Capturing deep-sky objects

While it’s possible to image the deep sky with a DSLR on a tripod, for the best results you’ll need a tracking mount to keep track of your target as it moves with the rotation of Earth. Without one, exposure times will be limited and the process of aligning and stacking multiple image files is complicated. Here we’ll look at considerations for DSLRs and deep-sky objects:

Shooting in darkness

Imaging deep-sky objects requires complete darkness. It’s worth getting to know your DSLR in daylight so that you can locate buttons by feel. On a Canon, well-used buttons include ‘Playback’, the ‘Q’ button, and ‘Live View’ and ‘Zoom’ functions (see our annotated image of settings buttons found on a Canon DSLR, right). The location of these buttons varies between models and knowing them will help you to confidently set up your DSLR after sunset. In our example, the ‘Live View’ (3) and ‘Zoom’ (4) buttons help us focus, the ‘Q’ (2) button lets us switch between settings on the LCD display screen and ‘Playback’ (1) shows our test images.

One challenge is focusing your DSLR for sharp stars, and this is where ‘Live View’ and ‘Zoom’ buttons are required. Before finding your desired target, use ‘Live

View’ to find a bright star; Vega, (Alpha (a)

Lyrae), Sirius (Alpha

(a) Canis Majoris) and Deneb (Alpha

(a) Cygni) are popular choices. The DSLR’s

‘Zoom’ buttons allow you to enlarge the star on your LCD screen and ensure your focus is precise.

The ‘500 Rule’

Imaging deep-sky objects requires far longer exposure times than the Moon, and if you are using a regular tripod the ability to capture the Milky Way or nebulae is limited by star trailing. To keep stars pin-sharp at longer exposures, a tracking mount is needed (see ‘Next steps: tracking and stacking’ box, opposite; and our feature on star tracker mounts in the April 2021 issue).

The ‘500 Rule’ gives a rough maximum exposure time depending on the focal length of the lens you’re using. The wider your field of view, the longer the exposure time you can apply before the stars begin to trail. If you are using a full-frame camera you divide

500 by the focal length of your camera lens. For crop sensors you apply the same rule, but multiply the focal length by 1.5 or 1.6 for Nikon and Canon DSLRs respectively. If we are using a 14mm lens to shoot a wide-field image with a full-frame DSLR, we’re limited to a maximum of 36 seconds per frame, but at 200mm this exposure time reduces to three seconds.

Signal noise

Managing ISO and exposure times is important for deep-sky objects, and both need to be increased for nebulae. The ‘500 Rule’ gives the longest exposure time on a static tripod and we can increase the ISO to pick up more detail from the exposure. The best ISO performance will vary from model to model, and it’s important to note that too high an exposure time will make the image noisy with a grainy appearance.

Find your camera’s ‘sweet spot’ where you can push the ISO to boost brightness without getting obvious electronic noise. When taking your test images there are times when setting a high ISO has its benefits: we often use ISO 6400 to find and position an object before switching to ISO 800 or 1600 for an imaging run. The process of locating a faint deep-sky object with a DSLR is tricky without a Go-To mount; this will not only do the job for but it will transform your results.

Capturing the Milky Way: great wide-field images can be obtained with a fast lens and tripod – this example was taken by a DSLR with a 24mm lens on a tripod – but for deeper shots you’ll need a star tracking mount ▲ Push the right buttons: get familiar with your DSLR camera’s settings buttons. They are similar on most models, but may vary in position. In this example, the settings are: 1) ‘Playback’ button; 2) ‘Q’ button; 3) ‘Live View’ button; 4) ‘Zoom’ function

ISO 12800 ISO 1600 Avoid noise (unwanted artefacts) in images by keeping ISO settings low

Charlotte Daniels is an amateur astronomer, astrophotographer and journalist