All Filters (Non-Solar)

All Filters (Non-Solar)

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Visual and photographic filters play a big role in how we see or capture the objects in the night sky.  There is a filter to enhance every situation.  Photographic color filters are an essential accessory to the astrophotographer, while visual color filters help to bring out amazing detail to the human eye.  There are broadband filters for boosting contrast or reducing the effects of light pollution and narrowband filters for concentrating on very specific wavelengths of light, including those only our cameras can see.  There is also a wide range of sizes available.  The size you'll need is usually determined either by the size of your eyepiece barrel (typically 1.25" or 2") or the size required by your filter wheel, carousel, or slider.  There are also specialized filters sizes that are made to clip into the body of your DSLR or attach directly to the back of your SCT!  OPT has divided these filters among several categories by type and other search options to make them easier for you to find exactly what you need. If you are looking for a solar filter, however, we have them housed in two categories: H-Alpha and Calcium K Solar Filters and White Light Solar Filters.   

 

 

 

Light is defined as the small portion of the eletromagnetic spectrum that is actually visible to humans.  The electromagnetic spectrum ranges from y-rays to long radio waves, with "light" living somewhere near the middle.  Astronomers are concerned with the visible spectrum and its two nearest neighbors.  The order of these wavelengths is as follows: UV (ultraviolet- nonvisible), violet, blue, green, yellow, orange, red, and IR (nonvisible).  IR is split further into two divisions for our use: near IR and IR.  The wavelengths are assigned numbers measured in nanometers; the visible spectrum lies between 400 and 700nm, although the eye will respond to wavelengths between 350 and 750nm.

In order to fully understand filters and how they work, we need to understand how the human eye gathers light versus a CCD chip, and how we preceive color versus how a chip sees that information.  If you can remember back to your grade school science class, you learned that stars are nearly every color of the rainbow.  However, stars appear white because your eye has two color receptors: cones and rods.  Rods detect brightness and cones detect color, but cones are not very sensitive; they rely on the rods to bring in the brightness they need to see the specific wavelength or color.  Since most stars are very dim and are viewed at night, your rods are doing most of the work.  What this means is that your eyes are not very sensitive at all.  For this reason, visual filters will be thinner and have higher transmission levels (meaning they let more light through).  

CCD chips are not limited by rods or cones; they can pull in any amount of brightness and any wavelengths, given enough time.  A camera can keep it's 'eyes' open for a very long time!  Filters are used most often with monochromatic chips, which will read the entire spectrum from the UV to the IR.  Most deep space objects send out a considerable amount of UV and IR, and the ability to capture these wavelengths can bring out an amazing amount of detail.

By blocking certain wavelengths or allowing only one wavelength range through, filters make different details apparent to the eye, or long exposures of specific details that are possible for a CCD chip.  As you navigate through this category, you'll find information about each filter type and exactly what they'll do for you.
 

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