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I’ve always been interested in astronomy, so there’s a short leap to combining it with my love of photography to get astrophotography – photographing astronomical objects. There’s a small set up on flickr with a few efforts, some nearly 20 years old.
The trouble with astrophotography is that everything is very far away, pretty dim, and moving. This makes it one of those areas where better gear can really make a difference. Apart from wide-angle star field views you need a fairly long focal length lens to get a decent result. The Moon is easy enough to get with a 2-300mm lens because it’s relatively nearby, very bright (sunny 16 rule is the guide for shooting the Moon), and pretty easy to find. You can get reasonably good shots of the whole disc of the moon with short exposures so that you don’t need to track it to compensate for the Earth’s rotation.

Other objects are a lot trickier. Recently I noticed Jupiter was getting prominent in the evening sky, and after a friend posted a photo of a somewhat overexposed planet and four little trails of light for the Galilean satellites, I thought I’d have a go myself. I wanted to try and show Jupiter itself as a disc rather than an overexposed mess, and avoid trails if possible. I broke out the Sigma 600mm mirror lens I have blogged about previously, and set up in the garden.

Galileans

My friend’s shot was a 15 second exposure a good exposure for the moons if they weren’t moving, but since the Earth rotates 1/4 of a degree every minute, they show quite a lot of movement when you use a long lens. For example, a 300mm lens on a APS-C camera has a field of view of about 5 degrees diagonally, so in 15 seconds a star or planet will move about 1% of the width of the field. That may not sound much but on my camera that’s about 40 pixels – quite a distinct trail. I decided to start at about a second and work down until Jupiter was on the limit of overexposure. At 1/15 it was about right but I found the moons were no longer visible, so I dropped to 1/8. since I was shooting RAW this gave me a little extra leeway for Jupiter so I was able to pull it back a little in post, and bump up the rest of the frame with curves to show all four moons faintly.

This is okay for a first attempt, but there’s a big noise problem when boosting the lower levels of a single frame like this, even shooting at ISO 100. So the next clear night I decided to try again, but this time take lots of identical exposures over a few minutes and this time stack the images together to eliminate noise. Most noise in a short exposure is random – the value of a given pixel fluctuates from frame to frame based on all sorts of factors. By combining multiple frames the fluctuations average out and you get a much reduced level of noise across the resulting image.

And yet it moves

I’ve done this before using Photoshop, but only for static scenes – whenever I tried it with a moving object, I had huge problems with alignment. So this time I downloaded RegiStax, a program specifically designed for astronomical image stacking. Now the interface is pretty obtuse, and I’m not entirely sure what was going on a lot of the time, but I did manage to muddle along, and combine 46 frames. RegiStax did a good job of automatically aligning them and averaged all the frames together to produce a single image with very low noise in the dark regions. This enabled me to boost the levels much more than the previous single image to make the moons much more obvious. They’re a little fuzzy, which is probably down to the inability to be totally sure about focus coupled with atmospheric turbulence.
Another result, which I wasn’t quite expecting, was the dark cloud band just about visible on the flattened disc of Jupiter itself. Amazing for such a small telescope – 95mm diameter is pretty paltry by astronomical standards.
I think I’ve hit the limit of what’s achievable with this set up, though when Saturn comes around again, I may have a stab at that. But all this has unfortunately fed the urge for more. There’s a lot of potential in this that if only I had a proper telescope I could have a lot of fun trying to achieve.

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