{"id":210,"date":"2024-09-20T16:34:56","date_gmt":"2024-09-20T14:34:56","guid":{"rendered":"https:\/\/deep-space-astronomy.geekworkers.dev\/?p=210"},"modified":"2025-07-08T20:56:16","modified_gmt":"2025-07-08T18:56:16","slug":"choisir-un-oculaire","status":"publish","type":"post","link":"https:\/\/deep-space-astronomy.ch\/en\/choisir-un-oculaire\/","title":{"rendered":"Choosing an eyepiece"},"content":{"rendered":"

How to choose an eyepiece for your telescope?<\/strong><\/p>\n\n\n\n

Choosing the right eyepieces is essential to getting the most out of your telescope. This guide explains what an eyepiece is, its role in the optical chain and how to use it. focal length<\/strong> determines magnification<\/strong>the importance of apparent and real field of view<\/strong>the differences between formats 1.25\" and 2<\/strong>I'll also give you some advice depending on the type of telescope (refractor, Newton reflector, maksutov-casegrain etc.). We'll also look at the specific needs of different targets (planets, Moon, deep sky), the impact of observing conditions (seeing<\/strong> and transparency) on the choice of focal lengths, the role of Barlow lenses<\/strong>. <\/p>\n\n\n\n

I'm also going to talk about a very important point: that of spectacle wearers. You need to pay attention to a few details to be able to observe without having to take off your glasses every time, and it's better to keep them on for astigmatism, for example. budgets<\/strong> (entry, mid-range, high-end.<\/p>\n\n\n\n

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On the program (it's dense but you'll really know everything):<\/strong>

1) What are eyepieces and what do they do?<\/strong>

2) Eyepiece focal length and magnification: understanding the relationship<\/strong>

3) Apparent field and real field: the extent of the visible sky<\/strong>

4) 1.25\" vs. 2\" eyepieces: advantages, disadvantages and compatibility<\/strong>

5) Select eyepieces according to telescope type<\/strong>

6) Choose an eyepiece to suit the targets being observed<\/strong>

7) Observing conditions: turbulence (seeing) and sky transparency<\/strong>

8) Barlow lenses: a magnification booster or a bad idea?<\/strong>

9) Wearing eyeglasses: EYE RELIEF, a crucial point<\/strong>

10) Eyepieces: recommendations by budget (entry-level, mid-range, high-end) all links in description.<\/strong><\/p>\n\n\n\n

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What are eyepieces and what do they do?<\/strong><\/h2>\n\n\n\n

A ocular<\/strong> is a set of lenses placed at the end of the telescope, at the level of the eyepiece holder. It is an optical part indispensable<\/strong> Without an eyepiece, the image formed by the mirror or lens could not be observed with the naked eye. The eyepiece acts like a magnifying glass, magnifying the image at the telescope's focus.<\/p>\n\n\n\n

The choice of eyepiece therefore directly influences the magnification<\/strong>the field of vision<\/strong> and the image quality<\/strong>. Eyepieces come in a wide range of optical designs, each with its own characteristics in terms of field of view, contrast, aberration correction, eye relief (the distance at which the eye is placed, which is essential for spectacle wearers) and, of course, price.<\/p>\n\n\n\n

Please note:<\/em> Most telescopes are supplied with one or two basic eyepieces (often a ~25 mm eyepiece for low magnifications and a ~10 mm eyepiece for high magnifications). The eyepieces supplied in kit form are generally of fairly average quality, sufficient for beginners but rapidly limiting. It is therefore advisable to consider buying new eyepieces for complete or replace<\/strong> to improve your observations.<\/p>\n\n\n\n

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Eyepiece focal length and magnification: understanding the relationship<\/strong><\/h2>\n\n\n\n

Eyepiece focal length<\/strong>expressed in millimeters (mm), is the main parameter that determines the magnification<\/strong> obtained with a telescope. The rule is simple:<\/p>\n\n\n\n

magnification = telescope focal length \/ eyepiece focal length<\/strong> .<\/p>\n\n\n\n

For example, with a 750 mm telescope, a 10 mm eyepiece will give a magnification of 75\u00d7 (750\/10) .<\/p>\n\n\n\n

How many magnifications do you need?<\/strong> Ideally, you should have a set of 3 to 4 eyepieces offering a range of magnifications spaced significantly apart. \u00a8A rule of thumb is to choose bearings with a factor of approx. 1,4 \u00e0 1,5\u00d7<\/strong> between each eyepiece. For example, you can aim for an eyepiece giving low magnification (for extended objects), intermediate and high magnification for planets or lunar details. A classic combination is 25 mm \/ 10 mm \/ 5 mm<\/strong>which \"covers all magnifications on almost all instruments\".<\/p>\n\n\n\n

Be careful not to seek the absolute maximum magnification.<\/strong> of your instrument. Beyond a certain limit, all you get is a blurred image, or one with no additional detail. On the one hand, theatmosphere<\/strong> blurs images when magnified too much (more on this later in the section Seeing<\/em>) . On the other hand, there is a physical limit to the diameter of your telescope: it is considered that you cannot usefully magnify beyond approximately 2\u00d7 diameter (in mm)<\/strong> . For example, a 130 mm telescope has a theoretical limit of ~260\u00d7 . Beyond this, the image darkens and becomes blurred (diffraction phenomenon).<\/p>\n\n\n\n

In practice, the Optimum magnification depends on the target<\/strong>. Low magnification is often ideal for extended objects<\/strong>These include large open clusters, diffuse nebulae, seeing the Moon in its entirety, comets drowned in a field of stars, etc., as they provide a luminous overview. Conversely high magnifications<\/strong> are essential for small or detailed targets<\/strong>Whether you're trying to distinguish planetary details, fine lunar craters, separate tightly-packed stars or resolve the heart of a globular cluster. Each situation has its \"optimum magnification\" where the object appears best: vary your eyepieces to find the best compromise.<\/p>\n\n\n\n

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Apparent field and real field: the extent of the visible sky<\/strong><\/h2>\n\n\n\n

In addition to magnification, a crucial criterion when choosing an eyepiece is the field of vision<\/strong> it offers. A distinction is made between apparent field<\/strong> of the eyepiece and the actual field<\/strong> in the sky:<\/p>\n\n\n\n

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  • The apparent field<\/strong> of an eyepiece is the angle, in degrees, at which the eye perceives the image through the eyepiece. This is a characteristic specific to each eyepiece model, usually supplied by the manufacturer. Standard eyepieces often offer an apparent field of view of around 40-50\u00b0<\/strong> . Wide-field\" eyepieces rise to ~.60-70\u00b0<\/strong>and ultra wide-field models offer 80\u00b0, 100\u00b0 or more<\/strong> .<\/li>\n\n\n\n
  • Then we have The real field<\/strong> which is the portion of the sky (expressed in degrees of arc) actually visible through the telescope with this eyepiece. It depends on the apparent field and<\/em> of the magnification according to the formula : real field = apparent field \/ magnification<\/strong> . Thus, the higher the magnification, the smaller the actual field observed in the sky.<\/li>\n<\/ul>\n\n\n\n

    A large apparent field<\/strong> gives the observer a very appreciable impression of \"spatial\" immersion - it's like looking through a large porthole - whereas a small-field eyepiece gives the impression of looking at the celestial body through a narrow tunnel. For example, with the same telescope and the same magnification, a 45\u00b0 eyepiece may show only a portion of the Moon, whereas an 82\u00b0 eyepiece will contain it entirely and give a much more panoramic visual sensation . (put a comparison)<\/p>\n\n\n\n

    Wide-field eyepieces therefore offer viewing comfort<\/strong> superior: you spend less time refocusing the object (particularly useful without motorized tracking) and the visual sensation is more immersive. On the other hand, these eyepieces are often more complex (containing more lenses) and therefore more expensive<\/strong>. They are also bulkier and heavier, which can sometimes unbalance a small telescope. From about 60\u00b0<\/strong> of apparent field, we enter the realm of wide field. An apparent field of 60-70\u00b0 is an excellent compromise<\/strong> for visual observation: immersion is already very pleasant, while weight and cost are moderate compared with the extremes.<\/p>\n\n\n\n

    In practice, to choose the apparent field of your eyepieces, consider the objects you observe most often and the actual field they require. Here's an example, would you like to see the Moon in its entirety<\/strong> in the eyepiece? If so, make sure that the actual field obtained is slightly greater than 0.5\u00b0 (lunar diameter). To see the entire Pleiades cluster (\u22482\u00b0 in extent), you need a real field of at least 2\u00b0 . You'll reach these real fields either with lower magnification, or with an eyepiece with a wider apparent field - ideally both. The formula real field = apparent field \/ magnification<\/em> can guide you: for example, an eyepiece giving 60\u00d7 with 60\u00b0 apparent field covers 1\u00b0 of real sky, while 60\u00d7 with 80\u00b0 apparent field would cover ~1.33\u00b0.<\/p>\n\n\n\n

    In a nutshell:<\/em> a large visible field<\/strong> (\u226560\u00b0) is particularly beneficial for extended deep-sky objects and for general viewing comfort (less frequent cropping, \"spacious\" impression). It also allows you to find the objects you're looking for and then start zooming in! A standard apparent field<\/strong> (40-50\u00b0) is sufficient for high magnifications on planets or small objects, especially if you have a motorized tracker that keeps the target in the center. It's common to have a mix of standard and wide-field eyepieces in your toolbox, depending on your intended use.<\/p>\n\n\n\n

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    1.25\" vs. 2\" eyepieces: advantages, disadvantages and compatibility<\/strong><\/h2>\n\n\n\n

    Astronomical eyepieces come in two standard diameters (called flowing<\/strong> or skirt<\/strong> of the eyepiece) : 31.75 mm<\/strong> (i.e. 1.25 inch<\/strong>) and 50.8 mm<\/strong> (i.e. 2 inches<\/strong>) . These dimensions correspond to the diameter of the metal cylinder inserted in the eyepiece holder.<\/p>\n\n\n\n

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    • The format 1.25\" (31.75 mm)<\/strong> is by far the most widespread: virtually all commercial telescopes use it. The majority of eyepieces on the market (especially short to medium focal lengths) are 1.25\". This is the standard on which you'll have the widest choice. This format has the advantage of being compact and lightweight<\/strong>It is generally less expensive for the same optical package than 2\". It perfectly covers most optical needs. medium and high magnifications<\/strong>.<\/li>\n\n\n\n
    • The format 2\" (50.8 mm)<\/strong> is mainly found on the eyepieces of long focal length and\/or very wide field of view<\/strong>. In fact, an eyepiece that combines a high focal length and<\/em> a wide apparent field produces a wide light beam<\/strong> output. Beyond a certain diameter, the 31.75 mm runner becomes too narrow and vignetterait<\/strong> (it would block some of the rays coming from the edges of the field).<\/li>\n\n\n\n
    • \u00a0The disadvantage is that these 2\" eyepieces are often bulky and heavy<\/strong>Some ultra-wide-field models can weigh over a kilogram! Make sure your telescope (and mount) can balance such a weight.<\/li>\n<\/ul>\n\n\n\n

      In a nutshell<\/em>for small-diameter entry\/mid-range telescopes, the 1.25\" covers 100% usual needs<\/strong>. The format 2\" becomes indispensable<\/strong> especially when it comes to exploring lower magnifications and wider fields<\/strong> that the instrument can provide (typically for the rich deep sky<\/strong>). Check the diameter of your eyepiece holder to see what it can accept. If in doubt, a telescope with a 2\" eyepiece holder is often a guarantee of upgradeability, as it will be able to accommodate the full range of existing eyepieces.<\/p>\n\n\n\n

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      Choosing eyepieces according to telescope type<\/strong><\/h2>\n\n\n\n

      Each telescope has its own specific features which influence the choice of eyepieces: focal length, aperture, type of focus, and compatibility of accessories.<\/p>\n\n\n\n

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      1. Astronomical telescopes (refractor)<\/strong><\/p>\n\n\n\n

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      • Long glasses (f\/D 10-15) :<\/strong> Ideal for planetary photography, they offer high magnifications even with medium focal length eyepieces (10-30 mm). Choose a wide-field eyepiece for overall views, especially if your eyepiece holder accepts the 2\" format. Simple eyepieces (Pl\u00f6ssl type) work well on these long focal length scopes.<\/li>\n\n\n\n
      • Short glasses (f\/D 5-7) :<\/strong> Perfect for deep sky: opt for wide-field eyepieces (> 30 mm, 2\") to explore large areas of the sky. For planetary photography, you'll need very short eyepieces (3-5 mm), or use a Barlow. Very open instruments require well-corrected eyepieces to avoid edge-of-field blur.<\/li>\n<\/ul>\n\n\n\n

        In a nutshell:<\/strong> On a telescope, combine a long focal length wide field eyepiece (for deep sky) and one or two short focal lengths (or a medium + Barlow) for planetary. On an achromat, expect a little chromatism at high magnification.<\/p>\n\n\n\n

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        2. Newton (and Dobson) reflecting telescope<\/strong><\/p>\n\n\n\n

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        • Typical focal length (~1000-1200 mm)<\/strong>A classic set covers 50\u00d7, 100-150\u00d7, and 200-250\u00d7.<\/li>\n\n\n\n
        • Eyepiece holder :<\/strong> Most Newtons \u2265 150 mm accept 2\" eyepieces, perfect for wide-field use.<\/li>\n\n\n\n
        • Coma and field:<\/strong> On open Newton (f\/4-f\/5), beware of coma: top-of-the-range eyepieces or a coma corrector will keep the image sharp right to the edge. Dobson lenses in particular benefit from very wide-field eyepieces (70-100\u00b0) to facilitate manual tracking and immersion.<\/li>\n<\/ul>\n\n\n\n

          In a nutshell:<\/strong> A good Newton with a 2\" wide field (25-30 mm), complemented by quality short focal lengths (5-10 mm), covers everything: nebulae, planets, the Moon.<\/p>\n\n\n\n

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          3. Catadioptric telescope (Schmidt-Cassegrain, Maksutov)<\/strong><\/p>\n\n\n\n

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          • Long focal length :<\/strong> These tubes quickly produce high magnifications: there's no need to go below 5-8 mm; prefer longer focal lengths for low magnifications (e.g. 32-40 mm).<\/li>\n\n\n\n
          • Limited field :<\/strong> SCT\/Maks are limited by their baffle diameter: a C8 (200 mm) benefits from the 2\" format, but below that (Maksutov < 150 mm), 1.25" is preferable.<\/li>\n\n\n\n
          • Optical quality :<\/strong> A simple Pl\u00f6ssl already gives a good image, but eyepieces with long eye relief and wide field are a real comfort (especially for planetary and spectacle wearers).<\/li>\n\n\n\n
          • Deep sky :<\/strong> These instruments are at home on compact, luminous objects. For large nebulae, we are limited by the actual field of view of the tube.<\/li>\n<\/ul>\n\n\n\n

            In a nutshell:<\/strong> Switch to longer focal length eyepieces for low magnification. Switch to 2\" on SCT 8\" or larger for deep sky. Intermediate focal lengths (15-25 mm, 68-82\u00b0 field of view) are versatile for the rest. High-end eyepieces improve contrast, but are not essential for beautiful images.<\/p>\n\n\n\n

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            General tip:<\/strong><\/p>\n\n\n\n

            For each instrument, mix and match your set:<\/p>\n\n\n\n

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            • a long focal length with a wide field of view (deep sky, spotting)<\/li>\n\n\n\n
            • medium focal length (clusters, nebulae, compact galaxies)<\/li>\n\n\n\n
            • quality short focal length (planetary, lunar details, double stars)<\/li>\n<\/ul>\n\n\n\n

              And adapt the quality of your eyepieces to the aperture and speed of your instrument, so you can get the most out of every session!<\/p>\n\n\n\n

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              Choosing the right eyepiece for the target<\/strong><\/h2>\n\n\n\n

              Eyepiece requirements vary depending on whether you're observing the planets, the Moon or objects in the sky. deep sky<\/strong> (galaxies, nebulae) or of the stars<\/strong> (star clusters, double stars). Here are a few tips to help you choose the right eyepiece for your specific needs intended targets<\/strong> :<\/p>\n\n\n\n

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              • Planetary and lunar observation :<\/strong> This field requires high magnifications<\/strong> to discern fine details (Saturn's rings, Jupiter's bands, lunar craters, Mars' polar caps, etc.). We therefore prefer short focal length<\/strong>. On a medium focal length instrument (1000-1200 mm), this corresponds to eyepieces of ~3 mm to 7 mm to cover 150\u00d7 to 300\u00d7. On an SCT\/Mak, slightly longer focal lengths (~8-15 mm) will suffice to achieve these same powers. The optical quality<\/strong> of the eyepiece is important here: contrast and image precision must be maximized. Visit orthoscopic eyepieces<\/strong> (~40\u00b0 field of view, short eye relief) have long been the purists' reference for the planetary, just like the monocentric<\/strong> (very narrow field but ultimate contrast). Today, more modern eyepieces offer excellent sharpness. and<\/em> a comfortable field of view, which is appreciable even for planetary work. A wide field is not mandatory for planetary work if you're tracking, but if you're pointing manually (Dobson), a 70\u00b0+ eyepiece is a real plus for keeping the planet in the field longer. For the Moon<\/strong>As long as the image remains sharp, you can go up in magnification, sometimes beyond 2\u00d7 the diameter of the instrument if the sky is exceptional. But be sure to bring a lunar filter<\/strong> if you're observing at low magnification, to avoid dazzling yourself. For planetary observations, it can be useful to use eyepieces with fairly fine magnification steps (e.g. 5 mm, 6 mm, 8 mm) to adapt precisely to the turbulence conditions prevailing at the time.<\/li>\n<\/ul>\n\n\n\n

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                • Deep-sky objects (diffuse nebulae, galaxies)<\/strong>These objects are often extended<\/strong> and dimly lit. To observe them in their entirety and with the maximum amount of light, we favour low magnifications<\/strong>therefore long focal length eyepieces<\/strong>. A classic is to use a ~30 mm eyepiece giving the largest reasonable exit pupil (4-7 mm depending on the instrument) to \"pick up\" a maximum number of photons. For example, the Andromeda galaxy (M31) or the Orion nebula (M42) are superb at 30-50\u00d7 in a 20 cm telescope, occupying a large part of the field. Aim for a actual field<\/strong> sufficient to contain the object: several degrees for the largest (e.g. the Pleiades are ~2\u00b0). This often requires a wide-field eyepiece<\/strong> combined with low magnification. With a 25 mm 68\u00b0 on a Dobson 250 mm f\/5, we get ~50\u00d7 and ~1.3\u00b0 of real field, already covering a lot of nebulae. A 2\" wide-field eyepiece<\/strong> (e.g. 40 mm 72\u00b0) can reach the maximum field of view (~2\u00b0) on the same instrument, enabling objects such as the Monoceros rosette or the Andromeda galaxy and its two companions M32\/M110 to be seen in their entirety in a single view. Note that under a polluted sky<\/strong> or veiled, diffuse objects suffer, and low magnification can make the sky background very bright, diluting contrast. In these conditions, it may be preferable toincrease magnification slightly<\/strong> to darken the sky background and make the object stand out. For example, under a suburban sky, going from 40\u00d7 to 80\u00d7 can darken the sky background enough to better see a galaxy, even if it means observing only part of it. It's a question of balance, depending on the sky you're looking at: under very dark skies, you'll get the most out of low magnification (dark sky backgrounds); under medium skies, moderate magnification can sometimes help. Finally, the use of filters<\/strong> (UHC, OIII, etc.) with the eyepiece can greatly improve the observation of certain nebulae - that's another subject, but think about it for diffuse deep sky.<\/li>\n<\/ul>\n\n\n\n

                  <\/p>\n\n\n\n

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                  • Star clusters and stellar objects :<\/strong> Star clusters are divided into open clusters<\/strong> (scattered, often extended stars) and globular clusters<\/strong> (very dense, compact).\n
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                    • For open clusters<\/strong>A low to medium magnification is recommended, so as to see the whole cluster in its stellar environment. Many open clusters (M45 the Pleiades, M44 the Creche, the Hercules cluster...) are 0.5\u00b0 to 2\u00b0 in apparent diameter, so you need an equivalent real field. A ~20 mm wide-angle eyepiece is often ideal for this on an average instrument. The wealth of stars<\/strong> of these objects stands out well, with good contrast and a wide field of view. We also appreciate a eye relief<\/strong> comfortable for this type of prolonged observation.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n\n\n\n

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                      • For globular clusters<\/strong>which are highly concentrated \"balls\" of distant stars, it is often necessary to go up in magnification<\/strong> to try and resolve the individual stars at the edges. High magnification (200-300\u00d7 on a 200 mm) can start to reveal the peripheral stars of a cluster like M13 or M22, where at 50\u00d7 you'd only see a blurred patch. However, you need enough aperture and a stable sky to get them really big, otherwise the image becomes blurred. You can alternate: a globular is pretty at low magnification (small luminous ball in a starry field) and impressive at high magnification (myriad of tiny stars).<\/li>\n<\/ul>\n\n\n\n

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                        • Visit double stars<\/strong> or multiple telescopes also require high magnifications, especially for close-up pairs. In the case of planets, we'll be looking for the instrument's maximum separating power, which means 5-10 mm eyepieces, or even less if the optics and sky allow. An eyepiece with weak diffusion<\/strong> A bright, well-corrected eyepiece will produce the most beautiful images (avoid low-end eyepieces that can generate stray reflections around bright stars). As with planets, the apparent field of view doesn't have to be enormous (we're aiming at a point object), but a little field of view and visual comfort don't hurt. For double stars with contrasting colors, observing at medium power may suffice and be more aesthetically pleasing, so don't hesitate to try out different magnifications.<\/li>\n<\/ul>\n\n\n\n

                          All in all, we can see that each target has its preferred eyepiece<\/strong> For example: planets\/moons like high-quality short focal lengths, extended nebulae like long wide-field focal lengths, open clusters like intermediate wide-field focal lengths, compact stellar objects like short to medium focal lengths, depending on the purpose. Ideally, build your eyepiece set to cover all these situations<\/strong>. With 3 or 4 well-chosen eyepieces (e.g. ~5 mm, ~10 mm, ~18 mm, ~30 mm), you can already do everything with a versatile instrument.<\/p>\n\n\n\n

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                          Influence of observation conditions: turbulence (seeing) and sky transparency<\/strong><\/h2>\n\n\n\n

                          Even the best eyepiece cannot overcome the limitations imposed by theatmosphere<\/strong> and the quality of the sky. Two main factors come into play: the atmospheric turbulence<\/strong> (seeing) and transparency<\/strong> of the sky. It's important to understand their impact, so as to adapt the choice of eyepiece (especially its focal length) to current conditions.<\/p>\n\n\n\n

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                          • Turbulence \/ Seeing :<\/strong> When the air is unstable (layers of warm\/cold air mixed together), high-magnification images become blurred and shaky. This phenomenon, well known to astronomers, effectively limits the useful magnification<\/strong> at a given moment. On a bad night, even a 300 mm telescope can be limited to 150\u00d7 or 200\u00d7 before the image degrades. It is therefore crucial to do not overweight<\/strong> when seeing is unfavorable. If you use an eyepiece that is too short (excessive magnification), the star you are observing will appear blurred, with no gain in detail<\/strong> - or even worse than at lower magnifications. On the contrary, at moderate magnification, the image will appear sharper and more stable. \"Atmospheric turbulence often spoils high-magnification images\".<\/em> Stelvision reminds us that there are only a few nights a year when the sky is stable enough to push an instrument to its limits. In practice, this means adapting your shortest eyepiece to the conditions: on an average night, you might only use your 8 mm, whereas on an excellent night, the 5 mm will finally reach its full potential. A sign<\/strong> if you magnify too much in relation to seeing, the image will look \"bubbly\" or permanently blurred, with no improvement in focus - in this case, switch back to an eyepiece with a slightly longer focal length. Remember also the 2\u00d7 diameter<\/em> (see previous section): this is a theoretical limit<\/strong> often unattainable in practice in the absence of a perfect sky. A 200 mm lens, for example, has a theoretical limit of ~400\u00d7, but in reality we rarely usefully exceed 250\u00d7. The optical quality<\/strong> of the eyepiece can play a secondary role here: a top-of-the-range eyepiece may deliver a slightly finer, higher-contrast image than a bottom-of-the-range one in turbulent conditions, but if the sky is very unstable, even the best eyepiece won't work miracles.<\/li>\n<\/ul>\n\n\n\n

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                            • Transparency \/ Light pollution :<\/strong> Transparency refers to the absence of haze, clouds, humidity, etc., which could attenuate the starlight. A transparent, dark sky allows you to take full advantage of low magnifications<\/strong> (large exit pupils) to observe very faint nebulae. On the other hand, under a sky with light pollution or a slight haze, using an eyepiece with too large an exit pupil (i.e. very low magnification) can make the background of the milky sky<\/strong> and drown the diffuse object in it. Under these conditions, it may be beneficial toincrease magnification<\/strong> moderately to darken the sky background (since the eyepiece distributes the same light over a wider retinal surface, the sky appears darker). Warning: this darkens also<\/em> the object itself, so there's a trade-off: for nebulae that aren't too faint (rich open clusters, certain galaxies), a little more magnification can increase the perceived contrast, whereas for a very faint nebula close to the threshold of visibility, any extra magnification will render it invisible. For example, under a suburban sky, the Dumbbell Nebula (M27) is better seen at 80\u00d7 than at 40\u00d7, as the background sky darkens more than the nebula fades. In heavily polluted areas, you can go even higher in magnification on small objects to improve contrast (even if it means observing only the heart of the nebula). In lightly polluted but foggy areas, choose moderate magnification. There is no single rule, but remember<\/strong> : if the background sky appears too bright through the eyepiece<\/em>For example, if you're looking at Jupiter, try an eyepiece with a slightly shorter focal length (magnification \u2191) to darken it. Finally, transparency has the greatest impact on diffuse objects; for planets or stars (bright, punctual objects), a slightly veiled sky has less impact - you can observe Jupiter through a thin cloud layer (there will just be less contrast). On the other hand, a veil or pollution aggravates the diffusion of stray light, and can create reflections<\/strong> in the eyepiece on bright objects. Top-of-the-range eyepieces with effective anti-reflective coatings will fare better in this respect, revealing more faint satellites around a bright planet, for example, than a bottom-of-the-range eyepiece dazzled by internal reflections.<\/li>\n<\/ul>\n\n\n\n

                              In conclusion<\/strong>When you're on the move, adapt your choice of focal lengths and magnifications to the state of the sky. On a good, stable night, take out the shortest eyepiece and use it to track down fine details. On an average or poor night, it's best to stick to moderate magnifications: the image will be cleaner and you'll be able to see more clearly. more details<\/strong> than by pushing too hard. And if the sky is milky, opt for modest magnifications on large, diffuse objects, or refocus on brighter targets. Bear in mind that experience is acquired<\/strong> The more you observe, the better you'll know which eyepiece is best suited to which object under which conditions, and the better you'll be able to interpret what you see (e.g. if the image of a star is shaky at 200\u00d7, it's not the eyepiece's fault, it's the turbulent sky).<\/p>\n\n\n\n

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                              Barlow lenses: a magnification booster or a bad idea?<\/strong><\/h2>\n\n\n\n

                              Visit Barlow lens<\/strong> is an optical accessory commonly used in astronomy. It consists of a group of divergent lenses, which we have just insert between eyepiece and eyepiece holder<\/strong> (or angled return). The Barlow has the effect of multiply the telescope's effective focal length<\/strong>and therefore multiply the magnification<\/strong> provided by a given eyepiece. For example, a Barlow 2\u00d7 doubles the magnification: a 20 mm eyepiece that gave 50\u00d7 on your instrument will give 100\u00d7 with the Barlow 2\u00d7. There are also Barlow 3\u00d7 (tripling) and other factors (1.5\u00d7, 5\u00d7, etc., including variable-factor zoom models).<\/p>\n\n\n\n

                              Interests of a Barlow :<\/strong> Barlow for high magnification without the need for short focal length eyepieces<\/strong>often uncomfortable. For example, it is more pleasant to observe with a 10 mm eyepiece plus a 2\u00d7 Barlow (which is equivalent to a 5 mm) than with a 5 mm eyepiece alone, because the 10 mm will generally have a higher angle of view. eye relief<\/strong> longer (greater eye-to-eye distance) and a possibly wider apparent field. The Barlow is therefore a means economical and practical<\/strong> increase your range of magnifications: with two eyepieces and a 2\u00d7 Barlow, you actually have four magnifications available. This is ideal for fine-tuning planetary observation, for example. What's more, on very open instruments (f\/5 and below), a good Barlow can help to \"soften\" the light beam for the eyepiece, improving its efficiency (some Barlows are used for this optical purpose, known as Barlow paracorr<\/strong> for Newton f\/3s, for example).<\/p>\n\n\n\n

                              Disadvantages and precautions :<\/strong> A Barlow adds lenses to the optical path, which can cause a slight loss of brightness<\/strong> and introduce aberrations if they are of poor quality. In general, mid-range\/high-end Barlows have good anti-reflective coatings and are virtually apochromatic (no added chromatism). But a Low-end Barlow<\/strong> can degrade the image: loss of sharpness, reduced contrast. What's more, if you accumulate magnification (e.g. very short eyepiece + Barlow, or worse, several Barlows in cascade), you quickly exceed the useful magnification<\/strong> of the instrument and the image becomes poor. The main advantage of the Barlow is that it provides good ocular comfort at high magnifications.<\/strong> but don't expect miracles: \"Few eyepiece\/Barlow pairs will be able to give an image equivalent to an eyepiece of equivalent focal length without Barlow\".<\/em> Pierro-Astro notes. In other words, an excellent 5 mm eyepiece will always provide a slightly better image than an average 10 mm + 2\u00d7 Barlow.<\/p>\n\n\n\n

                              Directions for use :<\/strong> If you're investing in a Barlow, choose a good optics<\/strong> (e.g. apochromatic 3-lens Barlows, or telecentric models of the Tele Vue Powermate<\/strong> which are high-end). The latter are more respectful of image quality, and many observers find them indispensable<\/strong> for high-resolution planetary imaging. On the other hand, avoid multiplying Barlows or using them on weak targets: each additional glass loses a little light and contrast, which penalizes deep-sky photography in particular. Finally, remember that a Barlow modifies the focusing range<\/strong> In rare cases, a telescope may not focus with certain combinations (especially in photography or binoculars). With visuals, however, there's usually no problem.<\/p>\n\n\n\n

                              In a nutshell<\/strong>the Barlow is a useful tool<\/strong> to extend the range of magnifications without too much expense, and to observe comfortably at high magnification. Used sparingly (just one good-calibre Barlow), it hardly alters the image at all, and offers even greater flexibility. But don't count on it to \"push\" your instrument beyond its limits: if the sky or the telescope don't allow 300\u00d7, adding a Barlow won't change a thing. Think of it as a practical multiplier<\/strong>not as a miracle cure.<\/p>\n\n\n\n

                              Example of a 2\u00d7 Barlow lens at 1.25\". Inserting this accessory before the eyepiece doubles the focal length of the instrument and therefore the magnification provided. Quality Barlow lenses are coated to minimize light loss and chromatism.<\/em><\/p>\n\n\n\n

                              \n\n\n\n

                              Wearing eyewear: the EYE LINK, a crucial point<\/strong><\/h2>\n\n\n\n

                              Do you wear glasses?<\/p>\n\n\n\n

                              Then a fundamental criterion comes into play: eye relief<\/strong>This is the distance at which the image is formed behind the eyepiece, where you place your eye.<\/p>\n\n\n\n

                              With many conventional eyepieces, especially short focal lengths and entry-level Pl\u00f6ssls, you need to glue your eye to the lens. If you keep your glasses on to observe (for example, if you have astigmatism: in this case, you MUST keep your glasses on), you need an eye relief of at least 15 to 20 mm<\/strong> to observe comfortably without hurting yourself.<\/p>\n\n\n\n

                              Fortunately, many modern eyepieces - planetary series, some wide-field (Baader Hyperion, Pentax XW, Tele Vue Delos, Explore Scientific LER...) - are designed for spectacle wearers. Always check this criterion before buying, as it is noted in the technical data sheets.<\/p>\n\n\n\n

                              A word of advice: if possible, test in a store or at a viewing party, because not everyone can stand having to take off their glasses or stick their eye on the lens.<\/p>\n\n\n\n

                              And beware: some eyepieces, even high-end ones, have a short relief at very short focal lengths!<\/p>\n\n\n\n

                              \n\n\n\n

                              Eyepieces: recommendations by budget (entry-level, mid-range, high-end) all links in description.<\/strong><\/h2>\n\n\n\n

                              The choice of eyepieces is vast, with prices ranging from around twenty euros to several hundred euros each. To find your way around, you can classify the offer into three budget ranges<\/strong> :<\/p>\n\n\n\n

                                \n
                              • Entry-level<\/strong> - Eyepieces at less than \u20ac50 each.<\/li>\n\n\n\n
                              • Mid-range<\/strong> - Approximately \u20ac50 to \u20ac200 each.<\/li>\n\n\n\n
                              • Top of the range<\/strong> - Over ~200 \u20ac each.<\/li>\n<\/ul>\n\n\n\n

                                Let's start with the entry-level range (between 50 and 100 euros per lens).<\/strong><\/p>\n\n\n\n

                                I'll be using Explore Scientifique 52\u00b0 LER (Long Eye Relief) eyepieces. Excellent quality, 52\u00b0 field of view, which is still very reasonable, especially if you use SC or Mak type optics. You have 2-inch options and a whole range of interesting focal lengths.<\/p>\n\n\n\n

                                Mid-range (\u20ac50 - \u20ac200 per lens)<\/strong><\/p>\n\n\n\n

                                Overall, in the \u20ac50-200 range, 60-70\u00b0 apparent field of view is standard<\/strong>and you can access the 82\u00b0<\/strong> at around \u20ac150-200. Optical quality (sharpness, correction) becomes satisfactory for most visual applications, even on fast instruments. The gain<\/strong> compared with basic eyepieces is clearly visible.<\/p>\n\n\n\n

                                I like to recommend the Baader Hyperion, sold as a kit. For around 600 euros, you get 4 focal lengths: 5, 10 , 17 and 24mm. You get a nice 68\u00b0 field of view, and the quality is of course top-notch. They're a bit like astronomy legends, too.<\/p>\n\n\n\n

                                We could mention the Celestron Luminos, which fit this budget for 7, 10 and 15mm focal lengths. Beyond these focal lengths, the budget is a little higher.<\/p>\n\n\n\n

                                High-end (> \u20ac200)<\/strong><\/p>\n\n\n\n

                                Here you'll find the highest-performance eyepieces used by the most demanding observers. Prices range from just over \u20ac200 to over \u20ac1,000 for some very wide fields. What does such an investment justify? Essentially: a maximum apparent field<\/strong>a impeccable image quality<\/strong> (sharpness, contrast, correction of the slightest optical defect right up to the edge) and a ergonomics<\/strong> often improved. These eyepieces can literally \"sublimate\" the performance of your telescope, giving images of incomparable purity and comfort - at least if your instrument itself is of high quality and your sky up to scratch. And if your telescope is properly adjusted!<\/p>\n\n\n\n

                                Top-of-the-range flagship series include :<\/p>\n\n\n\n

                                  \n
                                • Explore Scientific, 82\u00b0, 92\u00b0 and 100\u00b0 and even 120\u00b0.<\/strong> The ES brand also offers high-end wide-field lenses, often a little less expensive than Tele Vue for similar quality. For example, an ES 9 mm 120\u00b0 has been launched, or the ES 25 mm and 17 mm 92\u00b0 (very comfortable, ~600 \u20ac each).<\/li>\n\n\n\n
                                • Baader Morpheus<\/strong><\/li>\n<\/ul>\n\n\n\n
                                  \n\n\n\n

                                  To conclude<\/strong>the choice of eyepieces should be guided by your<\/em> observation requirements (desired magnifications, type of targets), the capacity of the your<\/em> instrument (diameter, focal length, aperture of the eyepiece holder), and your budget<\/em>. By knowing the principles outlined in this guide - the role of focal length, apparent vs. real field, constraints of your telescope, influences of the sky - you can put together a coherent eyepiece range that will maximize your pleasure under the stars. remember that a a good eyepiece is a lasting investment<\/strong> for years of observation. Happy eyepiece hunting, and happy skies!<\/p>\n\n\n\n

                                  You can of course buy through Deep Space Astronomy, and I'll be happy to advise you!<\/p>","protected":false},"excerpt":{"rendered":"

                                  Comment choisir un oculaire pour son t\u00e9lescope ? Choisir correctement ses oculaires est essentiel pour exploiter au mieux son t\u00e9lescope. Ce guide explique ce qu\u2019est un oculaire et son r\u00f4le dans la cha\u00eene optique, comment sa focale d\u00e9termine le grossissement, l\u2019importance du champ de vision apparent et r\u00e9el, les diff\u00e9rences entre formats 1,25\u201d et 2\u201d, […]<\/p>\n","protected":false},"author":2,"featured_media":8245,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[34],"tags":[],"class_list":["post-210","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-telescope"],"_links":{"self":[{"href":"https:\/\/deep-space-astronomy.ch\/en\/wp-json\/wp\/v2\/posts\/210","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/deep-space-astronomy.ch\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/deep-space-astronomy.ch\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/deep-space-astronomy.ch\/en\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/deep-space-astronomy.ch\/en\/wp-json\/wp\/v2\/comments?post=210"}],"version-history":[{"count":6,"href":"https:\/\/deep-space-astronomy.ch\/en\/wp-json\/wp\/v2\/posts\/210\/revisions"}],"predecessor-version":[{"id":9024,"href":"https:\/\/deep-space-astronomy.ch\/en\/wp-json\/wp\/v2\/posts\/210\/revisions\/9024"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/deep-space-astronomy.ch\/en\/wp-json\/wp\/v2\/media\/8245"}],"wp:attachment":[{"href":"https:\/\/deep-space-astronomy.ch\/en\/wp-json\/wp\/v2\/media?parent=210"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/deep-space-astronomy.ch\/en\/wp-json\/wp\/v2\/categories?post=210"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/deep-space-astronomy.ch\/en\/wp-json\/wp\/v2\/tags?post=210"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}