I just read an answer to a question regarding astrophotography and I wonder what filters (and other stuff that helps) I can use for astrophotography for reducing the "effect" of light pollution.

I would like to shoot objects at night near somewhat light-polluted areas, but with stars as visible as possible.

up vote 5 down vote accepted

Depending on the target you are interested in imaging and the sensor you are using, you may be able to use a variety of filters.

If you are interested in emission nebula, then usually a contrast-enhancing filter helps. These come in two strengths usually, a lower strength that lets more of the continuous light through (which helps keep star colors intact) and a high strength version which helps eliminate more of the background skyglow and reject more of the sodium and mercury vapor city lights.

If you are looking at a nebula that features light primarily in Hydrogen Alpha or Beta or Oxygen 3, then you can use a notch filter just for that specific wavelength. These are the best option for eliminating nearly all the skyglow and light pollution in an area. The narrower the bandwidth is better for these cases. Popular bandwidths are 7nm to 5nm.

There is a third option which is a very gentle filter based on didymium doped glass which can help reject some yellowish skyglow. It's a commonly used filter to enhance the red colors of fall foliage.

Choosing the right filter depends on your imaging sensor, too. If you are using a one-shot color sensor (DSLR and film are examples of this) then the former contrast enhancing filter would be more appropriate than a notch filter.

Note that if you are going after bright targets like the moon and planets, filters are usually not a big issue except to minimize telescope artifacts like CA and purple fringes. Then you would be considering IR and UV blocking filters and maybe minus violet filters.

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    CLS light pollution filter is also quite good. It blocks the light from mercury and sodium-vapor lamps and lets the most of the visible light and H-alpha emissions pass. (astronomik.com/en/photographic-filters/…) – OH6KVU Aug 14 '12 at 12:26
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    The CLS is an example of the lower-strength contrast filter which is good for moderate light pollution and skyglow applications. Other makers are Baader, Hutech, Lumicon, and a host of parts from Orion, Celestron, Meade, etc. – smigol Aug 14 '12 at 15:41

Distance and altitude are the two best ways to reduce the effects of light pollution in astrophotography.

If you like to use GIMP or Photoshop, there is a fairly easy trick to remove light pollution (and even clouds) from images. For this to work, the stars must still appear on the image - it can not and will not magically recover faint objects. It will just improve the looks of your picture.

This trick can also remove mist and fog to a certain degree.

The steps:

  1. Open the image in GIMP/Photoshop.
  2. Duplicate the layer.
  3. Use Gaussian blur on the duplicate layer. Use a radius that completely blurs the objects you still want to see (stars, nebulas, galaxies...). It should not affect the things (light pollution, clouds, fog...) you want to remove.
  4. Select "Subtract" as the layer mode. It should subtract the light pollution from the original image.
  5. Merge the layers.
  6. Carry on editing your images.

This has tremendously improved my pictures of the nightly sky.

Edit In my experience, it's best to edit pictures only after light pollution removal, not before.

  • it won't however make things that are completely washed out by the light pollution reappear... – jwenting Sep 30 '16 at 6:39
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    Yes. That's the "garbage in, garbage out" principle in action. It will improve the looks of the picture by a large margin, but if the original picture didn't catch the star, it won't magically reappear in GIMP. – user258532 Sep 30 '16 at 7:19
  • I have edited my answer to reflect that issue! – user258532 Sep 30 '16 at 11:12

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