above: the power of continuum subtraction is unrivalled for revealing faint nebulosity

Subtraction of stellar continuum from narrowband nebula images


Filters with a narrow passband tuned to an atomic emission line are widely used for imaging faint emission nebula. By only transmitting light corresponding an an emission line, these filters suppress the relative contribution from sources that emit over a wide range of wavelengths, such as light pollution. Since stars also emit over a wide wavelength interval they, too, are supressed relative to a nebula. This is especially useful for bringing out faint nebulosity in a rich field of stars. If the nebula is extremely faint then even the stellar contribution can overwhelm it. In such case another CCD image can be made though a so-called 'continuum filter', designed to transmit a band of wavelengths close to - but not overlapping - with the nebula emission line. By subtracting this 'continuum image' from the narrowband image the stars can in principle be made to vanish altogether. Simple in principle the process of subtraction is a bit more complicated in practice.

The importance of PSF matching

Below are closeup views images taken through an OIII narrowband and a green continuum filter:

At first glance everything looks fine: nebulosity + stellar continuum on one image; only stellar continuum on the other. However, the stellar point spread function (PSF) has a FWHM of ~2.0 pixels on the narrowband image and ~1.5 pixels on the continuum image. When subtracting the stellar continuum under such conditions bright or dark halo artifacts will then arise at each star position:

To investigate further I used AIP4WIN (extractive photometry) to map the stellar FWHM across the entire field of view of each image:

The stellar FWHM varies from roughly 1.5-3 pixels and it does so differently on each image. Differences in FWHM can arise due changes in seeing, focussing and lens mounting and are in my experience very hard to eliminate completely with fast lenses.

My strategy has been for each pixel to determine which image has the smallest FWHM and then blur it so that it matches the FWHM of the other image. I made a program for doing this in Mathematica; the FWHM distributions after this selective blurring are shown below:

The FWHM distributions are now very similar and the continuum subtraction will result in fewer halo artifacts:

The subtraction is still not perfect, which is mainly due to the fact that I have assumed circular PSF's. In reality the PSF is often slightly elongated and of course this, too, varies in magnitude and direction across both fields of view. With more advanced software this effect could be mapped automatically and an appropriate elongated blur be applied where needed. For now, though, I have not taken this effect into account. Saturated stars also lead to artifacts after continuum subtraction, as is also evident above.

Stellar flux reduction

In general it is nearly impossible to do a perfect continuum subtraction where all traces of stars vanish completely. Often, complete removal of stars is also not what is desired. Lowering the amount of star clutter helps bring out faint nebulosity, but some remaining stars help to bring a natural look to the final image. This effect can be achieved by subtracting a scaled version of the continuum image, as shown below:

Hence, the stellar signal can be continuously varied without affecting the nebulosity signal. If the reduction is pushed too far many artifacts will arise. I find that a stellar reduction up to ~70-85% can be acheived without getting too many artifacts.

Finally, to repair the artifacts that do arise I use a scaled version of the continuum image to replace pixels that have become negative:

For color imaging the procedures outlined above can be applied seperately to each channel. An example is shown at the top of this page. I am very interested in hearing details about how others have dealt with the challenges of continuum subtraction, so do not hestitate to mail me your comments.

Here's my main page where you can see more of the pictures I have taken so far.

Comments greatly appreciated! (mikael@leif.org)