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