High dynamic range Orion Nebula
Messier 42, the Orion Nebula, is a very popular target for astrophotography, and for good reason too. It’s bright, colourful, easy to find and shows a wealth of detail. If you’re not that experienced, the bright core region, known as the Thrust, provides a good practice target. Then, as experience grows, you can concentrate on the glowing outer wisps of nebulosity.
Here you’ll run into a problem; how can you retain the beautiful detail and colour in the Thrust region while pulling out the faint outer wisps? Modern cameras can go some way toward doing this when used in
conjunction with image processing techniques. These involve stretching the image to accentuate its dimmer parts while not losing the brighter core.
However, a similar, and possibly more visually natural, result can be achieved by sacrificial imaging. Here you concentrate on one area at the expense of the other. For example, longer exposures can be used to pull out the faint wispy stuff allowing the core to over-expose to white. It’s sometimes difficult to do this as an imager because it just feels wrong to let it happen. The core is even easier to image because you just need to make sure the inner region exposes correctly and not worry about capturing the outer stuff.
Both bright and dim images should ideally be approached using regular deep-sky imaging techniques. This requires taking multiple images, which are then calibrated using dark frames and flat fields to ensure the best quality. However, if you’re new to imaging and don’t feel comfortable with calibration, registration and stacking, there’s no reason why this can’t be done with single images to start with.
If you do decide to go the advanced route, basic calibration involves removing hot-pixels and vignetting from your light frames – that’s the term used to describe the images of the actual subject matter. Freeware such as DeepSkyStacker (deepskystacker.free.fr/ english/index. html) can do this for you but you still need to create the calibration data. Hot pixels – unnaturally light pixels caused by tiny sensor imperfections – can be corrected by covering the telescope aperture after you’ve taken a set of light frames. By taking another set of shots at exactly the same exposure, the resulting dark frames will isolate the hot pixels present in the light frames (they’re always in the same place); they can now be easily subtracted from the light frames.
Flats are a little more complex. You need to point the imaging telescope at an evenly illuminated light source and take images that saturate the camera sensor to about 50-70%. The saturation can be determined by the position of the peak spike in the histogram display for the image.
Random thermal noise in each image requires a number of darks and flats to be taken and averaged together. This reduces the background noise according to the square root of the number of images involved. For example, if four images are used, the noise is reduced to a half its original value. If nine images are used, the noise is reduced to one third.
Although this may sound complicated if you’re just starting out, programs such as DeepSkyStacker do much of the donkey work. Your job is to make sure the images you supply it are focused and tracked as accurately as your equipment will allow.
When different regions of a DSO require different exposures, you need to combine two shots