The absence of light

November 10, 2014 by grahamrob in General Astronomy, Imaging, Viewing, Widefield Imaging | Leave a comment

“Light thinks it travels faster than anything but it is wrong. No matter how fast it travels, it finds that darkness has got there first, and is waiting for it.” Terry Pratchet, Reaper Man.

It may seem something of a contradiction that as astronomers we seek very dark places and skies in order to see light, light that may have travelled millions of light years to get here – light travels 6 trillion miles in one year. For human beings the perception of darkness differs with the mere absence of light, due to the effect of afterimages that are produced by the unstimulated (by light) part of the eye. Typically our eyes will take between 20 and 30 minutes to fully adjust to darkness, at which time the eye becomes between ten thousand and a million times more sensitive than in daylight.

Objectively the Bortle Dark-Sky Scale describes nine levels of darkness and thereby quantifies the astronomical observability of celestial objects and impact of light pollution http://en.wikipedia.org/wiki/Bortle . With digital photography the colour of a point is described on the camera’s sensor by three RGB (red, green, blue) values, each ranging from 0 to 255. Thus when each pixel is fully illuminated each colour component measures 255 or for an RGB image 255,255,255. Conversely when all values are zero or 00,00,00, it appears black. However, the night sky is not black but measures somewhere between 10 and 30 when imaged.

Night sky image (Eastern Veil) with dark point set at 0,0,0

Dark sky image (Eastern Veil) with dark point set at 20,20,20. This approximates best to the natural darkness of the night sky.

There are even four subdivisions to describe approaching darkness at night:

Civil Twilight: begins at sunset and ends when the sun is 6^obelow the horizon or more practically, it can be described as the period after sunset during which terrestrial objects can still be clearly distinguished. Normally the end of civil twilight is usually 20 to 30 minutes after actual sunset.

Nautical Twilight: describes the period when the sun is between 6^o and 12^o below the horizon, during this time it is now possible to take reliable star sightings at sea. It may more commonly be described as nightfall but it is still not strictly dark yet.

Astronomical Twilight: defined as the period when the sun is now between 12^o and 18^obelow the horizon. To the casual observer this may be considered dark but it’s not, only when Deep Sky Objects such as nebulae and galaxies can be viewed is it fully dark.

Therefore, only after this sequence is completed, which takes almost two hours after sunset here at Fairvale Observatory at this time of the year, does true astronomical night or darkness occur. The excellent FLO Clear Outside weather forecast website, which is linked on the front page of this website, shows the current timings for each of these periods every day along the top horizontal bar, just below the hourly sub-division headings.

Obviously this has a major bearing for astronomers and perhaps more so for astrophotography. So sensitive is the camera’s sensor that when using long exposures the cumulative light recorded, even in a dark-sky environment, may result in a bright image that will need to be corrected during processing. Notwithstanding, the holy grail for astronomers is a dark, clear sky and the biggest enemy (other than bad weather and cloudy skies) is light pollution, which is spreading inexorably across the globe.

At the beginning of this post is a NASA picture of the Earth at night, produced as a composite of image data from the Suomi National Polar-orbiting Partnership (NPP) satellite, taken in April and October 2012 over a period of 312 orbits. NPP passes over any given point on Earth’s surface twice every day, flying 824 kilometres (512 miles) above the surface in a polar orbit, circling the planet about 14 times a day http://earthobservatory.nasa.gov/Features/IntotheBlack/ . Away from the cities much of the other light from wildfires, fishing boats, gas flares or mining operation is also visible. Whilst undeniably a beautiful picture, for astronomers it highlights one of the major obstacles we are up against, light, or more accurately light present here on Earth. The night sky before the invention of the commercial light bulb by Tomas Edison in 1878 must have been a wonderful sight; I doubt that Messier (1730-1817) would have successfully catalogued all his 110 objects as easily with today’s skies.

The dark side of the world: city lights of Europe, Africa, Middle East & Central Asia

The dark side of the world, with light just over the western horizon.

Time Travel

November 5, 2014 by grahamrob in Deep Sky Objects, Imaging | 1 Comment

In my opinion the current 12^th Time Lord, Peter Capaldi, is one of the best doctors yet but it’s all just a good yarn, isn’t it? At the level of quantum physics the potential of time travel has recently been shown to be feasible and even the paradox of Schrödinger’s cat has now been experimentally demonstrated at a quantum level i.e. the same thing can exist in two places at the same time. Still, intuitively time travel seems unlikely but nevertheless last week I travelled back 400 million years without moving from Fairvale Observatory!

As a result of good viewing conditions and excellent alignment of the mount and telescope, I sought to capture light that left on its journey 300 million years ago. This time marks the end of the late Carboniferous era, taking its name from the period of worldwide formation of coal deposits, which resulted in the highest atmospheric oxygen levels the Earth has ever experienced (35%) and lead to an abundance of giant insects and amphibians as the first reptiles also appeared on Earth.

Widefield view of Stephan’s Quintet (red circle) and NGC7331 + Deer Lick Group (red box)
WO GT81, Canon 700D + FF | 20 x 120 secs + darks/bias/flats @ ISO 1,600

Located in the constellation of Pegasus, Stephan’s Quintet is a group of four galaxies whose respective gravities lock them in a cosmic dance with each other that will inevitably lead to their coalescence. The fifth and brightest member of the group, NGC 7320, is in fact just 40 million light years away but viewed from Earth appears to be spatially associated with the aforementioned group and thus makes up the fifth member of the quintet. Unfortunately my 80 mm telescope only shows a smudge of light from Stephan’s Quintet but it is light that has just arrived here at Fairvale Observatory after making a 300 million year journey, it is literally looking back in time. A more substantial Hubble image shows us exactly what was happening to these galaxies at that moment – it seems probable that they have by now come together but we’ll have to wait another 300 million years to see that.

Stephan’s Quintet taken by the Hubble telescope

Stephan's Quintet (bottom left) and NGC 7331 & Deer Lick Group (top left)

Stephan’s Quintet (bottom left) and NGC 7331 + Deer Lick Group (top right)

One advantage of the smaller 80mm William Optics refractor telescope is that its field of view is quite large and whilst seeking to capture Stephan’s Quintet, I also inadvertently managed to image another group of galaxies. In this case the dominant NGC 7331 galaxy with, apparently close-by but actually located up to ten times further away, the Deer Lick Group of galaxies. The magnificent NGC 7331 is a mere 50 million light years from Earth and is thought to be similar to our very own Milky Way. The Deer Lick Group (indicated by four red arrows in the main picture above) is however some 400 million light years* away – thus corresponding to the mid-Devonian period or the Age of Fishes; named after the red rocks first identified in Devon, UK and particularly known for its plethora of fish that developed at this time. I am quite sure that even The Doctor would be impressed by the time travelled by the light from these objects as it arrives here on Earth after such a long journey and provides us with a glimpse of the past, today.

NGC 7331 spiral galaxy (foreground) and Deer Lick Group above (see main anotated picture for detailed location). Light form the Deer Lick Group of galaxies is 400 million years old.

NGC 7331 spiral galaxy (foreground) and Deer Lick Group above (see main anotated picture for detailed location). Light from the Deer Lick Group of galaxies is over 400 million years old.

* For the record, light travels 670 million miles in one hour or 6 trillion miles in one year.

It’s all about the wavelength

October 20, 2014 by grahamrob in General Astronomy, Imaging | 1 Comment

For the moment, putting aside the duality of light as a wave-particle (quantum) form, the behaviour of light in the visible spectrum can be both fascinating and a problem for the astronomer. As my tracking has improved (and hopefully will get even better) it has been possible to increase exposure times from a maximum of 40 seconds (and that was pushing it), to comfortably 90 seconds and sometimes more. The impact is that more light is gathered by the camera’s sensor and the outcome is greater detail and more colours – this all helps, a lot, when the light may travel for thousands or even millions of light years before hitting the sensor.

Unfortunately Fairvale Observatory is located on the southern edge of London and about 8 miles north of Gatwick airport, the result is that all that lovely light that has travelled from distant objects in the Universe has to compete with man-made light pollution. Last week I therefore invested in an Astronomik CLS filter http://www.astronomik.com/en/, which clips inside the camera, just behind the lens, and blocks the spectral lines of mercury and sodium-vapour lamps, letting the remaining part of the visible spectrum and H-alpha through.

The horizontal axisis the Wavelength in Nanometers (nm). 400nm is deep blue, at 520nm the human eye senses green and at 600nm red. At 656nm is the famous “H-Alpha” emission line of hydrogen.
The transmission in %is plotted on the vertical axis.
The redline shows the transmission of the filter.
Visual filters: The greyline in the background shows the relative sensitivity of the human eye at night. The maximum is at ~510nm and drops to longer and shorter wavelengths. You can easily see, that you can´t see anything of the H-alpha line at night (even if you can during daylight!) The sensitivity at 656nm is 0% at night!
Photographic filters: The grey line in the background shows the sensitivity of a typical CCD sensor.
The most important artificial emissionlines are shown in orange. The artificial light pollution is dominated by see mercury (Hg) and sodium (Na), which are used in nearly all streetlights.
The most important emission lines from nebulasare shown in green. The most important lines are from ionized Hydrogen (H-alpha and H-beta) and double ionized oxygen (OIII).

The major emission lines of nebulas:
H-β 486,1nm | OIII 495,9nm | OIII 500,7nm | H-α 656,3nm

Since my recent success imaging the Orion Nebula I’ve been frustrated by bad weather, cloudy skies and a full Moon. With a brief gap in the clouds last night I therefore couldn’t resist the opportunity to try out the new filter with the camera but without using the telescope. Shooting two sets of pictures, with and without the filter, and using a telephoto and 50 mm standard lens, the results were both successful and perplexing.

As expected it darkens the sky, a lot, but also skews the resulting image towards blue which subsequently has to be adjusted during processing, complicating the question of what is the right colour even more. It is a known fact that despite the obviously very large distances of astrophotography subjects, some camera lenses need to be focussed just short of the lens‘s infinity position. However, with the standard lens the focus point was very different when using the filter, noticeably less than without the filter. This seems quite strange and I have not quite worked out why this should be so, except the loss of and therefore change in wavelength of the light?

It will be interesting to see how the filter works when imaging deep sky objects using the telescope once the weather clears but the preliminary tests are promising. Certainly the ‘loss’ of light incurred will require longer exposures so I had better sort out autoguiding soon, my next challenge.

Without CLS filter, 8 secs @ ISO 800

With CLS filter, 8 secs @ ISO 800

Milky Way - in the Tarazed region - 60 seconds (only!) @ ISO 800

Milky Way (Tarazed region) without CLS filter 60 seconds (only!) @ ISO 800

Veil Nebula, with CLS filter, 90 secs @ ISO 800 (+ plane trace!) - unprocessed

Veil Nebula (and aircraft trace!) with CLS filter 90 secs @ ISO 800 – unprocessed

Veil Nebula Canon 700D + 50mm lens, with tracking, unguided & processed 15 x 90 secs @ ISO 800

Veil Nebula
Canon 700D + 50mm lens with CLS filter, tracking, unguided & processed 15 x 90 secs @ ISO 800

Gotcha!

October 7, 2014 by grahamrob in Deep Sky Objects, Imaging | 4 Comments

Two of astronomy’s most iconic images are Saturn and the Orion Nebula, M42 – one a highly distinctive planet of our Solar System, the other a trade mark of the winter sky as part of the Orion Constellation. Both therefore seem quite familiar but still need to be seen or better still captured on camera to personally experience their magic.

The Orion Nebula or Great Orion Nebula, is a diffuse nebula located just south of Orion’s belt in the constellation of Orion. It is approximately 1,344 light-years from earth and 24 light-years in diameter, which with an apparent magnitude of +4.0 is visible from Earth. Studies of the nebula have revealed much about how new stars and planetary systems are formed, indeed it is considered a stellar nursery for new ‘baby’ stars, typically only a few hundred thousand years old. Some 700 stars have been identified as formed from this nebula, most notably the ‘Trapezium’ asterism in the centre of the nebula, consisting of six bright stars. Spectacular red colours arise from hot hydrogen gas, whilst dust reflects the blue light from hot blue stars within the nebula.

The Orion Constellation from Fairvale Observatory last year – the Orion Nebula is just below the three central stars (Orion’s belt) in the centre of the three lower stars (Orion’s sword)

Due to its sheer beauty and notoriety I have previously dabbled with attempts to image the Orion Nebula before, initially by compact camera and subsequently by DSLR on the Skywatcher 150PL telescope, with limited success. Notwithstanding, the colours of the nebula were evident and even four of the main stars of the Trapezium could be seen – at the time I was quite pleased but equally frustrated as I was unable to capture this magnificent object at its best.

Afocal image of the Orion Nebula in 2013: I was pleased at the time with the colour is showed and even the Trapazium stars

Afocal image of the Orion Nebula in 2013: I was pleased at the time with the colour it showed and even the Trapezium stars

Orion Nebula later in 2013: DSLR & Skywatcher 150PL, single photograph, shows better colour and detail of the Trapezium

One year on, new equipment, new skills and a dark sky and all that has changed. Very early on last Sunday morning I succeeded in imaging the Orion Nebula in all its glory, in what must be my very best astro photograph to date. Gotcha!

The Orion Nebula October 2014 – the secondary feature in the top left corner is another nebula, M43. Orientated with equatorial North up and East to the left.
WO GT 81 Canon 700D + FF unguided | 20 x 90 secs + darks/bias/flats @ ISO 800

Seven Sisters

October 6, 2014 by grahamrob in Deep Sky Objects, Imaging | 2 Comments

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

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

The Witch’s Broom

October 4, 2014 by grahamrob in Deep Sky Objects, Imaging and tagged DSO Imaging, Nebulae | 2 Comments

With polar alignment and tracking now working quite well, I have been hunting around for potential new targets before moving on to the next challenges of computer control and auto-guiding. Within the constraints of my limited sight lines, light pollution, weather and a rapidly encroaching full moon, I decided to tackle the somewhat elusive Veil Nebula. Although the apparent magnitude of 7.0 is not unduly challenging, the delicate nature and low surface brightness of this very large ionized gas cloud can make it difficult to image.

Located in the Cygnus constellation, the Veil Nebula is a very large but feint supernova remnant about 1.400 ly from Earth that exploded between 5,000 and 8,000 years ago i.e. quite recently. The Veil Nebula, Cirrus and Filamentary Nebula usually refer to those parts that can be viewed, the rest of the feature not being in the visible spectrum; the Veil is one of the largest, brightest features in the x-ray sky. So big is the Veil that various sections are recorded as separate NGC numbers: 6960, 6992, 6995, 6974, 6979 and IC 1340.

Located close to the binary star system 52 Cygni, the classic view is of the Western Veil or NGC 6960, AKA the “Witch’s Broom”, “Finger of God or “Filamentary Nebula”, which spans across 35 light-years and I therefore set out to photograph. Following the recent success of the North America Nebula I undertook a test shot at the same settings: 90 seconds at ISO 1,600. However, the resulting picture looked excessively washed out and so changed to 90 seconds at ISO 800, which seemed to work better – though you never really know until the late stages of post-processing. Despite forecasts to the contrary, the cloud rolled in after just six shots but two hours later and still tracking, the clouds parted leaving a clear sky and just enough time to take another twenty shots.

Initial processing was not encouraging. Truth be told there’s still a lot to learn with this part of astro-imaging but, with some difficulty and courtesy of Mrs G, a good image of the Witch’s Broom was eventually teased from the data.

NGC 6960 AKA The Witch’s Broom
Canon 700D | 20 x 90 sec + darks/bias/ flats @ ISO 800

Star wars

October 2, 2014 by grahamrob in Deep Sky Objects, Imaging | Leave a comment

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

Vision Technician

October 1, 2014 by grahamrob in Equipment, Imaging | Leave a comment

Appearances can be deceiving. WYSIWYG was a term used in 1980’s computing (maybe it still is?) when printing, to indicate that what you saw on the computer screen e.g. formatting and layout, would be the same when printed: What You See Is What You Get. Being the early days of computing for the ordinary ‘man’, when Microsoft and Apple were just emerging from being start-up companies, the growing choices of hardware and software were not always fully compatible, frequently resulting in a lack of WYSIWYG’ness or put plainly, rubbish printing output. Even in the 21^st century everything is still not WYSIWYG.

I have just completed building and now started to use the new External Astronomy Control Centre (EACC) at Fairvale Observatory. I am unable to have a full blown, dedicated observatory here at Fairvale, so have to set-up and take down all the astronomy equipment on the patio outside each time I want to carry out viewing or imaging. It’s a chore, particularly since getting the new and much heavier AZ-EQ6 GT mount and many other pieces of equipment and gizmos; I know this accumulation of equipment and related computing will only continue for the foreseeable future – I am now certain this is an immutable law of astronomy!

Central to controlling all this equipment is the computer. Currently its main function is for planetarium software and image control and capture: camera settings-up and auto-capture, thus making the large number of images required (subs, darks, bias and flats) less of a chore and, in general, easier to carry out. I soon hope to extend such computer control to the mount, auto-guiding and tracking correction, plus linking all this to the planetarium for easier navigation.

For the moment this requires that I take my laptop computer outside in order to connect it to the various pieces of equipment, which as the longer, colder nights become more prevalent, exposes it to potentially damaging conditions e.g. dew; I may eventually be able to move the computer inside but for the moment it has to be outside. Furthermore, even using red-screen software to produce a red hue over the computer screen in order to reduce light spilling during image capture (it also helps the eyes to remain adjusted to night vision), the resulting red light is still quite bright and potentially detrimental to imaging. As a result of these issues I decided to embark on the construction of the aforesaid EACC.

The EACC design needed to be such that it is light, mobile and easy to set-up, whilst primarily achieving two main functions to combat the above problems: (i) protect the computer and leads from the environment – mostly cold and dew, and (ii) as much as possible, contain and thus restrict light spillage from the computer screen. After detailed consideration of possible designs, the resulting EACC has so far proved to be very successful in meeting these objectives. However, acronyms and fancy names can be deceptive – the EACC is a cardboard box!

EACC in development

EACC completed (there have been a few minor amendments)

Notwithstanding, it is turning out to be a very useful cardboard box.

Other recent cardboard box developments - in this case my daughter Alison's cat Alan looking out of a box, he is particularly keen in attacking and 'catching' feather dusters offered up to the hole.

Other recent cardboard box developments – in this case my daughter Alison’s cat Alan looking out of a box, he is particularly keen on attacking and ‘catching’ feather dusters offered up to the hole.

Of course, it won’t stand up to extreme weather conditions but then I won’t be carrying out astronomical viewing and imaging in such conditions. It has been very successful in restricting light, though it is still important that the AP (AP = access portal = front) is directed away from the telescope in order to direct light spillage away. Once I had established its usefulness, I sprayed it with a mat black paint to further reduce reflected light and make it a little more damp resistant – it is after all cardboard. All-in-all a very useful EACC (cardboard box), which however is not what it might at first appear to be – much like the vision technician, or window cleaner!

Last night the EACC in use

The cunning EACC design usefully deflects computer screen light away from the telescope & camera whilst imaging

WOW!

September 28, 2014 by grahamrob in Deep Sky Objects, Imaging | 2 Comments

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). Canon 700D ( unmodded) | 20x90secs @ ISO1,600 & darks + bias + flat frames, unguided

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 devil’s in the detail

September 27, 2014 by grahamrob in Deep Sky Objects, Imaging | Leave a comment

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

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