Optical accessories. The accessories which are actually useful and will not live permanently at the bottom of an accessory case.
1. Barlow lens. A Barlow lens is a negative lens group, that is to say it is a diverging lens. It works by increasing the focal length of the telescope objective, without the corresponding increase in the physical length of the light cone. A 2X Barlow will, with a telescope objective focal length of 1000mm, increase this to an effective focal length of 2000mm, with only a minor focus change position of a few millimetres for the eyepiece. A Barlow is positioned near focus, before the eyepiece. This 2X increase in focal length means that with an eyepiece placed into the Barlow, the eyepiece will give a magnification twice that of the eyepiece and telescope without the Barlow. Barlow lenses are commonly available from 1.6X up to 5X. They can be considered useful and convenient in that for example, three eyepieces and a Barlow can give six different magnifications, and a Barlow can be used to simply increase the focal length of the telescope objective to increase photographic image scale.
2. Focal reducer. A focal reducer is a positive lens group, that is to say it is a converging lens. It works by effectively decreasing the focal length of the telescope objective, without the corresponding decrease in the physical length of the light cone. Focal reducers can be used to effectively reduce the focal length for visual eyepiece observation at lower magnifications, but usually they are employed as photographic focal reducers to enable wider field imaging.
3. Star diagonal. A star diagonal is a useful accessory that permits a much more comfortable observing position than looking straight up through the telescope with an eyepiece. It is useful for any telescope whose focus is at the rear of the instrument. A star diagonal deviates the incident light cone by 90o so that when an eyepiece is placed into the diagonal, the observer looks down into the eyepiece. This deviation occurs from one optical surface, so star diagonals use either a flat mirror at 45o to the incident cone, or a right-angled prism whose hypotenuse is at 45o to the axis of the incident cone, and therefore totally reflective. Mirror diagonals can be over-coated aluminised flat mirrors, or many layered dielectric coated mirrors for greater reflectivity. A single deviation of 90o from a plane reflecting surface means that the image in the eyepiece is the correct way up, but reversed right to left.
a.) Full aperture solar filter. A filter in either a readymade holder or homemade holder that fits over the front aperture of the telescope, for looking directly at the Sun in white light. The most cost effective way of observing the Sun in normal white light is through a piece of Baader Astrosolar film. This is a thin plastic film coated to reflect over 99% of the Sun's light including infrared and ultraviolet. The tiny amount of light that reaches the eyepiece produces an image which is safe to view, neutral in colour and with a dark background sky. Sunspots, faculae and granulation detail on the surface of the Sun are all visible with this Astrosolar filter. It is important to note that this filter should only ever be used over the front aperture, and not at or near the focus of the telescope. In this way, the Sun's light is filtered prior to travelling through the telescope. It is also important to note that the Sun should only ever be viewed with any instrument with the correct approved filter. Severe trauma to the eye or complete and permanent blindness can occur, if the Sun is viewed without correct filtration.
b.) Small eyepiece and star diagonal photo/visual filters. The following are filters that are inserted into the bottom of an eyepiece or star diagonal by screw thread. They are available in 1.25" or 2" sizes.
Broadband Light Pollution Rejection filters. These are general filters for observers living in moderately light polluted areas, such as the edges of a town or village. These filters reject wavebands associated with sodium and mercury streetlights. They work by not transmitting the wavelengths that pollute and hence reduce the contrast of many deep-sky objects and the background sky. They are not recommended for observing very faint objects like some galaxies. Broadband LPR filters are recommended mainly for smaller aperture telescopes (<150mm). If they can be fitted to binocular eyepieces, they can increase the usefulness of hand-held binoculars for deep-sky viewing.
Narrowband Filters. These are sometimes called UHC (Ultra-High Contrast) filters. They transmit only narrow bands of wavelengths, and so are much more targeted at emission nebulae and planetary nebulae. They also work by increasing the contrast between object and background sky, so making a nebula more detectable. As much more of the visible spectrum is rejected by the filter, the background sky is very dark, so searching for star clusters and galaxies with a UHC filter will not be rewarding. Because of the light loss through filtration, UHC filters are best used with telescope apertures of >150mm. On certain bright emission and planetary nebula such as the Orion Nebula, Dumbbell Nebula and Ring Nebula, a UHC filter can provide spectacular images in the eyepiece.
Line Filters. These are single wavelength transmission filters for emission and planetary nebulae. Certain nebulae transmit strongly in specific wavelengths and this means that cutting out all other wavelengths results in clear visibility of structure within a nebula e.g. the Veil Nebula and Orion Nebula with an Oxygen III (O III) filter, or visibility of some nebula that remain invisible without the filter, e.g. The Horsehead Nebula or California Nebula with a Hydrogen-beta (H-beta) filter. These filters are recommended for use with telescope apertures of >150mm.
Polarizing filter. A polarizer is useful for astronomy in two ways. The first is that lunar surface detail is visible during daylight, as the filter cuts out the scattered skylight and transmits polarized light only. Without a polarizer, the image of the surface of the moon during the day is washed out and with low contrast. Secondly, it can be used as a weak lunar filter, by cutting down the image brightness of the moon at night.
Neutral density filter (ND). A filter for lunar observation. An ND filter reduces the brightness of the image of the moon so that it can be studied in the eyepiece comfortably. It reduces transmission of all wavelengths of the visible spectrum roughly equally, so there is no wavelength bias. ND filters are available in a variety of filtration strengths to suit the brightness of the lunar image. ND filters are much more effective than simple Moon filters, which are little more than coloured glass or plastic.
Colour filters. There are several filters available that transmit narrow colour wavebands. The aim of these filters is to increase contrast between, or detectability of, certain planetary features, such as the cloud belts and storms on Jupiter and Saturn, dust storms on Mars, faint markings in the Venusian upper atmosphere, and even faint detail in the lunar seas and crater floors. These filters work best with larger aperture telescopes, but can be used to slightly increase the contrast between prominent planetary features in some smaller telescopes.
Chromatic aberration reduction filters. These are filters that reject some short wavelengths associated with secondary spectrum common in achromatic refractors. They can reduce the violet colour fringing visible at high contrast boundaries in an image. They can increase the enjoyment of viewing brighter objects such as the moon or the brighter planets, with an achromatic refractor.
5. Coma Corrector. A coma corrector is a lens group that is placed near the focus of a telescope, before the eyepiece or camera. It corrects for off-axis coma in the image created by some telescope objective designs. It is most often used for deep-sky viewing or wide-field photography with a short focal ratio Newtonian.
6. Optical finderscope. A finderscope is a small device that points along the same axis as the telescope objective, thereby enabling accurate and fast pointing of the telescope at a specific object. It normally sits on the outside of the tube close to the eyepiece position. Many modern small telescopes are fitted with a Red Dot Finder by the manufacturer, as part of the telescope. This is a simple non-optical line-of-sight pointer which reflects a red LED off a small flat window, and can be seen through the glass against the background of stars. Some astronomers prefer the traditional optical finderscope. This is essentially a very small aperture, short focal length achromatic non-prismatic monocular, with a fixed cross-haired eyepiece of very low magnification. Examples are - 6X30, 7X50, 9X50. The benefit of an optical finder is that fainter objects than can be seen with the naked eye can be found, for centring in the main telescope eyepiece.