Half Full Moon – 29th May 2020
The Moon is probably the simplest astronomical object that people attempt to photograph as it requires no specialist equipment other than a long lens. The first thing most photographers discover is that it does not require a long exposure. Many photographers assume that because they are photographing at night, a longer exposure is required, but in fact The Moon receives the same amount of sunlight as the Earth, so when we photograph the illuminated Moon, it is equivalent to photographing a sunny day. For the above image, the exposure times were as short as 1/50s at F9, ISO200. As a consequence of the short exposure times, stars do not register and The Moon appears to be floating in a pure black sky. Another common mistake is to photograph The Moon when it is full. This often results in very little contrast on the Moon’s surface, meaning the craters and mountains do not show up very well. It is much better to photograph a partial moon so that plenty of shadows are thrown across the craters close to the terminus (the boundary between light and dark) , which makes for a more interesting image. To produce a large image of The Moon, a fairly long lens is required – for the image above a 600mm lens with a 1.4 tele-convertor on an APS-C sensor was used, giving an effective 35mm focal length of 1344mm. Obviously, with such a long lens a sturdy tripod is required to hold the lens steady, and it is best to use the self timer or an intervalometer to take the images to avoid camera shake, and lock the mirror up if using an DSLR.
The image above was created from a stack of 13 images. As the exposure times are very short, disturbances in the atmosphere can lead to distortions – a little like a heat haze effect. To counteract this, it is best to take multiple images, then the best image from the sequence can be selected, or the best group of images images can be stacked and averaged, so that disturbance in one part of a single image does not have too large an impact on the overall sharpness. This technique of stacking mages is called ‘lucky imaging’ where we rely on some images being sharp just by chance, and there are several programs that automate the process of selecting and stacking the best images, such as Registax or Autostackert. Note that this type of stacking is different to the stacking performed for dim deep space objects, where stacking is performed to increase overall exposure time and improve signal/noise ratio. In the case of this moon image, the stacking process produced only a marginal benefit over a single image.
In the image above, a couple of The Moons ‘seas’ appear clearly – the large round sea towards the top of the image is Mare Serenitatis, adjacent to that is Mare Tranquillitatis where thefirst lunal landing was made, and the smaller round sea towards the right side is Mare Crisium. The two prominent craters close to the centre of the moon are Hipparchus (the upper one with a smaller crater at its upper edge) and Albategnius (the lower one with a clear central peak). These are respectively 140km and 130km in diameter. Further south along the terminus and another crater wit a prominent peak is Walther, 135km in diameter. Towards the top of the image, the mountain ranges of Montes Apenninus and Montes Caucasus lie to the left of Mare Serenitatis and separate it from Mare Imbrium which is hidden in the shadows, whilst Mons Pico is the small white dot that can be seen on the flat plain of Mare Imbrium. This latter mountain rises 2450m above the surrounding plain, so is able to catch the sunlight when the surrounding area is in shadow.
Image Details
- Date: 29th May 2020
- Exposure Details: Stack of 13 x 1/50s, F9, ISO200
- Camera: Canon EOS80D (unmodified)
- Lens: Sigma 150-600mm + 1.4 Tele-Converter at 840mm focal length.
- Mount: Manfrotto MT190CXPro4 Tripod