Digital SLR Imaging
Using DSLRs for Astronomical Imaging
David Ratledge
STOP PRESS
An recent article in Astronomy Now (Jan 2008 - Tech Talk) erroneously explains the benefits of stacking many images digital SLR images together. The author has confused how signal to noise increases with increased exposure and states 100 1-minute exposures equates to one 10 minute exposure. Oh dear - signal to noise increases in proportion to the square root of the exposure whether this is a single exposure or a stack of multiple exposures. The single exposure is better because it has a single read-out noise component whereas the 100 1-minute ones will have 100 read-out noise components.
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 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.
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 withe the cost of swapping the filter you will still have a cheap colour camera with a massive chip.
Digital SLR Cameras - Canon and Pentax. Note TC-80N3 connected to Pentax
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 appears in the Starlight Xpress dedicated cooled CCD camera at around £4000. With DSLR cameras currently under £500 and conversion costs of about £200 it is easy to see why DSLRs are generating so much interest.
But do they work? They are uncooled so will they be bedevilled by noise? Most have automatic dark frame subtraction (a duplicate dark frame is taken straight after the image and subtracted) except the Canon which has to have this done manually. However it is better to switch off automatic dark frames - they waste imaging time. Much better is take them at the end of the evening when you are packing up.
I have used Canon's 300D/Rebel, 20Da and the Pentax *ist D. Nikon continues (even the new D3/D300) to have a problem with automatic noise reduction (even for raws), which cannot be switched off and deletes faint stars! See link. Canon and Pentax are therefore best for Astronomical Imaging.
How do the Canon and Pentax compare?
Both have an IR block filter which blocks
red emission nebula - the Pentax seems to block more (see below).
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The build quality of the Pentax appears better.
Both need careful processing to remove banding - horizontal in the Pentax and vertical in the Canon. Offsets and/or darks usually remove these.
- At first I thought the Pentax produced slightly sharper images. However, it is much easier to focus than the Canon and I think this is why Pentax images are usually very sharp. The problem with focusing the Canon is the very poor optical quality of the viewfinder - stars never focus to a point. Many use a PC for focus checking with the Canon but this means have always having a PC available.
- The Canon has better batteries (although proprietary) and has very low dark current when the temperature is near zero. The Pentax can use a cheap mains power supply (6.5 volt) and does not need the batteries removing to use it.
For unattended use a timer is required.
The PC software that comes with the cameras can command
a sequence of images but it is limited to a maximum of 30 seconds exposures. The remote timer (TC-80N3)
shoots a sequence of long exposures of any length automatically.
It also means you don't need a computer with the camera. Fortunately the Canon TC-80N3 is easily
modified to work with the Canon 300D/350D and the Pentax - yes it does work with the Pentax.
It is only sold with a connector for the 10D/20D but if this is cut off and replaced
with a 2.5mm stereo plug we will be up and running.
The following should make it clear how to do this:-
