CN FORUMS ARCHIVE Telescope Reviews: M42 using Skyglow Filter for Ha

A Hydrogen Alpha filter will cut out almost all light except light of wavelengths close to 656 nm. If you saw a bright pure light of this wavelength your eye would see it as very vivid red.

Some photographers prefer to desaturate their Hydrogen-Alpha photos and just display them in grayscale (black and white) although the reality is that what they have captured is a pure red light. One reason for doing this is that our brains respond well to black and white images and we can often pick out certain kinds of detail easier in grayscale than we can in red-scale. Visit a photographic exhibition and note how many photos are taken in black and white (grayscale), and how good they look to us. We are good at picking out the detail in them.

As you've correctly indicated an Orion Skyglow filter is not an Ha filter. Instead of a narrow band of Ha light it allows a broad band of wavelengths. It does however reject a wide band of color centred around the yellow. Pesky light pollution actually radiates at just about ALL wavelengths but is much stronger at some. The band of color that it rejects is where you'll find the more common emissions of street lamps (Mercury street lamps, etc..). The purpose of the Orion Skyglow filter is to cut out the reflected glow from the sky caused by street lighting.

By rejecting the wavelengths of common street lighting the contrast between what you want to see (nebulae) and the background sky is boosted. The nebula is the same brightness (actually a little reduced) but the background sky appears much darker so you can see more detail in the nebula.

The filter will strongly reject light at its "pollution" wavelengths including the small amount of light actually emitted by your target object at those same wavelengths. Doh!

The purpose of an Ha filter is to reject ALL light EXCEPT the very narrow range at which the Hydrogen Alpha molecules radiate.

Nebulae emit most of their light at very precise wavelengths, such as the Ha wavelength. There are others like Hydrogen Beta, Oxygen, etc.. Ha is a popular choice for a filter because many nebulae are strong radiators at this wavelength.

Using a Ha filter to view a Ha nebula the contrast you get between the sky and the object you are looking at is the maximum possible. The filter rejects almost all light except the light with a wavelength near 656 nm. This light will pass through the filter. Some of it will be from sky glow.

Even an Ha filter is not precise and transmits a narrow BAND of wavelengths (close to 656 nm). A Ha filter will tend to strongly reject every other wavelength so the contrast for a Ha nebula is the practical maximum that you can get.

The problem you create by limiting the photons captured, to include only those with a wavelength close to 656 nm, is that the total light you are capturing onto your sensor is drastically reduced. All the other signals from the nebula are also rejected. All the Hydrogen Beta, Oxygen and yes, even ordinary 'white' light from the nebula will also be strongly rejected.

So, in order to capture enough light to make a good photo you must therefore take a very much longer exposure than if you took a photo in a perfectly clean, transparent and dark sky with no filter (and captured all light from the nebula).

The original multi-colored image that you captured (the one showing blues as well as reds) is I believe the correct representation (ignoring for a moment the issue of color balance) of what you photographed. The Orion Sky Glow filter passes lots of colors (reds, greens, blues, etc.) but it will have reduced some colors in your photo.

Cranking up the red in software often does little to enhance a photo. The main purpose of using a Ha filter is to bring out detail that might otherwise be swamped by other light sources (i.e. by skyglow, or other light from the nebula, etc..). In fact one of the main reasons that folks capture Ha data is to use it as a Luminance (brightness) layer to enhance the detail of color (LRGB or RRGB) images.

The percentage of Ha that you capture by using a Sky Glow filter will certainly increase, but the total amount of it will remain the same (and will actually probably decrease slightly [say1%] because no filter is a perfect transmitter).

Hope this helps.

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Edited by 2020BC (02/23/06 06:55 AM)