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Thread: Maximum focal length

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    Default Maximum focal length



    My telescope is on it's way and it comes with a #32,25,17,13,6 mm eyepiece. I was reading something and if I got it right, there is a max mm of the eyepiece based on the apeture? It's a 2032, f10, so the biggest eyepiece I should use with this is 32 mm?

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    Default Re: Maximum focal length

    What size of focuser does the telescope have and what is the aperture of the telescope?
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    Default Re: Maximum focal length

    The largest EP that you could use with an 8"SCT is longer than 32mm, but with only a 1.25" focuser, a 32mm plossl EP (like you are getting) is the widest you will be able to go, so a longer EP is just a waste. But there are a couple of things you should do for this.

    First you will want a focal reducer for the scope. If it is a regular SCT then the much cheaper standard f/6.3 reducer will work fine. If it is a coma free designed SCT, you will need their much more expensive and scope specific focal reducer. This will convert the scope from f/10 to f/6.3 and greatly widen the view, dropping focal length from 2000mm to 1260mm.

    You will also want to upgrade to a 2" SCT diagonal and use 2" EPs where you can get EPs wider than the 32mm plossl. A 24mm 82 degree EP will be much wider than the 32mm plossl AND have greater magnification.
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    Default Re: Maximum focal length

    With a 2" visual back it is better optically and less expensive to use longer focal length eyepieces in an SCT. Less glass more throughput.

    The 2" 82 degree format eyepieces work fine but less expensive longer focal length simpler eyepiece designs work better for cheaper.

    The wide field 82 degree formats are all designed for use in fast scopes and that costs money that needn't be spent on an eyepiece. The GSO Erfles aka Superviews work just dandy in my experience with a 9.25 Edge and Intes MK66.

    If you want to maximize the true field of view then get the less expensive but longer focal lengths like the GSO 50, 40 and 30mm Superviews. If you want to maximize optical quality and TFOV then the Takahashi LE series has to be considered. I have some of both.

    Now if you want to maximize apparent field of view that's something else and maybe the 82 degree afov eyepiece route is for you. But what you will be giving up is light throughput and contrast for the wide field.
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    Default Re: Maximum focal length

    With my 2" mirror diagonal I can use a 56mm super plossl for super wide views with no issues on both my 8" and 10" SCT scopes. It helps to find and center things for me in a pinch.

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    Default Re: Maximum focal length

    Quote Originally Posted by Leveye View Post
    With my 2" mirror diagonal I can use a 56mm super plossl for super wide views with no issues on both my 8" and 10" SCT scopes. It helps to find and center things for me in a pinch.

    Russell Optics & Machine
    I got the same EP and like it. Just remember it has a field stop of ~40mm, so it does not give you the max possible FOV which EP with 46mm field stop would provide.

    Russell Optics 56mm Super Plossl - FOV 41 degree (AFV in my 8" SCT 1.13 deg)
    TV 55mm Plossl - FOV 48 deg (AFV in my 8" SCT 1.3 deg)
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    Default Re: Maximum focal length

    Quote Originally Posted by Apophis View Post
    My telescope is on it's way and it comes with a #32,25,17,13,6 mm eyepiece. I was reading something and if I got it right, there is a max mm of the eyepiece based on the apeture? It's a 2032, f10, so the biggest eyepiece I should use with this is 32 mm?
    For F10 scope with central abstraction the max useful EP focal length is ~50-56mm. Anything larger and the shadow of secondary mirror becomes visible. In fact even with 56mm I see a faint shadow looking at the full moon, but for DSOs it is perfectly usable.

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    Default Re: Maximum focal length

    Quote Originally Posted by Bigzmey View Post
    For F10 scope with central abstraction the max useful EP focal length is ~50-56mm. Anything larger and the shadow of secondary mirror becomes visible. In fact even with 56mm I see a faint shadow looking at the full moon, but for DSOs it is perfectly usable.
    That's true for an older person whose pupils can't dilate out to 7mm. If you can only dilate to 5mm then your experience is perfectly reasonable and understandable. But if this is a younger person whose eyes dilate to 6-7mm no harm is done with a 65mm.

    The human eye is part of the optical system. Wider eye pupil means wider exit pupils are OK and so longer focal lengths are OK.
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    Default Re: Maximum focal length

    Quote Originally Posted by not_Fritz_Argelander View Post
    That's true for an older person whose pupils can't dilate out to 7mm. If you can only dilate to 5mm then your experience is perfectly reasonable and understandable. But if this is a younger person whose eyes dilate to 6-7mm no harm is done with a 65mm.

    The human eye is part of the optical system. Wider eye pupil means wider exit pupils are OK and so longer focal lengths are OK.
    True for unobstructed scope. With DOBs and CATs if you push past 6 mm the shadow of central obstruction becomes visible.
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    Default Re: Maximum focal length

    Quote Originally Posted by Bigzmey View Post
    True for unobstructed scope. With DOBs and CATs if you push past 6 mm the shadow of central obstruction becomes visible.
    No. It depends on the eye AND whether the scope is obstructed. I've noticed that my ability to accommodate long focal length eyepieces without a doughnut shadow effect has declined over the years. When I was younger a 7mm exit pupil was not a problem in an obstructed scope. Now it's iffy. If you don't believe me check this article from Gary Seronik.

    Exploring Low-Power Limits – Gary Seronik

    He has no problems considering 7mm exit pupils for obstructed scopes. The shadow in the center in obstructed scopes is because the exit pupil is larger than the pupil of the eye. That's just what optics says, it's not an opinion.
    Last edited by not_Fritz_Argelander; 11-09-2017 at 09:23 PM.
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