1. ## 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. ## 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. ## 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. ## 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. ## 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. ## 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. ## 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. ## 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

Alvan Clark

9. ## 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. ## 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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