The paraphernalia of telescopes – The Luxuries

Well, now what are these luxuries? These are the accessories that you would like to use in order to get an enhanced view of the objects you wish to see. Without these accessories, some celestial objects might seem faint or too small or false-colored. There might also be certain objects that might not be visible at all without the use of these accessories, or you might not even dare to see some objects without an aid (e.g. the Sun).

Let’s see what enhancements are available.

Focal Reducer

A focal reducer (or telecompressor) is an optical element used to reduce the effective focal length of the objective.[1] This helps in decreasing the magnification and thus widening the field of view (FOV), and also increase lens speed. The focal reducer is inserted just before the eyepiece at the interface with the OTA. Remember the formula for magnification?

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where fo is the focal length of the objective and fe is the focal length of the eyepiece.

As can be seen in the formula, and as described quite a few times in the earlier posts, magnification is directly proportional to the focal length of the objective. Now, you might be wondering why, as discussed before in the sister post, The Indispensables, we cannot use an eyepiece with a high focal length, instead, to simply decrease the magnification. Well, that is certainly a way and is used commonly. But what that does not help achieve is increasing the lens speed, which using a focal reducer can. First, let’s see why we might want to do that and, then, how focal reducer helps doing that.

If you might remember from the post Rudiments of Telescopes, lens speed refers to the maximum aperture diameter, or minimum f-number, of a lens. A lens with a larger maximum aperture (that is, a smaller minimum f-number) is called a fast lens because it delivers more light intensity (illuminance) to the focal plane (fast, in relation to the shutter speed like in cameras, as the shutter needs to be open for a shorter time if the lens can take in more light). For telescopes, in general, let’s just say that we would need higher lens speed if we want to see objects brighter (helping in astrophotography is a bonus).

Now, what is f-number (or focal ratio) computed?

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where f is the focal length of the lens, and D is the diameter of the entrance pupil (effective aperture) of the lens.

Now, as can be seen, the f-number (N) is directly proportional to the focal length (f). And, for a high lens speed, we need lower f-number (that is achieved by lower focal length, given that we cannot change the aperture). So, that is where the focal reducer comes handy. You will, generally, see focal reducers characterized by a number like 0.5 (which means halving the focal length) and so on.

Barlow Lens

A Barlow lens is an optical element placed immediately before the eyepiece to effectively decrease the eyepiece’s focal length. Now, again, the formula for magnification comes in handy here.

Screen Shot 2016-03-05 at 10.30.47 AM

where fo is the focal length of the objective and fe is the focal length of the eyepiece.

Magnification, as can be seen, is directly proportional to the focal length of the objective and inversely proportional to that of the eyepiece. So, in order to increase magnification (narrowing the FOV, in turn), we can either decrease the focal length of the eyepiece or increase the focal length of the objective. The effective focal length of the objective can be increased using a teleconverter (a variation of Barlow) but that is usually used in cameras. For telescopes, we generally try decrease the focal length of the eyepiece instead. That’s where a Barlow lens can help. A Barlow lens, just like a focal reducer, is inserted just before the eyepiece at the interface with the OTA. Barlow lenses are generally rated for the amount of magnification they induce. Most commonly, Barlow lenses are 2x or 3x, but adjustable Barlows are also available. The power of an adjustable Barlow lens is changed by adding an extension tube between the Barlow and the eyepiece to increase the magnification.

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The amount of magnification is one more than the distance between the Barlow lens and the eyepiece lens, when the distance is measured in units of the focal length of the Barlow lens. A standard Barlow lens is housed in a tube that is one Barlow focal-length long, so that a focusing lens inserted into the end of the tube will be separated from the Barlow lens at the other end by one Barlow focal-length, and hence produce a 2x magnification over and above what the eyepiece would have produced alone. If the length of a standard 2x Barlow lens’ tube is doubled, then the lenses are separated by 2 Barlow focal lengths and it becomes a 3x Barlow. Similarly, if the tube length is tripled, then the lenses are separated by 3 Barlow focal lengths and it becomes a 4x Barlow, and so on.[2]


When we think of a filter, what comes to mind is something that blocks a constituent of the incident thing and lets the remaining through. Astronomical filters work on the same principle. For example, a certain type of these filters blocks certain portions of the color spectrum and lets the remaining pass. Now, why would we want to do that? By blocking some unwanted wavelengths (or portions of the spectrum), we can focus on the interesting ones. As an example, Jupiter has red colored gas bands which are contrasting with the rest of its surface color. If we want to analyze or contrast-up these red bands, we can block other colors in the light coming from the planet and just gather the red color, in layman’s terms. This process basically increases the signal-to-noise ratio of the required portion of the spectrum while blocking the remaining portions, thus increasing the contrast and details of the interesting part of the object.[3]

Astronomical filters can one of five major types – Solar, Color, Moon, Polarizing and Nebular.


Solar filters block most of the sunlight to avoid any damage to the observer (eyes and instrument).

Never ever try to view the sun without a solar filter. It could damage your eyes and the optical elements of the telescope.

These filters are usually made from a durable glass which transmits 1/100,000th of the light. They are used for observation, photography, and for viewing the sun as a yellow-orange disk. With a telescope, these filters help view the details of the sun safely, especially the sunspots and granulation of the surface. Another filter used for solar observing is the hydrogen-alpha filter which can help viewing the solar flares and prominences that are not visible in the normal solar filters. Unlike other filters described below, the solar filters are attached at the interface of air and the OTA and not between the OTA and the eyepiece.[4]


Color filters, as in mentioned in the introductory example, work by absorption/transmission of portions of the color. These filters can be used to increase contrast and enhance the details of the Moon and planets. All colors of the visible spectrum colors have a filter each, and every color filter is used to bring a certain lunar or planetary feature into prominence; for example, the #8 yellow filter is used to show Mars’s albedo features (dark and bright features) and Jupiter’s bands. The Wratten system is the standard number system used to refer to the color filter types (e.g. #8).

Some of common color filters and their uses are:

  • Chromatic aberration filters: Used for reduction of the purplish halo, caused by chromatic aberration of refracting telescopes. Such halo can obscure features of bright objects, especially Moon and planets. These filters have no effect on observing faint objects.
  • Red: Reduces sky brightness, particularly during daylight and twilight observations. Improves definition of albedo features, ice, and polar areas of Mars. Improves contrast of blue clouds against background of Jupiter and Saturn.
  • Deep yellow: Improves resolution of atmospheric features of Venus, Jupiter (especially in polar regions), and Saturn. Increases contrast of polar caps, clouds, ice and dust storms on Mars. Enhances comet tails.
  • Dark green: Improves cloud patterns on Venus. Reduces sky brightness during daylight observation of Venus. Increases contrast of ice and polar caps on Mars. Improves visibility of the Great Red Spot on Jupiter and other features in Jupiter atmosphere. Enhances white clouds and polar regions on Saturn.
  • Medium blue: Enhances contrast of Moon. Increases contrast of faint shading of Venus clouds. Enhances surface features, clouds, ice and dust storms on Mars. Enhances definition of boundaries between features in atmospheres of Jupiter and Saturn. Improves definition of comet gas tails.[5]

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Neutral Density

Neutral density filters (popularly known as Moon filters) are another approach for contrast enhancement and glare reduction. They work simply by blocking some of the object’s light to enhance the contrast. Although known as moon filters, they are used to enhance both lunar and planetary observations.[6]

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Polarizing filters (or polarizers) adjust the brightness of images to a better level for observing, but much less so than solar filters. With these types of filter, the range of transmission varies from 3% to 40%. They are mainly used for lunar observation, but may also be used for planetary observation. They consist of two polarizing layers in a rotating aluminum cell, which changes the amount of transmission of the filter by rotating them. This reduction in brightness and improvement in contrast can reveal the lunar surface features and details, especially when it is near full. Polarizing filters should not be used in place of solar filters designed specially for observing the sun.[7]

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As is evident in the name, the nebular filters are primarily used to observe nebulae. There are two types of nebular filters – Narrowband and Broadband.[8]


Narrowband filters transmit only a narrow band of spectral lines from the spectrum (usually 22 nm or less). Spectral lines are the dark or bright lines in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Emission nebulae (composed of ionized gases that emit light of various colors) mainly radiate the doubly ionized oxygen in the visible spectrum, which emits near 500 nm wavelength. These nebulae also radiate Hydrogen-beta line weakly at 486 nm. There are three main types of Narrowband filters: Ultra-high contrast (UHC), Oxygen-III & Hydrogen-beta, and Hydrogen-alpha, based on the spectral lines they transmit. The UHC filters range from 484 to 506 nm. It transmits both the O-III and H-beta spectral lines, blocks a large fraction of light pollution, and brings the details of planetary nebulae and most of emission nebulae under a dark sky. Let’s not get into the details of the spectral lines and these subtypes of the narrowband nebular filters. Let’s just think of these filters as those that block light pollution and help in enhancing the details of the nebulae.[9]


The broadband filters, or light pollution reduction (LPR) filters are nebular filters that block the light pollution in the sky and transmit the Hydrogen-alpha, Hydrogen-beta, and Oxygen-III spectral lines, which makes observing nebulae from the city and light polluted skies possible. These filters block the Sodium and Mercury vapor light, and also block the natural skyglow such as the auroral light. The broadband filters differ from the narrowband with the range of wavelengths transmission. LED lighting is more broadband so this is not blocked although white LEDs have from themselves a considerably lower output around 480 nm which is close to O-III and H-beta wavelength. The broadband filters have a wider range because the narrower transmission range causes a fainter image of sky objects, and since the work of these filters is revealing the details of nebulae from light polluted skies, it has a wider transmission for more brightness. These filters are particularly designed for nebulae observing, and not useful with other deep sky objects. However, it can improve the contrast between the DSOs and the background sky, which may clarify the image. Again, let’s limit our discussion on the filters as there is a whole science behind these which might not be very necessary to discuss here, in order to keep things simple.

In summary, while the color filters transmit certain colors from the spectrum and are usually used for observation of the planets and the Moon, the polarizing filters work by adjusting the brightness, and are usually used for the Moon. The Neutral density filters work by blocking some of the object’s light to enhance the contrast, and are generally used for planetary and lunar observation. The narrowband and broadband nebular filters transmit the wavelengths that are emitted by the nebulae (by the Hydrogen and Oxygen atoms), and are frequently used for reducing light pollution.[10]


Well, those are not the only accessories available for telescopes, there a lot more. In fact, there are so many that this post cannot possibly even cover a fraction of them. But the ones described here are easily the most important ones. Some accessories also depend on the telescope that you have. For example, a GoTo telescope might be enhanced with the use of an auto-alignment tool (computerized telescopes need to be aligned and calibrated in order to find the targets accurately), or a manual mount could be fitted with a motor and drives, and so on. I will try my best to touch up on other such accessories in some future posts.

Happy Stargazing!


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