At Astrofest we were delighted to be presented with the astrophotography book ‘In the Marlborough Night Garden… Volume 2’ by Gavin James and Jonathan Genton. The book is full of astronomical images and information, with the astrophotography being taken from Gavin’s back garden and with the help of the QSI 683.
About the camera
When discussing the camera in the book Gavin says:
“I have used a QSI 683-WSG8 for many years now and am continually astounded by the amazing quality of the data that it captures. It is a mono CCD camera with a chip size of 3326 x 2504 pixels, a pixel size of 5.4 µm and a total of 8.3 million pixels. This seems small in the current world of 20 million plus pixel camera sensors; however, the QSI has a trick up its sleeve to keep up with the competition and more. It is a mono chip, but has an array of red, green and blue filters across its surface. To create an image, the colour camera captures the red, green and blue light in one shot and the camera’s internal processing system combines the three data sets to make the colour image. The colour filter array used is a two by two matrix of one red, one blue and two green filters, so effectively it is using a quarter of the chip to capture the red light, another quarter for the blue light and half the chip for the green light.
Meanwhile, the QSI camera uses the whole of its chip to gather the wavelength of light passed by the filter placed in front of it. To make a colour image, I capture the light separately through a red filter, a green filter, a blue filter and a luminance filter, using 8.3 MP for each filter, so a total of 33.2 MP; I construct the full colour image in software on a computer afterwards.
You may be thinking that this is an incredibly laborious and pointless approach to imaging, no wonder astrophotography takes so long! You would, of course, be wrong. The huge advantage of using a filter system in this way is the ability to use narrowband filters. These are filters made to transmit only a very narrow band of wavelengths of light around a key wavelength, such as 656 nm, the wavelength of ionised hydrogen emissions, found all across the night sky in nebulae. It glows with a red colour, but is readily drowned out even by neighbouring red light in the broader spectrum of wavelengths. By using the narrowband filter, the areas of excited hydrogen can easily be imaged with otherwise unattainable levels of detail. I use a set of Astrodon filters comprising: luminance, red, green, blue, hydrgoen alpha and doubly ionised oxygen. The two narrowband filters have a 5 nm pass width around their main transmission wavelengths of 656 nm for Ha and 500 nm for OIII.
A key feature of the QSI camera is the chip cooling system that can cool the chip to 45°C below ambient temperature. One of the problems with long exposure astrophotography is the build up of thermal noise in the chip. The limited light pass of narrowband filters means that I use an exposure of 1800 seconds, half an hour, which is a very long exposure. Even the broadband filters can require an exposure of 1200 seconds with very faint targets. The cooling goes some way to combatting the noise. I have established that even on the warmest of Wiltshire summer nights, the ambient temperature rarely exceeds 20°C, so I can happily run the chip at -20°C throughout the year. this allows me to use a single library of dark calibration frames for all projects, which is a great time saver.”
To learn more about ‘In the Marlborough Night Garden… Volume 2’ visit Gavin James website by clicking here.