Digital SLR Imaging

Using DSLRs for Astronomical Imaging

David Ratledge

Introduction

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).

  • 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. However, recent cameras with Live View have made precise focusing easy.
  • 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:-


 

QE Graph

Quantum Efficiiency. Canon 40D and Kodak E-series CCD compared.

Quantum Efficiency

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.
 

IC 1805 Canon 300D Hutech and Takahashi 13 x 4mins

Andromeda Galaxy Pentax *istD un-modded and 300mm telephoto lens.

Andromeda Galaxy: Canon 300D modded and 300mm telephoto lens.

M31 Canon

 
 

Nebula Test

Flame Nebula/Horsehead Nebula

Pentax un-modded and 300mm lens (left) - Canon 300D modded and 530mm lens (right)

Orion Nebula - Hutech Canon 300D

15x1 min +10x2 min + 10x3 min .

The following are using the Takahashi FSQ106:-

Pleiades Hutech Canon Rebel/300D and Takahashi 106FSQ

This was 18 x 4mins.

Perseus Canon 20Da with 28mm lens at f/8

This was 6 x 4 minutes.

California Nebula - Canon 40D modded and Takahashi 106FSQ

This was 20 x 300 secs.

 

Lunar Geology - Canon 20Da

Sun in CAK light, August 2006 - Canon 300D modified and Baader CAK filter.
Note CAK image not recorded in standard 300D or 20Da

 

Narrowband Imaging with DSLRs

Using narrowband filters with DSLRs is possible and can give good results. In the case of H-alpha, as only I in 4 pixels sees red, they are somewhat inefficient but good results are possible.

Orion Ha

Orion and Barnard's Loop. Canon 300D (modded) with Baader H-alpha filter.

In the case of OIII, then both the blue and green pixels will record a signal but not at peak transmission. Still 3 out of 4 pixels will be recording so it is more efficient than H-alpha.

However, there is an alternative! That is to use a (visual) UHC type filter which passes both OIII (with H-beta) and H-alpha at the same time. Then the red pixels will record the H-alpha at the same time as the blue and green pixels are recording OIII. Modern DSLRs don't pass IR (even when modded) so visual UHC filters, which transmit in the IR, are not a problem. Using a UHC filter on a modded DSLR is a highly efficient way of maximising exposure time.

Conclusions

The images here have have been processed using IRIS. I have found this the best and quickest for dealing with these large colour images. I have tried shooting jpegs and raws and decided that that raws are the best way to go. IRIS handles the whole image processing operation including full calibration using darks, flats, offsets and cosmetic defects.

The most often asked question is what ISO and what exposure? I use ISO 400 where star colour are importand and 800 when nebulae are the main target. In my eperience the longer the exposure the better with perhaps 10 minutes being a noise limit for the Canon. I routinely use 5 minutes with my 300D but 10 minutes would probably be better with later Canon models (350D onwards) especially if temperatures are low.

Which is the best camera? If you want a camera that doubles for everyday use as well as astrophotography then the Canon 20Da is probably the best although it is somewhat pricey (still half the price of the Starlight Xpress) and has now been discontinued. For emission nebula then a modified camera (IR filter replaced) is best. I would personally recommend the Canon 300/350D modified by swapping the filter . Two firms in the UK now offer Canon conversion in the £175 to £250 range. The Pentax can be modified by Pentax Europe but is filter removal not filter swapping. This changes the focusing position so the screen cannot be used. As explained above Nikon cameras are best avoided.

Finally, one thing to remember: bigger pixels equals better signal to noise ratio. As pixels inevitably get smaller then cameras will not necessarily get better because they have higher pixel numbers. For this same reason the Canon 5D (with its filter swapped) could well be the best currently available - it has big pixels. So check those pixels sizes before you buy - this is one place where size matters!

Canon 40D, 450D: Despite pixels only 5.7 microns these cameras seems more a step forwards than backwards! They have 14 bit A-D converter giving raw files with a dynamic range of 16,000. They also have live view for easy focusing and this can be displayed live on an attached computer. It could be the best yet! The jury is still out on the 50D though and the 550D with 18 megapixels is probably one to avoid.

Sub-exposures and Signal-to-Noise

An article in Astronomy Now (Jan 2008 - Tech Talk) erroneously explains the benefits of stacking many 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 - there is a square root in there but not like that! Signal to noise actually 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. To minimse read noise then the longer the sub-exposure, without burning out bright details, the better.

Future

Some manufacturers are now introducing back-illuminated chips. Professional CCD chips use this technology to boost quantum efficieciency up to 95%. Hopefully this technology will appear in mainstream DSRLs soon. The future is bright!

 

Full colour book which includes DSLR imaging:
"Digital Astrophotography - The State of the Art" edited by David Ratledge Order Online

 
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