Sun, 10. November 2013
Astrophotography Without a Telescope - 23
Canon 300D, 10mm lens, 30 minutes exposure, f/8, ISO 800, cropped and Photoshopped.
This is a thirty minute processed exposure showing the apparent movement of stars about the South Celestial Pole (due to the Earth's rotation on it's axis). The Pole is at the centre of the apparent circles and is located at an altitude of thirty four degrees above the horizontal, facing due south (because the image was taken at latitude thirty-four degrees south). The stars all travelled 7.5 degrees during the exposure.
The Large Magellanic Cloud is at about the "8 o'clock" position; and the fainter Small Magellanic Cloud is at about "10 o'clock". The long bright star trail upper centre is Achernar. Aircraft & satellite trails are visible.
It was a thirty minute exposure @ f/8, ISO 800. I posted the full size image on my Panoramio page and received some remarks in response to my comment about unwanted 'noise' due to my choice of an older camera for this image. I was referring to electronic noise but the comments I received referred to the noise generated by the use of high ISO ratings. Yes, camera noise will result from high ISO, accompanied by long time exposure and wide aperture.
However, the noise I was referring to was not directly caused by the ISO setting. Long astro-exposures are very prone to what I would call electronic noise. Here is the original capture, untouched and uncropped:
Canon 300D, 10mm lens, 30 minutes exposure, f/8, ISO 800, uncropped and unprocessed.
(a) very bright coloured spots appear on the image, caused by photon overloading of some CCD pixels. They appear on every long exposure image and can be removed by software subtraction or by picking them off one by one in Photoshop. Here is an enlarged section of the upper left hand side of the image:
Left section of unprocessed image.
Look closely and you will see some large coloured dots. When colour enhancing the image they become even more obvious.
This is the right hand part of the original untouched image, showing the pink heat glow and some coloured pixel noise:
Right section of unprocessed image.
This was a test image to try out exposure levels. I was using my 60D for other imaging at the time.
The 300D is very prone to both these types of noise in astro-images. Clearly, I cannot use it again for long exposures like this. The 60D should handle noise somewhat better and will be used for my next star trail image.
Astro-Photography Without a Telescope.