Monday, 3 July 2017

Jesse Beaman - Astrophotography with a Tracker

I have recently purchased an astrophotography tracker - a piece of equipment used to improve the quality of night photography images. In this blog I will describe what a tracker is, how and why it’s used, and also show my progress so far.

So, what’s a tracker? The Earth rotates on it’s axis and as a result the stars appear to move across the sky. Expose a DSLR for over 30 seconds and any stars captured will appear as ‘trails.’ This means as an astrophotographer I’m limited to approximately a 30 second exposure. To compensate for the short exposure I ramp up my ISO to 6400. This can result in very noisy images and limits the amount of detail that can be captured at night.

A tracker overcomes this limited detail issue. Below is a single, 5 minute tracked exposure at ISO 800 of 'The Great Rift' in the Milky Way near the constellation Cygnus.

Earth’s axis points almost directly at the North Star, Polaris. This is why Polaris always stays in the same spot in the sky as all the other stars appears to move. Polaris is the astrophotography trackers’ best friend. The tracker is mounted on a tripod, and here’s the key; the tracker is then manually aligned with Earth’s North celestial pole (right next to Polaris). The tracker then rotates to match Earth’s rotation. Mounting a DSLR on the rotating tracker results in the DSLR tracking any part of the sky it is pointed at.

Providing I get my polar alignment right, I can expose for about 25 minutes before stars become distorted - much better than the previous 30 second limit.! This means I now have the ability to lower my ISO to around 800 and capture far more detail in the sky. In doing so I have increased something called my ‘signal to noise ratio.’ Basically my RAW image now contains more color pixels (signal) and less fuzzy, distorted, hot pixels (noise).

Another 5 minute exposure at ISO 800, this time showing the Winter constellations.

The images that compliment this blog have had a little RAW editing to increase detail, but essentially they represent what comes out of the camera. The key to tracked astrophotography is in the edit, something I have not yet experimented with. Skilled photographers capture hours worth of separate images and then stack them in the edit suite to push the signal to noise ratio as far as they can. Compare this to the 5 minute single exposures featured here and you’ll start to realise how far I have still to go!

Whilst the images here aren’t exactly finished pieces, they represent the next step in astrophotography for me. I’m looking forward to learning how to use these improved RAW files to take my work to the next level. Notice in the tracked image below that the foreground is blurred due to the tracker's rotation. This is another obstacle I'll have to overcome before producing any finished work with the tracker.

For anyone interested in trackers feel free to contact me with any questions.

Jesse Beaman

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