Imaging dark nebulae – the basics

A step by step guide to basic dark nebulae photography with a prime lens and DSLR or mirrorless camera

Fast, prime (ie not zoom), camera lenses used with a DSLR or mirrorless camera on a tracking mount offer a basic but highly effective way of shooting wide-field images of dark nebulae. Some lens models produce particularly sharp images and this, coupled with often tremendous light-gathering power at full aperture, means detailed pictures can be achieved in a relatively short amount of time. Here we look at the main steps you’ll need to get the shot.

Initial set up and aperture testing

Once your tracking mount and camera are set up, take a little time to set the aperture of your lens. By stopping down the lens slightly you may be able to achieve tighter star shapes and lessen vignetting. We’ll capture a flat field later to tackle this, but anything you can do at this stage to reduce such aberrations and even out the illumination will help when you eventually try to pull out the faint detail of the dark nebula later in processing. At this stage, note the configuration of the lens so it can be replicated exactly when we make the flat field.

Gather the data

Because dark nebulae are so faint we need to push the exposure lengths. How long you go is a balance between fogging the frame with light pollution and how much noise (unwanted artefacts) you’re happy with on one hand and the detail you’re picking up on the other. Experiment with ISO and exposure lengths. With that sorted, take as many exposures as you can. Get rid of those showing haze or cloud, even if it’s subtle. It can be easier to sift through these by flicking through the images on the rear camera screen, as gradients can show up more clearly there than on a large computer monitor.

Flat fielding and stacking

Next we need to create a flat field image. When using a camera lens you can capture very basic flat fields simply by replicating the original shooting configuration (focus point, lens aperture) and then pointing the camera at a bare patch of a smooth, white internal wall that’s evenly lit. Set your DSLR to auto-expose the image and take around 10 to 15 images. These images can then be used in stacking software to create a ‘master’ flat field for the main image data. Finally, stack the exposures you have and calibrate them with this master flat field.

Stretching the data

We now need to ‘stretch’ the stacked image to pull out faint features. One simple way to do this is to use the ‘curves’ tool found in many image editors. Bend the diagonal line of the curves tool so it takes on an ‘s’ shape; pull up the right side of the line to brighten the image and if necessary pull down the lower left part of the line, slightly, to increase contrast. Apply the adjustment and repeat to taste. You should gradually see faint details appearing. From here you can make the usual image-editor tweaks such as colour balancing and noise reduction.