Astrophotography

This is an album of images that I have taken of deep space objects using a telescope from my garden.

The nebulae shown here are diffuse clouds of inter-stellar gases and dust within our Milky Way galaxy. Some of them are huge, spanning hundreds of light years across. Areas where the gases and dust become dense enough will collapse under their own gravitational attraction, forming stars which ionise the surrounding gases, causing them to glow at specific frequencies, and illuminate the dust clouds, making these nebula visible with a telescope and long exposure photography.    
Other nebulae are the result of the end of a giant star's life when it runs out of fuel and explodes in a supernova event, throwing off material that expands outwards as a shock wave travelling at millions of kilometres per hour, compressing interstellar material as it goes, causing it to fluoresce.

Prints are available at https://www.etsy.com/uk/shop/HeavenlyBodiesArt 

I have an exhibition of my astrophotography in Bristol at the Tobacco Factory from 28th Feb to 27th March 2022. I'm holding an open day on 5th Match and again on 19th March where I will be there to discuss the images with anyone interested.

The exhibition details are at: https://tobaccofactory.com/whats-on/heavenly-bodies/

I have a pdf of my book that provides information on the images and the techniques used to capture them for download at https://1drv.ms/b/s!AsBXRx7nCMTThrguKUEIFINOjti3UQ 

Taking detailed images of faint nebulae and distant galaxies is not an easy task. To start with, these things are extremely faint such that all but the very brightest cannot be discerned with the unaided eye. Long exposure photography taken through a long lens or telescope solves that problem, capturing light photons over many minutes or hours to integrate into a visible image. 

To complicate matters, the Earth is rotating so exposures of the stars for more than a few seconds results in stars trailing on the image as they appear to move across the sky. This is resolved by using what is called an Equatorial Mount, which, when its axis is aligned with the celestial pole, rotates the camera at the same rate that the earth rotates to keep the target still on the camera sensor. This equipment has limitations in its accuracy though, so for exposures of more than a few minutes a second guiding camera, taking much faster images of around 3 seconds, is used to track the position of a star and make minuscule adjustments to the equatorial mount to correct errors to within a fraction of a pixel and maintain it exactly on target for long periods.

Light pollution is a problem in most populated areas. Light pollution is measured on a Bortle scale, with Bortle 1 being as dark as the sky gets and Bortle 9 being down town in a large city. Where I live, light pollution is about 'medium' at Bortle 5, so I can see plenty of stars but the Milky way is not that impressive. To reduce the effect of light pollution, narrow-band filters tuned to the wavelengths of spectral emissions from ionised Hydrogen, Oxygen and Sulphur gases within the nebulae are used to capture just this wanted light with greatly reduced light pollution.  

There are practical limits to how long an exposure of the nebulae can be taken. Factors such as light pollution, tracking accuracy, full-well capacity of the camera's pixels and of course intermittent cloud cover mean that exposures of more than 30 minutes become increasingly difficult. I tend to take 20 minute exposure through narrowband filters. A single exposure of that length can reveal the nebula after processing, but it will be very noisy. Taking multiple exposures, aligning and stacking them in software averages out the noise and increases the signal-to-noise ratio of the image, eventually enabling a much cleaner and more detailed image to be processed. Typically, a total of 7 to 45 hours of exposures taken over multiple nights are integrated for each of these images.

To capture these deep space objects I use a refractor telescope with cooled astro-camera , guide camera and accessories mounted on an equatorial mount.  To get consistency of results, and to avoid having to stay up all night, I automate as much of the image acquisition as I can, setting up schedule scripts in the NINA imaging program that I use. This deals with pointing the telescope in the right direction, keeping it precisely on target, focussing and switching in various filters, amongst other actions.

At the end of a successful night of imaging I will have multiple hours of images of one or more targets, depending on which targets are high enough in the sky at that time of year.  When I have enough hours of the required combination of filtered images the process of calibrating, aligning, integrating, optimising and then colour processing begins. I use a software package called PixInsight for this.

Images straight out of the camera are uninspiring - generally black with at most a few stars apparent. To reveal the image from the darkness, the image is 'stretched' which non-linearly increases the brightness of the darkest parts of the image while not blowing out the brighter parts.The single image is very noisy and may have unwanted issues such as satellite trails or dust specks on the optics.  To improve the signal to noise ratio of the wanted image, the image is first calibrated by subtracting master 'dark frames' which hold the residual pattern of signal that may be visible in the long exposure when stretched. 'Flat frames' are also used in the calibration to remove any dust motes or vignetting. The images from a colour camera appear monochrome at this point until they are debayered to appropriately assign the various pixels to the red, green and blue layers of a colour image.

The individual images, known as sub-frames, are graded and the best ones selected for alignment and stacking. Stacking the aligned sub-frames has the effect of averaging the noise in the image while enhancing the wanted signal. This also allows artefacts such as satellite trails to be detected and removed. The improved signal to noise ratio permits further processing to take place to minimise background gradients remaining from light pollution or moonlight. A stack of 37 sub-frames is shown below, notice the improved contrast and reduced noise compared to the single sub-frame.

For images taken with narrowband filters (to capture just the emissions from Hydrogen, Oxygen and/or Sulphur gases) the processing now splits the colour image(s) back to the Red, Green and Blue layers and the contributions from the various gases are isolated.