Beginnings and endings and the rise of the robot: cosmic clouds and bubbles

This is the third in a planned series of four image-rich posts within which I hope to share my progress in astrophotography; the first is here and the second is here. I decided to write these posts as a means of taking stock of what I’ve achieved in the years since taking up this challenging hobby in my mid-60s. There’s nothing like writing about something to sort out one’s thoughts. The 'backstory' to this current short series may be found spread through several earlier posts: they'll be obvious if you peruse the blog. 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’.

The type of astronomical target I’ll introduce here, via some more of my images, are the nebulæ (- nebulæ is the plural of nebula). A nebula is a cloud of gas and dust in space which, in the context of astrophotography, either
o blocks the light from the stars behind it getting to us or
o reflects the light of one or more bright stars between it and us or
o appears to emit light which has, in fact, been generated within it by bright stars.
These are dark, reflection and emission nebulæ respectively. I have captured images of all three types, together with a subset of nebulæ associated with material that has been ‘puffed’ out by an ageing star or in some cases thrown out violently as the star explodes as a supernova.

By the time you get to the fourth post in this series – I’m being optimistic you understand: of your reading stamina and of my ability to sit down and write it – it will have become obvious that there is a progression in scale. Every image I have shared up to and including in this post resides within our galaxy, the Milky Way; their distance from us is therefore limited to the size of the galaxy. The Milky Way has a diameter of approximately 100,000 light years (ly) across its disk but is only about 1,000 ly [i] thick. This constrains the distance from my back garden to the objects I’ve been imaging, be they binary star systems or star clusters or nebulæ. However, the size of the object itself is tending to increase and in the case of nebulæ we might be talking of objects that are over 100 light years across.

Following the pattern of my first two posts in this series, what follows is a collection of my images; the captions will provide additional information. The quality of the images varies simply because my equipment, software and associated proficiency and skill have evolved over time. One qualitative change came with the arrival of my entry-level pet robot this Summer – a fully automated imaging system having a wider field of view than my more conventional telescopes. One of its attributes is therefore the ability to capture larger-scale targets, especially when used to create a multi-panel mosaic. However, let’s start with images derived from the deep sky setup used for the previous post on star clusters …

This is NGC7023 [ii], the Iris Nebula – yes, they all have names – which resides within the constellation Cepheus. It’s about 1,300 ly away and is 6 ly across. Most of the nebula is dark; even with my limited image processing skills one can make out extensive patches of the sky with few visible stars or none. Even where some starlight gets through the stars take on a sort of reddy hue; this is caused by the same light scattering processes that gives us our red sunsets and dawns on Earth: the blue end of the visible spectrum being scattered in all directions whilst the longer wavelength red end is far less spread out. The bright blue colour at the nebula’s centre – which is what gives it its name – is reflected light from a star (HD200775) lying between us and the nebula.

The principal nebula in Orion, M42, can be seen with the naked eye in an area without light pollution (unless your eyes have reached the age mine have!) and certainly with binoculars. However, all you’ll see is a fuzzy white blob as our colour vision isn’t great in low light levels. Stick a telescope and astronomy camera in front of it and so much more is revealed. It’s about 1344 ly away and approximately 25 ly across; it covers twice the area of the Moon (or Sun) in the sky, so if we could make it out by eye it would surely be an awesome sight. The really bright core region is being illuminated by a group of four young hot stars, the Trapezium. Indeed, the Orion Nebula is our closest region of active star formation.

 

Two versions of the same thing taken a few months apart; the right hand image is oriented about 90° counter-clockwise compared to the image on the left. This is the Bubble Nebula, NGC7635, about 7,000 – 11,000 ly away (7-11 kly) in Cassiopeia. The radiation from a massive young hot star, SAO 20575, both illuminates and ‘blows’ away material [iii] which comprises the large expanse of dust and gas in which it sits: this forms the bubble we perceive. The boundary of the bubble is in essence the shockwave between the solar radiation and the nebula. By the way, the red glow, which is a characteristic of nebulæ, is due to the dominant emission colour of hydrogen. (For more – much more – on emission and absorption lines please see my videos on the topic here and here) Apart from their apparent rotation with respect to one another, which is partly a camera framing issue and partly due to the changing sky as the year progresses, you will notice a difference in the apparent star numbers and brightness. Because this patch of sky is in the direction of the Milky Way’s disk it’s particularly dense with stars; the further one looks out from the disk the less busy the view. This effect can obscure the object we’re focused on and to counteract this I employed a piece of software called StarXterminator which allows me to remove the stars digitally, process the fainter nebula and then add the original stars back in with reduced brightness. I use it as a plugin within AffinityPhoto2.

 

Enter the robot. My relatively recently acquired Dwarf3 sets the Bubble Nebula in its broader context within Cassiopeia. Not only do we get the wider nebula but two star clusters (M52 above left and NGC7510 bottom right), part of the Lobster Claw Nebula (NGC6357, bottom, left of NGC7510) and a bright region of massive star formation (NGC7538, upper right).

 

Another shockwave caused by the resistance of a cloud of dust and gas to the radiation, or solar wind, of a star (HD 192163) within it, which became a red giant ¼ million years ago. This is the Crescent Nebula, NGC 6888, in the constellation Cygnus. It’s about 5 kly away.

 

Now that we’ve established both the origin of the red colouration of hydrogen-rich emission nebulæ and the fact that my pet robot has a wide field of view, I could more easily image this extensive area of nebulosity in the vicinity of a bright star called Sadr in the constellation of Cygnus (γ Cygni, about 1,800 ly from us). The Crescent Nebula (see above) is nearby.

 

Still in Cygnus, the North America Nebula (NGC7000) is named because it resembles … guess what? To the right, in the lower half, is the Pelican Nebula: long beak, looking in the direction of the ‘Gulf of Mexico’.

If we crop in towards ‘California’ we can pick out more clearly the Cygnus Wall – a bright star-forming region which has been sculpted by solar radiation.

 

Moving from star-forming to star endings, this is The Eastern Veil Nebula (NGC6995). Together with its Western counterpart and a lot of other material in between it originated in the supernova explosion of a large star more than 10,000 years ago. The stellar material has expanded to cover approximately 3° of the sky, so ~6 times the Moon’s diameter. Its colour comes from hydrogen emission of course, but also from the oxygen and sulfur created in the original star. Nearby stars have sculpted its shape beautifully.

 

The Crab Nebula (M1, in the constellation Taurus) is the remnant of another supernova. This explosion was witnessed in recorded history: Mayan, Japanese, Arab and Chinese astronomers observed it in 1054 AD. It’s still expanding at around 1,500 km/s. It is approximately 6.5 kly away. This was the first nebula I attempted to image; I keep meaning to go back and do a better job but new shiny challenges seem to propel me forward instead.

A smaller aging star, the size of our Sun for example, might not explode as a supernova. Instead, it may swell up and slough off its outer layers as its hydrogen stocks deplete and then shrink back to become a dwarf of some kind. The ejected material becomes a planetary nebula. This example is the Ring Nebula, M57, in the constellation Lyra and at its centre now sits a white dwarf. It is 2,570 ly away and has been expanding for an estimated 1,610 years.

The Owl Nebula, M97, is another planetary nebula; it’s within the constellation Ursa Major. The qualitative difference between this and the Ring Nebula is that the star went through more than one period of throwing material off. The inner shell of material is more barrel-shaped than spherical and the result is this very distinct form to the nebula. It is ~2030 ly away and has been expanding for about 8,000 years.

The Dumbbell Nebula (M27), another planetary nebula, is expanding from a white dwarf ~1360 ly away and is in the constellation Vulpecula. From its measured rate of expansion, one may calculate its age as being ~10,000 years. Why, you might ask, is it still glowing after all this time? Well, space is effectively a giant vacuum flask: there is no conduction or convention to carry heat energy away only radiation – and this is exactly what we are detecting.

The image above provides an excellent segue into the fourth and final post in this short series, in which I’ll present my images of truly huge and distant objects imaged from my garden: galaxies. In the centre of the image above sits the spiral galaxy NGC7331, which is at a distance of almost 44 million light years (Mly). The reason I include my image of it here is that on 14th July this year there was a supernova observed in the galaxy. It was designated SN2025rbs. Unfortunately, the star that exploded sits close to the core of the galaxy and is therefore harder to resolve using amateur equipment such as mine. However, it was almost as bright as the galactic core so, if you look carefully at the enlarged inset, you should be able to make it out.

Until next time …

Endnotes:

[i]    In other words, light emitted during the reign of King Canute (or Cnut, ruler of England, Norway and Denmark) from a star at the ‘top’ of the galaxy will be reaching the ‘bottom’ about now. By comparison, our Sun’s nearest neighbour star is a little over four light years (ly) away whilst its most distant planet, Neptune, is a mere four light hours away. The Voyager probes, which have been travelling at prodigious speeds since their launch almost half a century ago, are still less than one light day away.

[ii]   There are several catalogues in existence which list notable objects. Arguably the most widely used within the amateur astronomy community are:
M, the Messier catalogue of non-cometary objects (Catalogue des Nébuleuses et des Amas d'Étoiles) begun in 1774 with the work of Charles Messier and
NGC, the New General Catalogue of Nebulae and Clusters of Stars compiled in 1888 by Johan Dreyer. 

There are analogous catalogues for stars, for example:
HD, the Henry Draper catalogue (1918–1924);
SAO, the Smithsonian Astrophysical Observatory catalogue (1966);
HR, the Harvard Revised Photometry Catalogue (1930 and 1983).

[iii]  As an aside, I note that there is a dynamic balance in the early life of a star between the rate at which material in a proto-planetary disk is pushed away by solar radiation and the chance that it clumps together and eventually forms a solar system. The outcome is decided within a relatively brief period after the star’s formation – a period measured in millions of years.

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 (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.

_____________________________________________________________

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) …

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.)

To Capture a Sunspot: solar filters

Occasionally I post on social media images captured using one of my telescopes in conjunction with a high framerate astro-camera. The image posted most recently was of the Sun, specifically of sunspots; I have collected a lot of these images in the past few years. In part this is because we’re nearing a period of maximum solar activity in the Sun’s eleven-year cycle and there’s simply more to see, but daytime astronomy also affords benefits when trying out new bits and pieces – it’s so much easier to learn how to handle new equipment in daylight. My recent foray was a case in point; I had bought a second-hand 80 mm refractor (see endnote [1] for more information and advice) and was keen to test its features after weeks of poor weather, ill-health and general busyness. This is definitely not a post on expensive astronomical equipment however, almost the opposite in fact: my aim is to share with you how I capture images of the Sun safely without spending a lot of additional money. It is a direct response to the questions posed to me by a member of a local amateur astronomy club: “Did you buy a solar filter cap or make one? What solar film did you use?”

The very first vaguely successful image of the Sun I managed to get. It was taken using my first telescope and an entry-level astro-camera and the combination of high magnification and small camera detector size meant that I captured only a small segment of the Sun’s disc. However, it remains in the slideshow of background images on my PC because it gives an impression of size and of the Sun’s neutral colour. Look closely and you’ll also see the ever-present convection cells as the Sun’s near-surface rises and falls. I’ve put a few more details into endnote [2].

This brings up to the matter of solar filters: how to reduce the amount of light entering our telescope to a level that will neither burn our eyeballs nor fry our camera’s detector chip. I should note at the outset that I am not discussing in this post the more dramatic phenomena associated with the Sun’s surface – no prominences or flares etc. of the sort shown in the images here. These require highly specialised (and expensive) equipment which limits the light entering the telescope to a specific wavelength only. The bits and pieces I describe here will allow all wavelengths (i.e. all colours) to pass through, but at very low intensity. Indeed, the solar safety filter material I use removes 99.999% of the light; which means that only one part in 100,000 reaches the telescope and your eye or camera.

Hopefully, the following series of images will explain it all …

Shown above is the setup in my garden I used to capture the image in question, a closer view of the solar filter fitted to the front of my telescope and an inset image of the filter’s rear face. Notice that there is a second solar filter fitted to the smaller finderscope, which is used to help find the ‘target’, shown to the upper right of the central image. The orange-coloured filter holder is a lid from an old food container (I think it was bought full of dry-roasted nuts!) with its central part removed using a hacksaw and the edges smoothed using sandpaper. The diameter of cut-out disk is as close as I could get it to the diameter of the telescope tube. The lid rim’s internal diameter has been reduced slightly using a strip of material cut from some anti-slip matting, fixed in place using double-sided tape. The final filter assembly was a snug fit over the front of the telescope.

The essence of the whole DIY project is to find a tube that has some rigidity (enough to hold its shape when picked up) and has an internal diameter just larger than the outside diameter of the telescope in question. In the example above I have used part of the thick cardboard tube in which one might find a bottle of a certain single malt whisky: it just happened to fit nicely onto the 72 mm refractor I now use for observational astronomy and in my visits to primary schools etc. I buy high-quality solar film in A4 sized sheets since it’s a cost-effective way of fabricating several filter assemblies; it’s available from many stockists but I happen to use this one. You’ll also need some epoxy resin adhesive. (In passing, I note that there are pre-made solar filter assemblies also available to buy; a quick scan online suggests that they retail for about £50 and upward each.)

Having found a suitable tube and cut it to an appropriate length, the key next step is to attach the solar safety film in such a way that it is not creased or scratched. I have found that a thoroughly clean sheet of glass is a great help; I keep an unused glass shelf for all such work but a smooth and flat ceramic tile might serve, as would a kitchen worktop if you can find a section that’ll not be needed for a day. Leaving the protective backing sheet on the foil, place it on the glass surface, foil upwards. Now mix enough epoxy resin to be able to run a thin thread around the end of the tube that’s going to take the solar safety filter; try not to get any epoxy on the tube’s inner surface, although small amounts aren’t critical. Carefully lower the end with the epoxy onto the foil sheet, avoiding any twisting or sliding motion: the foil should have remained flat against its backing sheet and the glass. There’s probably no need unless your tube is exceptionally light, but you could gently lay something like a small hardback book across the top in order to apply even downward pressure if you wish. Now walk away and leave the epoxy to set. Once all that’s done you can cut away the remaining foil with a pair of scissors and store it for another day.

Small adjustments are probably needed to ensure the filter assembly properly fits the end of the telescope. For reasons rooted only in habit, I tend to do this as a final step despite the fact that it’s probably wisest to get all this out of the way before attaching the safety film. There are all sorts of ways of achieving this, depending on how many millimetres larger the tube’s diameter is than the telescope. It may require only a layer of tape to the inside of the tube (- the end away from the foil of course as one doesn’t want to risk damaging the safety film if anything comes adrift). A layer or two of anti-slip matting fixed using double-sided adhesive tape works well, but it’s easy to find self-adhesive strips of felt or neoprene online and these can also be very useful. Remember, you are aiming for a fit snug enough that nothing’s going to fall off accidentally but not so tight that filter-destroying force is needed in order to slide it onto the telescope tube.

Here’s a selection of the filters made thus far for telescopes of diameters from 50 mm (finderscopes and guidescope) to 150 mm in the case of my Newtonian reflector (- in that case, I used the ring from a cake baking tin that had lost its base to rust); I’ve also made filters for both my grandsons’ telescopes. All this was from two (or three?) sheets of solar safety film. Also shown in the image is a scrap of anti-slip matting and lengths of self-adhesive neoprene and of felt.

Now for some images …

The above are some of the whole-disc images of the Sun I have captured prior to the one shown at the top of this post. The numbers and the sizes of sunspots vary enormously. We’re approaching the peak of the current 11-year solar activity cycle (expected in early 2024) so it’s unlikely you’ll look at the Sun at present without seeing any. Given that one could fit 109 Earths across the diameter of the Sun you’ll not be surprised that the largest sunspot in the image top left is several times the Earth’s diameter. Each sunspot cluster is given a unique identifying number – often preceded by ‘AR’ for active region; you can look this up here. Sunspots themselves are regions associated with the Sun’s magnetic field as it protrudes from the surface. Their dark appearance comes from the fact that they may be 2000°C cooler than their surroundings. Look closely and you’ll also notice brighter regions: those around the darker sunspots are called plages whereas the lighter patches often seen most easily near the edge of the solar disk are faculæ; these are associated with hotter regions in our field of view.

If we take a closer look you’ll see that sunspots have a central dark region, the umbra, and a less dark surrounding area, the penumbra where temperatures are at an intermediate level between the umbra and the surrounding surface.

Even the smallest scraps of solar film left over from making a telescope filter can have their uses. I captured this sequence of shots of a partial solar eclipse using my phone with a piece of safety film covering the phone’s camera lenses. The quality is what you’d expect from a handheld phone in a car park several miles from my house, but it was a fun thing to try. However, see below …

This is a better view. It’s another partial eclipse, this time captured using one of my telescopes and astro-cameras. You may be able to discern the silhouette of some of the Moon’s mountain ranges as it clipped the Sun. We are extraordinarily fortunate in the fact that the Sun and the Moon both appear to us on Earth as discs that are about ½° wide – which is why the Moon can cover the Sun when suitably aligned. (By the way, if you hold your little finger out at arm’s length the fingernail end covers about 1° of the sky so it’ll easily cover the Moon; see here.)

Happy sunspot hunting 😊

1700 words + endnotes

Endnotes
[1] For an overview of telescope types and what to consider and look for when buying try these web sites: here and here. I hasten to add that, like other second hand astronomy items, I bought the telescope mentioned in my opening paragraph from someone I knew to be trustworthy; one has to be careful.

Starting out in astronomy need not be prohibitively expensive – getting into photography, or off-road cycling, or many forms of sport, … might be comparable. However, amateur astronomers often talk in terms of ‘falling down the rabbit hole’: if you get hooked by the hobby you’ll find that there’s a never-ending series of spending opportunities ;-) My advice is to think about what it you most want to do/observe and start your search from there, being aware that as your aspirations evolve you may want to upgrade. The above links are only two of a multitude of places to get advice; read them in order to get an overview, but there’s a huge benefit to be had if you can try things out and talk to experienced people face-to-face. My suggestion is that you join your local amateur astronomy society. I’ve had loads of support from the lovely people here and also here and here. Most societies have websites and/or social media groups and you’ll find members only too keen to answer questions and offer informed advice. See here or here for a list containing many such societies in the UK. (Please note that these lists are not completely up to date, but they’ll get you started.) Once you have some equipment of your own you’ll find another slew of websites and helpful social media groups and online videos dedicated to users of similar kit.

[2] We all know that the Sun is both large and massive, and that it’s hot. In terms of size, the diameter at its equator is about 109 times that of our beautiful Earth; it represents 99.8% of the mass in our entire solar system. Its core temperature, which is where the fusion reactions occur that generate its output, has a temperature of about 15 million °C whereas the Sun’s surface temperature is about 5,500°C. As one rises into the corona (its outer atmosphere) the temperature rises again to about two million °C. (See here and here. Thus, what we perceive from Earth is the ‘cooler’ surface, referred to as the photosphere. In fact, the colour we see is strongly affected by the Earth’s atmosphere and by the limitations of our eyes: light from the blue end of the spectrum is preferentially scattered as the mix of wavelengths from the Sun passes through – this gives us our blue sky and leaves the Sun appearing yellow-orange-red as it nears the horizon but blindingly white when it’s high in the sky (- don’t look!). Our eyes fail to give us its intrinsic colour; if we could look at it through protective glasses from a space station our eyes would perceive the Sun as a white disc.

One of the simple calculations I used to set for students in their foundation year was to use something called Wein’s Law in order to estimate the temperature of the Sun’s photosphere. All that’s needed is the wavelength of light at the peak of the Sun’s emission, which we approximate to the wavelength of green-yellow light. If you’re that way inclined, try it out using the link above. The same formula may be used to estimate the surface temperature of other stars, or indeed the temperature within a furnace – the physics is identical. See also Video 13 in my lockdown series ‘Physics in the House’.