Introduction to Astro-Imaging
Astro-Imaging is one of my hobbies. I am a relative 'newbie' and only started this game in 2014 when we moved to a location with truly dark skies (Bortle 2). I am NO EXPERT ! There are many world class imagers whose rejects would put my efforts to shame but I enjoy it and having a seasonal business gives me the time in the winter to enjoy my passion.
Astro-imaging involves long exposures. When you look up at the night sky you can't see any of the objects on these pages, our eyes are just not sensitive enough. By letting the camera gather light over long periods we can see the wonders of the night sky.
To most people who have used a DSLR camera, a long exposure is maybe 1/2 a second or even 1 second. To an astro-imager a long exposure is 20 - 30 minutes.
Most of the images on this site have been made by taking many, many photographs of between 5 and 25 minutes (called 'subs' as in sub-exposures) then using a piece of software to 'stack' them on top of each other whilst carefully aligning them all. Total exposure can easily get to 12 - 15 hours per target.
You probably know that as time goes by the stars appear to move across the night sky - in reality they don't - they are still - its us that's moving! During a long exposure the stars would appear to move across the photograph. Sometimes this is desirable - have a look at the star trail photos in Widefield Images. Normally we want the stars to appear as single, well defined points of light. To achieve this the telescope is mounted to a motorised mount which is computer controlled. The telescopes track the chosen object across the night sky.
Dependent upon how bright or dim the object is this can go on for as little as 2 hours but often much longer. The image of the Wizard Nebula in Deep Space Images is 15 hours of data taken over 3 nights and it's still borderline and could really do with another 15 hours or so to really bring out the detail.
To track a star accurately all night in an arc across the sky to sub pixel accuracy is not easy. To help, you will see in the photos on the Equipment page that I am using two telescopes at the same time. The smaller one just takes photos of a single star and the computer works out where it is compared to where it should be and sends corrections to the mount. This is called 'guiding' and is a neat way of achieving great accuracy with a computer controlled mount. I usually try to achieve a pointing accuracy of about <0.3" (arc seconds) that's the width of a small orange - 2 miles away!!
Many people have said 'wow that must be some telescope' - its only a simple 4" (106mm) refractor! People also ask 'what can you see through it'? I don't know - I have never looked through it - that's 'visual' astronomy and not my 'thing' at all.
The camera I use is designed for astronomical use - you cant take daylight photos with it as its a mono camera - it only takes black and white images and it is cooled to about -30º. So how do you get colour images with a mono camera? In front of the camera is a filter wheel which is just a motorised carousel than holds 7 filters - only 4 are usually used for colour images - luminance, red, green and blue. The other 3 are Hydrogen Alpha, Sulphur II and Oxygen III - this is for 'narrowband imaging' which will come up later. About half the imaging time is given to luminance as this holds the detail and brightness. The other half is devoted to colour and is split between red, green and blue fairly equally but 'binned' 2 x 2 which makes the light gathering power much greater and therefore quicker at the expense of resolution - but the resolution is in the luminance layer !! These images are called 'lights' as I am capturing light!
Using the exact same setup I also take photos of absolutely nothing! Well not quite nothing; I take 30 - 50 images with the lens cap on at the same temperature and same exposure time as the lights - these are called 'darks'. I also take 30 - 50 photos at the shortest possible exposure time 1/1000 of a second - these are called 'bias' frames. Then I take 30 - 50 photos with the scope pointing at a 'flat' panel which is a light board, like you see in a hospital to view x-rays etc. these are called 'flats'. The darks, bias and flat frames are collectively known as calibration frames which might give a clue as to why these are taken. The darks are an image of any electronic 'noise' generated by the camera sensor. The bias do the same but in a different way and the flats show any dust, defects and vignetting in the optical system - vignetting is where the light falls away at the edges making an image brighter in the middle but darker round the edges.
All of this has to be temperature controlled - the camera is chilled to a constant -30º at the back of the scope whilst the front and all the mirrors and lenses are warmed to a few degrees above ambient to prevent dew forming and ruining the images.
Then the lights ( all the luminance, red, green and blue), darks, bias and flats are fed into an amazing programme called 'Pixinsight' which uses all the calibration frames to make all the subs as good as they can be, then stacks the images and aligns the stars and eventually produces 4 final images called luminance, red, green and blue (LRGB) which are all still mono images. These are individually processed to bring out details in faint areas and correct any remaining defects, reduce electronic noise, sharpen the image etc. and finally these 4 images are stacked to produce the final colour image. This final image is then tweaked some more in Pixinsight and Photoshop.
Earlier I mentioned 'narrowband' imaging. This is very similar to LRGB but instead of taking images with luminance, red, green and blue filters I use the three remaining slots on the carousel to take the same images with Hydrogen Alpha, Sulphur II (S²) and Oxygen III (O³) filters. These images are much less affected by visible light so a bright moon doesn't make much difference. The downside is that each sub needs about 3 times the exposure of the LRGB filters making it a really slow process. The upside is if the subs are combined in the right way you can achieve what is known as the 'Hubble palette' which is the wonderful teal blues and bright golds that many people associate with images taken by the Hubble telescope. For an example see the image of the Wizard Nebula.
Long winded - yes. Prone to failure - yes. Time consuming - very. Expensive - yes. Rewarding - oh yeah!!
People have often said 'but you could download far better images taken with the Hubble 'scope for free in seconds from the Internet' - well I could. I could also look at photos of food instead of eating - its not quite the same is it!!