How do I add color to monochrome astrophotography images?

When it comes to color imaging of the night sky with a monochrome camera, the use of color filters is usually implied. There are two major sets of color filters that are commonly used with monochrome sensors: LRGB and narrow band.

LRGB Imaging

Standard color imaging, or "broadband" imaging, makes use of LRGB or Luminance + RGB filters. Monochrome sensors are unfiltered, and as such are sensitive to both IR and UV (heavily sensitive to IR up to nearly 1000nm wavelengths). 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. Then broadband channels for red, green, and blue are captured separately, and later combined into a full color image.

This separate acquisition of L from RGB, and the use of an L filter in general, is important, for a couple of reasons. First, getting good SNR in astrophotography is very difficult. An otherwise unfiltered exposure gathers a lot more light than any color filter. Blocking IR is also important, as IR focuses differently than the visible spectrum, and can cause bloating of stars. 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. 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.

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). An additional 10 subs each of five to ten minute subs each for RGB channels are gathered. RGB data does not need the same integration time, and they can be noisier. 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.

Narrow Band Imaging

An alternative to LRGB imaging is Narrow Band, or NB imaging. LRGB imaging generally requires very dark skies to be effective. 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. 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.)

To combat light pollution when imaging from a suburban or urban location, narrow band imaging with monochrome sensors is another option. 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. The bandpass is anywhere from 15nm wide to as little as 3nm wide. 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 three primary bands are those I mentioned, and another for planetary nebula imaging is also common:

• Sulfur II (SII): 672.4nm, Deep Red
• Hydrogen Alpha (Ha): 656.3nm, Red
• Nitrogen II: 658.4nm, Red
• Oxygen III (OIII): 500.7, Cyan (Blue-Green)
• Hydrogen Beta (Hb): 486.1nm, Blue

A sufficiently wide Ha filter (5-6nm bandpass) will usually gather NII as well, however it is possible to get separate NII filters if you really enjoy planetary nebula imaging. Hydrogen Beta is the same emission as Hydrogen Alpha, just dimmer, so if you want to account for it, you can reuse Ha data for Hb.

Narrow band imaging can be used independently or in combination with LRGB. A common practice is to gather Ha as well as L data, combine the two for better contrast and detail into a super luminance channel, and sometimes blend a little Ha into the red channel. Narrow band filters can be used exclusively, and the two (Ha/OII) or three (SII/Ha/OIII) channels can be used to synthesize a variety of blends that bring out different details. Some imagers simply gather Hydrogen Alpha solo, and do grayscale imaging. Narrow band imaging presents a lot of opportunities.

Because of the narrow bandpass, narrow band filters allow imaging from heavily light polluted sites. The narrow bandpass also requires much longer exposures. Where LRGB can often be done in as little as five minutes a sub, maybe even less with a sufficiently fast scope, narrow band images generally require 20 minutes at least, and often 30, 45, 90 minutes or more depending on the channel and the surface brightness of the object being imaged. This tends to require more precise equipment. Exposing for 20 minutes can be a challenge, exposing for longer usually requires a fair amount of skill.

Choosing Filters

There are a relatively wide variety of filters on the market, of different price classes and camera compatibilities. There are two primary kinds of monochrome cameras: Purpose-built Mono CCD cameras, and modded "debayered" DSLR cameras. Mono CCD cameras have more filters available, and usually use 1.25" threaded filters or 2" threaded filters. CCD cameras come in a wide variety of sensor sizes, and some larger sensors may use 31mm, 52mm, or 65mm mounted or unmounted filters. DSLRs are often more complex. Some companies such as Astrodon offer "clip-ins" which are filter holders that can be dropped into a standard Canon APS-C DSLR, and used with either standard EF lenses (EF-S lenses can NOT be used), or with a T-adapter. You can also find T-thread filters that can be screwed onto the end of a T-adapter or into a T-ring.

There are a few key brands. Orion and Celestron offer a number of basic filters, however they are usually quite cheap and fairly low end. There are a number of other manufacturers in this class as well. These would be very entry level filters most of the time, with the exception of a couple Orion LP filters. Astronomik supplies a number of filters in screw-in and clip-in form for LRGB and NB imaging, both with DSLRs and CCDs. They are step up, as good as some of the higher grade filters I'll mention next in many cases, however some of their filters aren't of the best quality. The next step up would be Custom Scientific and Baader, both of which manufacture good quality LRGB and NB filters. CS and Baader filters are often wider bandpass (for NB filters), and usually not par-focal (so switching from one filter to the next requires refocusing), however they are cost effective. The top of the line filters would be Astrodon, the E-series Gen II LRGB filters and their 3nm narrow band filters. Astrodon filters are basically the best money can buy, they filter out some LP for the LRGB filters, they are par-focal for all filters, they offer the narrowest bandpass for NB filters (so highest contrast)...and they come with a matching price tag.

Using Filters with mono CCD

To use filters with a CCD, you usually require a filter wheel of some kind. There are a number of filter wheels out there. Some are generic, some are designed to work with specific brand or brands of CCD cameras. You can find filter wheels in 5-position, 7-position, 8-position, and 9/10-position (such as the case with the FLI Centerline filter wheels, which are fairly unique in the way they work.)

If you are just doing LRGB work or just NB work, a 5-position filter wheel will do. If you want to do LRGB and a couple narrow band filters, a 7-position will do. For a full constituent of LRGB and NB, you need an 8-position filter wheel. The extra position is either "clear", or blocked off for taking darks (for CCD cameras that do not have a shutter.)

You can find most of the necessary equipment for imaging with filters and a mono CCD camera at sites like OptCorp.