Up for Discussion – Astrophotography
For today’s Up for Discussion I thought I would ask John Holding to write a post for you on Astrophotography. I know many people are interested in this, me included, and I have been going out with John to do some of this photography and he has been teaching me a lot, so it seemed only natural to ask him to write a post for us.
Astrophotography by John Holding
Ever since my youth I have been fascinated by the stars. Many nights a friend and I looked through his telescope at various stars. A task somewhat easy as I lived in country town in South Australia. There were also many nights spent camping where one could lie out under the stars and just admire their glory.
Since taking up photography and having much more time available it has become one of my goals to take stunning Astro photographs. At first take this is inherently simple, just point the camera at the night sky and bump the ISO to greater than 3200 and exposure time to about 30 seconds and f stop as small a number was your lens will allow. Now assuming that one is in a fairly dark area one can get an OK shot. Much like this one.
This is a 30 second exposure at ISO >6400 on a F/4 fisheye lens.
However to get really stunning shots one has to go for longer exposures or start stacking the images, a somewhat more complex task. However there are some very good choices available in software specifically designed for such a job, Nebulosity being one but there are others. Stacking can also be done in Photoshop using an averaging technique. More on that later.
Like many areas of photography getting an image is a compromise. In this case much more so. For images that have minimal star trails the exposure time must be less than 30 seconds and if you are using a lens greater than 16mm (35mm equivalent) then the duration may have to be less. Additionally one will get less of the sky in the image. Which sometime makes the image less dramatic. Typical settings that will work are: set ISO to 3200 or greater, exposure time to 30 seconds and the aperture as wide as the lens will go. Oh of course you must have a tripod.
Out of this assuming that the sky was dark enough (no glare from city lights) one can get a reasonable image. From here one has to get much more determined and firstly start paying attention to the night sky rotation which changes what part of the Milky way is visible from your respective hemisphere. These days there are iPhone and android apps that do the job nicely. Once you know where the heart of the milky way is and what time it will appear over ones position one can set about planning the shoot. Additionally the moon should not be present or if it is then less than quarter moon.
Often the best astro photographs have an earthly reference, that is a close foreground object that anchors the viewers sense of scale and grandeur as well as adding interest to the shot.
Once you have your image/s then it is time to take an exposure of the landscape feature that you want to include. This is especially important if you are going to stack the images as the stars will move in every image and any landscape foreground when you align the stars will become blurred.
Next step is adjust each exposure in Adobe Camera Raw (assumes Photoshop Bridge is available ). Make the following adjustments: Increase contrast, decrease blacks, increase clarity, increase shadows, if needed adjust white balance and exposure until your image is where you like it. Below are examples of a before and after adjustment. You can still do this even if you have captured the image as a jpeg instead of RAW. Both images are 180 seconds at ISO 3200. (How I achieved 180 seconds is described below). Much less noise than the image above. If you look closely you can see the rotation effect in the lights in the lower left. PS that is Leanne’s camera in the image.
For stacking in photoshop each layer will need to be carefully aligned, pick a star or two near the middle of your image and with opacity set so you can see both layers move the top one until the stars align. Repeat for all layers. Then blend each layer to roughly the following formula: 1st layer 60%, second 33%, third layers 25% decreasing each layer opacity roughly in these proportions. Finally bring the first layer ground feature out by your favourite means (don’t ask me as I actually haven’t done much of that sort of work in photoshop). Finally you will get noise in the image but less than unstacked images.
For exposures greater than 30 seconds one has to get hold of an equatorial mount used for telescopes and mount you camera on that. Once properly aligned (alignment means the pivot point of the camera is parallel with the earths axis and points due south, two levels of alignment) you can drop your ISO and hence reduce noise and increase the exposure time into the minutes. I use a device called Astrotrac, which once aligned, and that is no different to an equatorial mount, allows image duration over several minutes. This means much less noise and no need to stack images. The downside is that the landscape will be blurred as the camera rotates at the same rate as the earth.
Good luck with your attempt at Astrophotography.
A few more images taken at various times and locations.
I would like to take this opportunity to thank John for writing this post, so thank you John, and I hope you will all do the same. I will put the above images, plus a couple more of his images into a gallery for you.