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.

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. 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. Note: Flat fields can be a problem with H-alpha and DSLRs - certainly with IRIS you need to make the master flat manually. 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. The Veil Nebula shot with an UHC filter (Canon 40D + Pentax 300mm lens) What ISO? For the 14-bit Canon cameras, then an ISO of 400 seems to work best - this is my experience over many years. This is also the closest ISO setting to unity gain ie 1 photon = 1 ADU number so there is a logic to it. For the 12-bit cameras then increaseing this to 800 or 1600 would appear to be sensible. A compromise would be setting these cameras to ISO800. This value certainly works well on the 350D although I always preferred ISO400 on the 300D. What exposure? I use ISO 400 as this preserves star colour yet is still fats enough for deep-sky objects. 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. 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. Which is the best camera? If you want a camera that doubles for everyday use as well as astrophotography then the Canon 60Da is probably the best although it is somewhat pricey. For emission nebula then a modified camera (IR filter replaced) is best. I would personally recommend the Canon 300/350D/450D/40D modified by swapping/removing the filter . Two firms in the UK now offer Canon conversion in the £150 to £250 range. The Pentax can be modified by Pentax Europe but is filter removal not filter swapping. This camera then becomes sensitive to IR which may or may not be an advantage. Finally, one thing to remember: bigger pixels equals better signal to noise ratio - all things being equal. 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 seem 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. Current recommended modded cameras: 40D, 450D, 1100D (all live view) - at the budget end: 350D but no live view and the 300D is still worth a punt if under £100. 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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