Jan 222013
 

astrotitle

HOW TO: Wide Field Astrophotography With a Camera and Tripod

Shooting with the Sony RX1 and Olympus OM-D

By Chris Malikoff

Hi Steve, I’ve been enjoying your site now for some time. Your reviews helped me change my mind about my heavy DSLRs, and as a consequence, I’ve bought into the Micro Four Thirds system with an OM-D. I couldn’t be happier. Recently, I’ve gone and taken the plunge and bought a Sony RX1 based on your reviews as well. Perfect!

Having now dropped my Canon 5D Mark-II and 40D, I thought that my astrophotography hobby was probably over. It’s pretty-well accepted within the general astro community that if you don’t use a high-end Canon you should forget it. Astrophotography needs cameras with super-sensitive sensors that display great high ISO performance and very low noise characteristics. None of the mirror less cameras are ready, say the pundits. I say, in response, not true.

1) What We’re After

The secret of taking decent wide field photographs of our night sky is TIME, and lots of it. You need to expose your sensor to very feint light coming in through your lens’ aperture and let the sensor wells soak up as many photons as possible before writing the data out to the processor and on to your memory stick as an image. The only way to do this is by employing bulb mode and letting the camera sit there for up to tens of minutes at a time – depending on your intended object or part of the sky. Throwing a spanner in the works, unfortunately, is this little problem we have with the sky at night. It, and everything it contains, seems to revolve around us as the Earth spins underneath it on its 23 degree axis once every 24 hours. This poses a curious problem to the average photographer – how long can I expose an image for before the stars and my brighter objects, such as “emission” nebulae, start to show blurred trails in the photo instead of presenting a nice clear image? This depends on a number of factors.

2) The Problem

First: The quality of your overhead sky really matters, especially down near the horizon if you want to incorporate a foreground in your shots. By this, I mean that the more light pollution there is in your neck of the woods, and as a consequence your contrast ratio is low. This means that in city areas the night sky is so bright from light reflected off the ground due street and other lights, that you’ll have almost no stars in view let alone the lovely wisps and gaseous tendrils of something as beautiful as the Great Orion Nebula or band of the Milky Way. From a location that suffers from a brightly-lit sky, you can’t expose for long periods of time because you’ll only get a washed-out white mess as a result. The tip is to get into your car and drive away from the city – as far as you can. Typically, I use a 100 kilometre (60 mile) rule that says you should be no closer to a city than this to see an “acceptably darkish” night sky in order to obtain a decent result. The further, the better. I’m lucky here in Australia – we have a lot of room. In the southern hemisphere we also have an advantage over our northern cousins in that our position on Earth lets us look in towards the galactic centre of our Milky Way galaxy, rather than seeing out towards the thinner edge. This means that our Milky Way is generally brighter than that which you get to see in the north.

Second: The moon is your enemy. Depending on what part of its cycle it’s at, it can range from nothing at all because it’s below the horizon, a dim sliver of light to a full-blown angry ball of white light. A full moon simply paints the atmosphere in visible white light that, like the previous point, serves to wash you out. Download a moon calendar app for your mobile device or computer which can show you what nights the moon is at its lowest output – and the best is when it’s not around. This is called the “new moon”. This phase lasts for two or three days every month. You really,really need to try astrophotography on these nights to get a good result. The moon is pretty – but it kills your chances of capturing decent photos of the night sky.

Third: The Earth’s rotation. Herein lies a choice you need to make, as you can take two distinctly different types of image of the same night sky.

3) Type of Image – Your Choice

The first, and most common images taken by astrophotographers, are of star trails. All you need is a statically positioned tripod and a camera fitted with a remote release or intervalometer to give you long (one minute) exposures. Simply point the camera towards either the north or south pole, depending what hemisphere you’re in, and watch as long trails of light start to appear in your images as the Earth rotates. Bright stars literally draw circular lines of coloured light on your sensor or film as they move around your local celestial pole within frame. There is freeware available called “StarTrails” that lets you stack these one minute images together which joins the sixty-second trails together into a circular mass of lines. These are great images, but they’re not what I’m after.

OMD_startrails

I prefer to see a still set of stars that show the bright patches of iridescent gas that burns as nebulae in between. To do this, you need to be able to counter the Earth’s rotation by moving your camera’s lens around the pole at what is termed the “sidereal” rate. By mounting the camera on a device who’s rotating axis is pointed directly at your local celestial pole, and that rotates in the opposite direction to the Earth’s spin at EXACTLY the same rate, you can “hold” the night sky still. This device is known as an “equatorial” mount. Normally, a decent computerised equatorial mount will set you back many hundreds or even tens of thousands of dollars. These are designed to carry a telescope payload that may or may not include a camera mounted at “prime” focus on the telescope. By using an equatorial (EQ) mount to place your camera and lens combination alone on, one can shoot the same patch of sky, literally all night, depending on the quality of the mount and how well it’s been aligned to the celestial pole in your region. There are usually, and necessarily, complex procedures involved in “polar alignment” that would take a few pages to explain. Unless your system is perfectly aligned with the pole, you will never see round stars appear in your long exposure images. Fact of life – nothing you can do except do the work.

Sony RX1 – 518 Seconds – f/4 – ISO 800

RX1_518secs_f4_ISO800_35mm

4) The Equipment

OK – so I don’t have a gazillion dollars to throw at a full-blown telescope EQ mount, but still want to take photos of the night sky without any star trails in evidence. Answer – purchase a portable EQ mount designed to sit on a common tripod. There are several varieties and brands available, and these are a fairly recent addition to the astrophotographer’s tool kit. They range in price from three hundreds-odd dollars to just over a thousand if you buy all the options. The unit I chose is called the Vixen “Polarie” – made in Japan by Vixen – a long-time supplier of premium telescopes and mounts. The Polarie will set you back around the $400-500 mark, depending on where you are. Others are “AstroTrac (UK) for just a little more, and the new iOptron SkyTracker which will cost you a fair bit less. Quality differs, but they’ll all do the same thing in the end – spin your camera around your polar axis.

Vixen: http://www.vixenoptics.com/mounts/polarie.html

AstroTrac: http://www.astrotrac.com

iOptron: http://www.ioptron.com/index.cfm?select=productdetails&phid=0193c9ab-b455-4fb1-9534-ef192192a93f

Olympus OM-D on the Vixen Polarie

OLYMPUS DIGITAL CAMERA

and the RX1…

OLYMPUS DIGITAL CAMERA

Once you have attached your chosen device to the top of your tripod using a geared head or very solid ball mount, you need to do two things. You must point it in the right direction relative to the horizon, and then you’ll need to point it up into the sky to the right elevation so that the central rotating axis of the unit is pointing as close to either the north or south celestial pole depending where you live. In the north – you have it easy. All you need to do is find the Pole Star, Polaris. (http://en.wikipedia.org/wiki/Polaris) This star is easy to find and closely marks the north celestial pole. All you need to do, with the Polarie for example, is use the sight tube built into the casing of the unit to sight this star through it. Lock your ball or geared head. Mount the camera and lens to the front of the unit on a second ball mount and point your camera to where you want to start shooting. Fire away. If you live in the south, as I do, then it’s a little more difficult. There is no star handily pointing out your local pole. You can use the optional “polar scope” to fine-tune which way you’re pointing after you use a compass (set to point to true south, not magnetic) and inclinometer (angle meter) to set the square faces of the unit in the right direction. If you’re in the south, then you have to know what your position’s latitude is, and use this to set the inclinometer so that you point high enough off the horizon to see the pole. I live in Sydney, which has a latitude of 34 degrees south. I set my inclinometer to 34 degrees and then set it against the front face of the Polarie so the it tilts back at 34 degrees. Then use the compass to point the front of the unit to true south. To do this you’ll need to know what the offset from magnetic south is for your area – it differs greatly depending on where you are. Use your smart phone and set it to show true, rather than magnetic south and it’ll work it our for you.

Olympus OM-D

OLYMPUS DIGITAL CAMERA

OK – so we’re nearly there. You now have your EQ mount sitting on your tripod and its main axis is pointing at your local pole. You’ve mounted your camera on the rotating front ring and it’s pointed somewhere interesting in the night sky that you’d like to photograph. What next? You need to set up your camera and decide on a field of view. Tip: The shorter the focal length, the wider the image and consequently the less critical your tracking needs to be. The longer the lens, the more critical your tracking is. My ideal length falls in the range 24mm to 50mm. Any longer and it’s starting to be a world of pain. Don’t be tempted to stick a 300mm tele on, because unless you have one of those huge telescope-grade EQ mounts, you’re going to end up with fuzzy, out of round stars. There is a weight limit on these small EQ mounts of around 2.5kg (6-7lb).

Olympus OM-D – 304 Seconds – f/5.6 – ISO 400 – 24mm

OLYMPUS DIGITAL CAMERA

5) Set up your Camera

Deep-sky wide field photos require exposure time. A few seconds simply doesn’t do it. You’ll capture a few of the brighter stars, but that’s all. You need to take exposures of two, three, five and even seven or eight minutes to get the “fluffy” stuff. Set, as a starting point, place your camera in full manual mode.

A) Focus: Set your ISO to 1600 or higher if you can. This is only temporary, and is needed to show you stars as bright as you can possibly see them in live view if you have it. Set your lens to manual focus. If you don’t have live view, set your focus to infinity as a starting point. With live view, you should be able to see these stars with your focus set to infinity. Adjust focus with live view’s zoom feature set to as close in as you can get. Canon gives you 10x, if you run an OM-D it’s 14x. You’ll see the star focus to a sharp point, with it becoming a soft disk either side of proper focus. Take it out of live view. Take a 10 second exposure. You should see sharp stars in your image.

B) Set your aperture to around f/2.8 – either via the lens or from a menu if it’s entirely electronic and fly by wire. Fast lenses are good here, as long as you don’t open them right up as you’ll start to see vignetting and/or spherical aberration creeping in. Stop it down a stop or two and just expose for longer. If you have a slower lens then don’t panic – time will fix it. I have an old f/4.5 tele that I use regularly and it works beautifully.

C) Switch on your EQ mount so that it starts moving at sidereal (star) rate – not lunar or any other rate that you may have on the dial.

C) Take a 30 second exposure. If you see round stars and no obvious trailing, then you’re good to go.

D) Now set your ISO value to around 400. Turn on ICNR (In-Camera Noise Reduction). This will help mitigate thermal noise in your image.

E) Set your exposure to 30 seconds and see what you get. If your camera can expose for longer than 30 seconds, like the OM-D at 60 seconds, try that. The OM-D’s brilliant “LiveTime” feature is phenomenal here. It will let you start exposing and you simply watch the image form on-screen in real-time. Brilliant for this job.

F) Now step up your exposures (if you don’t have LiveTime) to 60 seconds and beyond, with a cable or remote electronic intervalometer or release. I’ve managed 15 minute exposures with this setup, but you need REALLY dark skies to pull this off. Otherwise, you’ll start to get white-outs. Speaking of which, if you start to see this, simply decrease your ISO, step down the aperture another stop or two or reduct the exposure time. You’ll find a balance.

6) Final

Once you have a bunch of successive images of the same area, you can use any number of stacking programs, including Photoshop CS4 or newer, to stack them which results in better signal to noise ratio. This means that, by averaging-out the noise between stacked images, that you can push the levels of the image to increase the dynamic range – and suddenly your images will start to pop. That’s an entirely different subject for a different day.

Sony RX1 – 446 Seconds – f/4 – ISO 800

RX1_446secs_f4_ISO800_35mm

Have fun – and post your images somewhere where we can all see them.

Cheers

Chris Malikoff

Sydney, Australia

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