Deep-sky imaging
The most technical type of processing rests on how you capture your data
Summer means galaxy season, and even though the nights are lighter, it’s still a good time to begin imaging deep-sky objects (DSOs) such as galaxies, star clusters and nebulae. While a long focal length camera lens is fine for some targets, a telescope is the best option; you will need a T-ring and adaptor to use one with a DSLR.
Although DSLRs are a popular choice and achieve excellent results, the long exposures needed for DSOs mean that many astrophotographers opt for cooled CMOS or CCD cameras. These come in either mono or one shot colour (OSC) varieties.
Calibration frames (darks, bias and flats) are an integral part of DSO processing. The trade-off from long exposures is the potential for noise and vignetting to affect images, which makes processing challenging. Dark and bias frames reduce noise, while flat frames remove any vignetting.
Two popular stacking programs for deep-sky imaging are Sequator and DeepSkyStacker (DSS). Both are simple to use, although DSS recognises bias calibration frames (whereas Sequator does not) and is more advanced. Sequator is perhaps the best stacking program to start with until you become more comfortable. Once star and calibration images are added we suggest switching on ‘High dynamic range’ and ‘Remove dynamic noises’ (see figure 6).
Once calibrated, DSO images can be enhanced in Photoshop (see figure 7). Settings we recommend include ‘Levels’ and ‘Curves’, ‘Hide all’ and ’Reveal all’ masks, and the ‘Median’ and ‘High Pass’ filters. The masks help to adjust certain areas without affecting the whole image; these can then be blended using the ‘Gaussian Blur’ function. A ‘Median’ filter dims background stars and increases the impact of nebulae (find it under ‘Filter > Noise’). Lastly, the ‘High Pass’ filter (‘Filter > Other’) allows you to bring out further details from your object. Once applied, set the Layers mode from ‘Normal’ to ‘Soft light’ to blend.