Kit lenses, commonly bundled with DSLR cameras, offer a natural entry point into astrophotography due to availability and their wide field of view, allowing reasonable exposure times from a fixed tripod. The major difficulties for first time users are finding the correct camera settings and focusing on infinity. This guide offers a few tips based on my own experiences and blunders.
This was one of my first attempts at shooting the Milky Way, taken on holiday in Menorca. I didn’t have a tripod with me so I simply laid the camera down on the patio, pointing straight up. It’s a 30 second exposure at 18mm focal length, giving the widest field of view. To capture as much light as possible the camera iris was opened fully, giving a focal ratio of f3.5.
Camera and Lens Settings
It’s worth experimenting with these settings before heading out, to help avoid the frustration of fighting the camera in the dark.
When shooting from a fixed tripod it’s important to gather as much light as possible. Typically this means shooting with the lens wide open by selecting the lowest aperture setting available.
When taking the image above I forgot to open up the lens aperture, so it was shot at f5.6. As a result less light is reaching the sensor, giving a dim and grainy image dominated by electronic noise. Despite this I managed to accidentally capture the Andromeda galaxy at lower right. The aperture setting is something of a compromise, shooting wide open will result in distorted stars in the corners (coma) but this is often preferable to a noisy image. (If shooting with a faster lens or a particularly low-noise camera it may be worth stopping down slightly to improve the star shapes.)
At 18mm focal length, giving the widest field of view, exposure lengths of roughly 20-30 seconds are possible before star trailing becomes apparent. The maximum exposure time varies slightly depending on where the camera is pointed. Close to the north and south celestial poles the apparent motion of the stars is slower, so longer exposures can be used. I usually take 30 second shots regardless when imaging the Milky Way, trading a little bit of star trailing for a brighter image.
Users of entry and mid-level cameras will probably get the best results between ISO 800 and 3200. ISO 3200 will show up the Milky Way much more clearly but at the expense of a noisier image; however, this can be greatly reduced in post-processing.
Selecting 2 second timer mode helps prevent any vibration when releasing the shutter, to avoid producing streaky stars in the final image.
Lens Switches and Zoom
Manual focus must be selected on the lens. Also, some lens models have an image stabilisation/vibration reduction switch which may need to be disabled, depending on the model – some are not suitable for shooting from a tripod with this turned on. The zoom barrel should be set to the widest setting, typically 18mm focal length.
The next step is to take the camera outside, preferably somewhere dark. Finding a sharp focus with a kit lens can be challenging as they are quite slow lenses. Optical design is a compromise and the handy zoom ability results in lower light transmittance compared to a fixed focal length prime lens at the same focal ratio. Even at f3.5 only the brightest stars will be visible through the viewfinder or on liveview.
The first step is to find a rough infinity focus. Auto-focus lenses need to be able to focus past infinity so this won’t be quite at the limit of travel of the focus ring. It’s worth checking in daylight which way the focus ring needs to be turned to reach infinity. For a rough focus turn it to the stop and then back a very small amount.
Fine focusing is best achieved using the liveview feature if the camera supports it; I boost the ISO level to 3200 or 6400 after turning the display on to increase the number of visible stars. The next step is to find an object bright enough to focus on, which may not be in the same area of sky as that you wish to image. This is where some familiarity with the sky helps. A software planetarium such as Stellarium (a free download) will allow you to check what is visible from your location at any given time. Here are some suggested targets in order of brightness:
• The Moon. If the Moon is up you can even use auto-focus then click the lens back into manual mode, however this may not give the best possible focus across the whole frame. Also, if the Moon is too bright the sky will be washed out and fewer stars will be visible.
• The planets. Venus, Jupiter, Saturn and usually Mars are brighter than any stars and easier to focus on.
• Failing that, a bright star. Here’s a list of the brightest visible stars, if you aren’t sure where they are located in the sky you can check using Stellarium.
If you have a clear view of the horizon, a distant streetlight may be easier to focus on than a star.
When focusing on liveview it’s better to place the object a third of the way from the edge of frame rather than in the centre, this gives a better focus across the whole frame. This trick appears to work with all lenses and telescopes. Make small adjustments back and forth, the goal is to make the star as small and round as possible. Another approach is the ‘disappearing star’ trick. Find a star that is barely visible, as the focus ring is tweaked it will pop in and out of view.
Depending on the model the focus ring on kit lenses can slip slightly out of position while shooting, a piece of micropore tape or a blob of astronomical blu-tack can be used to fix it in place.
Framing the Shot
Once the lens is focused you’re ready to go. The Milky Way is the most obvious target but familiar constellations or asterisms also make pleasing images.
Shooting from a dark site will always help but moderate levels of light pollution can be incorporated into a composition. The glow of street lighting in this image taken from Hertfordshire masquerades as a sunset.
Some simple tweaks in an image processing program to brightness, contrast, colour balance and saturation can greatly enhance an image. For high ISO shots applying a de-noise filter can clean up a background considerably.
I usually present or print my kit-lens images at a fairly small size to hide any defects in the image.
With a little imagination the kit lens has plenty more to give. For example, multiple shots can be stitched together to make a panorama or mosaic; for example Microsoft ICE is a free download. It is also suitable for making star-trail images, something I haven’t yet experimented with.
A faster lens, such as the Samyang 14mm f2.8, would yield better results due to its superior light-gathering ability. Compare this shot of the Hurlers with the one at the top of the article:
Full-frame cameras produce brighter images with less noise due to their large sensors. They can be used at high ISO levels, making them the best equipment for fixed-tripod shooting. However, this performance comes at a price. An entry-level DSLR on a basic equatorial tracking mount is a more cost effective solution.
The image above has a total exposure time of 20 minutes, using a kit lens on a modded Canon 1100D (EOS Rebel T3 in North America) on an EQ3 mount. The mount can also be used with much longer lenses, bringing smaller deep sky objects into view.
Here's a quick animation from the night of 24th/25th March showing the movement of Comet 41P against the background stars.
Each frame is a 2 minute exposure taken with a 135mm lens, the time-lapse is two hours long. On the 24th March it was visible in the bowl of The Big Dipper, or business end of The Plough if you prefer. The bright stars of this famous asterism are just out of frame. Over the next few days it moved into the neighbouring constellation of Draco.
Comet 41P goes by the name of Tuttle–Giacobini–Kresák which is a bit of a mouthful. This is because comets are typically named after their discoverers, 41P was independently found in 1858, 1907 and 1951. It is a short period comet which approaches the Sun every 50 years or so and is thought to have a nucleus about a mile across. The comet has a circular appearance rather than showing the classic cometary tail because of the viewing angle.
Comet Lovejoy (C/2014 Q4, the amateur Terry Lovejoy has discovered several) did show a prominent tail. It was slightly brighter than 41P, the head being just visible to the naked eye as a Moon-sized disc. The image above was also taken with a 135mm lens and the same camera, so the scale is the same. The green colour is typical of dim comets and is caused by glowing carbon and cyanogens. Brighter comets release more water vapour into space, their large tails are white as they reflect sunlight.
Small or dim comets like these, visible in binoculars or a small telescope from a dark site, are frequent visitors to the inner solar system. Heavens Above maintains an active list. Great comets - visible in daylight - come calling far less often, perhaps once a decade or so.
The comet timelapse at the top of the page isn't my first. Back in August 2014 I accidently captured C/2014 E2 Jacques passing the Garnet Star and the nebula IC1396, home of The Elephant's Trunk. But I've only dabbled with cometary imaging, for some truly spectacular images I'd recommend taking a look at Damien Peach's website.
Practical astronomy with an almost-seven year old?
Spent some time with nephew-nearly-seven over Christmas and - despite the lure of lego ninjas and talking dinosaurs - he was quite happy to join me outside on a clear and frosty night. I started off by showing him a few landmarks of the sky - the Milky Way, most prominent stars and recognisable constellations - using a laser pointer to trace out their shapes. (After first explaining the importance of checking for aircraft, to avoid dazzling them. He certainly made a conscientious air traffic controller, for the rest of the evening I couldn't go near the the laser without a warning, including for planes fifty miles away and in the other direction.) Resting on the Pole star I tried to explain how the stars appear to rotate around it, as shown in the time-lapse video below.
Video by James Castelli, Cherry Springs, Pennsylvania - Aug 2008
Next we had a go at astrophotography, I set up a motorised tracking mount and attached a camera with a wide lens. I pointed the camera in roughly the right direction and disengaged the clutches so he could gently nudge it on to target. After a couple of attempts he got this shot of the Milky Way with a 2 minute exposure.
When thinking of deep sky objects (DSOs) - distant nebulae and star clusters within the Milky Way and even more distant galaxies - it's natural to assume that a large telescope is required to view them. In many cases this is true, where the vast sizes of these objects is trumped by the even greater distances to them. However, there are a number of DSOs - relative neighbours in cosmic terms - that occupy surprisingly large slices of our sky. The gallery below shows a number of fairly bright DSOs along with the Moon, to show their relative apparent sizes.
The montage shows the progression of the September 2015 lunar eclipse over a period of about two hours, as Earth's curved shadow passes across it. Unusually, this eclipse coincided with a so-called supermoon, where the full moon is at the closest point in its orbit and appears slightly larger to the eye - such an event will not occur again until 2033. At totality the Moon is still visible as some sunlight still reaches it by passing through the Earth's atmosphere. However, it appears much dimmer and redder than normal - at a second the final exposure in the sequence is 1,000 times longer than the first. The deep red colour is caused by the same phenomena that makes our sky blue - our atmosphere scatters blue light more readily so that more red light reaches the Moon's surface, and is reflected back to Earth.
Taken by Ken Bennett, with the aid of a 5" refracting telescope, DSLR camera and some choice Cornish cursing-words.