Today, I had an opportunity to go out and attempt my first solar imaging session. A couple of months ago I constructed a Baader Solar film filter for the objective end of my Starwave 102ED refractor. I’ve had a few good solar observing sessions with it, and so decided to have a go at imaging. I’ve seen varying results with ‘white light’ solar imaging. It tends to get overwhelmed by the absolutely stunning ‘hydrogen alpha’ imaging that gets done through dedicated solar telescopes like the Coronado range, or the Lunts. Take a look at the ‘solar’ section on the SEKAS website for examples : http://www.sekas.co.uk/SEKAS/Gallery/Pages/sun.html#2.
These amazing images are produced by telescopes that filer all but a tiny wedge of the electromagnetic spectrum, specifically around 656.28 nm in wavelength. These are the classic ‘red fiery’ images, with tons of surface details, and solar prominences showing. Unfortunately, you don’t get this with white light viewing. You do, however, get to see the sunspots, some granulation of the sun’s surface, and various other features. And best of all, it’s cheap to add to an existing scope.
At the time of writing, a sheet of Baader solar film costs £15-20 in the UK. You can make it into a filter yourself using some cereal boxes and tape. the one on the left is my effort. The second shot on the right shows it mounted on the scope itself.
These are easy to make, and there are countless tutorials available on the web about constructing them, so I won’t go over that again here. Just make sure you get the ND 5.0 version if you intend to use for visual use as well, as the imaging version (ND3.8) doesn’t block so much energy, and isn’t safe for visual use. However, the ND5.0 version is fine for imaging use…
Once mounted on the scope (and secured in place with a bit of tape to avoid it being knocked off or blown off by a gust of wind), you can safely observe the sun, and see sunspots and various other features.
Imaging with the solar film in place is basically the same as when doing planetary or lunar imaging. Generally I use my ZWO ASI120MC to take video, and then stack around 70-80% of the best resulting frames (usually out of at least a few thousand), in Autostakkert. I use fire capture to do the actual capture. Focusing isn’t too bad if you have a few sunspots in view, and you can use those, combined with the sharpness of the solar limb, to work out if you’re focused or not.
The close-up shown here is of a sunspot group, and was 2500 frames, of which the best 70% (as determined by Autostakkert’s quality estimator) were stacked. The resulting TIFF was then imported into Registax 6 and sharpened up using the Wavelets. Finally, I imported into Adobe Lightroom to finish it off. The image is not actually red as you’d expect, but is closer to bluey-white when captured in colour. The stacking process effectively works out which bits of each frame are shapest, and combines them all into one stacked frame, with much more detail. I’ve discussed this in the past for planetary imaging, but the same applies to the solar stuff too. In reality, I’d actually had a few chances over the preceding few months to go out and image the sun for the first time, but each time, the sun itself had been fairly ‘quiet’ (i.e. not much in the way of sunspots etc). I decided to wait until there was a bit more going on on the surface before I jumped in with the camera.
Once I’d managed to grab 2500 frames (in between the extremely patchy cloud), I decided to go for a whole-globe view using the Canon EOS 1100D. This is a single shot, imported into lightroom and tweaked to bring out as much detail as possible. The resulting image doesn’t show as much as the stacked image taken with the planetary cam, mostly as a single shot is more affected by the seeing conditions, but also because the features are simply smaller in the frame. Again, stacking enables the sharp bits of each frame to be extracted and combined to form the final, sharper, image.
The sunspot group in the closeup can be seen on the top left of the globe in the full shot. The closeup also shows some of the more delicate surface features around the sunspots, such as solar ‘plages’ – which are brighter regions often observed around sunspots (see http://en.wikipedia.org/wiki/Plage_(astronomy) for more information).
Overall, my setup looked like this today. The scope was mounted on the HEQ5, controlled just using the standard Synscan handset. There was no need for guiding, or for EQMOD or ASCOM, so I skipped that today, and just used the laptop for the purposes of actually capturing the images.
The red blanket over the laptop was the best way I had of seeing the screen when in full sunlight! Draped over the top of you and the laptop, like some 19th century photographer, it enables you to see what’s going on, most importantly when you’re focusing. I know there are more elegant ways of doing this, but this did for me today.
I lined up the scope with the sun by simply making the shadow of the tube as small a circle as possible on the ground. This got it more or less aimed correctly, and then I finished off with the handset. A finder scope wasn’t exactly an option with the sun, unless I didn’t value my sight, which funnily enough, I do.
Watch this space, as I’ll certainly be attempting more solar imagery in the future. The output from today’s session certainly surpassed my expectations…