“I have all the all the seven sisters that I need.

I am from Finsbury Park and am having a lark.”

Public Image Ltd (John Lydon et al), This is PiL 2012

The Seven Sisters chalk cliffs on the Sussex Heritage Coast, one of Britain’s finest unspoilt coastlines.

Seven Sisters London underground station on the Victoria line, in the borough of Haringey

Seven sisters – seven major oil companies, which formed the “Consortium for Iran” cartel that dominated the global petroleum industry from the mid-1940s to the 1970s.

What is it with seven sisters?  Mr Google returns 1,490,000 search results.

444 light years from Earth in the constellation of Taurus, with an apparent magnitude of +1.6, M45 or The Pleiades is one of the most prominent objects in the sky.  To the naked eye, the Pleiades look like a Little Dipper style asterism and with good eyesight it is possible to identify seven particularly bright blue stars.  This ‘young’ open star cluster actually contains over 1,700 stars, dominated by hot, blue stars.  M45 is currently passing through an interstellar dust cloud within the Milk Way, with the blue light from the brighter stars reflected off the dust, thus forming a distinctive blue nebulosity that can be seen surrounding the cluster.

M45 is generally considered to be a winter object in the Northern Hemisphere but, having just passed the Autumn Equinox at the end of September, it can already be seen in the late night / early morning sky.  Furthermore, as we leave the astronomical twilight of summer behind, the darkening skies are a real benefit to astro photographers; pity about the moon at the moment, which lingers until about 2.30am but thereafter leaves a still black sky, perfect for imaging.

Saturday night was the first time I have had to photograph the Pleiades using the new equipment so, given the prospect of a night long clear sky, there was no alternative but to get up early, very early – but it was worth the effort to capture this beautiful star group at its best: M45, the Pleiades AKA the Seven Sisters.

M45, Pleiades or Seven Sisters star cluster
Canon 700D unguided | 26 x 90 secs darks/bias/flats @ ISO 800

A big surprise to me since starting astronomy has been star clusters, which I was strangely unaware of before.  They come in two basic varieties – globular and open – their general nature is, as so many things astronomical, mind blowing.  The Milky Way has about 160 globular clusters, with highly elliptical orbits to the galaxy, whilst more distant galaxies such as M87 have over 13,000.  Each globular cluster typically contains hundreds or even millions of stars held together by gravitational forces in a roughly spherical form, generally packed into regions of ‘just’ 10 ly to 30 ly diameter.

Globular cluster stars are considered to be some of the oldest known objects in the Universe, formed just a few hundred million years after the formation of the Universe itself, and appear to be some of the first produced during galaxy formation.  Most of the stars are red and yellow Population II stars or ‘metal poor’, which have formed after a supernova.  More rare blue stars, known as blue stragglers, may also exist in globular clusters and are thought to be formed in the dense inner regions of stellar mergers.  Notwithstanding, the origin of globular star clusters is still poorly understood but research suggests they may be survivors of galactic mayhem 13 billion years ago.

http://www.mpa-garching.mpg.de/mpa/institute/news_archives/news1202_aaa/news1202_aaa-en-print.html

No known globular clusters display active star formation today, which is consistent with the view that globular clusters are typically among the oldest objects in the Universe and were some the first collection of stars to form.

And so the other evening I turned the camera on a globular cluster, M15 or NGC 7078, located by the constellation of Pegasus.  Estimated at 12 billion years old, it is one of the oldest globular clusters, 33,000 ly from Earth and one of the more densely packed clusters in the Milky Way,  containing some 100,000 stars.  Notably M15 contains a number of variable stars, pulsars, one neutron star and also unusually, a planetary nebula.  All-in-all quite a catch though I am still mystified and intrigued by their occurrence!

M15 Globular Cluster
Canon 700D unguided | 20 x 90 secs + darks / bias / flats @ ISO 800

Preparation + perseverance = progress, and what progress.

Another clear night last Wednesday, so with my new found success of polar alignment, I started early in the evening in order to try and photograph NGC 7000, or the North America Nebula (it looks like North America).  I had been inspired by images of NGC 7000 on SGL and had already tried a few times to capture it but without success.  With the much improved polar alignment (I went through two star and polar alignment sequences this time) and therefore better tracking, I figured it was also time to increase the stakes overall: a larger set of x20 images (previously 10), speed increased to ISO 1,600 (previously ISO 800), increased exposure time to 90 seconds (previously 30 to 40 seconds) and shooting a full set of additional dark, bias and flat images in order to reduce hot pixels and sensor noise.

And so it was that I managed to successfully photograph the mighty NGC 7000. The very nature of the nebula meant that I did not know if I had the picture until late in the processing phase but it was there.  This emphasises the importance of preparation and the set-up in order to subsequently rely on the scope’s orientation, focus and tracking – you are literally working blind whilst taking a photograph of such an object this way.

NGC 7000 or North America Nebula – after stacking and basic post processing in Photoshop (note aircraft trace, subsequently removed).
Canon 700D ( unmodded) | 20 x 90secs @ ISO 1,600 & darks + bias + flat frames, unguided

NGC7000 is located within the constellation of Cygnus, some 1,600 ly from Earth. The North America Nebula is an emission nebula and most of the light emitted is H-alpha (red), most of which is unfortunately filtered out by any normal camera, such as mine the Canon 700D DSLR, by an infra-red filter that is fixed over the sensor.  As a result the basic image captures predominantly OIII (Oxygen Three) light, which is a bluish green colour and is not removed by the camera’s filter.  Many DSO objects have such characteristics and I had been hoping to avoid this problem for a while.  There is a solution, which is to remove the filter, to modify or ”mod” the camera,  the resulting images would then reflect the full light spectrum.  The downside in doing this is twofold, which is why I have not done it to my camera: it’s not cheap to do and it renders the camera useless for normal, earth bound photography! Oh well, something else for the Christmas list.

In the meantime, the red has been put back into the image by using Photoshop. Either way it’s a great image and I am thrilled. Wow indeed!

NGC 7000 North America Nebula, with curves & levels adjustment in Photoshop

NGC 7000, North America Nembula, with curves, levels and colour balance Photoshop adjustment

The process of DSLR astrophotography can be broadly divided as four main steps:

1. Preparation – equipment, targets / photographic plan;
2. Set-up – mount, telescope, camera, control (mount & computer);
3. Capture – settings (exposure, ISO, f-stop), frames (Subs, darks, bias & flats), tracking;
4. Processing – stacking & post-processing.

I am only just starting to delve into the final phase, which is another of those black arts and can, which if understood and used well, unlock detail otherwise hidden in each picture.  This is where the difference between film and digital photography becomes most evident.

A digital photo is made up of a series of pixels. Each of the pixels in a digital photo corresponds to a photosite (also called a pixel) on the camera’s sensor.  When hit by light (a photon) the photosite generates a small electric current, which is measured by the camera and recorded in a file – commonly as JPEG or in DSLR astrophotography the RAW format.

JPEG files record the colour and brightness information for each pixel with three eight bit numbers, one for each of the red, green and blue channels.  DSLR cameras (like computers) use the binary system number system (a series of two digits – I or 0); the highest number in 8-bit notation is therefore 11111111.  As a result each eight bit channel records on a colour scale of 1 to 255, or a theoretical maximum of 16,777,216; the human eye can detect between 10 and 12 million colours maximum.

RAW files dedicate more bits to each pixel, which does not equate to more colours but greater tonal graduation – the image is said to have more colour or bit depth.  The theoretical number of tones recorded by my 700D 14bit DIGIC sensor is therefore 4.39 trillion!!!  Post processing such RAW files     therefore has potential access to vast amounts of information, resulting in the possibility of greater detail and subtlety.

At the moment my DSLR processing software is quite basic (relatively, it’s still very sophisticated):

Deep Sky Stacker – used to compile the sequence of original RAW images in order to produce a single, optimized picture containing the ‘best’ data set possible from all the images.  Other correction images may be also combined in this process to reduce such problems as sensor noise but, for the moment, I have limited these to just ‘darks’ (taken with the lens cap on) to help eliminate so-called hot pixels.

GIMP – free online post processing software use to finish the stacked image, by ‘stretching’ the colour ranges levels and adjusting tones and sharpness hitherto unseen detail emerges, often transforming the original photograph; the detail was originally captured by the camera in the RAW file but must be processed in this way to ‘release’ detail that would not otherwise be seen .

Through the application of these techniques modern astrophotography is able to reveal new and transform details of old wonders of the Universe.

Whilst GIMP is very good, a better (more detailed and expensive) post-processing software used in astrophotography and by photographers and graphic designers is Photoshop. Mrs G uses an old version of Photoshop and taking the previous images of M27 and M57 has teased further detail, in particular colour, from these images with great effect. With 4.39 trillion potential colour tones the devil is in the detail and is always worth looking for.

M57 – additional Photoshop post-processing brings out more colour (see previous blog for comparison)

M27 – Photoshop post processing has also ‘found’ more colour in this image too