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  1. #1
    chryso's Avatar
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    Default Slow scope vs fast scope



    I have seen people mention slow vs fast in regards to telescopes but I don't understand what that means. Can someone tell me what it means to say that a telescope is slow or fast?

  2. #2
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    Default

    Two telescopes
    150mm aperture, 1200mm focal length, f8 (slow)
    150mm aperture, 750mm focal length, f5 (fast)

    They both collect the same amout of light but if both were used with a 25mm EP, the slow scope would magnify the image more and make the subject look darker because the same light is spread over a larger area.
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    Default

    Fast and slow refer to the amount of time an exposure is exposed: fast scopes (f5 or lower) require a lot less time for proper exposures while slow scopes (f 6 or higher) require longer exposure times. For visual observing only, there is not much difference, though most observers say a slow scope will give better images with planet observing but with smaller fields of view. Also, fast scopes are shorter in length, cost much less to make and easier to move/store as well as having wider fields of view per equal magnification used on both.

    and yes, what Pederv says is also true.
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    Default

    They are photographic terms and are mostly only relevant to astrophotography. Fast scopes require shorter exposures; slow scopes require longer exposures.

    For visual use, it makes no difference to the view in the eyepiece. EXCEPT... Fast scopes are more demanding of eyepiece quality. A fast scope with a cheap eyepiece will make the view look like crap. The same scope with a high-end eyepiece will look much better. With slow scopes, the difference between cheap and fancy eyepieces is minimal.

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    The terms come from those used with cameras, where a camera with a large aperture could gather more light in a given amount of time, so you can take shorter exposures with a fast camera. The speed of an optical set-up is related to the focal ratio, which is the focal length divided by the aperture. More aperture lets in more light, but the optical set-up can get really long if the focal ratio is not adjusted to compensate for a larger aperture.

    Faster focal ratios are those with lower numbers. Typical fast telescopes run in the f/5 and f/4 range. Typical slow telescopes run around f/10 and higher. With telescopes a fast telescope will be shorter and will have lower magnification, wider FOV and brighter views with a given eyepiece. Fast telescopes also typically require better corrected (= more expensive) eyepieces. Dobs and other Newtonian reflectors are normally faster and SCTs and other designs are typically slower. As you can easily figure out, slower telescopes will have higher magnification, narrower FOV, and will work better with less expensive eyepieces.
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    Default

    To expand on KeithBC's comments -- faster scopes, because they have a shorter focal length, also have a more aggressively curved mirror. This difference is important at the focal plane -- the region of space just behind the eyepiece's objective lens.

    No scope has a flat focal plane, but slow scopes have a flatter one than fast scopes. Low-end, simple eyepieces (like the Plossls that usually come with telescopes) do not have anything to correct for a curved focal plane. In a slow scope, since the focal plane is nearly flat anyway, this isn't a problem. But in a fast scope, it means that when you have an object in the center of the field in focus, it may not be in focus at the edges, because the focal plane is curved. Running the focus slightly in and out you can actually see the focus point move from the center outward and back.

    Expensive high-end eyepieces like those sold by TeleVue (the Nagler, Panoptic, Ethos, Delos, etc. lines) are designed to handle curved focal planes gracefully, usually up to some limit (I believe the Explore Scientific 82 degree series can properly focus down to f/4 for example).

    The other (much less noticeable, in my experience) effect of a fast scope is called "coma". This is similar to spherical aberration, in that as you get further from the center of the field, the aberration gets worse. Stars will look like little "comets" that all point toward the center of the field. This is even with a high-end eyepiece that has corrected for the spherical focal plane and has the whole field in focus. There are corrector lenses (like the TeleVue Paracorr) that correct for this effect as well, but in my experience, if you're just using a scope for visual purposes, the effects of coma are difficult to see.

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