Digital SLR Imaging
Using DSLRs for Astronomical Imaging
The arrival of affordable digital SLR cameras on the consumer market has begun a revolution in astronomical digital images. It has brought the ease of use of an SLR camera - optical viewfinder, visual focusing and simple attachment of lenses - to the astro-imager. However, it is the sheer size of the CCD/CMOS chip in these devices that is causing the excitement. Typically they have at least 6 megapixels and in colour too! Their general sensor size is in the region of 24mm x 16mm - a size previously only dreamt of by the richest of amateur astronomers. Even full frame (36mm x 24mm) are now on the market and I would expect more. To puchase an astronomical CCD camera with a chip this size needs a serious investment in cash
Despite what an article in December 2005's Astronomy Now said, red nebula do not come out a funny blue/green colour! Red nebula just need either longer exposures or better still, the filter swapping for a clear IR block one. Even with the cost of swapping/removing the filter you will still have a cheap colour camera with a massive chip.
Note somewhere around the 40D Canon introduced double filters in front of the chip. One blocks deep red the other UV/IR. With these later cameras a filter swap is not actually needed - just the deep red blocking filter needs removing. This will change focus postion so camera lenses will not focus according to the distance scale on them and some will not even reach infinity but apart from that it seems the best solution if using it at the telescope.
First to break the £1000 barrier was Canon but all manufacturers, including Nikon, Pentax and have now followed. This puts them in price bracket of the cheaper (cooled) astronomical CCD cameras with their small sub-meagapixel chips. The chip that was used by Nikon and Pentax in their first DSLRs (a Sony chip) also appeared in the Starlight Xpress dedicated cooled CCD camera at around £4000. With DSLR cameras currently well under £500 and conversion costs under £200 it is easy to see why DSLRs offer so much.
But do they work? They are uncooled so will they be bedevilled by noise? Some have automatic dark frame subtraction (a duplicate dark frame is taken straight after the image and subtracted. However it is better to switch off automatic dark frames - they waste imaging time and a single dark frame has too much noise. Much better is to take plenty of them on cloudy nights.
I have used Canon's 300D/Rebel, 20Da, 40D and the Pentax *ist D. Nikon continues to have a problems with automatic noise reduction (even for raws), which cannot be switched off and deletes faint stars! See link. Currently Canon appears best for Astronomical Imaging with a very definite edge in lower noise - they seem to have built-in dark current adjustment/subtraction. They also have RAWs that, whilst they have been "adjusted", are capable of being calibrated correctly.
Quantum Efficiiency. Canon 40D and Kodak E-series CCD compared.
Where dedicated CCD cameras score over DSLRs is their higher quantum efficiency (QE). From the graph on the left, it is clear that a dedicated CCD camera, based on the popular Kodak e-series chips, has only marginally higher QE in the blue but considerably more in the red and H-alpha in particular. The Kodak E-series have a QE of 65% for H-alpha whereas for a modded Canon 40D it is around 25%. However, comparing the QE of mono CCDs and one-shot colour DSLRs is like comparing apples with pears - it is not as simple as it might appear!
In the early days of CCD imaging using mono cameras, colour images were produced by the RGB method ie 3 images sets were taken in turn with a R, G and B filters. Using this simple technique then there is little to choose between a dedicated mono camera and a one-shot colour type such as a DSLR. In fact some R filters used by mono CCD camera imagers reduce the amount of red light transmitted to balance exposures and so virtually wipe out the QE advantage they theoretically should have.
However, modern colour imaging with a mono CCD camera will more likely use the LRGB method. Here the L, luminance, has the maximum signal-to-noise ratio possible as it is using every pixel at every wavelength at full efficiency. However, having taken his luminance date, the mono CCD camera user will have to spend time collecting RGB data - time that does not improve the signal-to-noise of the L image - it only colourises it. The DSLR on the other hand does not have to waste time doing this and therefore, all his imaging time is productively spent improving his image. Swings and roundabouts!
For narrowband imaging, the mono CCD camera will use every pixel to collect the signal irrespective of what wavelength is being imaged. For example, for DSLR cameras, when taking H-alpha images only one pixel in 4 is recording a signal. However, narrowband imaging is still possible - see below for details.
Because we will be shooting many images each with their own read noise and stacking them together it is essential that the camera we choose has low read noise. Canon appear to be leaders in this respect. Left is a log scale of the read noise from one offset (bias) frame from a Canon 40D. Note it exhibits some fixed pattern noise (undesirable) but the histogram is close to a classic bell-shaped gaussian distribution (desirable). In reality this is a pretty good result for a non-specialist astronomical camera.
Note: a bias or offset frame is not the same as a read noise frame. A bias or offset frame actually measures the offset in a chip caused by the bias current. A bias (offset) frame contains read noise as do all frames read from the camera. The easiest way to produce a read noise frame is to subtract the master bias from a single bias frame.
The area around the Flaming Star Nebula - Pentax 300mm lens + Canon 40D
Andromeda Galaxy Pentax *istD un-modded and 300mm telephoto lens. The early Pentax cameras were very noise limited - the latest could have improved.
Andromeda Galaxy: Canon 300D modded and 300mm telephoto lens
Early Days Nebula Test
Flame Nebula/Horsehead Nebula
Pentax un-modded and 300mm lens (left) - Canon 300D modded and 530mm lens (right)
These were relatively short sub-exposures. With experience it became apparent that the Canons could easily handle 10 to 15 minute sub-exposures with consequently much superior results.
The difference shooting longer and using longer sub-exposures makes:
The following are intended to show the versatility of a DSLR:-
Pleiades Hutech Canon Rebel/300D and Takahashi 106FSQ
This was 18 x 4mins.
An early shot and today I would use much longer sub-exposures.
Perseus Canon 20Da with 28mm lens at f/8
This was 6 x 4 minutes.
California Nebula - Canon 40D modded and Pentax 300mm lens at F/4
Star Trails - 2 hour total exposure. Shooting on Christmas day means no aeroplane trails! Canon300D processed in IRIS using add max.
Lunar Geology - Canon 20Da
Sun in CAK light, August 2006 - Canon 300D modified and Baader CAK filter.
Full colour book which includes DSLR imaging:
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