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  1. #1
    Tenifer's Avatar
    Tenifer Guest

    Default close focus formula




    There was this scope which can focus object less than a meter
    from the object and you only have to move the focuser or mirror
    a little bit. It was the MTO 1000 focal length f/10 4" russian
    maksutov scope. I compared it with a 70mm F/8 560mm focal
    length refractor and i have to move the focus back one foot to
    achieve
    close focus 2 meters away (in the refractor). Is there a formula
    wherein I can calculate the exact back travel of the focuser to
    achieve focus of the target say 2 feet away from the objective
    of this 70mm f/8 560 focal length refractor given a
    15mm plossl with magnification of 37X? I used paper
    rolled into tube to extend the back focus a feet or a meter
    away from the refractor rear but I can't achieve focus. Also why
    is the maksotuv able to focus at one feet by just moving the
    back focus a little bit like an inch??

    Teni


  2. #2
    astrophotography@gmail.com's Avatar
    astrophotography@gmail.com Guest

    Default close focus formula

    On Nov 6, 11:40 pm, Tenifer <tensorsur...@yahoo.com> wrote:
    Also why

    If that Mak focuses by moving the main mirror then you are changing
    the system design by moving it giving it a different back focal
    length. If it is focused by just moving the eyepiece then you will
    have the same problem as with the refractor.

    Alvan Clark



  3. #3
    Tenifer's Avatar
    Tenifer Guest

    Default close focus formula

    On Nov 7, 9:40 pm, astrophotogra...@gmail.com wrote:

    In the SCT (such as Celetron's), one adjusts the focuser
    by moving the main mirror, so it means every focusing/eyepiece
    combo used would change the SCT to f/10, f/8, f/6 depending
    on how the mirror is moved (or focuser adjusted)? Duh.

    Tni


  4. #4
    Tenifer's Avatar
    Tenifer Guest

    Default close focus formula

    On Nov 7, 9:40 pm, astrophotogra...@gmail.com wrote:

    Uhm... in the SCT (such as Celestron's), one also focuses
    by moving the main mirror. So it changes the system
    design by every focusing and eyepiece combo such
    that it becomes f/9, f/10, f/7 depending on the combo?
    Duh.
    T



  5. #5
    dkelvey@hotmail.com's Avatar
    dkelvey@hotmail.com Guest

    Default close focus formula

    On Nov 6, 8:40 pm, Tenifer <tensorsur...@yahoo.com> wrote:

    Hi
    The refractor can be treated as a simple lens for this
    purpose:

    F.L. = 1/(1/D1+1/D2)

    F.L. is the focal length of lens
    D1 is the distance from the object to the center of the lens
    D2 is the distance from the lens to the eyepiece.

    The focal length of the eyepiece doesn't need to be involved.
    The distance from the lens is measured to the field stop in
    the eyepiece ( usually where the eyepiece makes the step
    to the larger diameter but may be in front of that some for
    short focal length eyepieces ).

    The Mak's primary is moving, as another mentioned. Since
    the primary is a short focal length, smaller amounts of
    movement are needed to move the focused image. It follows
    the same rules or fomula as the simple lens.
    What the Mak has that the refractor doesn't is that it
    also has a secondary ( the coated surface on the meniscus ).
    This is used to effectively extend or multiply the
    focal length of the primary.
    Most astronomical telescopes will have some issues with
    close focus because they are designed to work with a light
    source that provides parallel light beams from each point
    of the object. In a reflector, they'd use a parabolic surface
    to focus. If it was designed for a shorter focus, an elliptical
    surface would have been used. What this means is that
    one can not achieve a sharp focus when using such an astronomical
    telescope for close viewing.

    In the case of your refractor, you'd need to extend the
    focal point 0.217 meters ( 777 - 560 ) to focus at 2 meters,
    as calculated from above.
    Dwight


  6. #6
    Tenifer's Avatar
    Tenifer Guest

    Default close focus formula

    On Nov 8, 3:24 am, dkel...@hotmail.com wrote:

    You mean in the refractor. Even if I used 15mm or 7mm
    eyepiece, I can get the same image magnification when
    focusing at 2 meters by racking back the focuser 0.217
    meters??

    I plan to use the scope (besides sky viewing) as a 1 meter
    distant microscope. Do you know the theoretical
    maximum magnification that it can use.
    Also I own a 70mm f/8 apo refractor. I wonder what
    difference in details I can get when aiming the latter
    at 3 meters away versus the maksutov at one meter
    taking into account that the maksutov focal length
    (like becoming f/6?) is supposed to change as one
    racks the primary focuser in and out. What do you
    think?

    Teni




  7. #7
    dkelvey@hotmail.com's Avatar
    dkelvey@hotmail.com Guest

    Default close focus formula

    On Nov 7, 12:28 pm, Tenifer <tensorsur...@yahoo.com> wrote:

    No, I mean the focal distance isn't related to the eyepiece.
    The magnification is still the primary focal length divided
    by the eyepiece focal length.


    I don't know but I'd suspect it would even be noticed
    at typical lower powers.


    The smaller the primary, the sharper the image until
    diffraction limits take over. I'd suspect that the f/8
    would be better for most things.
    You might consider placing two refractors front to front.
    One refractor lens with your short distance to object
    and the other longer focal length to create magnification.
    Dwight




  8. #8
    astrophotography@gmail.com's Avatar
    astrophotography@gmail.com Guest

    Default close focus formula

    On Nov 7, 9:31 am, Tenifer <tensorsur...@yahoo.com> wrote:

    I'm sorry I don't know what "duh" means in this context. In any case,
    adjusting the focus by moving the primary doesn't change the focal
    ratio by the degree you suggest. There isn't a lot of movement in the
    tube. Moving the mirror the amount needed to change from f10 to f6
    would result in poor images because the system is designed to perform
    with one given spacing of the elements.

    Presumably a SCT is designed to focus perfectly at infinity. Once you
    change the spacing of the mirrors (by focusing) on a closer object the
    image degrades.

    Alvan Clark



  9. #9
    Tenifer's Avatar
    Tenifer Guest

    Default close focus formula

    On Nov 8, 6:49 am, dkel...@hotmail.com wrote:


    This is new. So it means I can change eyepieces from
    3mm to 25mm without changing or racking out
    the focuser provided I initially got it to the right focal
    distance (distance from lens to eyepiece) from the
    close-up or single lens approximation formula?

    Do you know the boundary or distance when the
    rays shift from parallel rays (from infinity) to when
    it starts to bend (from closeup)? How do you
    calculate for it?

    Thanks dude.

    Teni




  10. #10
    Rich's Avatar
    Rich Guest

    Default close focus formula

    On Nov 7, 9:31 am, Tenifer <tensorsur...@yahoo.com> wrote:

    But at some point, you are cutting into the main mirror by moving it
    closer to the secondary, thereby reducing the effective light
    gathering area and resolution.
    On some of the old C8s, you could literally place the focus point a
    couple feet behind the scope. Using a ft long extension, i was able
    to get down to the microscopic level focusing on objects just 10ft
    away or so. Definition wasn't the greatest however.


 

 
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