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Documentation / Astrophotography and the HDR image processing techniques

Photography of astronomical objects brings many problems as compared to the photography as most people know. The exposure times can be very long (even minutes) and the lenses, or telescopes used, often characterize with big focal lengths (hundreds to thousands of millimeters), while because of Earth's rotation (and not only), the celestial objects have to be tracked during the exposure. The quality of tracking, the optics (including aperture of the optical system), the imaging sensor (dynamic range, noise level, ...), the ambient temperature, wind, atmosphere's turbulences, thin clouds, light pollution, etc., that all makes astrophotography difficult.

What is more, there are the same problems as in the "daylight" photography. One of them is the high dynamic range of many of the photographed objects - comets, nebulae, galaxies, crescent Moon, partial lunar or total solar eclipses. All of these are hard to be photographed in a single shot. Because of long exposure times and high ISO gain that are often used, it is worth to take many photos and stack (average) them. That is one of the ways to dramatically increase the dynamic range - by making the SNR (signal to noise ratio) higher. Further dynamic range boost requires merging photographs taken at different exposure times. Having an image with elevated dynamic range, you still need to compress it (apply tone mapping) in order to make all the details visible on a screen, or a print.

If you're aiming for the best realism and quality, you should ensure the following:

• Photos should be taken in good conditions in terms of weather, light pollution, etc. You have to spend some time on setting the focus right and keep in mind the limitations of your telescope, guiding, lenses, cameras... The quality of equipment (expensive!) matters a lot, but let's be fair that getting maximum out of what you have and can afford is equally fun, even if the results are far from what others can achieve.
• If it's necessary (like in case of crescent moon, or nebulae like M42) take many different exposures at small exposure settings steps - preferably as small as 1EV. In many cases however you can just live with taking a series of photos with exactly the same settings - the more such images, the better (and it doesn't mean 10, but rather 50-100, depending on the quality).
• Always use raw photo formats - either RAWs from DSLR cameras, or FITS from astronomical cameras. In case of color RAWs fix the white balance at import and do not filter the noise too much - it should be done after stacking.
• The input photos, especially in case of deep sky imaging, need to be calibrated - that involves flat field and dark frame correction (available in easyHDR since version 3.16).
• Use "True HDR" method with "normal" selectivity when merging different exposures, or with "none" selectivity when doing a stack (alternatively just use the "Stacking" instead). If your input photos are spoiled by satellites or airplanes, use pixel rejection options for HDR generation (available with easyHDR 3.16 and above).
• Use very weak settings for "Local Contrast" tone mapping operator and try to compress the tone curve instead.
• In case of deep sky photos never clip background to black.

One of the examples is the Orion Nebula (M42). It is very difficult or impossible to take a photo with properly exposed core as well as the outer fine detail. The sample 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)

Another example is the Horsehead nebula (IC434), also in the constellation of Orion. Vincent Steinmetz, the author of the photograph, used a professional astrophotography camera to take twenty 10 minute exposures that were stacked together. The photos were taken under dark skies through a H-alpha narrow-band filter. That kind of filter blocks all light wavelengths, passing just a narrow window around Hydrogen-alpha emission line. The resulting stack, stored in FITS image format was directly loaded into easyHDR and tone mapped. The stack has a very wide dynamic range due to high quality of the single exposures taken by the camera, that was further increased by averaging the noise. There was no need to vary the exposure time with this setup and this particular object. EasyHDR's job was to boost fine details in the hydrogen clouds of the nebula, while not overexposing the brightest details.

Stacking is a powerful technique that can greatly reduce the noise on the image and allow stronger tone mapping that brings forward finest details. You can use specialized software for registering and stacking of the photos into 16 or 32-bit TIFF or FITS format (like Deep Sky Stacker), then load it into easyHDR for tone mapping. However easyHDR version 3.16 and above is also capable of HDR merging or stacking many RAWs and FITs images, providing white balancing, noise filtering as well as dark frame and flat field calibration.

The following sample photo of the North America Nebula in Cygnus is a stack of 41 RAW photos taken with astro-mod Canon 650D (IR filter removed), with additional UHC filter installed (cuts out some light pollution, passing through mostly Hydrogen-alpha and Oxygen-III light), the lens was Samyang 135mm @ f2.0 and the camera was guided with Star Adventurer Mini star tracker. Each photo was a 30 sec exposure at ISO 3200. EasyHDR imported the RAWs and corrected the white balance, dark frame and flat field (supported since easyHDR version 3.16). The photos were auto aligned and the merged result was tone mapped. Some additional post processing was done with Affinity Photo (secondary curve adjustment and further noise filtering).

The Deep Sky Stacker software was not able to align the following photo of the M57 nebula taken under suburban sky with a b&w camera (ZWO ASI178MM) with LRGB filters. It was because of too high noise and too small number of good quality stars. EasyHDR was used to load the input photos - each of the LRGB channels was a separate stack of 40 individual input photos in 16 bit FITS format. The dark frame was also subtracted by easyHDR. Then, the individual channels were "processed" with neutral settings and stored in 16 bit TIFF format for further combining into the LRGB composite with Affinity Photo software. EasyHDR was later used again with the color image to slightly boost the local contrasts. As you can see the photo could be much better if all inputs were as sharp as the sample single frame used here for comparison (problems with atmospheric seeing and focus). Also, there should be more photos taken at longer exposure.

The M31 Andromeda Galaxy processed solely with easyHDR - RAW development with dark and flat frame calibration, white balancing, auto alignment, noise filtering, stacking, further color correction and tone mapping.

Another sample processed only with easyHDR - RAW development with dark and flat frame calibration, white balancing, auto alignment, noise filtering, stacking with satellite and plane marks removal and tone mapping.

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