How to Photograph the Moon With a Telescope

The Moon is the easiest real astrophotography target there is, and it is the one that should come first. It is bright, which removes tracking, dark skies, light pollution, calibration frames and hours of stacking from the problem all at once. You can get a photograph you are pleased with on the first clear night with equipment you already own.

Two facts explain almost everything on this page, and both surprise people. The Moon is a sunlit daylight subject, lit by exactly the same Sun that lights a field at noon, which is why the exposure is a fraction of a second rather than minutes. And the full Moon is the worst night of the month to photograph it, because everything you want to see is a shadow and at full phase there are none.

What exposure does the Moon actually need?

Start with the Looney 11 rule: at f/11, use a shutter speed of one over the ISO. At ISO 100 that is 1/100 second. It is the lunar equivalent of the old Sunny 16 rule for daylight, and it works for the same reason, because the Moon is a rock in full sunlight and its distance from you does not change how brightly the Sun lights it.

Through a telescope you cannot choose an aperture. The tube has whatever focal ratio it has, so an f/10 Schmidt-Cassegrain sits close to Looney 11 already and needs roughly 1/100 second at ISO 100, while an f/6 Dobsonian is gathering light about three times faster and wants something nearer 1/320. Do not try to compensate with ISO. Set the lowest native ISO your camera has and change shutter speed, because ISO adds noise and there is absolutely no shortage of light here.

These are starting points rather than answers. Shoot a bracket of five exposures either side and check the histogram: you want the bright limb just short of the right hand edge, because a blown highlight has no data left to recover. Every camera meters the Moon differently, since most of the frame is black sky and the automatic meter tries to lift it.

Why is the full Moon the wrong night?

Because at full phase the Sun is almost directly behind you, so every shadow on the surface falls out of sight behind the feature that cast it. A crater a mile deep looks like a pale circular smear. The surface has no relief, and the photograph looks flat because it accurately records a flat looking scene.

What you want is the terminator, the line dividing the lit and unlit halves. At the terminator the Sun sits low on the lunar horizon, so crater walls throw shadows tens of miles long, mountain peaks catch light while the valley beside them stays dark, and the surface resolves into genuine three dimensional landscape. Three to ten days after new Moon puts the terminator across the middle of the disc in the evening sky, which is both the best light and the most convenient time of night.

This is also why the same crater is worth photographing repeatedly. Under a different Sun angle it is a different photograph, and a sequence of the same feature across several nights is one of the more satisfying projects available with modest equipment.

The one thing a full Moon does well is ray systems: the bright splash patterns radiating from Tycho and Copernicus are washed out at low Sun angles and obvious at full phase. So the full Moon is not useless, it just answers a different question.

How do you photograph the Moon with a phone?

Well, cheaply, and it is the right place to start. The Moon is bright enough that a phone sensor handles it comfortably, and modern phone processing is genuinely good at this subject. What is hard is purely mechanical: holding the phone camera exactly concentric with the eyepiece, perpendicular to it, and steady enough that the shutter does not shake it.

A three axis phone adapter clamps to the eyepiece and gives independent adjustment in each direction, which turns a lucky shot into a repeatable process. Three settings matter once it is mounted:

1. Lock the exposure manually if your phone allows it. Automatic metering sees a frame that is mostly black sky and lifts everything, which blows the Moon to a white disc. Tapping and holding on the Moon usually locks exposure and focus together.
2. Turn off digital zoom. It throws away resolution to enlarge a crop. Get the magnification from a shorter eyepiece instead, which is real optical magnification.
3. Use the timer or a voice shutter. Touching the screen at the moment of exposure is the largest single source of blur in phone astrophotography.

Shoot video rather than stills if the phone allows it, then extract the sharpest frames. That is the same lucky imaging principle described below, running on hardware you already own.

How do serious lunar images get so sharp?

Not with a better single exposure. With lucky imaging, which is a completely different technique and the reason amateur lunar photographs now routinely beat what professional observatories produced on film.

The atmosphere blurs the view continuously, but unevenly. Over a few thousand frames, some small fraction happen to be captured in a moment when the column of air above you was briefly still. Software sorts every frame by sharpness, keeps the best 10 to 25 percent, aligns them on surface features and averages them together. Averaging removes random noise, which then allows a sharpening pass that would only have amplified noise on a single frame.

The workflow in practice:

1. Capture video, not stills. A planetary camera drops into the focuser where an eyepiece goes and records uncompressed frames at high speed. This is the single purchase that changes lunar imaging most, and it costs about the same as one decent eyepiece.
2. Aim for 2,000 to 5,000 frames per panel. More frames means a larger pool to pick the sharpest from, and the sharpest few percent is where the whole result comes from.
3. Keep exposures short. A few milliseconds per frame freezes the atmosphere rather than averaging through it. This is also why field rotation on an alt-azimuth mount is irrelevant here.
4. Stack, then sharpen carefully. Wavelet or deconvolution sharpening on a stacked result brings out detail that is genuinely present. On a single frame the same slider only amplifies noise, which is what produces the crunchy over-processed look.
5. Mosaic if you want the whole disc at high resolution. At long focal length the Moon does not fit on a small sensor, so you capture overlapping panels and stitch them.

Focus is the other half, and it is worth more attention than most people give it. A Bahtinov mask on a bright star before you start turns focus from a judgement into a yes or no, and focus drifts as the tube cools, so it is worth rechecking during a long session.

How much detail can your telescope actually record?

Image scale decides this, and it is one calculation: arcseconds per pixel equals 206.265 times the pixel pitch in microns, divided by the focal length in millimetres. The Moon spans roughly 1800 arcseconds, so dividing gives how many pixels wide the disc will be.

Figures assume a 2.9 micron pixel, which is typical of a small planetary sensor. Two things follow. Long focal length gives more pixels across the disc, which is why an 8 inch Schmidt-Cassegrain is such a natural lunar imaging telescope. And beyond a point extra magnification only spreads the same detail across more pixels, because the atmosphere is the limit rather than the optics. A 2x Barlow doubles the focal length and is worth adding on a genuinely steady night, and worth leaving out on an average one. The magnification calculator and magnification explained cover where the real ceiling sits.

What about the same setup in daylight?

The Moon is often visible in a blue daytime sky, and photographing it there is genuinely worthwhile: a gibbous Moon against blue is a striking image and needs a slightly shorter exposure because the sky background lifts the whole frame.

The Sun is a different matter entirely, and the rules are absolute rather than cautious.

With a certified full-aperture solar filter clamped over the front of the tube, white light solar imaging uses exactly the same lucky imaging workflow described above, and sunspots change day to day in a way that makes a running record genuinely interesting. Read how to observe the Sun safely in full first, because the filter position is the whole of the safety argument and an eyepiece end filter is never acceptable.

What to buy, in order

1. Nothing. Hold your phone to the eyepiece tonight and see what you get. This step is not a joke, it is how you find out whether you enjoy this before spending.
2. A phone adapter . Turns the previous step into a repeatable one for about sixty dollars.
3. A planetary camera . The step that changes results most, because it unlocks video and therefore lucky imaging.
4. A Barlow , for the steady nights when the atmosphere will actually support more magnification.
5. A tracking mount, last, and only if undriven imaging is genuinely frustrating you. It buys capture time rather than image quality on this target.

That order is deliberate and it is roughly the reverse of what most people buy. If deep sky imaging rather than lunar work is where this is heading, read how to start astrophotography first, because it is a different, more expensive and more demanding hobby than this page describes, and the Moon is the honest place to find out whether you enjoy the process at all.

Related

• How to start astrophotography, and whether you should
• How to observe the Sun safely
• NPF rule exposure calculator
• Best telescopes for astrophotography
• What you can actually see with a telescope