Basic wide field imaging with a Canon DSLR
This is a basic guide to setting up and using a Canon EOS DSLR to take wide field images of the stars using camera lenses, for example of constellations, the Milky Way or nightscapes (starry skies with a landscape feature in the foreground).
The steps below are written for the Canon EOS 450d but are also relevant for the 1000d and 1100d. The same settings or similar should also work for the 500d, 550d etc. The 450d was the first in Canon’s consumer DSLR range to get Live View, which is a great aid to manual focusing in the dark. Older cameras like the 350d and 400d didn’t have Live View – you can still use them for astro-imaging, but getting perfect focus with them is more a matter of trial and error.
We’ll assume that you will just be putting the camera on a photographic tripod and will not be using a drive to track the movement of the stars, so the length of exposure will be limited to the exposure length at which trailing of the star images becomes noticeable.
The length of time you can expose for without seeing stars trailing in your image is determined mainly by the focal length of your lens. With a fairly long telephoto like a 200mm, exposures will be limited to only 2 or 3 seconds before stars trail noticeably. For wide field images, the standard Canon 18-55mm zoom kit lens is a good lens to start with as it is reasonably sharp and can give good results. Using this lens at 18mm you should be able to get about 25 second exposures before trailing becomes noticeable. A rough rule of thumb is maximum exposure length = 500 divided by the focal length of lens (mm). This is a starting point only, because the extent of trailing also depends on how near your target area is to the celestial equator (the closer it is, the longer the trails) – take a picture, zoom in on the played back image, then reduce the exposure length if trailing of stars is too obvious.
· Canon EOS 450d or similar. Make sure your battery is fully charged and preferably have a spare!
· Photographic tripod.
· Cable release or interval timer. If you don’t have one you can set the self-timer delay on the camera to 10 seconds so the camera doesn’t move when you press the shutter button. An interval timer (also known as an intervalometer or timer remote) is indispensable if you’re taking a lot of images one after the other, either to stack them to reduce noise (see later), or to create a star trails image (yet another guide!). Canon’s own TC-80N3 timer remote costs about £110, but you can buy a perfectly acceptable Chinese copy on ebay for about £15 – you’re choice!
· Red light head torch. Invaluable for changing camera settings in the dark without ruining your night vision. If you are going off the beaten track you will need a good white light head torch too.
Camera settings before you start
It’s worth changing as many camera settings as you can to what you are going to need before you go out imaging so you’re not fiddling with buttons in the dark. It’s also worth rehearsing a few times where all the buttons are for settings you will need when you’re out imaging until you can find them by feel - otherwise that red light torch is going to come in handy! So before you go out –
· Set the mode dial to ‘M’ for manual.
· Set the ISO speed to 800. The ISO speed is a measure of the camera sensor’s sensitivity to light – higher ISOs are more sensitive but lead to an image with more noise, which gives a grainy appearance. On the 450d ISO 800 is a good starting point. However, if you are trying to image the Milky Way using short exposures you might need to try ISO 1600 or even 3200, but your image may be ‘noisy’.
· On the Menu screen, set quality to ‘RAW + L’. When you get serious about imaging you will want to shoot in RAW all the time for the best quality images, but you can’t open RAW files in all software. You can with JPEG files, so the ‘RAW + L’ setting will save both a RAW file and a highest quality JPEG file to your memory card.
· On the Menu screen set ‘auto power off’ to something like 4 minutes. You don’t want the camera switching itself off in the middle of an imaging session.
· On the Menu screen set the LCD brightness to minimum, otherwise the glare of the screen will destroy your night vision.
· On the Menu screen set ‘live view’ to ‘Enable’
Right, you can go out now!
· Put the camera on the tripod.
· Plug in the cable release or timer remote and switch the camera on. If you’re not using one you will need to set the self-timer to 10 seconds.
· Set the zoom to 18mm. You can use any zoom setting you like, but you will get longer exposures without star trailing at the 18mm end of the zoom range.
· Set the aperture to wide open – f3.5 on the 18-55mm lens.
· Now the tricky bit – focusing in the dark. If the Moon is up or there are bright lights in the distance you can try auto-focusing on them. If this works and you get a beep confirming focus, turn the focus switch on the side of the lens from auto to manual, then make sure you don’t move the focus ring for the rest of your imaging session.
· If auto-focus doesn’t work you will need to focus manually. Switch the lens to manual focus, find a bright star and put it in the centre of the viewfinder. Turn the focus ring to the infinity stop then back a bit – auto-focus lenses will focus past infinity (strange but true!). Get the star image as sharp as you can in the viewfinder, then turn on Live View. Press the zoom button twice to give 10x zoom. You might need to centre the star in the LCD screen using the cross keys. Now fine tune your focus until the star looks as sharp as possible. Turn Live View off.
· Set the shutter speed to 5 sec and take a shot. Play back the image you have taken and zoom all the way in on it. If you are in focus, the fainter stars in the image will be as small, or smaller, than the little white rectangle that indicates the portion of the image you are viewing. If you’re not sure, repeat until you are satisfied that you’ve got the best focus possible. It’s worth taking your time with this – you’ll be disappointed when you get home if all your images are out of focus.
· Frame your subject. Easier said than done when you’re looking through a dark viewfinder. Make sure the LCD screen is turned off so it’s not affecting your night vision.
· Take a 5 sec shot and check the framing. If you’re not happy with it, repeat until you are.
· At last, ready to take an image! If the sky is fairly dark, try 25 sec.
· Now play back your image and bring up the histogram. Aim to have the 'hump' of the histogram, depicting the pixels corresponding to the night sky background, at around 30 to 50%. If the hump is too far to the right you will need to either reduce your shutter speed – try 15 or 20 sec, or stop down the lens to f4 or f4.5, or reduce the ISO. It's unlikely that the hump of the histogram will be too far to the left, but if it is, you could try increasing the ISO, or increasing the exposure in post processing. If you zoom right in on the image you will probably see that the stars have trailed a bit. If you think this will be too noticeable in your photo (depending on what size you intend to display at) you will need to reduce the length of exposure – again try 15 or 20 sec.
It's normal to edit night sky images, e.g in Lightroom and / or Photoshop. Typical adjustments which may help to improve the image include noise reduction, colour temperature, levels, contrast, sharpening.
Hopefully you will have taken some photos you are pleased with. There are other things you can do to improve your results -
· Take lots of sub-exposures (‘subs’ or ‘light frames’) and stack them in a stacking programme like DeepSkyStacker (DSS). This improves the signal to noise ratio (S/N) in the final image. The S/N improves by the square root of the number of subs used – 4 subs gives twice as good S/N, 9 subs = 3 times as good, 16 subs = 4 times as good etc. About 20 is a good starting point. You won’t be able to do this if your image is a nightscape, with a landscape foreground, because DSS will align the stars in your subs but blur the foreground. You need to be using your RAW frames in DSS, then save the stacked image as a TIF file for further processing.
· Take dark frames. These are used to remove hot pixels – the brightly coloured little specks in long exposure images. Using the same camera settings, with the camera at the same temperature and without moving the focus ring, put the lens cap on and take 5 or more dark frames. Load the darks into DSS with your light frames and the programme will subtract them from your light frames to remove the hot pixels.
· Recommended books
Making Every Photon Count by Steve Richards – the best general intro to deep sky imaging.
Digital SLR Astrophotography by Michael A Covington– more DSLR specific and some good stuff on using camera lenses.
That’s about it. Using DSS and editing software like The GIMP or Photoshop will have to wait for another How To, or there are plenty of tutorials on the web – have a look on www.stargazerslounge.com.