Editing Filters (section)

== Filter types ==
There are two main distinctions between filters used in astronomy, those that are used to see things (visual filters), and those that are use to help image things (photographic filters).  Since this is an AP subreddit, only photographic filters will be described here.

When imaging with a non-modified DSLR (or any OSC cam for that matter), light is already being filtered into wavelengths by the CFA and so no additional filters are necessary.  Of course, since many of us live in light polluted locations, “light pollution filters” are often used.  A light pollution filter is a photographic filter designed to remove certain wavelengths of light that are common in areas of civilization.  Because many towns use mercury and sodium lights to light their streets, houses, parking lots, etc, the amount of light around is typically flooded by wavelengths of light that match that of mercury (chemical symbol: Hg) and sodium (chemical symbol: Na).  Since intelligent chemists have figured out which wavelengths those are, other intelligent people have found a way to filter them out using a light pollution filter.  See figure 2.<blockquote>Figure 2: Figure 2: Light pollution lines and common filters, Baader.</blockquote>Figure 2 is complex, but focus on the orange lines.  The orange lines in figure demonstrate which specific wavelengths of light are given off by mercury (Hg) and sodium (Na) lights.  Therefore, when using a light pollution filter, such as the Baader UHC-S filter shown as the blue line in figure 2, the amount of light that gets past the filter and makes it to the sensor is essentially 0 for many of the mercury and sodium wavelengths (such as 546nm, 577nm, 578nm, etc).  This is how light pollution suppression works.

Of course, one could always add a more than one filter in front of a OSC camera.  OSC cameras often come with more than one filter built in.  We have already seen how they must include a Bayer array (otherwise they wouldn't be a OSC camera), but many also include an ultraviolet (UV) IR cut filter.

=== UV-IR cut filters ===
----A UV-IR filter is a filter designed to limit the amount of light that makes it to the sensor to a certain predefined area of the visible and infrared spectrum.  Recall figure 1.  There is still plenty of light that has a wavelength less than 400nm (we call this ultra-violet), or greater than 700nm (we call this infrared, sometimes referred to as “deep infrared” or “deep IR”).  Since the astrophotographer likely does not want this light to hit his/her sensor (for various reasons that are beyond the scope of this entry), it needs to be filtered out.  That filtering is done by the UV-IR cut filter.  This filter is a single filter that, just like a light pollution filter, will not allow light with a wavelength less than 400nm, or greater than 700nm to pass.  Therefore, all light that DOES pass through this filter must be in an appropriate wavelength range, which is what the astrophotographer desires.  There are many UV-IR cut filters available, many of which will “cut out” light at different values.  For example, some will allow light of 699nm to pass, but 700nm is not allowed, while other filters will allow 700nm to pass, but 715nm is not allowed.  The varying values of the “cut off” point is a user selectable option.

Of course, there is no reason the UV-IR cut filter must be one piece of glass.  It is completely possible to only use an IR cut filter which will “cut out” all IR light (i.e. light with a wavelength greater than 700nm), but let all light less than 700nm pass.  The same can be said for the UV side of the spectrum.

=== Broadband vs Narrowband ===
----When imaging with a mono camera, there are two main methods by which to obtain color.  One method is through “broadband” imaging (also called RGB imaging), and the other is through “narrowband” imaging.  In a broadband image, color is collected through filters that are provided by the astrophotographer, and mimick the Bayer array.  The color filters used are red, green, and blue.  Broadband imaging provides a significant increase in resolution of a given sensor, because during an exposure the entire chip is being utilized to collect the light.  In a OSC camera, only the pixels under a given color are being used, which, depending on color, is 25% or 50% of the total number of pixels.  So, by broadband imaging with a red, green, and blue filter, you are increasing the effective resolution of your image by 25%, 50%, and 25%, respectively, as compared to a OSC camera with the same sensor.  Of course, using this method, the filters need to be purchased and maintained separately from the camera, more for the astrophotographer.

Narrowband imaging is an entirely different technique.  Whereas broadband imaging says “I'm going to collect ALL wavelengths of light between 600-700nm and call that red” in order to get color, narrowband imaging says “I'm going to pick ONE wavelength, say 656.3nm, and call that red.”  Therefore, when narrowband imaging, the astrophotographer is essentially picked one wavelength of light he/she would like to record, and providing a filter that does exactly that.  The most common wavelength of light that is imaged in this method is hydrogen alpha (Ha) which gives off light of 656.3nm.  There are many astronomical objects that give off this light, and so there are many “Ha” targets available for a narrowband imager.  Unfortunately, since this light is limited to a single wavelength, often very long exposures are needed in order to collect enough light such that the signal to noise ratio is high enough to generate a good image.  This is why an expensive, high quality, mono-CCD is used with narrowband imaging.  Other common narrowband wavelengths are shown in figure 2, such as oxygen three (OIII) at 495.nm, and 500.7nm.  Do note there is some 'width' to which what wavelengths a narrowband filter lets through. I.e. 7nm, 3nm.

=== Planetary Filters ===
----As with deep sky imaging, planetary can also be done with either an osc camera or mono. Some OSC cameras such as the ASI 224MC offer high IR sensitivity which can be of benefit. When imaging the moon and not intending to saturate the soil colour, an IR Pass filter may be employed to cut through the worst of the atmospheric seeing which affects the bluer end of the spectrum. Alternatively you may require an IR cut filter when imaging mars if you wish to attain colour accuracy.

Other specialty filters also exist such as Ultra violet bandpasses, or Methane CH4. These allow for imaging of specific cloud structure detail in planets which may not be otherwise visible in broadband RGB.