Most astro photographers start out with a DSLR camera. They are relatively cheap and come with a large sensor and being colour, save a lot of time and effort in regards to filters. Then, most will at some point modify the camera to remove the ” hot filter ” that sits in front of the camera sensor to increase the camera’s sensitivity to infra red Ha light. Doing this makes the camera all but useless for daytime photography as the colour is all wrong and the auto focus will not work properly after the modification. The problem still remains that although the camera is more sensitive to IR light, you still have to take very long exposures especially on fainter objects. The reason is that DSLR camera sensors have a Bayer matrix on top of the sensor surface to create colour images – the sensor itself is actually a monochrome ( black and white ) sensor. Being able to remove this matrix of coloured lenses would be ideal for astro imaging as the Bayer matrix actually reduces the sensor’s sensitivity. As the matrix is usually in an RGGB pattern ( for Canon cameras anyway ), this means only a quarter of the red light makes it through the matrix to the sensor, only half the green light and only a quarter of blue light. This is an obvious drawback for astro photography especially when the object you’re imaging is rich in Hydrogen Alpha ( Ha ) light. Some enterprising imagers have successfully removed this colour array but it is very delicate and time consuming work and can all too easily result in a damaged – non working sensor. ( I know, I have tried twice on an old Canon 350D, both attempts resulting in a ruined sensor ). So if you really want the extra sensitivity and be able to capture more of the interesting stuff, you will have to get an astro dedicated monochrome camera.
Keep your eyes open, as always, for second hand equipment. You will need a camera of at least 6 megapixels (MP’s) – you never know, you might find a bargain, but expect to pay £800 or more even for second hand cameras and the more MP’s the sensor has, the costlier it will be.
Having a mono camera opens up a whole new sky for you to image. If you could see in Ha, OIII or SII ( I’ll explain these later ), the night sky would be crisscrossed in a filigree of webs, with knots and clumps of bright gas clouds and dark patches of dust hiding the light behind. All of this can be imaged with the right filter in front of the camera.
These images were captured using a Hydrogen Alpha ( Ha ) filter mounted inside a filter wheel before the camera. Other filters most commonly used are Hydrogen Beta ( Hβ ), Oxygen III ( OIII ) and Sulphur II ( SII ). These filters are themselves expensive to make as they use exotic coatings on the optically polished glass surface that only allow a very specific narrow band of the light spectrum to pass through ,hence they are called narrow band filters and they are expensive, so again try and get good second hand ones if possible and look after them. I use Baader 2 inch narrow band filters Ha, OIII, SII also a Baader 2 inch clear focusing filter and a Baader UV/IR blocking L filter ( Luminance ). These 2 inch filters are physically much larger than the mono sensor for two reasons, they keep vignetting ( a falling off of image brightness towards the outside edge of the image ) to a minimum and they give me some future proofing – incase I ever get a camera with a larger sensor.
There are also the Broadband Red, Green and Blue filters. Again, I use 2 inch Baader filters. So why use these colour filters and not a DSLR ? The reason is, if you use a red filter in front of the mono camera, all the red light from the object will pass through to the sensor not just a quarter of it – obviously the same goes for the other filters.
By using these different filters to image a specific part of the spectrum great detail can be captured and by using image editing software a very detailed colour image can be produced.
When these narrowband images are combined in editing software, PhotoShop for instance and the colour channels are manipulated to mimic the ‘ Hubble Palette ’ the results are spectacular.
The vast majority of CCD or CMOS mono astro cameras also have the capability of cooling the sensor; in fact most of the camera body is dedicated to cooling. The majority of cameras will cool to -45C below ambient temperature so in the summer they will not cool the sensor as much as in the colder winter months. This cooling of the sensor means that the long exposure image will have a much lower noise level present in it than an uncooled camera will. I set mine to -20C every time and it reaches that temperature within a few minutes. This ability to cool the camera to set levels ‘ set point cooling ’ means you can create what is called a Dark Library at any time just by covering the sensor up. For instance you can have a dark folder on your PC with 60, 120, 180, 240, 300, 600, 900 second dark exposures taken at -10C, -20C etc, so when you process your images you would use the corresponding darks for the exposure length and temperature the image was taken at.
Another option is binning, that is, combining adjacent pixels. So 1×1 bin means that every pixel’s readout is counted individually, 2×2 means 4 adjacent pixels readout are combined, 3×3 means combining 9 adjacent pixels readout and 4×4 means combining 16 adjacent pixels readout to make a ‘ super pixel ’. This binning in effect makes the camera more sensitive but does reduce the image size accordingly, your dark library will now have to take binning into account e.g Dark, 4x4bin, 300s, -20C. There is also ‘ gain ’ which is the equivalent of DSLR iso speed. The figure used for gain will depend on the camera make used but in essence, the higher the gain the more sensitive, but also the more noisy the image will be and the dark library will have to take gain into account also e.g Dark, 4x4bin, 300s, -20C, 139 gain. Once you have created the dark library it should hold every exposure length, temperature setting, binning factor and gain setting you are likely to use, but you will find in reality you will only use a few combinations. I mostly use 1x1bin, 300s, -20C, 139gain or 4x4bin, 300s, -20C, 139gain. The gain setting of 139 is also called ‘ unity ’ and is the mid point of the gain scale.
These calibration frames will include Bias and Flat frames.
Bias frames are also taken with the cover on but are taken at the fastest shutter speed of your camera ( on the 1600mm this is a zero length exposure ), this records the readout noise. Again these have to be taken at the same binning factor as your images. e.g Bias 4x4bin, 0 , -20C, 139 gain
Flat frames are images taken of an evenly illuminated light source, the sky at twilight using a white T-shirt stretched over the front of the telescope ( I’m not a lover of this method as it tends to get T-shirt fibres on the mirrors or lens of the telescope ) or a home-made light box. This records any imperfections in the optical train, dust etc. Also uneven illumination or vignetting. Expose the flat to get your histogram about half way without it being too dark or too light, APT ( Astro Photography Tool ) has a flats wizard called CCD Flats Aid that helps work out the exposure to reach a given ADU (Analog to Digital Unit), start at 20,000 ADU’s and increase or decrease exposure to centre the histogram. Again these have to be taken at the same binning factor as your images. e.g Flat, 4x4bin, -20C, 139 gain. These should be taken at the end of the imaging session for each filter used, though I’m finding that flats for my 2 inch narrow band filter’s are not really necessary.
So now we know how to name our calibration folder’s correctly, it stands to reason that the image folders have to be similarly named and include the name or designation of the object you imaged and what filter if any was used e.g Ha, NGC7000, 4x4bin, 300s, -20C, 139 gain.