Astrophotography

25-04-2016

I've been into astrophotography for a few years now. Whenever I get chance I try to get out with a camera and take some shots. Living in London however, this is problematic, though I've still given it a try. In the past I've had some good shots from a few dark spots. I've attempted to photograph The Moon from London several times, and even the Great Nebula in Orion, but it's only since going to Morocco where I got chance to properly have a go for reals.

The place we stayed at near Zagora, was called Sahara Sky. I spoke of this place before but their telescope setup is what I will focus on here. The conditions in Morocco are particularly good. So good in fact, that at certain times and places, you can see shadows cast by Jupiter, not just the Sun! The desert means clear skies and low turbulence, though dust storms can, and did, make things tricky. But that is comparatively rare. Certainly, most observing sites in the UK, while great in terms of dark skies, suffer from the weather problem.

Whilst at Zagora, I decided to focus on the deep-sky objects. In order to do that, you need a good telescope, on a solid, tracking mount. Exposure times need to be in the 30 seconds to 5 minutes range so theres no room for wobble or incorrect tracking. To take deep sky shots, it is my understanding that one needs to battle noise more than anything. Longer exposures mean more noise in the sensor. This became apparent quite early on. Ultimately, all these images are stacked - which is to say they are compared and the noise removed whilst the details are multiplied. There are a great many tricks in the arsenal of image processing that can be applied. You can find more about the various frames and stacking at the DeepSkyStacker FAQ page

For the deep sky setup, I used two separate setups, the Vixen Polarie mounted on a Velbon Portable tripod. The second setup used a Takehashi FSQ-106ED on a very nice, German equatorial mount. The Polarie is aimed manually with a polar scope and is very portable but wobbles a little too much and is nowhere near as accurate. The Takehashi requires an extension tube, and a T-Tube adapter, fitted with the larger 2 inch diameter tube (as oppose the smaller 1 & 1/4 which is typical for smaller scopes - I was very fortunate to be loaned both of these things). The Takehashi has a focal length of effectively 540mm.

takehashi

I had two cameras with me: my venerable Nikon D90 and the newer, but more budget Nikon D5300. The latter I choice because of the low noise and high sensitivity of the sensor, and it's relatively cheap price. In addition I bought a second-hand Tokina 100-400mm telephoto lens. I'd tried a couple of cheaper 500mm prime lens, that required a T-mount adapter. With these lenses I found they couldn't focus at infinity properly. I learned later that this is because the flange distance on the Nikon body is longer than that of a Canon. So, if you want a good lens for cheaps for astro work, get youself a Canon camera and a 500 prime instead. With a day to go I was lucky to find one at Aperture - a camera shop in London. I used a couple of different lenses with the D90 on the Velbon, saving the D5300 for the telescope.

Deep stacking with the telescope allowed for shots of faint objects. I decided to aim for a variety of different targets

whirlpool

This is the Whirlpool Galaxy and probably one of the better shots. CX Settings. This was stacked using PixInsight and largely followed the tutorial here. PixInsight I've found to be slightly superior to DeepSkyStacker though the former costs around £300. I used the free 30 day trial to get this shot. I say slightly better, there are many, MANY more functions but I never got near to using any of them.

nebula bad

This image shows the results from the D90 and the Polarie. Although the Nebula has come out and there are colours there, it doesn't seem too great. I used the highest ISO on the D90, with the Polarie and took about 80 shots. This is using the 400 f5.6 Tokina telephoto lens.

bodes Nebula

This shot of Bode's Galaxy from the Takehashi is quite pleasing. It uses 80 Light shots at 12500 ISO, at 15 second exposures. I used 20 Dark frames and 20 Dark offset frames. I couldn't take any light frames as I had no way of taking pure light shots, so I had to do without these sadly, which is a shame.

mooooon

This shot of the moon was quite fortuitous. There were a number of more standard telescopes around the roof and this one was trained on the Moon for viewing with the eye. I managed to attach the D5300 to it and took this shot. It's the best shot of the Moon I've ever taken.

Owl Nebula

This shot of the Owl Nebula looks much better in the original 6000 x 4000 image. This was stacked with Deep Sky Stacker as oppose to PixInsight.

horsehead nebula

This image of the Horsehead Nebula is just about visible in the centre of the frame. I love this image because it such an iconic object in the sky; I'm really glad to have captured it! Stacked with PixInsight, this was around 80 frames with 20 darks and offsets, at 12500 ISO of about 15 seconds.

sbig

Some of the setups at the hotel were quite involved. Above is an S-BIG CCD based camera. Last I checked, such a device cost around £4000 or thereabouts. You can get quite fancy in the imaging department. I had, at one point, considered a DSLR cooling case, which apparently lowers the noise considerably. Another adaptation is to literally grind away the IR filter on your camera in order to make it more sensitive to the wavelengths that deep sky objects emit. People start talking about wells, quantum and all manner of scary things. I decided not to do any of these this time around - I wanted to see how far one could get with basic gear.

milkyway

This shot is perhaps my favourite of the entire trip! It requires a totally different setup to the deepsky rig. I used my portable tripod, D5300 at 12500 ISO and all the noise reduction settings turned off. The lens I used is an older, Tokina 11-16mm f2.8 lens. I've had so much use out of this lens - it's the best I've bought. I took around 40 shots but I only ended up using around 20. I stitched them together using a program called Hugin which is a lovely program. Each image was pre-processed in Lightroom first, then stitched, with the final image being processed again in Lightroom. Each exposure is around 30 seconds in length. This gives some trailing in the stars, but not so much that it detracts from the overall look.

You can use the 500 rule to work out star trails. Simply take the number 500 and divide it by your focal length. For some reason, I had the focal length at 16mm rather than 11, but this gave me around 30 seconds to work with. I'm not convinced this rule works all that well in practice but never-mind. The ISO was also perhaps a little high, especially with the light pollution from the nearby town. If I'd have had a proper panoramic head, I could have taken fewer photos, or maybe multiple exposures for HDR. A filter may have also helped as the original shots were quite yellow.

Overall, I was quite happy with the panoramic shot, but the setup with the Vixen Polarie needs a lot of work, especially with the longer focal lengths. Deep-Sky-Photography is more of a question of how much money you want to spend. I actually think there is little in the way of skill in these photos - it's simply being a good technician. If you have the money for better gear then you'll get a better shot. I'd make a better contribution by writing a replacement for DeepSkyStacker as it's beginning to show it's age. You can spend a lot of money on the software too, with MaximDL being regarded as the best of all.

I would advise anyone getting into astrophotography to think outside of the box. Artistic landscape shots, excellent timing and different technologies are the way forward. Look at the Humans and Space category in Astrophotographer of the Year and you'll see what I mean. We've already seen photos taken from high-altitude balloons becoming winners; I think such creative uses of technology will only increase.

All of these photos can be found in their full glory on my astrophotography flickr album