For the last year or so, the Covid-19 pandemic has stopped me from being able to travel to spectacular locations to pursue my passion for astrophotography. However, the time at home has allowed me to continue my practice of deep-sky astrophotography. The great thing about shooting deep-sky images is that you can shoot them anywhere in the world; with the correct kit, like the Sony Alpha 7R III, light pollution isn't an issue.
Astrophotography vs Deep Sky photography
I think of astrophotography images as being those that combine the landscape with the elements of the sky we are all familiar with - the moon, the stars and, of course, The Milky Way. Whereas when you shoot deep-sky photographs, you photograph a subject in the night sky using a telephoto lens, such as the Sony FE 200-600mm f/5.6-6.3 G OSS lens and usually a 2x teleconverter. This combination gives you a super-telephoto lens with up to a 1200mm focal length. Apps, such as Stellarium, which I use, can then be used to help you locate your chosen subject in the night sky.
Because of how light is emitted from these distant bodies in space, it is possible to shoot them in your garden. You don't need to travel to a dark remote location to take spectacular images. The moon is the perfect subject to start with, as you can photograph it with very accessible cameras and lenses.
Digital cameras have a filter in front of the sensor that blocks specific frequencies of light that our eyes can't see, such as infrared and Ultraviolet light. I have a modified Sony Alpha 7R III that has had this filter professionally removed so it can capture those light frequencies emitted from deep space that our eyes can't see.*
But to only see the light from deep space, I need to block out almost all visible light and only let specific wavelengths through, so I use another specialist Astro filter which I temporarily place in front of my sensor when shooting.
As the Earth rotates, the stars in the sky appear to move. I mount my camera and lens to a sky tracker to counter this. This device slowly moves the camera relative to the Earth's rotation to avoid blurred images or star trails, even when exposing shots for minutes at a time.
Exposing the Image
With very little light coming into my Sony Alpha 7R III camera, I set my lens to the widest possible aperture and the camera sensitivity to the highest ISO with a low amount of noise, which in the case of my Sony Alpha 7R III is ISO 800. Then I expose the image for as long as possible, but depending on the situation, it could be 1 minute or 10 minutes.
The trick to deep-sky photography is that you don't take a single image. You take as many photos as you can; hundreds, even thousands, of images. I can sit in my garden all night shooting the same subject over and over. Then the final image exposure is then created by stacking the images together using software such as Astro Pixel Processor or Deep Sky Stacker.
The software looks for details in the image that are consistent across the thousands of photos. It filters out those details that aren't consistent, such as noise or atmospheric conditions, and allows for the creation of incredibly sharp, detailed images, all whilst shooting in your back garden.
If you are just starting, you can concentrate on doing something basic, such as photographing the moon. You don't need a modified camera to do this; you can do it with a camera like the Sony Alpha 7C or Alpha 7 III and a telephoto lens such as the affordable Sony FE 200-600mm f/5.6-6.3 G OSS with a FE 2x teleconverter.
The exposure depends on what you see on your screen; you want to make the image as bright as possible, using the camera's histogram to ensure you don't overexpose the highlights. Then shoot a series of images to create a stack.
It is best to create a stack for the moon because there is a lot of atmospheric turbulence in our atmosphere. If you take a single image of the moon, you may only have one area that is sharp. By taking multiple shots and stacking them, you can create a final image where the whole surface of the moon is sharp and detailed.
And when shooting the moon, you don't need a star tracker. Just make sure that the moon is in the centre of the shot each time, and the stacking software will take care of the alignment of each image.
* Please note that camera modifications cannot be provided by Sony or its authorised service centres. Hardware modifications are not advised by Sony and are entirely at the owner’s own risk.
"It's a very special kind of photography: Not visible to the human eye, but real"