So… You’re curious about telescope filters! What exactly are they? They may be called telescope filters, eyepiece filters or optical filters, but what it all narrows down to is kind of like adding a special set of “glasses” to your telescope to help you see things better! Unlike prescription eyeglasses, telescope filters don’t enhance the image itself – it enhances the light that reaches your eye. If you go outside on a bright, sunny day in clear prescription eyeglasses, you might have difficulty seeing details in some objects because of reflections or an overabundance of light. The same is true of your telescope! The primary lens or mirror gathers light just like your cornea. How much better can you see in high light situations when you’re wearing sunglasses? How does what you see change if your glasses are yellow, red, green or even polarized? Ah… You’re beginning to understand!
If you take a close look at the open end of your telescope’s eyepiece barrel, you will notice it has fine, machined threads inside. This is for a filter cell – “special glasses” for your telescope. The human eye can only understand certain wavelengths of light that run between the ultra-violet and infra-red ends of the visual spectrum. By using a special filter, we can either eliminate or enhance certain wavelengths of light which allows our eyes to see more details. We are processing the light we see in “real time”. Let’s check out the different kinds of telescope filters….
Telescope Filters for Imaging and Photography
Filters for Imaging and Photography work differently. A camera, even for a fraction of a second, can gather wavelengths of light the human eye cannot see – and begins concentrating the ones it can. For this reason, special filters are made for photographic or CCD imaging use – and, like visual filters – can eliminate or enhance. While some of these filters can also be used optically, others will have no effect on the visual end of the spectrum. These are often labeled “for photographic use only”.
H-Alpha CCD Filters block all unwanted light, especially the Mercury (Hg) and Sodium (Na) emissions from streetlights and the IR region where CCDs are often most sensitive. This provides maximal increase in contrast even under poor observing conditions for objects such as planetary nebulae and faint supernova remnants. H-Alpha CCD Filters narrowband delivers the absolute highest contrast deep sky images of H-II regions, all with the shortest exposures possible. Imaging with narrowband filters, specifically hydrogen- or H-alpha, is an ideal tool to observe structure in emission nebula and is a wonderful means to enjoy the astrophotography hobby even when the moon is up.
Oxygen III (OIII) CCD Filters offers total blocking of all unwanted light-pollution, especially the Mercury (Hg) and Sodium (Na) emissions from streetlights and the IR region where CCD’s are often most sensitive. Oxygen III (OIII) CCD Filters were designed for CCD imaging of gas and planetary nebulae. Due to the narrow bandpass of the OIII line at 501 nm, the filter provides maximal increase in contrast even under poor observing conditions.
Sulfur II (SII) Filters work together with the H-Alpha-CCD filters and OIII-CCD filters to alllow tricolor-imaging of emission line objects even from very heavy light polluted places. Sulfur II (SII) Filters are for photographic use only and are designed to isolate the SII emission at about 672 nm in the red portion of the visible spectrum. It is just a little further out in the red than the more well-known Hydrogen-alpha emission at 656 nm.
UBVRI Filters and Filter Sets are interference filters in optical system instrumentation for astronomy, microscopy, high-performance imaging and spectroscopy. UBVRI Filters and Filter Sets are research grade UBVRI filters are specifically designed for photometry with CCD cameras.
UV and IR Filters improve the color balance of the single shot color CCD by blocking the near IR light that causes stars to look bloated with using a typical 35mm camera lens to shoot wide field images. UV and IR Filters provide planetary images that are less disturbed by wavefront distortions in the atmosphere (seeing).
Telescope Filters for Visual Use
These can often be called telescope filters, visual filters, optical filters or astronomy filters. What it all adds up to is that each one of them are designed to enhance or eliminate certain wavelengths of light. Let’s take a look at filters for visual use and just a little more about each type…
Color Correction Filters are meant specifically for certain types of equipment (notably achromatic refractors) which produce an unwanted color haze around bright objects – called “fringing”. If you thought this was a problem with your telescope, don’t worry! It’s perfectly normal for chromatic aberration to occur. It merely means that not all wavelengths of light are focused to a single point and it can be corrected. Color Correction Filters are sometimes called “minus violet” filters for their elimination properties and a wide range of these filters are available to suit every budget and need.
Contrast Boosting Filters help to eliminate color fringing and in some circumstances also improve visual observation. Check on the specifications on each one and you may just discover that they can improve your lunar and planetary views as well!
Hydrogen-Beta Filters or H-Beta are designed for visual observation of extremely faint nebulae, as as the Horsehead, the Cocoon, or the California Nebula. They isolate just the Hydrogen-beta line of the spectrum, and block most everything else. We also stock a Passband Modifier Filter which can be used with your own medium to narrowband nebula filter to isolate the H-Beta emission line.
Light Pollution Reduction (LPR) Filters are sometimes referred to as Narrowband Filters or Deep Sky Filters. These filters not only block mercury and sodium vapor lights, but put the hurt on fluorescent, incandescent and neon light as well – at the same time enhancing desirable wavelengths such as hydrogen-beta and ionized oxygen. The Light Pollution Reduction (LPR) Filters Category also includes Moon and Sky Glow filters which help reduce the effects of moonlight and light pollution on our own atmosphere’s neutral oxygen. This filter will significantly improve contrast for faint nebulae.
Moon and Neutral Density Filters also include Polarizing Filters. Moon filters are an essential for viewing fine lunar detail, while polarizing filters act like a “dimmer switch for your telescope”. Some models enhance red, green and blue light!
Oxygen III (OIII) Filters also belong in a class by themselves. Students of astronomy know that nebula produces light from doubly ionized oxygen and this filter enhances it beyond your wildest dreams. If you’d like to see detail in nebula that rivals even photographs, Oxygen III (OIII) Filters are for you!
Sky Light / Protective Filters are designed specifically for the Schmidt-Cassegrain telescope. It screws on to the rear cell and is a general purpose filter which improves color saturation and balance, increases visual and photographic contrast, helps to decrease glare in lunar and planetary observing, and penetrates atmospheric haze. It also doubles as a protective intervention to keep dust from getting inside! It is also known to help penetrate haze.
Ultra High Contrast Filters are also classed with Narrowband Filters, but also deserve their own category. It acts as a light pollution filter while enhancing the three nebula wavelengths. This is a boost for smaller aperture telescopes looking for challenging deep sky objects!
Violet Reduction Filters are also known as anti-fringing filters and help to eliminate unwanted color halos produced by achromatic refractor telescopes. This eliminates the purple fringe you sometimes see around images and works with daylight images as well as astronomical applications.
Telescope Filters – Solar Filters
Solar Filters are also divided into categories… Again, dependent of the wavelength of light which passes through the filter. It is very important to remember right from the beginning that you must use a proper solar filter for visual use or permanent eye damage will result. There are old-style solar filters cells still out there that attach to the eyepiece, but please don’t use them. These type of solar filters have not been tested to today’s safety standards and are known to crack. Other materials such as mylar, smoked glass, exposed film, etc. are also unsafe. While eclipse viewing glasses are proper solar filters, remember they were designed to protect the unaided eye – not the concentrated light from a telescope or binoculars! Just like other filters, there are also solar filters meant for visual and photographic use. If a solar filter is labeled “For Photographic Use ONLY”, take their word for it. These style filters can also allow certain wavelengths of light to pass through that are not harmful to imaging equipment, but can be to the human eye.
Hydrogen-Alpha (H-Alpha) Solar Filters fit a very specific style of solar observing and may not be well-suited to the beginner. The Coronado Solar Max 40, 60 and 90 Series H-Alpha filters are meant to be mounted with a special plate on a high quality refractor with a specific focal length for each Series. The filter is then “tuned” (tuner supplied) to a specific h-alpha wavelength. While this may seem too technical, some events happen on the Sun (like coronal mass ejections) which occur quickly and this changes the doppler shift of the wavelength. By being able to adjust the frequency, you are able to follow events which otherwise might not be visible! In addition a blocking filter is also supplied. The blocking filter is a secondary interference filter which mounts at the eyepiece and allows a full disc image. Neither can be used separately of each other.
Other Hydrogen-Alpha (H-Alpha) Solar Filters include those made by the SolarScope Company which are custom built and tested on the Isle of Man. These models are dedicated tuned requiring a tilt for moving events and also require the use of a supplied blocking filter to stop interference before the final image. Looking at the Sun with the use of an Hydrogen-Alpha (H-Alpha) Solar Filter is nothing short of an incredible experience. When the Sun is active, massive flares tower and dance along the limb… On the surface granulation swims and swirls to the changing magnetic heartbeat of the Sun. Features such as prominences, the chromosphere, detail in and around sunspots, plagues and filaments come to dimensional life!
White Light Solar Filters are also another dimension of solar observing. This highly affordable and less technical method is no less exciting. These filters produce an amazingly high-resolution, bluish-white image of our Sun. Fitted exclusively by telescope aperture size, any telescope can become a safe solar scope. With white light, you can very plainly see sunspots and activity in both the umbra and penumbra regions. Granulation is also present and occasionally a white light solar flare can be spotted shooting off the limb! As a sunspot nears the limb, watch in fascination as high magnetism produces the Wilson Effect and the solar surface appears to dimple! Full aperture solar filters are a fantastic way to introduce the beginner to the world of solar activity and add a new dimension to your telescope’s uses!
The Herschel Wedge is a refractor only form of White Light Solar Filter. Put quite simply, the Herschel Wedge is the finest, most detail oriented solar filter ever designed. While H-alpha is great, the Herschel Wedge enhances light across the full spectrum. Here at OPT, we have your safety 100% in mind, and that’s why we only carry the Baader Herschel Wedge. It has a critical safety feature which ensures that the ND filter will always be present, with no chance of accidentally viewing the unfiltered light as with some other herschel wedges. So what can you see? Unlike a full aperture white light filter, the Herschel Wedge does not block any transmission and that precise Zeiss prism optical quality delivers back views that will “WOW” even the most experienced solar observer. Details that you once thought were lost due to tube currents or atmosphere turbulence jump to ultimate life. No more tweaking images… It’s laid bare before your eyes. The effects of magnetism in umbra and penumbra regions become as crisp and clear as if you were looking at iron filings under a microscope. Granulation is clean and well defined! Combine with a Baader Solar Continuum Filter and you’ll completely exclude the red and blue wavelengths. Welcome to the peak of the visual wavelength where the telescope optics are sharpest and free from chromatic aberrations. For both visual, photographic and imaging purposes, it simply cannot be beat. You’ll produce award winning images with stunning details!
And now you know all about telescope filters!
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