A dark sky is not really necessary for most lunar photography. The moon is very bright compared to most objects, so lunar photography can be done just about anywhere - even a car dealership. Transparency is also less important for the same brightness reason. The moon can easily punch through slight haze, although the darker areas might sacrifice some contrast. Good transparency is better than bad transparency, but often less than perfect transparency has more stable air and hence better seeing. Seeing is a very important for lunar imaging. It's even more important if your optical/imaging system doesn't let you get very short exposure lengths. If you examine images (e.g. watch a captured video file) you'll never complain about sky brightness or transparency, but you are likely to say, "What rotten seeing!" If seeing is bad, it's not a good night for shooting the moon.
The other part of the "best time" issue is the motion of the moon. We always prefer to observer and photograph objects as close to the zenith as possible. This reduces the issues with seeing (and transparency). The closer to the horizon and object is, the more likely it will be quivering in the sky. Better to wait until it gets overhead.
Of course, the moon never gets overhead at this latitude. The highest altitude we can expect for the moon is a little above 70 degrees. That's because the moon (and sun and other planets) travel roughly along the ecliptic. If you track the ecliptic you'll see it's closest to the North Pole between the constellations Gemini and Taurus. If we want to see the moon at its highest, that will be during the time it spends moving between Gemini and Taurus (the highest ecliptic) at the time of night were the moon crosses meridian (culminates). At other times of the month - when the moon is outside of that peak - the highest altitude will be less and we'll be imaging through more atmosphere. On the other hand, when the moon is passing through the lowest portion of the ecliptic - the area between Sagittarius and Scorpio - it has the "lowest highs" and it's the least appealing time for imaging.
So when the moon is near Gemini and Taurus and it's crossing the meridian, that's a great time to observe and image it. Of course, crescent moons pass the meridian during the day. If you want to find the best time to capture them after dark, you need to think about how the ecliptic crosses the horizon. During some months, the ecliptic approaches the horizon at a low angle. During other months it approaches at a high angle. The high angle months are preferable because that means for a given phase of the moon it will be higher at sunset. The steepest approach months are December and January. The most shallow approach months are June and July.
This means that the best chance to capture a crescent moon is in the cold winter months as soon after sunset as possible. The best conditions to capture a gibbous moon are during its monthly pass near Gemini or Taurus at the culmination point for that night. These prime times aren't the only times, but you also want to avoid the opposite - taking images through more atmosphere than necessary.
So use your time wisely and do some vent capping for practice. Make time to adjust your big telescope to ambient temperatures. Pick your exposure time correctly for the best dynamic range in your images. Take your images at a time when the moon is looking its best. Most of all - have a good time.Published in the December 2005 issue of the NightTimes