Take the perfect astrophoto with our step-by-step guide
ASTROPHOTOGRAPHY
CAPTURE
Lunar occultations of planets
How to image a far-from-easy Uranus disappearing behind the Moon on 14 September
Occultations of the planets by the Moon aren’t very common and when they happen, all fingers are crossed for clear skies. If the weather holds good for the lunar occultation of Uranus on 14 September, there will be a lot of interest in the little planet, certainly more than it normally gets.
Unlike the main planets closer to the Sun, Uranus appears dim, right on the threshold of naked-eye visibility. It’s also rather distant and looks small even through a sizable telescope. This presents issues with visually observing the event as well as some awkward problems when trying to image it.
These will be exacerbated by the fact that the start of the occultation will involve the Moon’s bright limb passing in front of the planet. Setting up captures for Uranus will certainly overexpose the Moon, while exposing for the lunar surface may mean Uranus is lost from the shot altogether.
Fix upon framing
There are various imaging setups that can be used for this. First consider how you want to record the view: a wide field to cover the whole Moon or a riskier narrow field zoomed in on the planet? The latter isn’t that hard to organise for the disappearance event, but adds uncertainty in when Uranus reappears from behind the Moon’s dark limb: here you’ll need your best guess as to where the planet will reappear.
Then there is the choice of whether to produce an image that is visually and scientifically accurate, or one that is an enhanced version of reality. The latter is perfectly acceptable as long as you state what you have done and the steps that led to the final image. Better still, consider combining both techniques to show the scientific aspects together with an enhanced version to better show what’s happening.
This will entail performing the best capture of Uranus you can manage before the occultation, along with a separate capture optimised for the lunar surface. Ideally these should be taken around 30 minutes before or after the occultation. Or, to play it safe, both before and after the occultation – just to be sure!
During the event, the emphasis should be on exposing for Uranus: disregard the appearance of the lunar surface, which will almost certainly be over-exposed. The purpose of this capture will be to provide accurate positional information for the event. If possible, when the planet and Moon are in the same shot, make a note of the settings for Uranus and reduce the exposure to take a quick image of the Moon. This will help you figure out just where Uranus is in respect to the features on the lunar surface.
With a capture like this, practice makes perfect. If you get clear weather on any of the nights in the run up to the occultation, use them to practise imaging Uranus and the Moon so that on the night you’ll know exactly what to do. But if something does go wrong, remember there’s a similar event due to occur in the early evening of 5 December.
Equipment: camera, telescope, polar-aligned equatorial tracking mount
Send your images to: gallery@skyatnightmagazine.com
Step by step
STEP 1
Decide how to image the event. A wide field can show the whole Moon but Uranus, at 3.7 arcseconds across, will look like a star. Its disc will appear 1/500th the apparent size of the Moon’s 1,834-arcsecond disc. A close-up shot that allows you to capture Uranus as a disc will only fit a small portion of the Moon.
STEP 2
Consider capturing the Moon correctly exposed in one shot. Then, increase exposure so that Uranus shows, preferably with a well-defined edge of the Moon. Note the camera settings. Take shots at regular intervals to show the relative position of Uranus next to the Moon both before and after the occultation.
STEP 3
Load the over-exposed Moon shots into layers in a layer-based photo editor. Align them on the Moon’s edge. Set all but the bottom layer’s blend modes to lighten so Uranus shines through. Flatten, then add the correctly exposed Moon as a new layer, aligned on the bottom layer. Delete the sky around the correctly exposed Moon.
STEP 4
If you can make high-frame-rate captures at a large image scale, consider a multi-frame capture of Uranus before the occultation. With a mono camera, an IR-pass or long-pass filter (eg 610nm) may give a steadier result. Once the planet has been captured, consider increasing the exposure to try for Uranus’s moons.
STEP 5
For large image scale setups, consider doing an extended lunar mosaic of the limb region where Uranus will disappear. If time is short before the occultation, this can be done while Uranus is hidden or after. If it’s done before, simply follow the mosaic capture with correctly exposed shots of Uranus at regular intervals to record its position, as described in step 2.
STEP 6
For the reappearance, reset the camera to the correct settings for Uranus (see Step 2). If you have an equatorial tracking mount, stay centred on Uranus from disappearance until reappearance. When done, compose the positional shots (see Step 3), adding enhanced images of the Moon and Uranus as you have them.
Pete Lawrence is an expert astro-imager and a presenter on The Sky at Night