Colours of the Sun (2)

 

Part Two: pretty pictures and a little nerdy stuff

For my own peace of mind I must begin this post as I began Part One with a warning that might seem blindingly (!) obvious to you but which I ought to spell out nevertheless: please never, ever look at the Sun without adequate (certified) protective filters. Even more crucially, make absolutely sure that your binoculars’/telescope’s field of view doesn’t even stray close to the Sun. The filters I used to generate the images shown below removed, at a minimum, 99.999% of the intensity of the sunlight.

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It’s a fascinating time to be imaging the Sun as we’re near the peak of solar activity in its eleven-year cycle. When I started imaging the Sun a few years ago several days could go by with little or no activity; as 2024 progressed it became a rare day when there weren’t several large and/or complex active regions. The Sun’s disc has a diameter of approximately 109 times that of the Earth so in the images below you’ll realise immediately just how extensive these active regions can be.

Shown above is the neutral/white-light imaging setup I use in order to gather images of the photosphere; I introduced it in Part One. With this I can capture the whole of the Sun’s disc within a 3k x 3k (i.e. 9 megapixel) colour image which gives a decent level of detail. Sunspot active regions and the associated faculæ are easily observed. This is not like taking a snapshot using ones phone or handheld camera unfortunately. The issue is one of atmospheric turbulence: the higher the magnification the more distortions a given exposure may suffer from (I uploaded to my YouTube channel a very short video taken through another of my telescopes in order to illustrate this phenomenon.) To get around this I’ll typically collect up to 2000 frames using the software cited in Part 1 then use a free software package called Autostakkert! in order to select the best – usually the least distorted ten or fifteen percent only – before stacking them to create a single optimised image. This will go into another free package called Registax for sharpening and perhaps basic colour balancing before sending it to an image processing package like AffinityPhoto or Photoshop for final ‘polishing’.

Now, as discussed in Part 1, ‘colour’ is a term that needs a bit of thought. When I use my colour astro-camera the frames that emerge have a green tint because the detector chip inside isn’t the standard RGB of a smartphone or other digital camera but it’s a pattern of RGGB (for reasons I’ll not go into here). Correcting for this is trivial, but in truth one could choose almost any colour for the final image simply by selectively altering the colour balance or colour saturation at the image processing stage. A ‘correct’ balance will yield a white photosphere, but if it’s desired then a yellow-coloured Sun is easily possible. I’ve done both, although I’m increasingly tending to the more neutral/natural white.

When it comes to the remotely accessed setup in Grenada, Spain I mentioned towards the close of Part 1  the whole point of the exercise is to observe at specific wavelengths, using in particular H-alpha light. One is therefore using precision narrow-band filters to select out that one colour associated with the emission from excited hydrogen. A colour astrocam is very inefficient for this – there will be nothing at all recorded in the green or blue channels of course – and it’s far better to use a monochrome camera and then add any desired colour digitally during processing. The rig is shown above; it’s a screenshot from the Zoom-enabled session (- details here; my excellent guide and teacher for the experience was Gary Palmer). The setup we used was the one on the left hand side: a Williams Optics 120 mm refractor, with a ZWO-ASI1600MM astrocam, all on an IOPTRON CEM120 mount.

After a couple of false starts due to the weather, my session took place on 4th July. In order to ‘ground’ the images I used my own backyard setup to capture the Sun’s whole disk in white light using my homemade solar filter. It’s shown below, with the active regions identified using the internationally accepted conventions which one can find online. I find the Space Weather website useful in this regard, and you can find images and associated information for any specified date in their archive (see here). Also useful is the NASA Solar Dynamics Observatory, which includes video sequences for particular dates (see here). I’ll share the rest of the story using the images I later generated from all the data that was gathered on the day. Each of these was generated from original data files containing 2000 exposures which were graded by quality and the best stacked and refined using the packages mentioned above.

My backyard image of the photosphere as it appeared on 4th July, collected and processed as described above.

A couple of composite images overlaid onto my white-light image of the photosphere are shown above. I have marked on my image the active region under observation; superimposed on the side are three narrow-band filter images of that same region. Notice the difference in appearance of the sunspots and the area around them depending on which element we’re utilising: hydrogen, sodium and magnesium. The faculæ (regions of higher temperature) show up well in H-alpha light, sunspots less so; the violence of the Sun’s magnetic field changes in an active region can be seen in the contorted surface patterns. Surface granulation shows up well in sodium light, and the sunspots seem increasingly more clearly revealed as one progresses through Na to Mg. Please note again that the colouration is added digitally – hence the variation: I have been learning how to process the narrow-band monochrome such as these for the first time and therefore trying out various methods. During this period, as an additional complication, I began to migrate from using Photoshop to AffinityPhoto. Unfortunately, there are relatively few online tutorials based on the use of AffinityPhoto although this one was useful; the short books by Dave Eagle were particularly helpful.

In the pair of H-alpha images shown above one can readily see prominences reaching out into space. The one on the left is associated with an active region not yet in view from the Earth – i.e. it is being generated around the rim. The Sun rotates once in approximately four weeks at the latitude of these prominences, so this one would have appeared within days. (The Sun is a ball of gas, hence the fluid-like variation of a solar ‘day’ depending on where one is looking between the poles and the equator; see here for more information.) Also apparent in these images are the ‘grass-like’ spicules in the near-surface region of the chromosphere. Each of the two images is itself a composite: one set of 2000 short-duration exposures for the surface and another set with longer exposure times for the spicules and prominences. These two sets were separately processed and then combined digitally to yield the final composite images. The original two layers of the final composite image shown above on the right are reproduced below after stacking the selected subset of 2000 exposures for each of them.

Although filaments – a prominence as seen from above – may also be discerned in the coloured images above, particularly the one on the left, I’ve inserted an image below in which I have accentuated them during processing.

Given the novelty of all this I confess that it took a long while to get to grips with the remotely collected data files I was sent by my guide Dave Palmer. However, in the process I’ve learnt a lot, enjoyed myself and am pleased with what I have to show for all the effort. I hope you like the result.