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Astrophotography and the HDR image processing techniques
Photography of astronomical objects brings many difficult problems as compared to the photography as most people know. The exposure times can be very long (even tens of minutes) and the lenses, or telescopes used, typically characterize with big focal lengths (thousands of millimeters). This means that the photographed objects must be well guided during the exposure and that the noise (that increases with the exposure time) can spoil the efforts. When we add to this problems with light pollution, quality of optics (even smallest imperfections are clearly visible in case of photographing stars) and the weather, astrophotography appears to be very difficult.
What is more, there are the same problems as in the "normal" photography. One of them is the high dynamic range of the photographed objects - comets, nebulae, galaxies, crescent Moon, partial lunar or total solar eclipses.
A tone mapped HDR photo of a crescent Moon. Visible are both the earthshine and the sunlit lunar surface.
This kind of photo cannot be taken without using HDR techniques because of very high luminance ratio.
The photo has been processed with easyHDR of a series of 18 photos taken at exposure times ranging
from 1/100 sec (at ISO 200) up to 2 sec (at ISO 400). Some of the shortest exposure photos were taken
at the same settings to further reduce the noise.
HDR photo of a partial lunar eclipse - September 28th 2015, about 15 minutes before totality. Seven photos processed with easyHDR - all taken at ISO 400, exp. times: 1/100 sec, 1/30 sec, 1/15 sec, 0.4 sec, 3 x 1 sec.
One of the examples is the Orion Nebula (M42). It is very difficult (top shelf class equipment is needed) to take a photo with properly exposed core as well as the outer fine detail.
The sample M42 (Orion Nebula) photos, shown below, were taken by Luke Bellani with Meade DSI Pro 2 astrophotography camera attached to Skywatcher ED80 telescope. The photos were taken at 4, 15, 60 and 340 seconds of exposure. EasyHDR directly imported the FITS image format (that is used by the camera to store image data) along with the exposure parameters, which were used to calculate the exposure values, necessary for true HDR radiance map generation.
4 seconds (EV: -3.91)
15 seconds (EV: -2.00)
60 seconds (EV: 0.00)
340 seconds (EV: 2.50)
The HDR generation window with the loaded M42 sequence (True-HDR method is used)
Below are the results achieved with various HDR generation methods and different FITS contrast correction options.
True-HDR method, selectivity: "normal".
If you're aiming for the best realism, you should ensure the following:
- Photos should have dark background (no light pollution), so you don't have to alter the contrast of input photos.
- Take many exposures at small exposure settings steps - preferably as small as 1EV (the above M42 example lacks at least one photo between 60 secs and 340 secs of exposure). Some photos can be even taken at the same settings - by doing this you won't gain dynamic range, but you'll lower the noise level.
- Use True-HDR with "normal" selectivity.
- Use very weak settings for "Local Contrast" tone mapping operator.
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