Re: close focus formula

On Nov 8, 11:35 pm, Rich <rander3...@xxxxxxxxx> wrote:
On Nov 7, 9:31 am, Tenifer <tensorsur...@xxxxxxxxx> wrote:





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

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

is the maksotuv able to focus at one feet by just moving the
back focus a little bit like an inch??

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

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

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.- Hide quoted text -

- Show quoted text -

1. . know where I could get a 5 inch extension tube (2" diameter)?
Televue is selling a 3.5" extension but I need at least 5 inches
for my 70mm refractor.

2. I read that when you focus closer than the
focal length, the refractor would become a magnifier
glass. But it doesn't appear to work. Any clue why.

3. Also when a refractor is said to be f/8. It means one can say
remove
the lens and reverse it and put it back to the cell.. meaning both
sides is f/8?

4. I understood an astronomical scope is said to work at
infinity target.. so when the target is focus closer,
there is more spherical aberrations, how much is the abberations
(like 1/8 wave at infinity turning to 1/4 wave at close focus)?

5 .Lastly. Using eyepiece extension, the image distance is said to be
longer
and the view dimmer. This means the exit pupil coming from the
end is thinner for the same eyepiece at normal parallel infinity
target imaging?

Dwight? anyone? Thanks.

Teni

.

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