Geeky Stuff
Astrophotography can be really challenging, and it’s often hard to realise just how much work goes into creating a final astro image. There are so many things to consider — from choosing the right equipment and overcoming technical issues, to dealing with weather and light conditions and processing the individual exposures, which is almost a completely separate field to master. But despite all of this, astrophotography is incredibly rewarding.
I don’t want to go too far into the technical details, but let’s start with choosing the right equipment.
When imaging deep-sky objects, it’s often not necessary to have a telescope with a long focal length. Many nebulae are enormous, so a wide-field telescope can be a great choice for capturing them. For smaller planetary nebulae or galaxies, a telescope with more reach is preferable. Aperture is also important, particularly for planetary astrophotography, where a larger aperture can provide greater resolution and allow more detail to be captured. Cameras also need to be well matched to the chosen OTA (optical tube assembly).
The most important piece of equipment, however, is a decent telescope mount. Even the best telescope won’t perform well without a stable mount capable of carrying the equipment without wobbling. An EQ (equatorial) mount needs to be precisely aligned with the celestial pole and tracks the sky as the Earth rotates, allowing the telescope to stay pointed at the target without it drifting out of the FOV (field of view).
It’s essential to keep everything as stable as possible during an exposure, as even tiny movements can result in streaky stars and blurred images. Using a guide scope and guide camera helps compensate for small tracking errors, but there are limits to what guiding can correct — especially when wind starts shaking the whole setup!
This brings me to weather and light conditions. Here in the UK, astrophotography can be challenging enough with our famously cloudy skies. Living on Scotland’s West Coast, I also often have to battle strong winds, which can make imaging nearly impossible. And when the forecast finally looks perfect — clear skies and no wind — there’s a good chance you’ll discover that it’s a full moon, lighting up the entire sky and making deep-sky imaging challenging all over again!
The darker the sky, the better the contrast between faint celestial objects and the background sky. Dark skies therefore allow much fainter details to be captured and can significantly reduce the amount of imaging time needed.
Light pollution is another major issue. People often don’t realise how much artificial light affects the night-time environment. If only essential lights were switched on at night, or better still, if more appropriately designed and directed lighting were used, not only would astrophotographers benefit, but nocturnal wildlife would too. People living in cities might even be able to see the Milky Way from their own back gardens — and that would be quite a stunning sight!
Once the conditions are finally playing ball, all technical issues are resolved and my dog hasn’t decided to run into the tripod, it’s finally time to collect some decent sub-exposures. 😄
I always use my monochrome camera for deep-sky imaging, with total integration ranging from around 10 to nearly 100 hours depending on the target, although 30-40 hours is fairly typical for my more recent images. My favourite targets are narrowband objects, particularly emission nebulae. I use specialist filters for Ha (Hydrogen-alpha), OIII (Oxygen III) and SII (Sulphur II). These filters only allow very narrow wavelengths of light through, making it possible to capture faint emission from specific elements while reducing the impact of moonlight and light pollution.
I’m currently using 5- or 10-minute exposures for these narrowband filters. Another option is LRGB — Luminance for detail, combined with Red, Green and Blue for colour. These filters are generally used for broadband targets such as galaxies, reflection nebulae and dark nebulae. I often use 40- or 60-second exposures for Luminance and 100- or 120-second exposures for Red, Green and Blue.
Hydrogen-alpha can also be added to some broadband targets, such as galaxies, to highlight the emission nebulae within them.
After collecting all this data — usually over several weeks, months or even years — the individual sub-frames first need to be calibrated and pre-processed before they can be stacked into their respective channels: Ha, OIII, SII or L, R, G and B.
Only the best sub-frames make it into the final stack. Good focus, sharp stars, useful detail and contrast are all important, while frames affected by clouds, poor guiding or aircraft trails may have to be discarded. The selected sub-frames are then combined with calibration frames such as darks, flats and dark flats to produce clean master images for each channel.
From there, the real processing begins.
Each individual filter initially produces a monochrome image. The colour only appears when the different channels are eventually combined.
For narrowband images, a Hubble-style colour palette is often used, mapping Ha to green, OIII to blue and SII to red. Ha and SII actually lie in the red part of the spectrum, while OIII is predominantly in the blue-green region, so assigning these colours is where a little artistic licence comes in — and where the fun really starts!
LRGB images are much closer to the colours we would expect to see if our eyes were sensitive enough to detect these faint objects. Colour calibration can bring the result even closer to their natural appearance, although I sometimes use a little artistic licence here too. I always try to keep my images looking natural rather than pushing the colours too far.
Once the individual channels have been combined, I see a colour image for the first time. It’s always a mesmerising moment!
There is still plenty of work to do afterwards. Colours need adjusting, especially as Ha can be very dominant and easily produce an overly green image. Details and contrast are carefully enhanced using the information captured in the original dataset. I always try to keep things natural and avoid going overboard with sharpening or other processing tools. For me, subtle adjustments usually produce a much more pleasing result.
Click here for more detailed information about image processing
There are also some targets I would love to photograph but unfortunately they sit very low in the sky from my garden, with my house blocking the view — yet another astrophotography challenge! Houses, trees and bushes can all get in the way.
I’m hoping to get to a nice dark location here in Scotland one day, with clear skies, no wind and a new moon, so I can finally capture some of these elusive targets.
Take a look at Rho Ophiuchi — it’s absolutely stunning and very high on my imaging list!
So please keep checking back for new astrophotographs. 😊