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Thread: Novice Questions about Focal Ratio and focal length and aperture

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    Default Novice Questions about Focal Ratio and focal length and aperture



    As I've been getting to know my scopes better I notice that each has strengths and weaknesses. In general how much does the focal ratio dictate what kind of viewing the scope is suited for? Can aperture make up for f/ratio, or is it focal length that makes all the difference? Is it a rule of thumb that smaller f/ratio's are best for wide-field/deepsky viewing and larger f/ratio's are best for planetary viewing? I've listed the specs of the scopes below.

    Also, are there rules of thumb about what eyepieces work with what f/ratio, or does it really depend on aperture? Would I, in general, want to stick to narrow FOV eyepieces for planets and broader FOV for deep sky viewing? Is there a general rule of how much magnification I can use according to aperture, f/ratio, or focal length?

    My experience with my scopes is below.
    • The vixen is nice because it's so small and seems to be a decent scope for moon and planets at lower powers. There is very little if any chromatic aberration, but it does not get as tight focus as a refractor. Also it starts to get weak with eyepieces below 10mm
    • The ES AR102 is better than the Vixen at deeper field/broad area viewing. It gets a nice tight focus, but can get a lot of chromatic aberration, especially on bright objects and at higher powers. For instance, viewing Jupiter through a 10mm eyepiece has a lot of fringe.
    • The Zhumell seems to be the best general all around, but I'm wondering if that is just because of its aperture.



    Vixen vmc110L: Focal Length 1035mm; Focal Ratio f/9.4; Aperture 110mm
    Explore Scientific AR102: Focal length 663mm; Focal Ratio f/6.5; Aperture 102mm
    Zhumell Z8 Dob: Focal Length 1200mm; Focal Ratio f/5.9; Aperture 203mm

    Thanks
    Peter
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    Default Re: Novice Questions about Focal Ratio and focal length and aperture

    For visual use it is the combination of objective and eyepiece, not just the F ratio based on the objective lens or primary mirror. The combined focal length is determined by 1/f_combined = 1/f_objective + 1/f_eyepiece. Combinations with the same ratio of f_combined divided by the objective diameter will give the same light flux.
    ... Henk. Telescopes: 6" Mak-Newt (Comet Hunter), ES ED127CF, ES ED80, Zhumell Z12, Coulter Odyssey 10, AT6RC, Venture RX-7, Celestron Skymaster 20x80, Mounts and tripod: Losmandy G11S, AVX, LXD55, Tiltall, Cameras: Fuji X-a1, Canon SX40, Xt, XSi, T6, ELPH 100HS, DIY: Dob and camera barndoor trackers, afocal adapter, Dob with foldable base and Az/Alt setting circles, Accessories: SSAG, Plossls, Barlows, Telrad, laser collimators (Seben LK1, Z12, Howie Glatter), Cheshire, 2 Orion RACIs 8x50, Software: DSS, ImageMagick, PHD, Nebulosity, Photo Gallery, Gimp, CHDK

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    Default Re: Novice Questions about Focal Ratio and focal length and aperture

    Quote Originally Posted by CamelHat View Post
    For visual use it is the combination of objective and eyepiece, not just the F ratio based on the objective lens or primary mirror. The combined focal length is determined by 1/f_combined = 1/f_objective + 1/f_eyepiece. Combinations with the same ratio of f_combined divided by the objective diameter will give the same light flux.
    Is it true, or urban myth that long focal length newtonians are capable of refractor like quality, or better? Some say yes...some say no....Part of me wants to believe it is possible.....I am just learning about all of this stuff....I am tempted to chime in, but sometimes a little bit of knowledge mixed with a greater degree of ignorance and no experience is a very dangerous combination.....Austin gave me a new link for a calculator that projects different aspects of laying out a design for newtonian reflectors......however, I'm totally mathematically challenged, and calculator doesn't give hard numbers for the loss of light from the influence of different dimensions of secondaries...well, it does in some way, by referencing loss as a percentage of the obstruction of the surface area of the primary mirror by the secondary...which isn't actually a "light" measurement per se'. It did give calculated dimensions and locations for internal baffles though, which I thought was pretty cool....First time I have seen that done, and it had been a question on my mind already for a while......

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    Default Re: Novice Questions about Focal Ratio and focal length and aperture

    <quote> In general how much does the focal ratio dictate what kind of viewing the scope is suited for?
    As larger scopes have become more affordable, and the need often to move scopes to dark sites for serious imaging and observation, the f/ratio of the scope has become an important " convenience" determining factor....

    Aperture still rules as the factor that determines a scopes light gathering and resolving power...
    the f/ratio is important in keeping OTA sizes reasonable and more or less portable, at the expense of some distortions<like coma> hence the need for more corrected ep's or coma correctors, or at some point, <with very fast mirrors>both......

    A longer f/ratio will develop more power with any given lens than a faster mirror will. The magnification/afov is affected by fixed formulas that CamelHat was making reference too in a fixed way, thus fast scopes will naturally have a wider afov and lower magnification given the same ep.

    A wide field ep needs a certain width of light on it to illuminate fully, so larger secondary mirrors are used in faster scopes to deal with the wider cone coming from the primary, and achieve the correct illumination for wider field stops at the ep...
    slower scopes usually use a smaller secondary, which limits how big the cone of light is, so their widest FOV and lowest magnification may be less that a fast scope because of the size of the field stop in the ep itself and the decreased width of the light cone that is available to illuminate it.........

    larger secondary mirrors affect<reduce> the efficiency of the primary mirror, but it is required by the size of the light cone created by the steeper angles<wider/fatter> of the deeper parabolic figuring in the primary mirror needed to make the f/ratio shorter<faster>.

    which does what better? a slow scope is pretty much limited to an 8 or 10 <max> primary just due to the length of the OTA for all practical purposes....too long to carry, and the observing position at the EP is already at 6' high at zenith for an 8" f/9 scope.....
    For visual use, the same attributes that make a slow scope a great planet killer<contrast and detail> lend themselves well to seeing the really dim fuzzy dso's....the primary size is the limiting factor....the thing that makes fast scopes the market favorites? Again, size.....bigger primaries, and portability......convenience......






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    Default Re: Novice Questions about Focal Ratio and focal length and aperture

    MoosBros, your post makes a lot of sense to me. Thanks. I've got my eye on the Orion 180mm Mak for this very reason. It's focal length is 2700mm with a decent amount of aperture. At f/15, it should be really great for planets and dim fuzzies like nebulae and galaxies. It also looks like the secondary mirror is a smaller obstruction than in the Celestron SCT's.
    Happy gazing

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    Default Novice Questions about Focal Ratio and focal length and aperture

    Hmm. An f15 scope should be nice for planets, but isn't going to be as good for DSOs - you won't be able to get as wide or as bright a view as you can with a lower f ratio one.

    Even with a 40mm eyepiece, you'd get less than a 3mm exit pupil... Ok for brighter objects, but not ideal if you're hunting for really faint objects.

    And with a given eyepiece, you'll get 3x the magnification, 1/9th the brightness on extended objects, and 1/3 the true field that you would in an f5 scope - handy for smaller, brighter objects like planets but not so good for fitting a large DSO in (you'd be at the same sort of focal length as my 11" SCT - and that won't fit all of some of the messier open clusters in at once without a focal reducer).

    (Edit - adding a focal reducer would probably help for the DSOs; the main point is that while for visual use, it's the aperture and final magnification that counts, the f-ratio sets a limit on how low a magnification and how wide a true field you can get with available eyepieces).
    Last edited by jerryTheC; 04-23-2014 at 07:35 AM.
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    Default Re: Novice Questions about Focal Ratio and focal length and aperture

    Quote Originally Posted by Peterg707 View Post
    MoosBros, your post makes a lot of sense to me. Thanks. I've got my eye on the Orion 180mm Mak for this very reason. It's focal length is 2700mm with a decent amount of aperture. At f/15, it should be really great for planets and dim fuzzies like nebulae and galaxies. It also looks like the secondary mirror is a smaller obstruction than in the Celestron SCT's.
    You caught me a little off guard there Peter..I am still kind of "stuck" in the newtonian dark ages in understanding how all the science comes together to solve reflector telescope issues......I had not even thought about the Maksutov Newtonian reflectors when I posted that..I do know they are regarded well corrected, even the faster scope versions..I wonder if that's how the RC astrographs developed...I need more info, lots more info......I know what I'll be reading about today, and tomorrow, and maybe the next day.....hmmmmmm.
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    Default Re: Novice Questions about Focal Ratio and focal length and aperture

    Maks are just plain Planet killers as well as lunar killers ...and that is the reason I just purchased one...BUT with the very narrow FOV and the small aperture they " generally suck" on DSO's ... Only real drawback with Maks is that they take some time cooling down but honestly that is no big deal...(I'm talking about Mak-Cassarians in my over 50 years in this hobby I admit I have never used a Mak-Newt

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    Default Re: Novice Questions about Focal Ratio and focal length and aperture

    Quote Originally Posted by jerrythec View Post
    hmm. An f15 scope should be nice for planets, but isn't going to be as good for dsos - you won't be able to get as wide or as bright a view as you can with a lower f ratio one.

    Even with a 40mm eyepiece, you'd get less than a 3mm exit pupil... Ok for brighter objects, but not ideal if you're hunting for really faint objects.

    And with a given eyepiece, you'll get 3x the magnification, 1/9th the brightness on extended objects, and 1/3 the true field that you would in an f5 scope - handy for smaller, brighter objects like planets but not so good for fitting a large dso in (you'd be at the same sort of focal length as my 11" sct - and that won't fit all of some of the messier open clusters in at once without a focal reducer).

    (edit - adding a focal reducer would probably help for the dsos; the main point is that while for visual use, it's the aperture and final magnification that counts, the f-ratio sets a limit on how low a magnification and how wide a true field you can get with available eyepieces).
    Amen!

    I totally agree. I added the bold font to Jerry's post.

    One can play and say all the mumbo-jumbo, but in the end for DSO visual observing aperture rules.

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    Default Re: Novice Questions about Focal Ratio and focal length and aperture

    Quote Originally Posted by MoosBros View Post
    Is it true, or urban myth that long focal length newtonians are capable of refractor like quality, or better? ......
    Like most other things, there's some truth to it. But just some. A slow scope suffers less from coma, and the narrow light cone is easier for el cheapo eyepieces to deal with. ON THE OTHER HAND...contrary to popular belief, very good long focal ratio mirrors are actually MORE DIFFICULT to make than very good short focal length mirrors. As I found out as a teenager back in the early 70s when I got up the gumption to do a LIGHTNING FAST f/primary...I'd been told that it was oh-so-hard to figure a faster than f/8 mirror. It was EASIER than the f/10 I did for my first attempt as a sprout.
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