Astrophotography
Imaging the lunar eclipse.
The total lunar eclipse on 21 January is well placed for UK viewing. Our last one, on 27 July 2018, was a damp squib. After days of cloudless summer skies, the weather closed in just in time to ruin the event, obscuring it from virtually all of the UK. Even if we could have seen it, we’d only have enjoyed part of the spectacle; totality was already underway even before the Moon rose over the horizon. Fortunately the 21 January
eclipse will take place entirely in UK skies so all we have to hope for are no clouds.
The eclipse starts at 02:37 UT when the Moon’s disc enters the weak outer penumbral shadow. This month’s Sky Guide Challenge (on page 61) is to see how early on you can determine the effects of the penumbral shadow. Photographically, this shouldn’t be too difficult as long as exposures and settings are maintained. It’s important to ensure that any image of the Moon’s disc is correctly exposed. A low ISO and short exposure should cope well with this bright Moon. Comparing images taken at, say, five-minute intervals should reveal the subtle advance.
The main visual part of the eclipse occurs when the Moon enters the darker umbral shadow. This starts at 03:34 UT and continues until 06:51 UT. The period of totality – when the umbral shadow fully covers the Moon’s disc – is between 04:41 UT and 05:43 UT. Totality is somewhat easier to deal with than the two partial eclipses either side of it. A partially eclipsed Moon presents part of the Moon’s disc darkened by the umbral shadow and part at normal brightness. A typical DSLR doesn’t cope well with this and you’ll normally end up with shots that show either a black shadow with correctly exposed bright surface or a white surface with a beautifully coloured eclipse shadow.
Depending on your exposure frequency – ie, one shot every two minutes – it’s possible to jump between two camera settings to take alternate shots revealing either shadow or surface. A camera attached to a telescope works well if you’re after lunar detail. A focal length around 1m provides a good balance between image scale and ease of tracking and framing.
Normal camera lenses can be used too. For reasonable disc detail, a 200mm or longer lens is recommended, but the essence of the eclipse can still be achieved with shorter lenses. Using a wide-angle lens it’s possible to produce a sequence shot showing the Moon’s brightness changing over time. For this eclipse we recommend a 15mm focal length for a non-full-frame or 25mm for a full-frame DSLR. Mount the camera on a fixed tripod pointing at azimuth 255° (position Betelgeuse on the frame’s vertical centre line at 02:34 UT). Adjust so the horizon runs along the bottom (long) edge of the frame and ensure the Moon’s in the image. Take shots at intervals of two to three minutes and add the images in a layer-based editor with upper layer blend modes set to Lighten. The initial images of the Moon’s disc will typically need to show it over-exposed to white to cope with the dimming that occurs during the main part of the eclipse.