Narrowband filters can be used to enhance images of nebulas by isolating light given off by the nebula while blocking light pollution. The most common filter to use is a hydrogen-alpha (H-alpha) filter. This filter passes a wavelength of 656nm, which is in the red part of the spectrum. As implied by the filter's name, this wavelength is emitted by hydrogen gas, which is the dominant component of most nebulas.

Since a narrowband filter passes only one wavelength, what you end up with is a monochrome image. To create a color image, you need images taken at three different wavelengths. For regular color images with a monochrome camera, this is done by using red, green, and blue filters. A one-shot-color camera uses an array of red, green, and blue sensitive pixels to create a color image, but the idea is the same.

There are two common techniques for narrowband imaging: one is to create false-color images by using three different narrowband filters, usually hydrogen-alpha, oxygen-III, and sulfur-II. This is the filter scheme used in many Hubble Space Telescope images, including the famous Pillars of Creation. The second common technique is to use only an H-alpha filter to captrue a monochrome image, then combine this with a normal color image, using the H-alpha as a luminance image to enhance the nebula detail. This is the technique that works best with a color camera so it will be the one described here. The processing is done in two stages: first in MaxIm DL, then in Photoshop.

Capturing Narrowband Images

Narrowband filters get their name from the fact that they pass a very narrow portion (bandpass) of the spectrum. The filters are specified by both the wavelength they transmit (H-alpha, OIII, SII, etc.) and the width of the bandpass. The bandpass is given in nanometers (nm). For example, a typical narrowband filter might have a bandpass of 6nm. The narrower tha bandpass, the more the filter isolates the desired wavelength. However, since it transmits less light, a narrower filter requires a longer exposure time. Also, at very fast focal ratios (such as using a HyperStar system at f/2), the bandpass of a narrowband filter will shift away from the desired wavelength. This means a wider bandpass is necessary.

When using a one-shot-color camera for narrowband imaging, the bandpass is a critical consideration. In H-alpha, for example, only red light is transmitted. This means only the red-sensitive pixels in the camera will detect any light. This effectively reduces the overall sensitivity of the camera. Between this fact and the fact that a narrowband filter passes much less light than a broadband filter or no filter, the required exposure times are much longer. Use of a wider bandpass is essential. 12nm filters work for one-shot-color cameras at slower focal ratios, while with a HyperStar system, using a 35nm bandpass filter can work well.

The example image used below of the Lagoon Nebula was taken with a Starlight Xpress SXVF-M25C, a Celestron NexStar 11 GPS, and a HyperStar lens at f/2. A 2" Baader 35nm H-alpha filter was used for the narrowband exposures, and the normal color exposures were taken unfiltered. Even at f/2 and with a broader H-alpha filter, the required sub-exposure time with the one-shot M25C was 5 minutes. The color exposures were 2 minutes each.

Processing in MaxIm DL

Once the images have been captured, you will combine them in the normal manner. For these images, the Stack command was used with Auto Calibrate and Auto Color Convert checked. In this case, flat field images and bias frames were applied to remove vignetting.

The result of color conversion on an H-alpha image is a bit startling. As one might expect, the image is completely red as this was the only wavelength of light that the H-alpha filter let pass.

Above: The combined and color-converted H-alpha image

The first thing to do with the H-alpha image is convert it to monochrome. This can be done simply by direct conversion from color to monochrome, but there is a better way. Go to Color > Split Tricolor. You will now have four images: the original color image, and one monochrome image for each channel, red, green, and blue. You may note that there is some information (mostly bright stars) visible in the blue and green channels. This is normal because all one-shot-color CCDs have some leakage between colors; the red pixels are actually very slightly sensitive to blue and green, for example. At this point you can close the color image and the blue- and green-channel images. The data we want is all in the red-channel image.

Above: The H-alpha image after splitting the color channels and keeping only the red data

Next, we need to align the H-alpha image (now monochrome red-channel only) with the color data. With both images open, select Process > Align. While using an auto-star alignment works well for the individual frames within a stack, aligned two different stacked images is often more successful using a manual 2-star alignment. Select a star in the upper left of each image, then one in the lower right. When finished the color image will now be aligned with the H-alpha image.

The last step in MaxIm DL is to stretch the images appropriately then save them as TIFF files for use in Photoshop.

To stretch the color image, begin by setting the minimum value in the Screen Stretch window to zero. Then move the green white-point slider to increase the maximum value. Go until the brightest part of the object of interest (in this case the core of the Lagoon Nebula) no longer appears washed out. This gives the full dynamic range we want. Now save the image as a 16-bit TIFF using a manual stretch set to Linear, Screen Stretch and 16-bit.

Above: The color image after alignment and applying a screen stretch

For the H-alpha image, this will become our luminance channel in Photoshop, so this is where the real detail of the image will come from. For this we probably want to apply a non-linear stretch of some kind. One option is to use a log stretch. Begin by repeating the Screen Stretch procedure you did on the color image: set the minimum to zero and move the green slider to preserve the brightest part of the nebula. Now go to Process > Stretch and select Log, Screen Stretch, and 16-bit for the settings. After applying, bring the red slider in the Screen Stretch back up near the left edge of the histogram curve. (As an alternative to a log stretch, you can try using Digital Development. See this tutorial for details.) Finally, save the image as a 16-bit TIFF using a manual stretch set to Linear, Screen Stretch, and 16-bit.

Above: The H-alpha image after applying a non-linear stretch

Processing in Photoshop

Begin by opening your two TIFF files in Photoshop. Make the H-alpha (monochrome) image the active image. Go to Select > All (or press Ctrl-A). Then go to Edit > Copy (Ctrl-C). Now make the color image the active image and go to Edit > Paste (Ctrl-V). This will place the H-alpha image in a new layer on top of the color image.

For now, we want to work on the bottom (color) layer. Click the eye icon next to the top layer in the Layers palatte to hide it, then click on the bottom layer to make it active (it will turn blue).

We now need to make some fairly dramatic changes to the color layer to make it work when we apply the H-alpha image as a luminance layer. Things will look pretty funny for a bit, but trust that it will all work out in the end!

First, we will use curves to brighten up the color image. Go to Image > Adjustments > Curves (Ctrl-M). If the histogram curve shown in the background of the Curves window is shifted to the right, bring the lower left point on the curve to the right until it just touches the edge of the histogram. Next, move the middle part of the curve upward about halfway to the top of the window. Then bring a point halfway between the middle and upper right back down a bit to keep the bright areas from washing out. When done, the curve will resemble the one shown below.

Now open Curves again and repeat the same curve shape. The color image should now be quite a bit brighter than it was before.

Next, we will use Shadows and Highlights to strongly enhance the color and contrast. Go to Image > Adjustments > Shadows/Highlights. Click the Show More options checkbox to expand the window. Begin by setting the Shadows as follows: Amount = 50, Tonal Width = 50, Radius = 30. Next, move the Highlights Amount slider to zero (the other Highlights sliders won't matter). Increase the color correction slider to 100%, and leave the Midtone Contrast at zero.

Above: Shadows and Highlights settings

Above: The color image after Shadows and Highlights

The last adjustment on the color layer is a Levels adjustments (Image > Adjustment > Levels, or Ctrl-L). We are going to adjust each color channel individually to get a neutral color balance and darken the background. In the Channel pull-down menu at the top of the Levels window, select Red. Move the black-point slider (on the left) toward the right until it reaches the left edge of the histogram curve as shown below.

Repeat this procedure for the green and blue channels. You have now made the background a dark neutral color. If necessary, you can adjust the midpoint sliders in each channel as well to get a good color balance. Since the Lagoon Nebula appears low in the sky from the northern hemisphere, this image had a little red subtracted (by moving the midpoint slider in that channel to the right) and some blue added (by moving the midpoint slider in that channel to the left). This compensates for the atmospheric extinction of imaging at low altitudes.

Above: The color image after Levels adjustment

Now you can turn the top layer back on by clicking the eye icon next to it in the Layers palatte. Click on the top layer to make it active. Change the blend mode from Normal to Luminosity in the pull-down menu at the top of the Layers palatte.

It is often a good idea to check the histogram for the top layer at this point using Levels and adjust the black point to darken the background if necessary. When finished, go to Layer > Flatten Image to merge the two layers into one.

Above: The merged image after the H-alpha layer has been set to Luminosity blend mode and flattened

From here on out, you can process the image exactly as you normally would. The final image below (flipped left-to-right for the proper orientation) was enhanced using several of the techniques described in the processing tutorials on this site.

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