What are Telescope Filters?
Telescope filters are used by professional and budding astronomers alike to improve the clarity of objects in the sky when viewed through a telescope eyepiece or camera. Filters can reduce the appearance of specific colors and wavelengths of light. By doing this, the filter enhances visual or photographic contrast and brings out the details of stars, planets and other astronomical objects. There is a telescope filter for just about every application. A light pollution filter is one of the most popular — no filter can block all forms of light pollution, but they can reduce its effects to provide a better viewing experience.
How to Choose & Use Telescope Filters
If you are new to the world of telescope filters, the huge variety of filter types and sizes can be overwhelming. However, if you filter your results by the "filter size" and either the "filter type" or "primary purpose" in our left-hand navigation, it will go a long way towards narrowing down the field.
Filter Size: Let's say you are looking for a moon filter. Before you start shopping, you'll need to know what size moon filter you need. The most popular moon filter comes in a threaded 1.25" cell, like this Celestron moon filter, and if you plan to use a standard 1.25" eyepiece to look at the Moon, then this is the filter size you need. The same is true if you are using any other type of filter that is designed with a 1.25" or 2" threaded cell, just remember you need to match the filter size to the eyepiece barrel size and you'll be good.
Since we look through an eyepiece when doing visual astronomy, it makes sense that any filters meant to be used in conjunction with an eyepiece would have compatible threads so the two can be safely connected. However, when doing astrophotography, threaded filters are not always necessary. There are exceptions, such as when using a standard entry-level filter wheel or filter carousel that accepts 1.25" filters, but most other filter wheels use filters that are not threaded. Different filter wheels use different sized filters, which is why you see so many sizes in the same filter type. If you have a filter wheel or camera with an integrated filter wheel, then you know what size filter you need, so again, filtering in our left-hand navigation by Filter Size will narrow down your choices.
Telescope Filter Types
There are many different telescope filter types available to suit a wide range of stargazing and astrophotography needs. Some of the most common types of telescope filters are:
- Moon Filter: A moon filter or neutral density filter is used to dim the light of the amazingly bright moon. While it may seem contradictory, this actually makes it easier to view the moon's surface, bringing out its details and allowing you to look at it comfortably. An upgrade to a standard moon filter is a variable polarizing filter, which rotates to let you adjust the brightness level and can also be used on Venus or to split double stars. If you'd like a good moon filter that also reduces the visual effects of light pollution, check out the Baader Moon and Skyglow Filter. This multi-tasking filter can also be used in astrophotography and is available in a 2" threaded cell as well as the standard 1.25" model.
- Planetary Filters: Color filters can be used to enhance specific features on the planets by narrowing in on certain parts of the spectrum. For instance, a blue filter will highlight details in the cloud belts and Red Spot on Jupiter, while a yellow-green filter (Wratten #11) provides better contrast when viewing Mars and Jupiter and the Cassini Division in the rings of Saturn. Many manufacturers offer a planetary filter set to give you a nice range of choices, but keep an eye on the percentage of light transmission on each filter so you choose those that work best with your telescope's aperture. To see a list of different filters, their transmission and recommended telescope size, take a look at the High Point Scientific Filter Guide.
- Light Pollution Filter:Both professional and amateur astronomers understand the importance of dark skies when it comes to astronomy. Observatories are built on mountaintops and remote areas in an attempt to escape light pollution, and amateurs love to attend star parties held in "dark sky" locations. For most of us, however, finding a truly dark sky for normal telescope sessions is not an option, and we resort to using a light pollution filter, aka LPR (light pollution reduction) filter to remove wavelengths of the worst offenders (sodium, mercury-vapor, etc.) while allowing wavelengths of light emitted by celestial objects to enter our eyepiece or land on our camera's sensor. Light pollution filters reduce the effects of light pollution when viewing objects, but they do not eliminate them. To do so would block out the good stuff as well as the bad...so it is a balancing act, but one that results in darker background skies that reveal many objects that would otherwise be washed out by light pollution. Check out this article to read more about the effects of light pollution on visual astronomy and astrophotography.
- Filters for Astrophotography: There is a huge selection of filters specifically designed for astrophotography available today, from LRGB filter sets that add color to images taken with a monochrome sensor to super narrowband filters that only allow a small wavelength of light to reach the image sensor. Popular narrowband filters include OIII, or Oxygen III, a Hydrogen Alpha filter, or HA, or a Sulphur II, or SII filter - and that is just the beginning! Astrophotography filters come in various sizes to fit different filter wheels or camera filter carousels, and many are also available in a special holder called a "clip" filter that can be used inside specific DSLR cameras, such as Nikon and Canon.
- Solar Filters: White light solar filters are fairly inexpensive and fit over the end of your telescope tube to block harmful solar rays that would damage your eyes. When installed correctly and used carefully, white light solar filters will allow you to watch solar eclipses safely, and you can also see active sunspots as they form, grow, and disappear in the Sun's photosphere.
To learn more about the different telescope filter types, make sure to check out the High Point Scientific Filter Guide.
A Note About Broadband, Midband & Narrowband Filters
While these aren't necessarily telescope filter types in and of themselves, "broadband", "midband" and "narrowband" are terms used to describe the wavelength of light transmission that each filter will allow, with narrowband allowing the smallest portion of the light spectrum through and broadband allowing the widest.
Buy the Best Telescope Filters Online at High Point Scientific
As always, we would be more than happy to help you pick the best filter for your telescope and applications. If you have any questions, please contact us — our non-commissioned experts are ready to help you get the best stargazing experience possible.
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