Re: close focus formula

On Nov 11, 6:08 pm, Roger Hamlett
<rogerspamigno...@xxxxxxxxxxxxxxxxxxx> wrote:
On Sun, 11 Nov 2007 00:37:00 -0800, Tenifer <tensorsur...@xxxxxxxxx>
wrote:





On Nov 11, 12:04 am, Roger Hamlett
<rogerspamigno...@xxxxxxxxxxxxxxxxxxx> wrote:
On Sat, 10 Nov 2007 04:09:53 -0800, Tenifer <tensorsur...@xxxxxxxxx>
wrote:

On Nov 10, 1:58 pm, dkel...@xxxxxxxxxxx wrote:
On Nov 8, 6:48 am, Tenifer <tensorsur...@xxxxxxxxx> wrote:

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

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

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

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- Hide quoted text -

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??

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.

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?

No eyepiece that I've seen is made perfect but switching
eyepeices should only require small readjustments to
get back to focus, assuming the object hasn't moved
relative to the objective lens.
I've seen some eyepeices that might be as much
as 3/4 inches off but that is the exception.
Plane of focus of the objective doesn't move. The eyepiece
just looks at this plane by its effective focal length. This is
suppose to be where the field stop is but like I said,
no eyepiece is perfect. The amount you'd change the
focus would be the same for a close object, between
eyepieces, as it would have been for looking at a star.
If the two eyepieces required one turn of the focuser knob
for a star, it would be exactly the same for a close object
( not totally true, this assumes your eye is focused at
infinity and most eyes are not but it shouldn't be enough
difference to notice ).

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?

I'd expect it would depend on how fussy one was.
If it were a problem, stopping the objective down
should help.
Dwight

Maybe you know the answer to this question that
astronomy scope users may not able to answer
because they only focus at infinity and can't imagine
getting close to infinity which can't be calculated.

Given say a close focus of 37X magnification at a
distance of 2 meters between object and objective lens.
What magnfication must be used when the distance
become mere 6" from the object? Scope is a
70mm f/8 560 focal length refractor using 15mm EP.
Do you know what formula can be used in the above.
Thanks.

The fornula for a simple refractor, is the standard lens formula:

1/u+1/v = 1/f

'f' here is the focal length of the refractor. 'v', and 'u' are the
distances to the focal points, in front of, and behind the lens. When
focussed at 'infinity', the term for this becomes zero, so you are
left with the distance to the focal point behind the lens, being the
same as the focal length. So:

1/u +0 =1/f
Hence u=f.

Now for the object 2m in front of the lens, you need to solve for u,
so you get:

1/u = 1/f - 1/v

With f=560mm, and v=2000mm

1/u = 1/560 - 1/2000

This then gives 'u' = 777.77mm from the lens. Compared to the focal
point for the stars, you will need to move the eyepiece 'out', by
217.77mm. So you need something like an 8" extension tube, to have any
hope of focussing the scope.

Best Wishes- Hide quoted text -

- Show quoted text -

I know the above. But I can't seem to get the relationship
between field of view and magnification. Say I use the
70mm f/8 560 focal length refractor at a target 6 meters
away using 37X. If I'd get nearer to the object at 2.5
meters. How much magnification I must use to see
the SAME DETAIL or RESOLUTION (sorry for the
capitalization as it is to emphasize it). It has to do with
field of view as one gets nearer the object but what is the
exact formula to calculate it??

This principle would finally make me decide whether
to get a 4" maksutov or retain 70mm for long distance
microscopic work such as viewing ant or bee colonies.

teni

It is simply linear.
A 'rule of thumb'. At 100yards, 1", is 1 minute of arc.
If you had a 'view' of something at 10 yards away, that is 1" across,
it is 10 minutes of arc, 'naked eye'. If you use 40* magnification,
you will 'see' it as covering 400 minutes of arc. If you instead move
it to 100yards away, you will have to use 400* magnification, to see
it as the same size.
Now, there are caveats on this, because of the diffraction effects and
atmosphere, the object will _not_ show as much detail at 100 yards
away, and 400* magnification, as it would at 1/10th the distance, and
1/10th the magnification, but it's apparent size, _would_ remain the
same.


So it's proportional. If the 70mm f/8 refractor at 6 meters from the
object at 40X magnification is put 3 meters from the object,
the magnfication simply needs to be 20X see the same detail.
Agree?

Going back to the 4" maksutov vs 70mm refractor in long
distance microscopy. This means the maksotuv at 4 feet
(nearest that can focus from the 1 meter focal length)
would produce twice the magnificaiton possible in the
70mm refractor at 8 feet away. Therefore the 4"
maksutov wins in long distance microscopy because
you also get a brigher image because you are using
the mirror to focus and not extending it to the eyepiece
end.

But a complication. I wonder what's the true magnification
in the mak (or SCT) since one is moving the primary mirror.
So what one thinks as a magnificaiton of 50X in the Mak
is not really 50X because the focal ratio changes and
so the value in the (aperture x focal ratio) / eyepiece
= magnification and I wonder how to calculate for it
(put in a separate thread...)

Teni



Best Wishes- Hide quoted text -

- Show quoted text -


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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. ...
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  • 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)