Clustered

This is the second in a planned short series of image-rich posts within which I hope to share my progress in astrophotography; the first, on binary star systems, is here. Although I’ve been a stargazer since my childhood, this more serious-minded hobby started two or three years after I retired and has progressed haltingly in the six or seven years since. Now, as I approach my mid-seventies, I’d like to take stock of what I’ve achieved and there’s nothing like writing about it to sort out one’s thoughts.

The advent of being able to capture images of star clusters dawned when I finally managed to use plate solving methods to find and identify specific targets and then employ guiding to lock onto them for long exposures. Plate solving is, in essence, a little like using the features on a map to help one navigate to a destination; in this case the map is of star positions and it’s my laptop doing all the work. The plate solving software uses an image of whatever the telescope-astrocam is ‘seeing’ and from the relative positions of the stars in that image calculates what bit of sky the telescope must be pointing at. From there it’s a simple task to work out what correction to that direction is needed to get to the designated target. Indeed, within the software I’m currently using (SharpCap – other packages do similar things) it’s then possible to use the correction identified automatically to slew the telescope to the right direction. However, the Earth is still rotating beneath the night sky so our target will progressively drift off centre unless the telescope is moved in the opposite sense. (A setting-up process called Polar Alignment is a prerequisite for all this; the telescope’s mount is tilted to the observer’s latitude and aligned to the Celestial Pole – near Polaris.) This is where guiding comes in. A small secondary telescope and astrocam is used with appropriate software (I use PHD2) that locks onto the position of stars in its field of view and controls the motion of the telescope’s mount such that they stay in the same position: in other words, it ensures that the telescope accurately tracks the stars. Now we’re in a position to capture objects for extended periods of time rather than rely solely on polar alignment and the need to add together (stack) multiple images of only a few seconds duration. I’ve written about these steps in an earlier post, here.

What follows is a set of images showing the star clusters I have imaged thus far. The quality is mixed, to say the least, which is exactly what you’d expect given the continuing – indeed, never-ending – learning curve I’m on. There are two very distinct types of star clusters: globular and open. The names say it all really, globular clusters comprise groups of stars in a spheroidal arrangement – a ball of stars if you will. Open clusters comprise a group of stars with a separation large enough that each of them can be resolved … assuming they are bright enough to be seen at all. The one thing they have in common is that they exist and orbit within the Milky Way as a group; they are gravitationally bound to one another.

On the left is the best I could do with my first telescope – as described in the previous post; it shows the brightest star in the Pleiades and its near-neighbours. I have inverted the image, which is akin to creating a negative of the sort used by astronomers before digital cameras; it can make it easier to pick out the fainter details. The numbers shown refer to the apparent magnitudes of the stars – a term I outlined in the previous post when discussing brightness. On the right is a more recent view of the entire visible open cluster, Alcyone is central and about 1/3rd down from the top. This is high on my list for a return visit when the conditions are right since it’s travelling through a cloud of dust and gas and the cluster’s members illuminate it at a faint level. I have written at length about the beguiling Pleiades in an earlier post, here.

There seems to be nothing astronomers like better than giving particular groups of stars a descriptive name. The tendency began, I suppose, with the classic constellations – most of which comprise stars not actually held together by gravity but merely ‘lined up’ in the view of the observer. Thus, the Pleiades or any of the clusters shown here would be a defined cluster from whatever viewpoint one might have within the Milky Way but the Plough, for example, only has that shape when viewed from our solar system – it is an asterism. Making out why the above open cluster is called the Pyramid is challenging, so I have included an image on the right which might help 😉.

My final open cluster is called the Beehive – a name that continues to evade explanation to my mind. Although not visible to my setup, there are probably about 1,000 stars in the cluster in total. Its more formal designations include M44 or NGC2632. The ‘M’ denotes the eighteenth century astronomical catalogue begun by Charles Messier, who devoted much time and effort in the compilation of a list of ‘fuzzy’ objects which were not comets. ‘NGC’ stands for New General Catalogue of Nebulæ and Clusters of Stars, although the term new is relative since the catalogue and its supplements were put together in the late nineteenth and early twentieth centuries.

M56 is an interesting globular cluster in theat the stars have a particulalrly low metallic content (compared to our Sun for example). This suggests that the stars are unusually old: few new stars incorporating the heavier elements released in the death of earlier-generation stars. In addition, the cluster is moving counter to the overall rotation of the Milky Way and that suggests that it might have form outside our galaxy and then been captured at some time in the past.

M92 is a globular cluster which has a mass equivalent to ~330,000 Suns. Like many globular clusters its metallicity is relatively low, leading to estimates of its age in the region of 11 billion years.

M3 has a similar morphology to M92 but a higher mass (~½ million Suns) and although still low by galactic standards, its metallicity is a little higher than M92’s. It sits quite a long way above the plane of the Milky Way, which sets it apart. M3 contains an unusually large number of variable stars.

M13, the Hercules cluster, is arguably the most stunning globular cluster visible from the northern hemisphere. It probably has as many stars as M3, but is far closer to us.

The next post I plan to write in this series considers parts of the Milky Way which require us to go up in scale. Indeed, the targets won't all fit into my expanded field-of-view so we'll be viewing specific regions of them. These are the nebulæ: vast clouds of dust and gas. However, the idiosyncratic naming beloved of amateur astrophotographers will continue unabated.

There are several earlier posts which will give you the 'backstory' to this current short series. They'll be obvious if you peruse the blog. The first one I wrote on this post-retirement hobby of mine was uploaded a little over five years ago and focused on imaging the Moon and planets, as were several that came after. However, an alternative to reading those would be to watch a recording of the talk I gave in January 2025 which summarises the earlier stages of my journey: there’s a link to the YouTube capture in the first paragraph of ‘Climbing over Failure’.

Binary

This is, if I can carve out the time, hang on to my present motivation and avoid the distraction of the many other ‘shiny things’ in my world, the first in a short series of image-rich posts. I’d like to share my progress in astrophotography – a hobby started two or three years after I retired – and in the process to try to order my own thoughts and plans as I approach my mid-seventies. I currently envisage a post on the binary star systems I’ve looked at (this one), a follow-on post focused on star clusters, then a look at stars either in the process of ‘dying’ or via their post-explosion remnants. Somewhere in that mix, most probably in the latter post, I’ll bring in my images of vast clouds of gas and dust, and the shockwaves one can occasionally see within them. All of these astro images relate to sights within our own galaxy, The Milky Way, but the final post I want to write and share will focus on my images of other galaxies ranging in distance from our nearest neighbour, Andromeda, to those at distances large enough that their light took hundreds of millions of years to reach the little telescope in my garden.

We’ll start with binary star systems simply because I have already said a lot about them in posts written early on in my astrophotography hobby. These systems were of the necessarily easy-to-find variety given that my equipment, at that stage, was relatively simple and therefore limited: the Mizar-Alcor system, which is part of the Plough, and the Castor system within the Gemini constellation.

Mizar is one of the stars in the ‘handle’ of the Plough. It’s dimmer companion Alcor can be spotted by the naked eye under suitable conditions (i.e. a relatively young eye and a sky without a lot of light pollution!) and is easily spotted using binoculars. This is arguably the easiest binary system to start with. Add a telescope and entry-level astro-camera into the mix and it’s possible to make out that there are three stars in the system: a closer binary, Mizar A and Mizar B, and Alcor. Indeed, this deceptively simple and easy-to-spot system is even more fascinating because each of those three stars is itself a binary – six stars in total dancing around as pairs within pairs within a pair. Although a teeny bit more difficult to locate, the Castor system has a very similar makeup: Castor A, B and C – each of which is itself a binary. (Almost all binary star systems comprise a larger, gravitationally dominant star and a smaller partner. They’ll rotate around a common point in space, which will be closer to the large star. This is driven by the same physics which defines the barycentre between Earth and Moon, and also the Sun and the rest of the solar system. In the case of the Earth-Moon system, both bodies orbit around a point approximately 4,670 km from the Earth’s centre. I wrote about it here and there’s a useful YouTube animation here if you’d like to know more.)

My next step involved looking for colourful binaries – typically binary systems with a blue, white or yellow partners. The perceived colours are affected by our brain’s interpretation of adjacent points of light, but that doesn’t detract from the beautiful sight. It is unfortunate that at this point in my journey I hadn’t realised just how much the mirrors in my Newtonian reflector telescope were in need of re-alignment and calibration. One day I ought to retrace my steps and image them again, but …

The yellow-blue binary Almach (in the constellation of Andromeda) and Cor Caroli (in Canes Venatici) are two such systems, and very pretty they are too.

There is a caution to offer at this point: not all pairs of near-neighbours in the night sky are binary star systems. Yellow Albireo (in Cygnus) and its blue companion, for example, is definitely a pretty sight. However, the jury is out in terms of whether they are actually a gravitationally linked binary system or simply an unconnected pair of stars moving independently within the galaxy which, for a time, appear as a double when observed from our viewpoint. If there is a gravitational link then it’s tenuous.

Albireo: binary or double?

Having an overwhelmingly bright star in a binary system can be a problem when trying to obtain an image of the pair. Regulus is a good example of this; in order to see Regulus B I had to over-expose Regulus A. This is a good juncture for a working definition of a star’s apparent magnitude (- actually, every celestial object has an apparent magnitude when viewed from the Earth). The magnitude represents the object’s brightness; it’s a logarithmic scale: each step of 1 in magnitude represents a change in brightness of approximately 2½ times (more accurately, 2.515). Although it’s not immediately obvious, the more negative the magnitude, the brighter the object. Thus, the Sun has an apparent magnitude of -27, the brightest star visible from the northern hemisphere, Sirius, has a magnitude of -1.5 and Polaris, the Pole Star, sits at +2. Under good conditions the human eye can see stars as faint as magnitude +5. Regulus A has an apparent magnitude of +1.35 (so brighter than Polaris) but Regulus B sits at 8.1 – the difference in brightness on this logarithmic scale is a factor of over 500!

Regulus A & B (the latter at the 7 0’clock position) in the constellation Leo.

One of the peripheral uses of these images was to estimate the ‘performance’ of my telescope-astrocam combination. All of the above images were captured through a 150 mm diameter Newtonian reflector having a focal length of 1200 mm (Skywatcher 150PL) and an ‘entry-level’ camera (Altair Astro 290c). It was a fun thing to do (certainly for a geek) and it gave me confidence that everything was behaving as it ought to be.

Using the Mizar-Alcor system it was possible to verify that my camera’s field-of-view was indeed very small: only ¼° on its long axis! However, it could resolve objects a mere 0.002° apart. The actual distances shown, 1 lightyear between Mizar and Alcor and 55 light-hours between Mizar A and B, are quite short on the cosmic scale – the nearest star to the Sun, for instance, is over 4 ly away. Neptune is about 4¼ light-hours from the Sun.

Newtonian and beginning astrocam on the left, more recent 'deep sky' setup on the right.

When I started getting deeper into the hobby, buying a shorter focal length refractor telescope (Altair Astro 80EDR, focal length 440 mm with 0.8 reducing lens) and an astrocam with a far larger chip (Altair Astro 533c) – both bought second-hand from people I knew – imaging binary stars became a useful ‘filler’ between long runs on larger and fainter objects like galaxies and nebulae. The images below are all taken with this new setup, which has a square field-of-view of 1.4°. There are binary star systems pretty much everywhere one looks …

Even Polaris is a binary system

Next stop, star clusters.

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P.s. there are several earlier posts which will give you the 'backstory' to this current short series. They'll be pretty obvious if you peruse the list of posts on my blog. The first one I wrote on this post-retirement hobby of mine was uploaded a little over five years ago. 

Climbing Over Failure

On Saturday 18th January I gave a talk at the monthly meeting my local Beacon Astronomy Group (here, or here). I was given a gentle brief a couple of months prior – as befits my very amateur status – by the co-organisers Dirk Froebrich and Tim Long: “Tells us about your journey into astrophotography”. It’s a brief that risks self-indulgence from the speaker; I’m sure I succumbed now and again, for which I apologise the those who came to listen, as well as to those who watch Tim’s recording of the talk. The questions, after I’d stopped talking, came thick and fast. I hope this was a good sign – at least people were listening. There were several more open-ended one-to-one conversations as I was packing up; these continued all the way to the car park. I spotted smartphones capturing the odd slide as well which, again hopefully, tells me that there were snippets of information folk found interesting or useful. All good. More than once I promised to complement the talk with a blog post containing the salient bits and pieces: this is it.

In truth, much of what I’ll include here has already been covered in earlier blog posts; I have been writing about aspects of my retirement hobby since the Summer of 2020. However, it’s probably easier all round to extract, condense and augment that material into one brief post than to expect you to follow a load of links and distil everything yourself. I’ll take you through the slides, including a few screen-captures, like the opening slide above, if that helps us navigate. As is customary, I began with an overview and tried to make the point that, although stargazing and photography have interested me since my early teens in the 1960s, it wasn’t until I retired that I could attempt to marry the two. Indeed, I didn’t add a dedicated camera to my first ‘proper’ telescope for almost a year after buying it; I had an enormous amount of fun simply looking through an eyepiece at the beauty above my head. That first telescope, bought a few months after I retired, reflected the advice to beginners I remembered from an episode of the BBC’s ‘Sky at Night’ programme (- which was broadcast in B&W until I was 17 years old!): it was a 150 mm (6") Newtonian, the Skywatcher 150PL. Had I tried harder to update my knowledge or, better still, talked to a few more experienced amateurs, I might have made a different selection. But, then again, perhaps I wouldn’t …

I know others have succeeded, but when I tried to capture images of what I was looking at using my smartphone, it wasn’t very satisfying.

At this point in the timeline I give huge credit to Dirk Froebrich for the opportunity to experience astronomical imaging at a professional level. Dirk, a colleague of mine before I retired, needed someone to look after the Beacon Observatory during the periods he was away over one particular summer. This was a time when his Citizen Science project ‘HOYS’ was in its infancy – there are many more volunteers and partners now. (This is a wonderful project and I heartily recommend it.) I was trained in the basics of running the observatory remotely from my PC at home and sat through quite a few clear nights following the observation script I had been given. I confess to being little more than a ‘trained monkey’ at that point, and I know I made more mistakes than I ought, but I did collect usable data which found its way into a full-blown research paper. I was hooked.

My first dedicated astro-camera was a simple ‘entry-level’ colour model, the Altair Astro 290c, and with it I managed to image all the planets other than Mercury (which I can only ever see through binoculars from an upstairs window due to surrounding trees, houses and so on). I could also image bright/easy-to-find individual stars and a few binary star systems. When trying to image the Moon or Sun (using homemade solar filters – take care!) the limitations of my little astrocam were immediately apparent: it enabled great images, but of only a small portion of these larger targets.

The field of view, FoV, from my reflector telescope and astrocam combination is indicated by the yellow rectangles shown in this smartphone-captured image of the Plough (part of the constellation Ursa Major). This was adequate for binary stars and planets but less so for the Moon and Sun and far too restricted for larger targets such as nebulæ and nearby galaxies: the Andromeda galaxy, for example, covers five times as much of the sky as the Moon (- although it’s so faint that we don’t see it sitting beyond the foreground stars of our own galaxy, the Milky Way). 'FL' in the light blue insert above refers to Focal Length by the way.

Even relatively simple astrocams (or DSLR cameras) need specialist kit and software if one is to gather imaging data successfully. The tripod and mount on which the telescope sits must be able to compensate for the Earth’s rotation: to keep pace with the stars as there appear to move across the sky and thereby to keep the chosen target in your system’s field of view. A key first step is to be able to polar align the mount, which means aligning it to the celestial pole – near to Polaris. I mostly use a Skywatcher HEQ5 mount. Beyond that, software is needed to run the camera and thereafter to process the exposures collected.

For pre-planning and target identification I use Stellarium and Carte du Ciel on my PC, along with useful tools such as those found here, here and here. Useful phone apps include SkyMap, Lunar Map HD and Stellarium along with your favourite weather apps (I use the UK Met Office app alongside ‘Weather & Radar’). For capturing data from my astrocam I use Sharpcap and in order to process that data through to a final image I’ll typically use packages like Autostakkert, Registax, Photoshop / Affinity Photo2 and PIPP. A quick online search will lead you all of these. There are a great many ‘tutorials’ on YouTube, enough of which are sufficiently useful to get you started; better still, join your local amateur astronomy club and tap into the expertise of more experienced people. I’m a member of two that meet in my part of the country, and although I can no longer attend meetings in person I can nevertheless ask questions – and sometimes answer them – via social media, typically Facebook.

There’s one important aspect of all this I’ve yet to mention: the need to mitigate the effects of atmospheric turbulence. The higher the magnification, the more your target will appear to ‘wobble’ as it’s distorted by changes in the atmosphere; it’s the same issue that leads us to talk about the twinkling of stars. The way forward is to use a statistical approach: a method referred to as ‘lucky imaging’. We collect many individual exposures (hundreds, often thousands, using Sharpcap in my case) pick only the best, perhaps the best 10-20%, and then stack them (integrate them in essence; this is where Autostakkert comes in) to generate a decent image. This may be sharpened, colour-balanced or even colour-enhanced, labelled and so on using packages such as Registax and AffinityPhoto.

So far, so good. We still have the issue of a narrow field of view, FoV, and the matter of locating those fainter objects like galaxies and nebulæ that we can’t actually see.

I solved my immediate FoV problem by buying a telescope having a shorter focal length (Altair Astro 80EDR to complement the Newtonian) and an astrocam with a significantly larger detector chip (Altair Astro 533c). With an optical add-on called a reducing lens, this enabled me to increase my FoV to a whopping 1.46 x 1.46°. Whilst a huge improvement, this still represents about as much of the heavens as you can cover with your thumbnail held out at arm’s length: there’s a lot to see up there!

So-called GoTo mounts help circumvent the issue of navigating to faint objects, although I confess to having only modest success using the default control pad that comes with them – especially when limited to my original narrow FoV. I now rely on using Carte du Ciel, which controls my Skywatcher HEQ5 telescope mount from my laptop via ASCOM-EQMOD software … and a game-changing process called plate-solving. The beauty of plate-solving is that the various software packages offering it can use an image of whatever patch of sky you’re pointing at and employ the pattern of the stars present to identify exactly where you are. Integrate that utility within a program like Sharpcap (or APT, or …) and it becomes straightforward to designate a target within Carte du Ciel, which will slew the telescope to somewhere in its close vicinity, then use the plate-solving utility in Sharpcap to refine your targeting.

Having navigated to our target object, with a properly polar-aligned telescope+astrocam combination offering a suitable field of view, we can begin taking the longer exposures necessary for fainter objects. Depending on the mount’s alignment and the quality of its tracking motion one might expect to achieve exposure times of the order of ten seconds or so. It’s still necessary to collect many frames and then to stack them, although the software for this is somewhat different to that typically used in the context of ‘lucky imaging’. I use Deep Sky Stacker, DSS, for this. The image above, unfortunately in monochrome rather than colour due to an error on my part, is derived from 132 4s exposures. In addition, calibration files now become desirable in order to avoid problems with electronic camera noise, bits of dust in the optics etc. – these may become noticeable for longer exposures. Atmospheric turbulence is still present of course, but with exposures very much longer than those typically used for solar system ‘lucky’ imaging, we have by default averaged these distortions. (Thus, light from stars will appear to be spread across several pixels.)

The next important step is to increase exposure times from seconds to minutes. Even with excellent polar alignment and a good tracking mount this isn’t practicable without the use of guiding. In essence, one fits a small secondary telescope, the guidescope, to the main one such that it’s pointing in the same direction. A more modest camera is fitted to the guidescope (ideally monochrome, but that’s not essential) and this is connected to the laptop and to the tracking mount. Using a piece of software called PHD2, which apparently stands for ‘push here dummy’ v.2, our guidescope and camera may be set up to lock onto the positions of a number of the stars in its field of view – meaning that the tracking mount’s position can be corrected in real-time. In theory, and mostly in practice, this means that we’re now safe to use exposures running to hundreds of seconds.

In the screenshot above you’ll notice the circled stars that are being used; there’s a ‘target’ on the right illustrating the scatter in position around the designated centre – it’s calibrated is seconds of arc (one arcsecond is a sixtieth of an arcminute, which itself is a sixtieth of a degree). The interesting bit is the plot at the bottom: this shows the error in Right Ascension (RA, East-West) and Declination (Dec, horizon-pole; see here) – and the corrective pulses sent to the RA and Dec motors in the tracking mount to bring it back to the axis. 

The end result, after lots of image processing which I’m not going to write about here, is an image like this …

My penultimate slide sums it all up:

Whatever else you do, please continue to treat yourself by looking upwards on those beautiful clear nights whenever they come along.

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.

Colours of the Sun (1)

Part One: a bit of light science

For my own peace of mind I must begin this post 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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What colour is the Sun?

Red and orange, as it rises or sets? Yellow, when it’s low in the sky – and in pretty much every child’s painting? What about in the middle of the day when it’s too bright to stare at, and what does it look like to the astronauts on the International Space Station? Just what is the true colour of the Sun?

It depends …

What we perceive is an admixture of the light that the Sun emits (all the various colours, or wavelengths, it generates), the effect of whatever that light travels through and finally our ability to detect it. Our eyes – actually, our eye and brain in combination and assuming the absence of colour blindness – see reds, oranges and yellows when the Sun is nearer the horizon and its light is therefore passing through a lot of Earth’s atmosphere. The shorter wavelengths of light, towards the blue-violet end of the rainbow, have been preferentially spread out by process called Rayleigh Scattering. This describes the scattering, or spreading, of light by particles smaller than the wavelength of the light: the scattering increases as the wavelength of light shortens. Thus, the blue-violet end of the rainbow spectrum is scattered widely, giving us a blue-coloured sky, whilst the redder colours, with longer wavelengths, are scattered less and are therefore more likely to reach us from the Sun’s direction.

If we can reduce the intensity of the sunlight to safe levels when the Sun is high in the sky and thus passing through much less atmosphere, we’d see the Sun as a whitish disk. Our friendly local astronauts, free of our atmosphere altogether, would also tell us that it looks white. This is because the Sun is emitting light across an incredibly wide range of wavelengths. Although its emission is most intense at wavelengths corresponding to green light, the presence of wavelengths to either side add together to give us white (see here and the figure below for a little more information). Ben Harding, a long-time member of a local amateur astronomy society, SEKAS, and a source of considerable useful advice, pointed out to me an excellent way to demonstrate the Sun's colour when it's high in the sky and therefore not going through a lot of atmosphere. At the many public events SEKAS participates in he projects a suitably attenuated image of the Sun from a telescope onto card in order to show everyone that it is indeed white - see image below, on the left. This image is used with his kind permission. It turns out that we can also get a hint of this from our Earth-bound vantage point even without equipment. Light summer clouds appear as near-white in colour due to a process called Mie Scattering which describes the scatter of light from spheres of a size comparable to the wavelength (colour) of the light – microscopic droplets of water in a cloud fit the bill. In this case, there is no significant variation in the strength of the scattering process with the wavelength of light: thus, all colours are affected approximately equally and we therefore get an idea of the actual colour of the Sun, white.

Solar projection revealing the Sun's true midday colour. On the right is a graph showing the intensity of light emitted by the Sun across the visible wavelengths. It approximates to something called Black Body Radiation, with a peak intensity in the green part of the visible colours but significant intensity to either side and out towards ultra-violet and x-rays and towards the far infra-red; see here. Note the sharp spikes in intensity at specific wavelengths; these tell us about the chemical make-up of the Sun; we’ll return to them below.

This is my first ever image of the Sun. It was generated from a stack of frames collected by my entry-level astrocam (Altair Astro gpcam2 290c) and my first ‘proper’ telescope bought post-retirement (Skywatcher 150PL on an EQ3/2 mount with retro-fitted drive motors; see here for the full story) and a homemade solar filter (see here). My field of view was limited to 0.27º x 0.15º so I only captured a portion of the Sun’s disk which has an apparent diameter of about ½º. However, it’s enough to illustrate both the colour of the Sun in visible light and the granulation – the result of thousands and thousands of convection currents, like the swirl of cold milk poured slowly into hot coffee. In fact, the Sun’s surface is reminiscent of the shell of a chicken’s egg. There are lots more images in Part 2 of this post.

This is my current setup for imaging the whole disc of the Sun. The telescope is an AA Starwave 80edr fitted with a Baader 2x Barlow lens and an AA533c camera cooled to -10°C; the mount is a Skywatcher HEQ5, controlled from a laptop via ASCOM/EQMOD using Carte du Ciel and SharpCap (URLs for the software are in the appendix to an earlier post, here, together with installation notes).

Having made a start at describing the colours of the Sun according to our eyesight we now need to dig a little deeper. We tend to assume that we’re looking at the surface of the Sun, but it turns out that things are not entirely straightforward in that regard. Remember, the Sun is a huge ball of gas; it’s a pretty good sphere, which one might expect since both the gravitational force pulling it in and the pressure trying to push it out are both acting in all directions uniformly. However, what we think of as the surface of that sphere is in fact the deepest of the three principal outer layers of the Sun: the Photosphere  (It’s called that because it’s where we perceive most of the Sun’s light to come from: hence, ‘sphere of light’.) The photosphere is at about 4000-6000°C depending on altitude. This is what we see if we look at the Sun through a neutral, or white-light, safety filter. It’s also the layer in which we can see sunspots (patches with a slightly lower temperature which, as a result, appear dark) and faculæ (typically, nearby areas of slightly raised temperature which therefore look brighter).

Above the photosphere is the Chromosphere – literally, sphere of colour. Its temperature rises with altitude to over 8000°C and it glows with a red colour derived from the hydrogen plasma that makes it up. We don’t normally see the red because the chromosphere is of much lower density than the photosphere and the intensity of the light generated is therefore swamped by the denser photosphere. It is however possible to see it when the brighter photosphere is blocked out at the peak of a total solar eclipse. The chromosphere is the layer of the Sun’s atmosphere associated with giant prominences that may extend thousands of kilometres into space, and filaments (essentially, prominences seen from above) and spicules. The latter are relatively short spikes of red-glowing hydrogen that can give the Sun a sort of ‘grassy’ look. There’ll be several of my images in Part 2. Further out still we get to the Corona, but I’m not going to dwell on that in this post.

So far, we’ve established that the colour we see depends on the temperatures present in the Sun’s outer layers (Black Body Radiation),what the sunlight has travelled through (e.g. Earth’s atmosphere) and the workings of our eyes (or our cameras). We’ve also had a hint that the chemical make-up of the Sun has an influence (e.g. the red colour of the chromosphere being due to hydrogen). The additional important factor arises from the fact that, whilst the Sun is mostly composed of hydrogen (H) and some helium (He), there are many other elements present in small quantities. We can use them to look at the Sun using particular wavelengths/colours of light – these are the emission lines uniquely associated with each given chemical element. For example, if we put energy into a hydrogen atom – and there’s no shortage of energy in the Sun – it enters what’s called an excited state. The natural next step is for that atom to shed the excess energy, typically by emitting a packet of light (a photon); the beauty of that process is that every element has its own ‘fingerprint’ of emitted photon wavelengths or colours. Thus, a principal colour emitted by hydrogen is red; it’s often labelled as Hα, or H-alpha, and has a wavelength of 656 nm (nanometres, 10^-9 m). If we observe the Sun through a filter that transmits only this wavelength of light then we’ll see a red Sun. Moreover, by picking up that one particular wavelength alone we’re effectively looking specifically at how hydrogen, to the exclusion of all other chemical elements, is behaving within this solar environment.

We can track through the ‘rainbow colours’ by using filters that single out the wavelengths emitted by other elements present on the Sun. Sodium, Na, emits light primarily at 589 nm, which is yellow; magnesium, Mg, emits photons at 517 nm and that’s a green light; calcium, Ca, emits blue light at 393/396 nm. For each colour (or wavelength of light) we are focusing on a single element and thus on a slightly different aspect of the Sun’s behaviour; in effect, we can use these selected elements as an internal solar probe in order to complement observations in ‘white’ light (i.e. all the wavelengths together – our view of the photosphere). There is a caveat: the effect of Rayleigh scattering (see above) is to scatter the shorter wavelengths preferentially and getting good Earth-bound images using the blue light from Ca is therefore hampered. As a consequence I show no Ca-light images in this post.

It is in fact possible, with the right equipment either on Earth or on a satellite, to focus on the light emitted from excited atoms of each of the elements in the periodic table up to iron, Fe. (There is insufficient energy in the Sun to make anything heavier than iron – that requires the sorts of energies associated with a supernova explosion. Remember that the next time you look at gold jewellery; it exists because earlier generations of giant stars have died 😉). There is additional information here, here and here.

For we amateurs, however, the bright emission wavelengths of hydrogen, sodium, magnesium and calcium are accessible. The cost of the kit is, however, a very long way beyond my budget. This is particularly true for those wishing to collect images using the light from hydrogen-alpha emission: thousands of pounds. Hence my own ‘white light’ setup which depends only on reducing all the incoming wavelengths to leave only one part in 100,000; I can image only the photosphere: sunspots, faculæ and granulation. Although I’m mostly content with this state-of-affairs I did treat myself to a relatively inexpensive gift during the Summer: I paid for 90-minutes of guided access to a solar imaging setup in Spain. The details are here. I recommend it to anyone with an interest in this area. 

I’ll share some pretty pictures in Part 2 of this post, along with a bit of nerdy stuff on image processing. Until then, keep looking up (safely) …

The Last Moon

Every year for several years I’ve had an invitation spend the morning with two classes of Year 5 pupils at the Churchill School during their ‘Space’ curriculum topic. Not once has this been less than a delight; it’s always great fun (see here and here to get a flavour). This year’s trip took place in late June – timed such that their preferred target of the Moon was up in the daytime and was no more than about half in sun. The reason for not wanting either a crescent Moon or a full Moon is that we want a chance of seeing a selection of craters, lava basins and mountain ranges but we also want some decent shadows arising from a low Sun in the lunar sky. On that latter point, the ‘best’ shadows are those near the terminator – the line that divides night from day. There are several options for such planning, but the website I tend to use is here.

This is my small, i.e. reasonably transportable, telescope and mount: a Skywatcher 72ED on a Skywatcher EQ3/2 mount (- the image above wasn’t taken during this visit). I’ve retro-fitted motors to the mount in order to track the motion of objects across the sky as the Earth rotates; indeed, this is an enormous help when attempting a ‘mass observing event’ since the target – the Moon – isn’t continually drifting out of the eyepiece’s field of view.

Having learnt the hard way that the variation in height amongst ~60 ten year olds can make viewing tricky I’d purchased a right-angle for the eyepiece which could easily be rotated to suit all-comers. That worked well. Inserting an additional lens (a 2X Barlow for those who wish to know) which, in effect, doubles the magnification was less successful since it has the effect of ‘dulling’ the image. At night this isn’t a problem when viewing the Moon, but on a bright and relatively humid morning in mid-summer when the contrast is already reduced it had the effect of ‘washing out’ the details on the lunar surface. However, once committed to the setup there was no time available to reconfigure everything once observing began. One lives and learns.

This wasn’t taken on the day – it’s an image I prepared earlier ;-) However, it does show the sort of view I had hoped to show everyone – albeit somewhat degraded by the daytime conditions. For anyone wishing to identify features on the Moon when observing by eye, using binoculars or a telescope I’d recommend a phone app called ‘Lunar Map HD’ – although there’s a lot of other online resource also available.

Unfortunately, this is the sort of view we all got: the moon on a bright, humid day in the middle of summer - washed out.

Everyone saw the Moon through my telescope as far as I know, although it took some people less time than it did others to get their eye in just the right position to see it. (One person did come back for a second attempt right at the end, having failed to see much when it was their turn. I’m glad that they did; it would have been such a shame to have said nothing and as a result missed the experience.) After we’d finished there was an hour of Q&A back in the classroom. It’s rewarding to be able to show people a view of the Moon that they might not have had before, but it’s definitely huge fun to attempt to field an apparently never-ending stream of testing questions. Every year I’ve been invited to visit this school I’ve been blown away by the energy and insight behind the questions. Yes, there are some that might be written off as left-field or off-piste – but that would be to miss a key point: these are all young people who’re engaged with the wider subject matter, and many were genuinely well-informed. Enthusiasm is worth a lot.

Nothing trumps break time, and nor should it – particularly at that age – and the time rapidly arrived for me to take my rig apart again and return it to the boot of my car. I was helped by my two wonderful Year 5 host teachers, in the course of which they told me about their new teaching assignments for next year. Apparently neither of them will be teaching Year 5 – so my astronomy/space mornings have in all probability come to an end. That visit may well prove to be the last observing session I assist with at the school; no more shared Moon ...

Over my head

[I confess: this is a hybrid post in which I have broken the golden rule of 'knowing my audience'. As a result, you may well choose to read only part of it. The opening few paragraphs speak of my experience in the planning and presentation of a talk on astrophotography to a non-expert audience. After a couple of tiny footnotes you'll get to two appendices. The first of these was written in response to requests to provide a synopsis of the above talk; it includes a lot of links to various packages etc. So far, so good. The second appendix was added as a note of my parallel struggles trying to get a replacement laptop to run all my astrophotography needs in the way my rapidly-fading older one did; it's distinctly nerdy and is also replete with links.]

House of (more) Treasures

 

In a recent post (here) I waxed lyrical on the subject of my first visit to the Canterbury Cathedral Library and Archives: “[sitting] at a desk surrounded by old wood with light filtering in through handmade glass, and to hear at one point the cathedral’s bells drifting through high ceilings”. I have now embarked on a six-session u3a course in the history of printing in Europe, led by Dr David Shaw, so get to spend even more time there. Such a joy.

The course itself, which is proving to be a delight, has thrown up several nuggets of information to be nestled in the memory, awaiting their time. For instance, did you know that the terms ‘lower case’ and ‘upper case’ derive from the days when a compositor – the person who set each letter of a font in place in order that a page might be printed using a manual printing press – had to select the next letter in a given word: their font cases were arranged such that the more common letters, ‘e’ for example, were close at hand (literally in the lower of the usual arrangement of two font cases) and those less commonly required, capital ‘Z’ perhaps, were in the more distant or upper case. When working with the speed allowed by ‘muscle memory’ this could save a lot of time and effort, rather like touch-typing – a skill I have, regrettably, never properly acquired. It’s no wonder the apprenticeship lasted seven years. This would often be followed by a period as a ‘journeyman’ during which the person would travel to various printing works in order to expand their experience and expertise. The size of a font was also defined at this stage, with 72-point corresponding to one inch (1″) – thus, a 12pt font corresponds to letter/number heights that fit within 1/6th of an inch or a little over 4 mm. This was, evidently, an early example of industrial standardisation; paper sizes were similarly standardised.

However, my principal focus in this post is to mention one of the books that David thoughtfully made available for us to marvel at during our mid-session break: Robert Boyle’s 1660 work on what we would now think of as air pressure and the like. Robert Boyle was a founding member of the Royal Society and made seminal contributions to the physical sciences; indeed, the slightly younger (but perhaps nowadays more famous) Isaac Newton used some of Robert Boyle’s work in order to derive an equation for the speed of sound in air. It is a personal pleasure to be able to turn the pages of this beautiful book; moreover, in a straw poll of the twelve other u3a members with me on this course, I discovered several people had retained a memory of hearing about ‘Boyle’s Law’ from their school days – a testament to his legacy.

Having discovered – whilst drafting the earlier blog post referred to above – the extent of the time and energy required of the Cathedral’s hard-pressed Archive & Library staff to generate and supply images of old documents in their collection, I was delighted to find online a ready-made image of another copy of the this very book. The above title page and example illustration comes courtesy of the Science History Institute and is made available under Creative Commons Public Domain Mark 1.0 Universal.

Not only was it fascinating simply to see and touch this original copy of Robert Boyle’s ground-breaking work, but I learnt from David that there was a (tenuous) personal connection to the story of how this rare book came to be in the Cathedral’s collections. As may be seen in the document reproduced below, the book was a part of the collection of a former Rector of a parish not far from where I live. The Rev. Richard Forster was born only seven years after Robert Boyle’s death; he amassed an impressive library containing many hundred books and much else beside. As one might expect, works on theology were prominent but alongside those were mathematical and scientific books. In his will he left the library to his successors in the role. At some point the task of being Rector of Crundale was combined with that of being Vicar of the nearby parish of Godmersham and the library was transferred to the beautiful vicarage at Godmersham.

Reproduced from a document on David's website (here).

The vicarage was situated just outside the boundary wall of Godmersham Park which, as her fans will know, has a close connection with the writer Jane Austin. It also commanded enviable views across the River Stour and as a result of this proximity and its age, suffered from damp at ground level; its library was accessed via an impressive spiral staircase. My parents-in-law lived in the Godmersham parish; indeed, my father-in-law served as churchwarden for fifty years. Moreover, my wife and I were married in Godmersham church in the late 1970s. The wedding was conducted by the then Vicar/Rector, Canon Graham Brade-Birks – who has been mentioned with affection in a previous post on this blog site, here. I’m pretty sure that he was already eligible for retirement when he conducted our wedding service but he was a man with the clear conviction of his calling, and retirement was postponed for as long as was practicable. However, after he eventually retired, the vicarage was sold (to ‘someone in television’ as I recall) in order to raise funds for the Church of England and there followed an extended interregnum: there was, therefore, no successor to whom the library’s contents could be passed. Thus, although non-stipendiary (unpaid, usually part-time) vicars/rectors were subsequently appointed, Canon Brade-Birks was indeed, in effect, the last of the line. Hoping to ensure the survival of the library’s contents, he left it to the nearby Wye Agricultural College where he had taught the odd course on soil science. (I have also written a post mentioning Wye College, where I was employed for a year after leaving school – see here). At the time, the College was a constituent part of the University of London. It was later subsumed into Imperial College and then closed and sold off; the collection that had been looked after by Canon Brade-Birks was eventually handed into the care of Canterbury Cathedral’s Archive & Library.

As I admitted, this is a tenuous link. For such links I am, however, grateful.

 

 (I acknowledge with thanks the editorial suggestions offered by David Shaw.)