Re: close focus formula

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

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

.

Relevant Pages

  • Re: close focus formula
    ... from the object and you only have to move the focuser or mirror ... D1 is the distance from the object to the center of the lens ... D2 is the distance from the lens to the eyepiece. ...
    (sci.astro.amateur)
  • Re: close focus formula
    ... from the object and you only have to move the focuser or mirror ... D1 is the distance from the object to the center of the lens ... D2 is the distance from the lens to the eyepiece. ...
    (sci.astro.amateur)
  • Re: close focus formula
    ... from the object and you only have to move the focuser or mirror ... D1 is the distance from the object to the center of the lens ... D2 is the distance from the lens to the eyepiece. ...
    (sci.astro.amateur)
  • Re: close focus formula
    ... from the object and you only have to move the focuser or mirror ... D1 is the distance from the object to the center of the lens ... D2 is the distance from the lens to the eyepiece. ...
    (sci.astro.amateur)
  • Re: close focus formula
    ... from the object and you only have to move the focuser or mirror ... D1 is the distance from the object to the center of the lens ... D2 is the distance from the lens to the eyepiece. ...
    (sci.astro.amateur)