Prep for the Lunar Eclipse

A lunar eclipse with over 60 minutes of totality will be visible from BC and most of North America on Sunday Jan 20th, 2019 – weather permitting.

RASC volunteers will be at Vancouver’s MacMillan Space Centre for a special eclipse viewing event. If the weather cooperates then the Trottier Observatory at SFU will also be open for a special Starry Nights event.

Image of lunar exclipse

Composite image of the total lunar eclipse on Sept 28th, 2015. Image Credit: Sky&Telescope  / Jamie Cooper

A total lunar eclipse occurs when the Sun, the Earth, and the Moon line up so that the Earth’s full (umbral) shadow falls on the moon. The moon doesn’t completely disappear but it will be cast in an eerie darkness and take on a reddish hue. The eclipse starts at 7:35 pm PST with the Moon in the south-east sky slightly more than 20° above the horizon. The mid-point of totality occurs at 9:13 pm PST and by then the Moon will have moved more towards the south and climbed to an altitude of 42°.

You can watch the the eclipse with the naked eye but binoculars make eclipse colours more vivid and give great 3D views. Lunar eclipses are easy to observe even in light-polluted city skies.

Lunar Eclipse from Sep 2015 over the lights at Percy Perry Stadium in Coquitlam

A telescope allows you to watch the eclipse encroaching on craters one after another.

If you are interested in photography then check out Alan Dyer’s fantastic article on how to photograph the lunar eclipse. I am going to try to get a shot of the eclipsed moon near the open star cluster M44 – the Beehive Cluster – in the constellation Cancer. I have been doing a bit of planning and practice to improve my chances of getting a good shot.

The Moon will be located a bit over 6° from M44 so it will be a widefield shot. I entered the data for my Nikon D5100 DSLR with a 55-200mm zoom lens at 105mm and a focal ratio of F4.5 into the SkySafari app to verify the field of view will nicely frame both the cluster and the Moon.

Image of field of view for DSLR with 105mm lens
A rectangle that shows the field of view expected for my Nikon DSLR and 105mm lens

But what about the exposure time? Dyer notes that “the crescent Moon with Earthshine on the dark side of the Moon is a good stand-in for the eclipsed Moon.” Fortunately, there was a clear patch tonight so I took a few test images at ISO 400 to practice and get a rough idea of the exposure settings – some Earthshine is visible with a 2 second exposure but stars only start to become visible with a 5 second exposure.

Click on the 5-second exposure image to see a larger version – the star that is visible a little above and to the right of the Moon is the 4.8 magnitude star 20-Cetus and most of the stars in the Beehive cluster are much fainter with magnitudes above 6.3. So I’ll need to take images at different exposures: about 1-2 seconds to capture the eclipsed moon and 20 seconds or more to capture the faint stars in the cluster. I’ll try using HDR techniques  (high dynamic range) to combine the several exposures into a single image – either using the D5100’s builtin HDR setting when taking the shots or during post-processing.

The rule of 500 says that the maximum exposure time to avoid star trailing is 500 / effective-focal-length. My Nikon 5100 is an APS-C crop sensor with a multiplier of 1.5X which gives me at maximum exposure of 500 / (105 * 1.5) which is just 3.2 seconds. So, I am planning to piggyback the camera on my motorized/HEQ 5 Pro mount to track the stars and avoid trailing with the 20+ seconds needed to capture the stars in the cluster.

The hardest part may be getting a clear sky, in January, in Vancouver!