It all starts here

20 September 2016 - 1 Comments

Posted by David Woods

ELEMENTAL IMAGING

M31 602sec Star Adventurer single frame

It all started here.... actually.

In simple terms – just looking at the night sky, I guess. But since your eyeballs are about the same as looking through an 8mm telescope, you're gonna need some help if you want to reveal the mysteries of the universe.

Now I'm a big lover of small refractors so something like a 70mm refractor will give you 100 times more light gathering power than your own eyes. This is great news if you want to start looking at things in the night sky. But what makes it even better (and some say this is what saved astronomy!) is that mysterious world of astro-imaging.

But fear not – for it's still elemental. Also cheap. You need four good things – a good mount with a sturdy tripod (or pier), a half decent telescope, a camera of some sort and somewhere to process your images, normally PC or laptop. All the other stuff like filters, guiding and processing software blah, blah, blah, will come later and these just add refinements to what is in essence a very simple operation.

Unfortunately human biology limits us – no matter how long and hard you stare at a dark image it won't get any brighter. Your eyes perform constant Live View. This is where imaging cameras, be it DSLR or CCD, have the upper hand. They can expose and absorb photons of light over many minutes helping to build up an feint image.

The trick is – keeping it all simple. But none of this matters unless you know how to find what you want to photograph and can track it, without blurring, through the night sky. So for me a computerised equatorial mount is the most important element, if you want a pain free experience.

Sure – you can video planets and even use it with a Dobsonian if you wish, but you're limited to 30 seconds. However, on a camera with a higher frame rate that's not an issue.

Our tuned mounts regularly run in excess of 20 minutes per exposure. A guide camera will help to achieve this but having a mount that's responsive and accurate is more benefit than a fancy expensive telescope.

That's why I started tuning mounts - I knew they could do the job but many astro-imagers were throwing money in the wrong direction, adding complexity and layers of possible mishap for what essentially is an arc through the night sky at a precise sidereal speed.

Plus, let’s admit it, Telescopes are sexy. Mounts are just, well, Mounts.

But they do a simple yet sophisticated job. Close your eyes and imagine yourself on the Earth’s surface, spinning at 900mph at 23º (with a wobble at both poles), travelling 3000mph through space. Your mount then has to track to within a pixel on a galaxy thousands of light years away…

So, is just a camera and a tripod elemental imaging? Yes, but you’re limited to several seconds. But any motorised mount will do and even unguided you can do a lot with 30-60 secs at ISO1600, and that’s where most of us start off. Start there. From a tracker to a mid-range mount, just use your mount practice polar alignment and just use a camera and say a 200mm lens and a remote shutter. Turn off your autofocus, pick a bright star and use the 5x10x zoom on a bright star for find 'star infinity'. Then start stretching those exposure times. It’ll be hit and miss at first, but you’ll get better.

What I do is polar align using my Sky-Watcher Star Adventurer and take a 1min image at ISO1600. If it’s OK and not trailing then I take a 3 minute and check again. If still OK then 6 minutes, and then 10 minutes. If there’s too much trailing just dial back the trailing, or better still redo your polar alignment. Don’t waste time doing incremental 1 min exposures.

This image above is a single 602 sec unguided 200mm exposure on that mount, which I then dialled back to 6-8 minutes. Look closely and you'll see the stars trail a bit. 3-5mins should be fine for UK skies and repeatable with a bit of practice.

Job done.

Your mission is pushing more imaging than 60 secs unguided on any mount. That’s when stuff starts to get interesting.

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