Deep sky imaging has been called a dark art, believe me it’s not. It is a bit more complicated than other types of astro imaging because most of the time we are dealing with incredibly faint objects.
To get around this lack of light, longer exposures are used, 5 – 20 minutes are not uncommon. This presents a few problems but not unsolvable ones. Longer exposures require some means to ensure that the object’s light stays still on the camera sensor or the image will be blurred. This is done by auto-guiding. The other issue depends upon your local light pollution levels – longer exposures will exaggerate the light pollution in your image. Light pollution filters are available which filter out the common sources of this pollution such as sodium and mercury vapour street lights *. I use an IDAS D1 Light Pollution Suppression filter and it’s expensive, but it works incredibly well for my local light pollution levels.
* This will change. With the ever increasing number of Blue White LED street lights, it will be impossible to filter this light pollution out. The only option then will be to change to Mono cameras and Narrow Band filters.
Depending upon what type of telescope you will be using, you may need a coma corrector / field flattener otherwise stars away from the centre of the image will appear stretched out like comets. This coma affects Newtonian reflectors of around f7 and lower. The other issue with these telescopes is lack of inwards focuser travel making it impossible to focus properly with DSLR cameras. I got around this by carefully cutting 20mm from the bottom of the optical tube thus moving the primary mirror closer to the secondary mirror thereby bringing the point of focus 20mm further outside the focuser: problem solved.
You will need a properly polar aligned tracking mount that can accept guiding commands and be capable of carrying the imaging telescope and camera, guide scope and guide camera and the counter weights to balance the setup. I use a modified Skywatcher NEQ6 Pro. You will also need a laptop computer to control the guiding and imaging software ( I use Backyard EOS ). Install ASCOM Platform 6.3 this will ensure all your software works together. You will by now notice that you have a spaghetti – like amount of power, usb and other cables lying around: keep these as tidy as you can to avoid tripping over them in the dark.
Once you’re ready, locate a fairly bright star and get the best focus using the live view function of your camera – a Bahtinov or Hartmann mask will help you focus correctly.
Locate the object you want to image; some people frown on using an object locator handset but once you’re 3 star aligned it saves a lot of time finding what you want to image. Take a test exposure to make sure your object is centred properly and start the auto guiding software. This will send a series of commands to the mount moving it in Right Ascension ( R.A ) and Declination ( DEC ). The software will note how much the chosen guide star moves and in what direction, so that when the calibration is complete it can send corrective commands to the mount keeping the guide star centred on its cross hairs. I use the free software PHD2. The software can also perform a star cross test, this moves the mount while you’re taking an exposure of the night sky. Revealing how well behaved the mount is and if there is excesive backlash in the drive gears.
I also use a GPUSB interface that uses optocouplers to electrically isolate the computer from the telescope, eliminating a possible source of electrical interference. The software uses a guide camera to see what is going on. The nice thing about Backyard EOS, is that between exposures it talks to PHD2 and performs a dither. Moving the mount very slightly so that the light from the object falls on a different part of the camera sensor. When the images are processed colour mottle from the sensor is then averaged out.
Start taking exposures using either a remote shutter release with intervalometer or the image capture software of your choice; take at least 20 – 30 exposures for stacking later. You should, if all has gone well, have a nice set of images to work on, these are called Light frames.
To have a really nice astro image it has to be calibrated. To do this you will need to take a set of calibration frames, Dark, Bias and Flat frames. Take at least 20 of each.
Dark frames are images taken with the cover on the end of the telescope to keep out light, they are also taken at the same exposure length as the Light frames – this records the sensor noise, hot and dead pixels and should be taken at the same temperature as the Light Frames. Sensor noise depends on temperature, the cooler the camera the lower the noise.
Bias frames are also taken with the cover on but are taken at the fastest shutter speed of your camera ( 1/8000 of a second for my Canon EOS 40D ), this records the readout noise.
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. Set your camera to Av mode, shutter speed to 1/125 of a second and iso to 400. ( In Backyard EOS I expose until the histogram is just over 3/4 to the right ).
I feed all these frames into a free program called Deep Sky Stacker which will crunch through the Light, Dark, Bias and Flat frames – giving you a calibrated image for further processing in Photoshop for example.
I use a modified Canon EOS 40D DSLR. It’s had the IR Blocking filter removed to make it more sensitive to the red (Hydrogen alpha) end of the spectrum. I did the modification myself but there are companies and people who will do the job for you.