RigTonightAstrophotography and the moon: when you can still shoot, and what to shoot
The moon is the brightest thing in the night sky by far, and moonlight scattered by the atmosphere brightens the whole sky. Around full moon a dark rural sky can look like a suburban one. But "wait for new moon" throws away half of every month. With the right targets and filters, you can image through much of it.
What moonlight does
Moonlight is reflected sunlight, so it covers the whole visible spectrum. Scattered in the air, it adds a bright, often uneven background to every frame. That has two effects:
- Less contrast. Faint detail sinks into the brighter background, and you need much more total exposure to pull it back out.
- Gradients. The sky is brightest near the moon, so frames get a brightness slope that has to be removed in processing.
How much it hurts depends on three things: how full the moon is, whether it's above the horizon, and how close it is to your target.
The lunar month at a glance
- New moon: the moon is near the sun and invisible all night. The best nights for everything.
- Waxing crescent and first quarter: the moon is in the evening sky and sets around or before midnight. After moonset, it's fully dark.
- Full moon: up all night, opposite the sun. The hardest week for broadband imaging.
- Waning gibbous and last quarter: the moon rises later each night, about 50 minutes later on average, leaving the evening dark.
So in any month, there are usually dark hours somewhere: early evening after full moon, or late night before it.
Which targets cope with moonlight
Emission nebulae through narrowband filters cope best. Hydrogen-alpha (Hα, deep red) and sulphur II filters pass a slice of spectrum only a few nanometres wide, which blocks almost all the moonlight. Many imagers shoot Hα through bright moonlit nights. Oxygen III (OIII, blue-green) is more affected, because scattered moonlight is bluer, so save it for darker nights. Dual-band filters on colour cameras sit in between.
Broadband targets suffer most: galaxies, reflection nebulae like the Pleiades, dust clouds, and the colour data for any target. Save these for the week around new moon.
Bright star clusters cope reasonably well; they're bright enough to stand out against a brighter sky.
Distance from the moon matters
The scattered glow is strongest close to the moon and fades with distance. A target 90° or more from a half moon can still give usable data, while the same target 20° from it won't. When the moon is up, pick targets on the opposite side of the sky.
A practical plan for the month
- Week of new moon: galaxies, reflection nebulae, faint broadband targets, and RGB colour data.
- Crescent and quarter weeks: broadband in the dark hours before moonrise or after moonset; narrowband while the moon is up.
- Full moon week: narrowband emission nebulae, especially Hα and SII, or a night off to process.
Let RigTonight plan around it
RigTonight's Tonight tab shows the moon's phase and when it's up, and works out how much it affects each target: brightness, height and distance from the target through the night. Targets get a moon impact of none, low, moderate or high, and the ranking takes your filters into account, so a rig with a dual-band filter sees emission nebulae rise on moonlit nights.