Starting out: imaging the Moon
The Moon is a great object to test your DSLR and get to know its settings. It’s easy to find and focus on, and the DSLR can capture a lot of detail just from a single shot. For most Canon models, the ‘Q’ button allows you to move around the main capture settings (see our DSLR settings image on page 64). These are some of the settings to consider for lunar astrophotography:
ISO
This setting changes your DSLR’s sensitivity to light and is one of the most important astrophotography settings to get used to. ISO ranges vary between models; some go from 100 to 12600 and beyond.
The lower the ISO setting, the less sensitive your camera will be to light and vice versa. However, this sensitivity is also dependant on the exposure setting: a camera on a low ISO setting but long exposure will pick up more detail than one on the same ISO setting, but with a shorter exposure time. For the Moon, which is bright, you don’t want a high ISO as it will wash out the image and mean you lose detail. For Moon images, select the lowest level, ISO 100, from the ISO menu. We don’t tend to use ‘Auto’ functions in astrophotography, instead opting to manually choose the best setting.
Aperture/f-stop number
The f-stop number that your DSLR is set at is another important factor. It affects the depth of field and how ‘wide open’ your camera aperture is. A low f-stop number means the aperture is wider (and is often referred to as a ‘fast’ setting): the DSLR will receive
more light, but the depth of field in the image is shallow. This isn’t important for lunar imaging, but it becomes apparent in wide-field imaging since a shallow depth of field will affect the focus of the foreground.
We don’t want a low f-stop number to image the Moon because not only will it allow light to flood the camera sensor, but it will also limit lens performance. Every lens is different, however it’s best to start at f/7 and work your way upwards to see what produces the best results, stopping when the image looks sharp. Both pictures of the Moon above are captured at f/7.1 aperture on a lens of 400mm, f/5.7 focal length.
Exposure
Understanding the effect of exposure time is crucial for astro imaging: depending on the brightness of the object imaged, a longer exposure usually means more detail. Deep-sky exposures can be up to 15 minutes per frame, though this depends on the camera and its sensor, which can heat up creating image noise.
Because the Moon is so bright, exposures that long would completely wash out the image, so this type of photography needs shorter frames. You want to set a length that captures the detail in darker regions without overexposing lighter areas. For our lunar image, we started at 1/200 seconds and found the image too bright. We then tried 1/250 and 1/400 and captured the two images above. 1/400 seconds was a little too fast: the Moon looks dull and less detailed, even after processing. Of all the images taken, a combination of 1/250 seconds exposure, with an ISO of 100 and and aperture of f/7.1 proved to be best for the DSLR we used, a Canon EOS 700D.
White balance
White balance (WB) is an important factor for daytime photography, but less so for astro imaging because we are shooting in RAW image format. We do this to give the most control in post-processing, so the white balance of our image can be adjusted later using photo editing software. For this reason, white balance can be left in ‘Auto’ mode, or ‘AWB’. We can recommend Canon’s ‘Tungsten’ WB setting, however, because it can benefit wide-field images by removing the reddish glow of light pollution.
A custom white balance may be needed for astro-modified cameras. This is because once the infrared (IR) filter is removed all images have a red hue. This can still be managed in processing, but it’s easier to set up a custom white balance that you can apply from the camera if imaging without filters.
Noise reduction
A common beginner’s mistake is to have the noise reduction setting switched on for astrophotography, in an effort to remove the noise (unwanted artefacts) from frames. But this setting is counterproductive when imaging at night; if it’s turned on, it effectively takes two photos for every exposure: a normal ‘light’ frame (which is your regular image), and a ‘dark frame’ designed to remove noise. The camera then subtracts this dark frame from the light frame, meaning each exposure takes twice as long. It’s best to ensure noise reduction is switched off; on Canon DSLRs, this can be done from the main settings menu.
The best way to remove noise is to take ‘calibration frames’. Dark frames are one type of calibration frame. Creating and applying calibration frames allows you to remove most forms of noise in post-processing (see ‘Next steps: tracking and stacking’ box, on page 65). ▶
The effect of exposure time is crucial for astrophotography: depending on the brightness of the object imaged, a longer exposure means more detail