Photographing 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:

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.

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.


 The Moon, just past full

Here is a view through my 254mm Newtonian, with 2x Barlow lens.

Canon EOS 5DMkII 254mm Newtonian @2400mm f/9.6 ISO800 1/400s 2011-9-15 00:24:19UT
From Rookhope 54.8N 2.1W 330m asl. Rural, almost no light pollution (3 Bortles)


 The Moon in daylight

(Feb 2013) Clear Moon-free nights for deep sky photography have been very scarce this winter, so in desperation I tried photographing the Moon in daylight! This photo was taken from Rookhope. It is much sharper than the best I took later, at 8:30 in the evening. The late afternoon air was much steadier (less turbulent) than later in the evening. Processed in my own software, only to remove the blue channel (dominated by sky), convert to monochrome and enhance the contrast by tone curves.

Canon EOS 5D MkII ISO200 1/60s
254mm aperture Newtonian, f=1200mm (f/4.8)
x2 Barlow lens -> f=2400mm (f/9.6)
2013 Feb 17 16:46:20 UT

Here is a larger scale crop of the northern portion of that image, showing how sharp it really is:


 Earthshine on the crescent Moon

Canon EOS5DMkII 254mm Newtonian @ 1200mm f/4.8 2s & 1/100s ISO6400 2011-03-07 20:07 & 20:05 UT
From Rookhope 54.8N 2.1W 330m asl. Rural, almost no light pollution (3 Bortles)

The upper photo is deliberately overexposed, 2s at ISO 6400, in order to show the Earthshine. That is, sunlight that has first been reflected by the Earth onto the dark part of the Moon. The lower photo is a more correct exposure for the crescent Moon, 1/100s at ISO 6400. The ratio of exposure settings suggests that the Earth reflects about 0.5% of incident sunlight back into space.


 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.

There are several things to note about this.

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

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.

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