Take the perfect astrophoto with our step-by-step guide
ASTROPHOTOGRAPHY
CAPTURE
Bright Moon? Not a problem
The Moon may wash out this year’s Perseid meteor shower, but there’s always hope
Visually, this year’s Perseid meteor shower will be something of a washout due to the presence of a bright Moon around the period of peak activity. Photographically, however, there is hope.
A camera set up to do a multi-second exposure of the night sky will typically return a bright, overexposed frame if the Moon is nearby. However, with careful tuning of the camera’s settings, it is possible to reduce the intensity of the recorded background sky so that it doesn’t overexpose. For example, reducing the camera’s sensitivity (low ISO, small aperture) will deliver a multi-second exposure without overexposure. Under normal nighttime conditions, such settings probably wouldn’t record many meteors.
To work here, the camera needs to be set to the settings you would normally use for meteor imaging – high ISO and wide aperture – but the exposure time needs to be reduced to prevent sky overexposure. In this way, if a meteor trail passes through the camera frame, it should record just as it would if the sky were darker and the exposure longer. There are caveats hough. Shorter exposures mean more shots will be taken. The ideal filetype here is your camera’s RAW image format, and such files tend to be on the large side. Lots of large image files means you need plenty of storage available, but for modern cameras this shouldn’t be too much of an issue. More frames also require more time to check afterwards.
Mind the gap
The use of RAW means that at the end of each shot a fair amount of data needs to be transferred to your camera’s storage card. This can create a ‘gap’ in your camera’s imaging capability, a period when no image is being taken. Under normal ‘dark sky’ meteor exposure settings, this gap will be far shorter than the exposure time. However, when the Moon is about and exposure times need to be radically shortened, the gap becomes significant.
For example, a one-second gap vs a 29-second exposure means that only two seconds of imaging time is lost per minute (3.3%). However, if the exposure time is reduced to one second, this equals the gap time. Consequently, 30 seconds are lost per minute of imaging time (50%). A shorter exposure also raises the probability of truncating a meteor trail mid-flight, something that increases for brighter and longer meteor trails.
Although not ideal, it is still possible to set up a camera to record this year’s Perseid shower and, given clear weather, there’s every chance that you will be able to record some trails. And as it’s likely there will be fewer people out having a go on the night of the 12th/13th, if you do capture a bright trail you may well be the only one to do so. Follow our How To guide opposite and see what you can catch.
Equipment: DSLR camera, MILC camera, tripod or tracking mount, remote shutter release
Send your images to: gallery@skyatnightmagazine.com
Step by step
STEP 1
Choose a lens that will give you a good field of view but avoid going too wide as the trails will appear small and unimpressive. Something around the 14–18mm mark would be a good compromise. Have a set of charged batteries ready as well as plenty of storage cards and a lockable remote shutter cable.
STEP 2
The camera will need to be mounted on a stable platform. A tripod will keep the camera still as the sky moves through the field of view. A tracking mount will keep the camera pointing at the same area of sky. If using a tracking mount, make sure the camera doesn’t end up pointing at a foreground object.
STEP 3
Set camera and lens to manual and check the camera clock.
Pre-focus at infinity using a bright target such as Jupiter. Older cameras should be set to an ISO value of 3200 or 6400, while more modern bodies can go further, eg 5000-10,000. Fully open the aperture, reducing by a stop or two if edge stars distort.
STEP 4
Aim at an area of sky that won’t bring the Moon into frame. Set a test exposure of 5s and take a shot. Examine the sky. If it’s overexposed, reduce the exposure. If it appears dark, consider increasing exposure. Reduce ISO or aperture only if you can’t get a non-overexposed sky with less than a 1s exposure.
STEP 5
Set the camera to continuous shutter mode and lock the button down on a connected remote shutter cable. The camera should continuously take shots at the pre-set exposure. Routinely check the lens for moisture, using a 12V hairdryer to remove any. A 12V heater band is a recommended alternative if you have one.
STEP 6
Capture for as long as you can. Download the results. Using a viewing app, examine each image in turn. Renaming any suspected trail images with a prefix (eg ‘meteor_’) makes them easier to find later on. Perseid trails should align with the radiant and often show green-pink coloration.
Pete Lawrence is an expert astro-imager and a presenter on The Sky at Night