![]() The narrower the bandpass, the higher your contrast will usually be, as more and more stray light not coming from that specific emission band will be blocked. The bandpass is anywhere from 15nm wide to as little as 3nm wide. Narrow band imaging uses filters that block out everything except a narrow band around a very specific emission, such as Hydrogen Alpha, or Oxygen III, or Sulfur II. To combat light pollution when imaging from a suburban or urban location, narrow band imaging with monochrome sensors is another option. For best results, you need to find a dark site where overhead emissions are around 20 magnitudes/square arcsecond or darker (20-22.5mag/sq" is usually considered a good dark site, and usually 25-45x darker than your average suburban or city skies.) Some high end modern filters, such as the Astrodon E-series Gen II, make some attempt to block out primary sources of light pollution (namely low pressure sodium vapor emission bands) in the R and G hannels, but as LRGB is broadband imaging, you can't really do much about light pollution. LRGB imaging generally requires very dark skies to be effective. The human eye is less sensitive to spatial resolution in color, so heavy NR can be applied to the RGB channels, while more careful NR and enhancement is done to the L channel to bring out all the detail.Īn alternative to LRGB imaging is Narrow Band, or NB imaging. RGB data does not need the same integration time, and they can be noisier. An additional 10 subs each of five to ten minute subs each for RGB channels are gathered. Typical integration times with LRGB may be anywhere from a few hours to as much as ten or twenty hours of L data using three minute to ten minute subs (for your average f/4-f/7 scope). After L is gathered, much shorter integration times for R, G, and B channels can be gathered for later combination with the high SNR L image. As such, the L filter is usually where most of your exposure time is done, to gather as much high SNR data as possible, or as much "integration time" as possible. Blocking IR is also important, as IR focuses differently than the visible spectrum, and can cause bloating of stars. An otherwise unfiltered exposure gathers a lot more light than any color filter. First, getting good SNR in astrophotography is very difficult. This separate acquisition of L from RGB, and the use of an L filter in general, is important, for a couple of reasons. Then broadband channels for red, green, and blue are captured separately, and later combined into a full color image. For maximum detail, an L or luminance filter is used to capture high resolution, high SNR detail across the full visual spectrum, while blocking out IR and UV. ![]() Monochrome sensors are unfiltered, and as such are sensitive to both IR and UV (heavily sensitive to IR up to nearly 1000nm wavelengths). Standard color imaging, or "broadband" imaging, makes use of LRGB or Luminance + RGB filters. There are two major sets of color filters that are commonly used with monochrome sensors: LRGB and narrow band. ![]() When it comes to color imaging of the night sky with a monochrome camera, the use of color filters is usually implied. ![]()
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