I've not yet had the pleasure of an astro club visit to try any other scope/eyepiece combination, although I did try to attend the Regents Park  venue in July, and with only the  Celestron 127EQ to compare, I will always recommend an  8" -  200P as a reliable and easy  to use, first time/life time telescope. By Found inside – Page 35Focal ratio If you divide a telescope's focal length by its aperture, you get its focal ratio. For example, one of the telescopes above, with the 100-mm aperture and the 500-mm focal length, would have a focal ratio of five, ... and using the focal ratio to determine my high power EP, I chose the 6mm providing a power of 200x, to match the design of the telescope. Focal Length - Telescope Focal Length Focal Ratio - Telescope Focal Ratio Go-To - If its a Go-To Mount "Yes" blank if not. The 400mm scope gives a magnification of x20 the image, the 500mm scope x25 the image and the longest focal length scope at 700mm gives a view at x35 the image. Precision refractors to easy to use Dobsonian telescopes, Schmidt Cassegrain telescopes perfectly suited for observing and imaging and professional level Astrographs - telescopes.net has it all. I have to say that no single scope will provide everything you need to see all objects although the scope mentioned comes close. The telescope will be more compact, lighter and easier to transport. This is the 'speed' of a telescope's optics, found by dividing the focal length by the aperture. The focal ratio of a telescope, whether "slow" or "fast", most certainly has everything to do with observing, and through a static eyepiece. With a 200P 8" telescope and a 6mm eyepiece or smaller, the Moon will totally amaze you. Found insideFocal ratio is the ratio of the focal length of the telescope to its aperture. To calculate, divide the focal length (in mm) by the aperture (in mm). For example, a telescope with a 2030 mm focal length and an aperture of 8 inches (203 ... If you want to gaze at the moon or the planets, a telescope with a focal ratio of f/10 is perfect. It is usually written, for example, as f/6, where the focal length is six times longer than the aperture, and for this reason is sometimes referred to as the f-number.. Formula: Aperture X Focal Ratio . Let's say you have a reflecting telescope with an aperture of 6 inches (152.4mm) and a focal length of 1000mm, your focal ratio is 1000/152.4= 6.56! If you enter an actual focal length and ratio then any selected telescope will be ignored and the extender/reducer factor will be applied to this entered value. A fixed focal length lens, also known as a conventional or entocentric lens, is a lens with a fixed angular field of view (AFOV). Copyright © 2021 Stargazers Lounge Found inside – Page 66camera and telescope system is exactly whatever is the fratio and focal length of the telescope. One merely has to think in terms of ... To find the focal ratio of your telescope, divide the instrument aperture into the focal length. We use cookies to ensure that we give you the best experience on our website. Insert a 15mm into either and you get magnifications of. If you want to see something in more detail with less in the frame, such as a planet, you are going to want a high focal ratio. The LX200 Series 8" ACF™ Telescope features a 8" aperture, 2000mm focal length, and f/10 focal ratio with Dual-Fork Mounting. Eg: f5 telescope with 40mm eyepiece = 8mm exit pupil. A great compact grab-and-go telescope designed for entry-level and intermediate astronomy enthusiasts; Substantial 4.5" aperture and fast f/4 focal ratio provides bright, detailed views of solar system targets like the Moon and planets, as well as wide-field celestial objects like nebulas and star clusters The focal ratio provides an indication of the brightness of images. Therefore its f-ratio is 2,000 / 203 = f/9.85. The focal ratio is a number that relates the focal length and the aperture. Up North (yep up a little more, right a bit, wait! Focal ratio is nonetheless a determinant among like-apertured telescopes. The formula for the focal ratio is: Focal length in mm / Aperture diameter in mm. For planetary observation, an aperture of 8 inches and some additional magnification power should give you satisfying results. For example, if you are standing on a vista overlooking a mountain range and a valley you can look at it in a few different ways. Its just a numbers game, how you interpret them , is up to you? Focal Ratio - Faster, Brighter, Smaller. The focal length divided by the aperture of a mirror or lens gives you the number known as the focal ratio. If we go from a scope with a larger f-ratio to one with a smaller f-ratio, we can get the same surface brightness with a shorter focal length eyepiece. The primary mirror focal ratio is f/1. Next time focal ratio is brought up in conversation, just remember, it is a simple relationship between focal length and aperture size which determines what kind of contrast you can get out of your telescope. Our intent is therefore only to provide a comprehensive introduction to the essential aspects of telescope design and construction. This book will not replace specialized scienti?c and technical texts. This workbook provides over fifty unique stargazing challenges, allowing you to track your progress and record your observations. For example, if we have a telescope with a focal length of 800mm and an aperture diameter of 80mm you would get a focal ratio of 10. You are correct that high focal ratios are easier on the EP, this is because the angle of light is shallower. Incidentally, notice how I characterize the scope and the eyepiece. Sign up for a new account in our community. Like focal length, focal ratio can tell you a lot about a telescope: larger f/numbers imply higher magnification with a given eyepiece and a narrower field of view, smaller f/numbers the opposite. This is related to the brightness of image you see, so the larger aperture scope is able to deliver the same image scale at higher brightness, or the same brightness at larger image scale. They are described by two parameters, aperture and Focal Ratio.The diameter of the light collecting surface is called the aperture of the telescope - the bigger the aperture, the brighter the image. In practical astronomy, these are referred to as 'slow' telescopes. Telescopes with a low "f number" (less than 4) are better for watching distant and faint objects, like a distant . Found inside – Page 250Telescopes come with fixed focal ratios. A low ratio, such as an f/2, has a short focal length compared to the primary lens diameter and are called fast. This means that the telescope tube is short and the telescope will see a large ... Put simply, look for a high focal ratio if you want to observe the planets of the Solar System. focal ratio = focal length of telescope / aperture of telescope. Focal Ratio. This book presents a complete summary of the author's twenty five years of experience in telescope design. The "ideal" focal ratio ranges from one telescope to another. The cons are: Fast mirrors are much harder to nake well, and because of that, they are more expensive than slower mirrors. Large aperture dob, 6" slow dob and small wide field refractor. Here, too, are details of choosing new and used optics and accessories, along with enhancements you can make to extend their versatility and useful lifetime. This book is for you. Really. I could probably get away with three though. Still another prefers observing only galaxies, nothing more, and picks a 12" f/4-f/5 Newtonian for the task. All I know is that the focal ration affects zoom and the sharpness of the image. If you want to watch all the videos in the correct order, please visit my website at http://www.jasonkendall.comNow we can see the last bits of basic telescopes, with the focal ratio, magnification, plate scale and so on.Supplement the videos with \"OpenStax Astronomy\"https://openstax.org/books/astronomy/pages/6-thinking-ahead6: Astronomical Instrumentshttps://en.wikipedia.org/wiki/MagnificationMagnificationhttp://www.televue.com/engine/TV3b_page.asp?id=212\u0026plain=TRUEEyepiece Calculatorhttps://en.wikipedia.org/wiki/List_of_telescope_typesTelescope Typeshttps://en.wikipedia.org/wiki/Plate_scalePlate Scalehttps://en.wikipedia.org/wiki/F-numberf-ratioThis is part of Module 5 of 14 which details an entire online introductory college course. If you want higher power for viewing the shadow-transits of Jupiter's moons across the face of the planet, or a close-up of a lunar crater, then combine a 2x barlow with a 9mm eyepiece, and for a simulated 4.5mm: 222x. This book provides extensive guidance for amateurs on observing and imaging equipment and demonstrates how to best use them. I totally agree with the last post. Focal ratio and length. The unique 3-lens field flattener allows wide flat focal planes, 80 mm or even more (!) Refractor telescopes use an optical design that relies on a series of lenses to focus light to a single point. Focal ratio in telescopes. Exit Pupil The exit pupil of an instrument is the cylinder of light . So. The flip side, if you wanted to see the whole scene, the valley, the multiple mountain peaks (including the one beautiful one) you will want to use a scope with a lower focal ratio. If we express the coma blur length in arc minutes, it comes to 645h/DF 3. The smaller the f/number, the lower the magnification, the wider the field, and the brighter the image with any given eyepiece or camera. As the name implies, the primary mirror is 30 meters (98 feet) in diameter, and because it is f/1 it has a focal length of 30 meters. The only general rule is get as large a diameter scope as you can afford so that it will gather more light so you can have brighter views of objects, especially those elusive nebula and galaxies. Found inside – Page 183Telescope. Focal. Ratio. When using a combined (or in this case, a smaller separate) chip, both chips are in the ... As a consequence, the number of potential guide stars will be reduced at longer focal ratios (such as f8 and above). Your examples gives two scopes with the same aperture but with different focal ratios, mine gives two scopes with different apertures with the same focal ratio. It is the ratio of the focal distance to the aperture size. A major feature of this book is the way it compares not only different classes of refractor, but also telescopes of each class that are sold by various commercial manufacturers. 900mm Focal Length, f/13 Focal Ratio. The Schmidt-Cassegrain telescope is a member of the catadioptric optical family. We look at many wavelength bands and how the sky looks at all those wavelengths. Formula: ( Pixel Size / Telescope Focal Length ) X 206.265. Easy enough right? If you don't plan on doing any astrophotography then it is not generally something to worry about. Available in other Styles, Configurations & Kits. The reflector's 8" aperture and fast focal ratio will amaze you with high-contrast views of the cosmos. Coma is a function of both off-axis distance and focal ratio, meaning faster-focal-ratio (smaller f-number) telescopes will have more coma than a similar size but slower telescope. Found inside – Page 103fixed telescope mirror aperture is Ep a 5-2 « f1 ? where we used s « fi from ( 1 ) . ... The focal ratio is usually indicated with the notation “ f / R ” ; i.e. , a focal ratio of 6 is written “ f / 6 ” , when , in fact , it is ... The telescope's design is exquisite given the cost and we are impressed with the optical performance. As an example, a scope with an aperture of 4.5 inches and focal length of 45 inches will have a focal ratio of f/10. The fast telescope is mostly used for observing deep-sky objects like nebulas because it has a wider field of view, and the image is brighter with any given eyepiece or camera. If the scope is too heavy to be bothered with or you need to do yoga/gymnastics every time you use it then it will soon get boring. For instance, a telescope with an 80-mm-wide lens and a 400-mm focal length has a focal ratio of f/5. Both offer excellent color and contrast, but APO or apochromatic telescopes boast perfectly refined color and superior contrast. So a telescope with an aperture of 130mm and focal length of 650mm will have a f/ratio of f/5. When beginners and "newbies" begin searching for a telescope, it's the focal-ratio that they see advertised, hence the title of this thread. We look at it from a hardware point of view, and we see that the current way of doing images relies on tiny boxes. this can help get focus quickly at higher magnifications with slower scopes. Focal Ratio Calculator. These telescopes are sealed tube instruments, typically f11 to f15 in focal ratio. November 17, 2015 in Getting Started General Help and Advice. Trying to take the same picture with a low focal ratio just won’t work for photography. Focal Ratio (f/number) = Telescope focal length / Aperture in mm. If the field of view calculation just above the box readers less than about 30', you won't be able to fit the whole moon into the FoV, and you'll need to reduce the magnification of the eyepiece you are using (try a 10mm eyepiece) or just change scope. The quickest way to calculate this is to divide the focal length of the eyepiece by the focal ratio of the scope. The second edition of Electronic Imaging in Astronomy: Detectors and Instrumentation describes the remarkable developments that have taken place in astronomical detectors and instrumentation in recent years – from the invention of the ... Focal ratio - The larger its ratio is, the sharper image you'll get. In astronomy, the f-number is commonly referred to as the focal ratio (or f-ratio) notated as . Found inside – Page 20A telescope with a fast focal ratio can image in just about any kind of seeing conditions. The faster lightgathering ability also makes it easier for your mount to track your targeted object, since shorter exposures are able to be used ... A scope with an f/5 ratio will show an image almost four times brighter than an f/10 scope but half as large. A thorough complement of accessories comes with the 8" f/3.9 Newtonian Astrograph telescope. Type: GoTo Truss Tube Dobsonian Telescope Aperture: 16" Focal length: 1800 mm Focal ratio: f/4.4 Limiting stellar magnitude: 15.7 Weight: 174.0 lbs. On this first page alone... http://agenaastro.com/telescopes/refractor-telescopes.html. In our three example scopes this would give 1mm, 2mm and 4mm respectively. Field of view - The field of view is the circle of sky visible through the eyepiece. For example, if you are looking at a distant nebula through a telescope with a focal ratio of 4, you will see it much faster than through a telescope with a focal ratio of 8 but the image will be smaller and less defined. The magnification only depends on the ratio of the focal length of the objective f and the focal length of the eyepiece f e: MP = f / f e. Common eyepieces for telescopes are 26 mm and 12.5 mm. I'm showing off their websites and what the instruments look like. Unfortunately, at large focal ratios, you'll have a much fainter image. The system gets better when at a darker site, although that's been so long, I've almost forgotten the feeling. The focal ratio is not relevant to that particular calculation. Historically, the rule of thumb for guide scope focal lengths was about ⅓ of the focal length of the primary imaging telescope. The focal ratio is the focal length divided by the telescope's aperture - it is usually written as f/number. Formula: Focal Length / Aperture . Consider... An 8" f/10 Schmidt-Cassegrain has a focal length of 2032mm, whilst an 8" f/5 Newtonian has a focal length of only 1000mm. If one wishes to talk about contrast, then focal ratio may be one of the factors to consider, but I would suggest more aperture would be the way forwards rather than worrying about an f/8 vs an f/10 scope for example. Last, what can they see when they work well? Telescope with the low focal ratio is fast, and one with the high focal ratio is slow. No simple answers necessarily, but two things to finish with. A higher focal ratio is going to produce cleaner more defined images at the expense of a wide field of view. Beginners are generally reluctant or unable to sink a lot of cash into a first telescope, but they want their money's worth, and yes, "the best bang for the buck", hence... "May I interest you in an 8" f/6 Dobsonian, sir?". Therefore, an 8" f/4 Newtonian has more coma than an 8" f/6 Newtonian. A 32mm would offer 31x; slightly high in my opinion, but still an ideal per its 8" aperture for deep-sky, and for scanning the star-fields of the Milky Way even. the mirror has a diameter of 203mm). This value is found by dividing the telescope's . Good luck, it is an exciting time buying a telescope. the scope by its diameter and f-ratio, the eyepiece by its focal length. Is there an ideal focal ratio for a NOOB telescope ? So for example, my old Meade SCT has a focal length of 2,000mm and it has an aperture of 203mm (i.e. What is more important about the focal length of a telescope is that the longer the focal length of the telescope the more powerful the scope will be. This book provides an introduction to the design of a variety of telescopes, mounts, and drives suitable for the home-constructor. We'll also lament the loss of Arecibo. From there, we go on to some of the more prominent space-based observatories, what they see, and how they do what they do. By contrast a 200-mm astrograph (a telescope designed for astrophotography) that has a focal length of 400 mm will have a focal ratio of f/2. For example, if you are looking at a distant nebula through a telescope with a focal ratio of 4, you will see it much faster than through a telescope with a focal ratio of 8 but the image will be smaller and less defined. it takes a little practice to maintain track on the smaller rocks/hills in the valleys? Beginners not looking to do AP are better to avoid fast scopes and the inherent collimation, focusing and choice of eye pieces they bring. Whithout getting into the technicalities and the math used to determine FOV or magnification, as a general rule all my three 70mm refractors (too many I know!) 4. Found inside – Page 282As F/D is the focal ratio of the telescope this equation gives the focal ratio at which the image scale equals the ... So, for a 20-cm telescope, which has a resolution of 0.6′′, a focal ratio of 1718.87 would give an image scale of 0.6 ... what was the magnification I was getting with this scope? Found inside – Page 23Consider two telescopes, one with a focal ratio of f/5 and the other with a focal ratio of f/10. ... Refractor and Newtonian telescopes in use today can have focal ratios as low as f/4, while nearly all Schmidt Cassegrain telescopes ... Mathematically it's the focal length divided by aperture. Found inside – Page 55Where the optical design of the eyepiece becomes important is when the focal ratio of the telescope is short, particularly for telescope focal ratios of f/5 or less. As an example, if the intent is to use a wider field eyepiece, ... This is part of my complete intro Astronomy class. So, that means the rule of thumb is 300X useful magnification. It is the f ocal length of the telescope divided by its aperture. My 11" f/10 SCT has a vastly narrower field of view than 5" f/10 SCT. Calculate the focal length of your telescope. Back a little and your there! It may not seem like that big of a deal but in terms of getting good photographs out of your telescope, focal ratio plays a large and critical role. A 32mm ocular would result in a magnification of 64x; a bit high-powered for wide-field deep-sky. Telescope Type. This, normally, gives us a very neat number to use for the focal ratio. So when comparing any telescopes, whether they are "similar" in terms of aperture or focal ratio, the only reliable way to calculate (or predict) their field of view is to look at the focal length. Focal ratio sounds complicated and scientific but it is actually quite simple. a decent aperture and is also reasonably light with great optics. Others desire only to observe the Moon and the planets, and then might choose a 4" or 5" f/9-f/12 refractor. A key aspect of telescopes is their angular resolution. In the first lecture, Refraction and Reflection, I discuss the obvious point that all telescopes rely on some ability to focus light to a point. Next, I go over angular resolution and seeing. Regardless of the expense of eyepiece used, fast scopes (below about f4.5 or perhaps f4) definitely benefit from a coma corrector to avoid stars appearing as little comets towards the edge of the field. You calculate focal ratio, which refers to a telescope's speed, by dividing the focal length by the aperture. figure goes down rapidly as the f/ratio diminishes, you can see that excellent collimation becomes a necessity with shorter f/ratio scopes because of rapidly decreasing tolerances for miscollimation.
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