Photographing the moon

Sections in this page

Using a telephoto lens
Using a small telescope
Eclipses of the moon

Using a telephoto lens

As the brightest object in the night sky, many photographers like to try to photograph the moon. It is an obvious target after obtaining a new, more powerful, telephoto lens. The results are often disappointing though. The apparent diameter of the moon from earth is only half a degree. The field of view of a 300mm lens on a standard 35mm camera is 7 degrees (APS-C-sized detectors help a bit because their field of view is reduced by a factor of 1/1.6.) So the moon through a 300mm lens looks like this:

The moon through 300mm lens: the whole frame

Canon EOS5D 100-400mm lens at 285mm 1/1000s f/5.6 ISO400 2008:2:11 17:52:05

However, cropping the central portion of the image so it can be displayed at full size (one pixel in the camera corresponding to one pixel on the screen) we get the following, in which it is possible to see mountains and craters.

The moon through 300mm lens at 1-to-1 pixel size

Canon EOS5D 100-400mm lens at 285mm 1/1000s f/5.6 ISO400 2008:2:11 17:52:05

Galileo first saw such things 400 years ago, in 1609. That's why 2009 has been designated the International Year of Astronomy.

It is of course essential to photograph the moon at a phase other than full. At full moon there are no shadows, so mountains and craters cannot be seen and the image is much less interesting.

Using a small telescope

Using my Meade ETX-125C which, with the camera at the prime focus, acts as a 1900mm lens, I get a view like this:

Canon EOS D60 ETX-125C 1900mm 1/90s f/15 ISO400 2003:1:10 17:28:31

Through the eyepiece of the telescope I get a more magnified view but then I am at the mercy of atmospheric turbulence. Sometimes I get a clear view but often not. It is a matter of chance whether the moment of exposing the camera corresponds with a moment of clear seeing, to get a sharp result. There is also a problem of dust on various glass surfaces when viewing through eyepieces. It is virtually impossible to avoid these causing blemishes unless I first take a flat-field image under exactly the same conditions: without moving the eyepiece or camera in any way, use my flat-field box to take another shot. Then use GRIP to correct the photo with the flat field.

Eclipses of the moon

On Saturday 3rd March 2007 there was a very favourable lunar eclipse visible from my home. It was at a reasonable hour (around midnight) and the weather turned out, for once, to be perfect. The moon was well placed in the sky, below the constellation of Leo, and Saturn was visible nearby, as shown in the following map based on a photo I took in mid-eclipse.

I took a sequence of images at roughly 5-minute intervals throughout the eclipse and turned them into an animation, shown here.

Animated sequence of lunar eclipse, 3/3/09.

There are several things to note about this.

Firstly that during the darker stages of the eclipse it was possible to see stars in the photos even when the moon was correctly exposed. That is not usually possible because the moon is far too bright.
Secondly, by watching those stars you can clearly see the moon moving across in the animation (though I have set it up so the moon stays fixed and the stars move relative to it).
The moon also rotates slightly during the animation. That's because my camera was on an ordinary photographic tripod, so the mounting was altazimuth rather than equatorial.
Most importantly, I had to change the exposure as the eclipse progressed, in order to keep the moon correctly exposed. Because the camera keeps a record of its settings as metadata in each image file, I can track how the brightness of the moon varied during the eclipse, and plot a graph of that. This has some scientific value because the variation of the moon's brightness does indicate something about the transparency or otherwise of the earth's atmosphere at the time.

Here is the graph of brightness against time from my photos of the eclipse on 3/3/07:

Graph of camera EV settings during eclipse.

It was plotted by making use of the fact that a single stop change in exposure time, aperture or ISO sensitivity is a doubling or halving of the amount of light exposed. That is not the full story because we also need to use GRIP to measure the average brightness level of the moon in each image, and also calibrate detected levels against amount of light, in order to get the exact brightness change.

I searched the internet for a photo taken from the moon during such an eclipse but I could only find an artist's impression, so maybe it has not yet been done. It will be most interesting to see. The earth should look like a ring of fire.

Detailed information about predicted eclipses, both of sun and moon, can be found on NASA's eclipse page.