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Astrophotography Advice & Answers

We chat with ePz member chedd about astrophotography, picking a few tips up along the way.

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If you take a look around ePz member chedd's portfolio you'll soon find a showcase of images which are quite literally, out of this world. The images we're talking about are of the various galaxies that decorate our sky and we had a chat with chedd to see exactly how you go about taking these sorts of images and how he got hooked on astrophotography in the first place.

For those wanting further in-depth tips on astrophotography, take a look at chedd's blog - digitalrust.co.uk

NGC 434 / Horsehead nebula - 19/01/14
NGC 434 / Horsehead nebula - 19/01/14 © chedd. 

How did you get into Astrophotography? 
"I have always been interested in astronomy since I was a child. With the advent of digital cameras and ever decreasing prices in telescopes and hardware I began the almost vertical learning curve around 6 years ago. I joined my local astronomical society shortly afterwards and hundreds of hours later, here we are!"

If someone wants to start out in Astrophotography what's the basic set of equipment they will need?
"This entirely depends on what you are interested in photographing. Rich, widefield photographs of space, the planets including our own moon or deep sky objects such as those in the Messier catalogue. Starting out in astrophotography can be very exciting due to the virtually limitless number of stunning and beautiful objects to capture. After a time you will find yourself gravitating towards certain favourite types of object.

At it’s most basic, a solid tripod and any camera capable of manual control over shutter speed and focus is all that is required. It is possible to photograph many astronomical deep space objects using this equipment alone; for example the Orion nebula, the Andromeda galaxy, the milky way and of course several planets in our own solar system.

A DSLR with a 200-300 mm lens on a motorised 'tracking' mount is a good next step. This could either be a purpose built device that connects between your camera and tripod ( such as the astrotrac ) or a similarly priced 'equatorial' telescope mount ( EQ5 etc ). These types of devices allow you to take exposures of several minutes and dramatically increase the quality of your pictures.

The next step up is a telescope and adapter for your camera along with a 'goto' astronomical mount, combined with an autoguiding system, that allows you to track an object in space for many hours. This is generally the preferred method and one that I use."

For more information on equipment, take a look at chedd's blog where an overview of his equipment can be found.  

Is there a particular Astrophotography subject that's easier to capture than others?
"The moon is perhaps the easiest and most obvious target to start with, however to really dip your toe in the water you could practice on the astonishingly beautiful Messier 42 in the constellation of Orion, the magnificent Andromeda Galaxy (which is due to collide with our own galaxy in a couple of billion years) or perhaps Messier 45 most commonly known as The Pleiades. Remarkable results can be obtained from 30 second exposures of these later objects (try taking 20 or 30 photographs and stacking them in software)."

How do you know what stars / galaxies will be where and when in the night's sky? 
"There are numerous online and printed publications showing the sky at any particular night of the year. My favourite piece of free software is Stellarium which will show you a complete skydome from your location anywhere in the world with thousands of objects to photograph. There are also some very good Apps available of which my favourite is 'Sky Safari'." 

Is there a lot of preparation that takes place before you begin shooting your images?
"Like the vast majority of amateur astrophotographers I do not have the luxury of a permanent observatory. This means assembling all my gear outside, ensuring the battery tanks are fully charged, polar aligning the telescope tripod and connecting it all up every time I wish to take photographs. This takes between thirty minutes to an hour. I then set up the camera / telescope autoguiding system (P.H.D) as accurately as possible before starting to finally capture the object." 

Rosette Nebula
Rosette Nebula © chedd.

What method do you use for capturing your images?
"As I alluded to earlier there are many different ways to photograph astronomical objects. For many the preferred method is to acquire lots of relatively short exposures, combine or stack them in software and perform noise reduction, sharpening and filtering with software to produce the final image.

Typically for a 'deep sky' object such as a nebula or galaxy I will set my DSLR to an ISO of 1600 ( noise is not an issue as we will see later ), my exposure length to 8 minutes and try to capture around 2 hours worth of these 8 minute 'subs' as they are better known.  Having acquired the 'subs' I will put the lens cap on the front of the telescope and take three 8 minute 'dark frames' to assist with noise reduction later. With the lens cap still on the front I will capture 21 'bias frames' at the shortest exposure time my DSLR will allow which is 1/4000th sec. The last step is to acquire 'Flat' frames with the lens cap removed. These are very important as they are used to help remove optical imperfections in the imaging train such as the effects of coma, dust bunnies and so on. I use a homemade lightbox, place the camera / telescope in front of this and take 21 exposures at 1/50th sec.

The final stage is to combine the 8 minute sub exposures of the object I have photographed with the dark, bias and flat frames in a software package such as MaximDL or the free software Deep Sky Stacker.

After a while (depending on the speed of your computer) an image will be generated that can be imported into Photoshop for post processing. Typically this will involve stretching, filtering, noise reduction and colour balance."

How do you produce such sharp and still images of stars which move through the night's sky?
"As you know the Earth is rotating relative to stars and other objects in space, often used to great effect in Startrail style photos. This is very bad for taking long exposures of a particular galaxy or nebula because they will drift across your cameras frame. The longer the focal length you are using the faster objects will move across your camera frame and blur the detail. This is why you really need a method to track an object in the sky for many minutes or you will be limited to around 15-30 seconds per exposure."

Lunar mosaic Lunar mosaic © chedd.

What method's used for capturing images of the moon?
"It is not possible to get a nice sharp image of the moon at a good focal length with a single photograph. Air turbulence and atmospheric perturbations generate too much distortion. The preferred method is to set your camera to video mode, capture at least 90 seconds of video and feed this video file into a software package such as the free program 'Registax'. The program will examine each of the video frames you shot and output the clearest as an image file. If you do this many times for different sections of the moon and stitch them together in Photoshop, you can create a mosaic image such as the one above." 

How do you deal with light pollution? 
"If you live away from the city light pollution is not such an issue however near the city you can buy filters to help reduce the negative impact. These will either clip onto your camera directly or between your camera body and lens."

Do you have a favourite image you've taken and if so what is it and why do you like it? 
"Perhaps my favourite image is one I took with my Canon 7D and stock lens of the moon and its 22 degree halo. If you look carefully at the 4 o'clock position you can see the planet Jupiter shining. The phenomena is similar to sundog formation where moonlight is shining through ice crystals in Cirrus clouds ( 25 seconds @ ISO 400, f/14)."
Take a look at chedd's ePz portfolio to see more of his images. 
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