Astro Tutorial #1.6: THE F-RATIO "fast" vs "slow" scopes

hi everyone this is Chris welcome back to another episode of catching photons last time we discovered the focal length and the aperture has two main properties of a scope this time we want to bring those two units together and see what scopes we can possibly create so the reasoning is fairly simple you take an image of a given field of view then you collect the life from this patch of the sky onto a given sensor and that gives you some level X of luminosity but if you then use a longer focal length but fix the aperture then you project a smaller patch of the sky with less light onto the same chip and hence get a lower luminosity so images tend to get darker if you increase the focal length but fix everything else on the other hand if you increase the aperture of the scope then you will get more light from the same patch of the sky and the image will be brighter again so longer focal length means darker image but larger aperture means brighter image so it is possible for a long scope with a wide aperture to produce a brighter image that has a shorter scope with a very narrow aperture and to get an idea of what the scope can deliver we introduced than the F ratio the F ratio is simply the focal length divided by the aperture so if you have a long scope say thousand millimeters and a small aperture say 50 millimeters then you get an F ratio of thousand divided by 50 equals 20 and then we write F 20 to indicate that this is a ratio the higher the F ratio the less light the scope collects the lower the F ratio the more light it collects and thus the brighter the appearance of the images our F 20 therefore means that the scope can only collect very little light just to clarify things when we say collect less light or more light you all know that we mean light per time because of course you could produce a totally overexposed image just by turning up the exposure time to 11 but in our case we of course want to compare always the same comparable time and lots yep here are examples of reasonable F ratios on the market we start with ie rather cheap scope it's quite long focal length 800 millimeters and rather narrow with 50 millimeters so the F ratio is f-16 and therefore we call this scope slow why do we call this scope slow because if you compare two scopes one short the other long and both have the same aperture if you then want to create an equally bright image with both scopes then these go with the higher f ratio we'll need a longer exposure to meet the brightness level of the shortest cope and the scope with a higher F ratio is slower yeah so then we have a vixen lens go bad thousand millimeters focal length so longer than the first one but because the aperture with 105 millimeters is much wider the F ratio is actually lower at F 9.5 so the scope is faster than the first one and hence can collect more light up down we go the small ng scope has 360 millimeters focal length so a rather short scope and an aperture of 50 millimeter so the same as the first cope but because it's shorter the field of view is much wider and hence it collects much more light from this bigger section of the sky the F ratio is therefore significantly lower with F 7.2 next scope enters the range of what we can call fast scopes it's my beloved Sky Watcher Newtonian with 750 millimeter focal length its intermediate long but with an aperture of 150 millimeter it can collect lots of photons that gives us f5 and then we have the brass or refractor with short 450 millimeter focal length having nearly the same aperture as the long vixen that was the second scope its f ratio is down to F 4.5 and then we end the line with the incredible Raza from celestron whereas I focal length of only 400 millimetres but still an aperture of over 200 millimetres so more than half of the focal length that gives the arroz an incredible view on a big chunk of the sky while collecting as much light as possible the focal length is down to f/2 with f/4 at five or even the raza with f2 we are getting in the range of DSLR lenses but with focal lengths over 400 millimeters and apertures up to 200 millimetres you can take incredible images with ie resign yes it's rather cost e such scopes are called astra graphs as they are intentionally designed for taking images of the sky with a rather it's not even possible to attach a normal eye piece so it's imaging only know what is a high F ratio always bad nah I wouldn't say so c4 astra imaging you always tend towards the lowest F ratio just because this cuts your imaging time down because the scope can get a light faster there can be disadvantages low F ratios and some reflection scopes can for example lead to aberrations at the outer borders of the image so here you might not step down too far but that's far ahead but first and foremost the scope needs to fit the requirements and the focal length must fit the objects being observed and cost is a major factor to consider - also if you plan to capture or observe planets in the moon only they tend to be so bright and you can easily observe them with f10 or even f-16 scopes that's fine for them but then never try to capture a faint galaxy with that scope okay watch out never try to talk to a down-to-earth daylight photographer they use a different way of using the terms above if I say focal lengths like us but then they really use the F ratio instead of the aperture so they tend to say I shot at aperture F - thats kind of strange but I do so to clarify the amount of light they are working with we have a fixed aperture and a fixed focal length so that's not our business we stay with focal lengths the aperture and the ratio between them that's cool and the truth okay homework find some cheap scopes online say on a major online selling platform work out their f ratio and compare the results with the beautiful images of fine nebulas distant galaxies and so on in the commercials around that scope it's it's funny what they want to sell us sometimes so now you are equipped with the first tools to work out the basic optical qualities and capabilities for the thousands of scopes out there long and short focal lengths wide and narrow aperture and the resulting fastness of a scope all in your hands and next video so we will open up these scopes and study their inner structure we will discover lens and mirror scopes different architectures and even some mixed types so stay tuned like this video hit the like button but more importantly guide newcomers to this tutorial if you think they can hear find a safe haven for starting with this wonderful hobby and tip of the day I added a link below it leads to a webpage where you can calculate the fov of a given telescope using its specifications know you are an expert on this so go on and play around and as always I say clear skies everyone until next time here on catching photons you

Channel: catchingphotons

Views: 30,519

Keywords: Astro, Astronomy, Astrophotography, Telescope, Tutorial, night sky, sky, visual astronomy, beginners, f-ratio, classification, focal length, aperture, fast scope, slow scope, lens scope, astrograph, RASA11, RASA

Id: v3wgG1gcZt4

Length: 8min 7sec (487 seconds)

Published: Fri Jan 10 2020