With monochrome CCD cameras, the best imaging technique, in a sky with significant light pollution, is achieved with the use of narrowband filters, also known as interference filters. This technique is only used for emission nebulae and planetary nebulae. Indeed, in the majority of cases, the luminance of these is outside the light emitted by the lighting of cities. By using interference filters, we only capture the luminance of nebulae while effectively blocking that coming from light pollution. These filters cannot be used for photographing star clusters and galaxies, because the majority of the luminance of these objects is outside the narrow bandwidth of each of these filters. For the preprocessing and processing of the images acquired with these filters, the techniques described in this site are used, in particular the LRGB technique. Read this section if you are not familiar with this technique (LRGB). It will then be easier to understand the following presentation.
With monochrome CCD cameras, three narrow band filters are used; Ha (alpha hydrogen line in red), OIII (oxygen line in green-blue) and SII (sulfur line in dark red). There are different possible associations for these filters. I will present some of them. Before using them, you must know the emission line of the nebula you want to photograph. Its radiation will then be made to correspond with the passband of one of these filters or the association of two or all of the three filters. This bandwidth is expressed in nanometers (nm) on a wavelength scale (Wavelenght in English). Here is the bandwidth of each of the three filters:
- Ha: 656,3 nm (red)
- OIII: 500,7 nm (blue-green)
- SII: 672,4 nm (dark red)
The function of these filters is to capture these emission lines as closely as possible. In general, the narrow bandwidth is on the order of 7 or 5nm (note that there are other widths of bandwidths commercially, eg 8nm, 3nm). Here is the graphic representation of this whole statement:
The graph shows the typical transmission lines of different filters
The vertical scale represents the percentage of transmission of the different filters. The horizontal scale represents the bandwidth of the different filters expressed in nanometers. The bandwidth of the standard Red, Green and Blue filters is shown in the pale colors of these filters on the graph. The bandwidth of narrowband filters is shown in darker colors. It can be seen that standard RGB filters completely cover the light spectrum visible to the human eye during the day (at night, human vision cannot see colors, because it is limited to wavelengths close to 555 nm) . Narrow band filters cover a tiny part of the light spectrum in its transmission line. In the filter specifications, the passband is rated at 50% of the transmission of these filters. For example, if the filter passband is 7nm, it means that the filter passband at 50% transmission is 7nm (+/- 3,5nm from the center of the emission line). We also see that the bandwidth of the RGB filters is around 100 nm, compared to the narrowband filters which are 7 or 5 nm.
With the use of narrow band filters, it is recommended to use a narrower focal aperture, f / 4 and higher, i.e. f / 5, f / 6… If we use a more open focal length, lower than f / 4, ie. f / 3, f / 2…, there will be a shift in the transmission line of the narrowband filters, making them less efficient. The more the focal length is opened, the more the displacement will be important. So, if you intend to use these filters with an open focal length, prefer purchasing filters with a bandwidth of 10nm and above.
Here is the presentation of different combinations of filters:
The Hubble palette
Narrow band photography was developed initially for scientific purposes. The first uses were designed for the Hubble telescope. The combination of filters served to bring out in the images the chemical distribution of the main elements of the nebula. The association is as follows; SII for the red layer, Ha for the green layer and OIII for the blue layer. The Ha filter being the most dynamic layer, green will dominate in the image. The red layer (SII) and the blue layer (OIII) will be used to give relief to the photo by revealing the regions of the nebula rich in sulfur and oxygen. We now denote this association of filters by la hubble pallet. The image produced will therefore be in false colors. Here is an example of a color image produced using the Hubble palette:
IC434 - The Horse's Head - Ha- (SII-Ha-OIII) - Green dominates
To convert this dominant green into copper colors, typical of the Hubble palette on several images produced by fans of this SII-Ha-OIII association, I personally use the selective color correction Photoshop software. Here is an example image showing this conversion:
NGC2237 - La Rosette - Ha- (SII-Ha-OIII) by applying selective color correction.
Click on the image to view it full size.
This image was taken at my personal observatory located in Longueuil, Quebec, Canada, in a site of extreme light pollution (white area).
We can appreciate in this image the great variety of colors, thus helping to bring out even more the nuances of the nebula. Converting the green cast (Ha layer) to copper color provides a more pleasing image.
Using interference filters to produce more realistic color images
The Ha- association (Ha-OIII-OIII)
To produce more realistic color images, we'll use the following association:
- Red layer: Ha
- Green layer: OIII
- Blue layer: OIII
The Ha filter (656,3 nm) being in the red, it is associated with the red layer. The OIII filter (500,7 nm) which is between green and blue, is associated with the green and blue layers. I use this palette for predominantly red nebulae. Most of the signal will be revealed by the Ha filter. The images taken with the OIII filter will mainly be used to present the stars with more realistic colors. As the nebula is red dominant, I use the image taken with the Ha filter as the Luminance image. Here is an example of an image taken with this association:
NGC6820 - Ha- (Ha-OIII-OIII) - Click on the image to view it full size
This image was taken at my personal observatory located in Longueuil, Quebec, Canada, in a site of extreme light pollution (white area). After examining the image taken with the OIII filter, no signal from the nebula appeared in the image. The luminance of the nebula was therefore entirely in hydrogen (Ha filter). I could then have used the following association: Ha- (HaR) VB (the luminance image in Ha and the color image with standard RGB filters by associating the Ha image with the Red image in the Red channel to find the color of the nebula at the same time as the red component of the stars). I would have thus obtained stars with even more realistic colors. But it is difficult to know in advance that the nebula does not emit any signal in oxygen (OIII).
The association (Ha-OIII) - (Ha-OIII-OIII)
A variant of the previous association is the use of the Ha and OIII images as a luminance image. When the nebula emits a lot of signals in hydrogen (Ha) and oxygen (OIII), the use of a luminance image with the Ha and OIII images will bring out all the details of these two emission lines. Here is an example :
NGC6960 - The Witch's Broom Nebula - (Ha-OIII) - (Ha-OIII-OIII) - Click on image to view full size
Other combinations of filters can be made, for example Ha-SII-OIII or Ha-OIII-SII. These associations will be favored when there is a lot of sulfur (SII) in the image. It should be noted that the colors of the images will appear less realistic than the Ha-OIII-OIII association.
The recommended exposure time for each filter
Imaging with narrow band filters requires long exposure times to obtain a sufficient Signal / Noise ratio. Indeed, although each filter allows more than 95% of the signal to pass through its transmission line, the latter must pass through the filter. I recommend using the mode Bin 2 × 2 to produce these images, which will make it possible to obtain four times more signal for the same exposure time as the Bin 1 × 1. An exposure time per photo of 5 minutes (at f / 6) in Bin 2 × 2 is considered a minimum. Here are the exposure times I use for each filter at the focal length f / 6 (and f / 6,3) in Bin 2 × 2; 10 images of 10 minutes (1,67 hours per filter, equivalent to 6,68 hours in Bin 1 × 1). If we use 2 filters, the total exposure time will be more than 3,33 hours (13,32 hours in Bin 1 × 1) and with the 3 filters more than 5 hours (20 hours in Bin 1 × 1). It may be necessary to consider imaging more than one night.
To find out the equivalence of the exposure time for other focal lengths, go to the chapter Astronomical calculations to section The focal aperture and the exposure time.
Richard Beauregard
Sky Astro - CCD
Revise on 2021/03/17