Imaging with MULTI-BAND FILTERS

Over the last few years there has been an explosion of multi-band filters for astrophotographers.

- Gary Palmer

Used with a colour astro camera, they help to combat light pollution and make it easier to image deep-sky targets from cities. Compared to mono cameras, one-shot colour cameras also help in this regard as they save time imaging. Combining data captured with a multi-band filter and with a standard light-pollution filter allows some good creativity when it comes to processing; the colour can be changed, and stars moved from one image to another to give some very impressive results.

Here we have examined a selection of multi-band filters to look at the differences in the wavelength of light they let pass through to the camera sensor. There are dual-band filters, which mainly pass the light emission from ionised hydrogen alpha (Ha) and oxygen III (OIII) (though some are now passing sulphur II (SII) and Olll emission), which can enhance the detail within nebulae. There are tri-band filters too, which generally have the addition of passing hydrogen beta emission, useful for adding detail to areas that contain OllI.

The target we chose to image with the eight filters was the Orion Nebula; it has a good mix of gases to image and, in the outer areas beyond the Trapezium, great detail which is harder to capture with standard light-pollution filters. For each multi-band filter, we captured 40x 180-second exposures, processing this with a full set of calibration frames in PixInsight. Further processing was limited to noise control and gentle stretching, but no colour calibration, as we wanted to show what each filter produced. 

Optolong L-Ultimate dual-band 3nm filter 
2-inch, £389