Re: close focus formula

On Nov 8, 6:43 pm, Tenifer <tensorsur...@xxxxxxxxx> wrote:
On Nov 9, 7:11 am, Rich <rander3...@xxxxxxxxx> wrote:



On Nov 8, 3:50 pm, Tenifer <tensorsur...@xxxxxxxxx> wrote:

On Nov 9, 4:25 am, Rich <rander3...@xxxxxxxxx> wrote:

On Nov 8, 11:05 am, Tenifer <tensorsur...@xxxxxxxxx> wrote:

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.

I'd got to a local machine shop and have it made. You'll likely pay
less if any astronomical equipment dealer has one.

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What kind of tube did you use with the above possibility wherein
a smaller 1.8" can be inserted to the scope 2" focuser?

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?

Yes, that won't change, the lens still behaves as a positive f8 lens,
but it won't work very well.

What do you mean it won't work very well. So if I reverse my lens,
what aberrations would be introduced?

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)?

Well, Questar (Maksutovs) used to supply a ring to stop down the 3.5"
Mak to around 80mm when it was used at distances closer than 100ft,
but honestly, they were too picky, the degradation in image quality
wasn't that pronounced even using it close in. I'm not sure how much
the aberration would be.

I'm also picky. I want to know the exact comparison between my
current 70mm f/8 flourite refractor at a distance of 2 meters versus
a russian 4" maksutov (stil planning to get) at distance of 1 meter.
My 70mm apo is 1/7 wave undercorrected. If the 4" maksutov is
1/4 wave, what would produce better image when the maksotuv
is 1 meter away from target while the 70mm flourite 2 meters away
from target. Any idea? The scope would be used to analyzed wall
latex painting texture and bugs at zoo in addition to astronomy which
my cloudy skies won't permit a lot of times.

I personally do not trust the optics on those Russian Maks. Some are
good (Intes) some are bad. A reducer-compressor lens used on the
refractor would likely provide a better image.

Supposed I used a 15mm Plossl in the 70mm F/8 refractor with
magnification of 560/15=37.33X. What would a reducer-compressor
lens do to the image supposed the target is 2 meters away from
the objective?

But a better solution would be a digital camera and a 180mm macro
lens, unless you need extreme magnification.
If you're really picky, Questar makes long distance microscopes based
on their Maks to image from ranges like 10cm to 200cm.
About $8000.

10cm is about 4". Can their 3.5 Questar focuses that close?
This means the back focus is moved about a foot or so?
And why the insanely $8000?


No, the regular 3.5" Questar can focus to about 10 ft. They make
specialized long distance microscopes using their Maksutovs that can
focus within a narrrow close range. They are used in hazardous places
where regular microscopes can't be used.




This image was done at about 1.8 meters with a 66mm apo refractor
388mm f.l. with a 6" extension. I reduced it by half in software.http://www.pbase.com/andersonrm/image/88648575

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?

It would be proportional to the distance from the objective. So, if
exit pupil was 5mm at infinity and the focal length was (500mm) then
using it to image closer object would mean the focal point moves
further from the back of the lens and the exit pupil would be
proportional to distance. Therefore if the new focus point was 1000mm
from the back of the lens, the exit pupil would be 2.5mm.
I think...- Hide quoted text -

What aberrations would be produced by this unique situation
where normally 5mm exit pupils become 2.5mm at further back
focus? Coma? Astigmatism? What theoretical aberrations
would be introduced?

Teni

Spherical aberration, with the apo, lack of a flat field might be an
issue. But it's possible you might see no image degradation. When a
barlow lens is used (which effectively shrinks the exit pupil and
moves it further back in the optical train, it sometimes improves
images, more than just using a more powerful positive (eyepiece) lens.- Hide quoted text -

If I'd use a barlow to the already 5" Extension tube, the scope would
become
very long and hard to balance. So you mean I'd get a big eyepiece like
32mm and put it in barlow to the end of the 5" extension tube... hmm..
wouldn't this look awkward.

I don't see how you'll get around using a long extension if you want
to focus close with your scope, unless you find some kind of custom
lens that will allow a close focus.


In your close up experiments. What magnification have you used the
highest in say 1.8 meters with still good contrast in your 66mm
refractor?

80x or so.


About the 4" russian mak. The following review says that even in
maximum
bright target, no glare would be detected when a ETX-90 would be
flooded
with those glare. So the design of the 4" maksutov seems to be such
that coatings are not even needed. I wonder if the meade 7" is also
such anti-glare. What do you think...

I find some Cloudynights reviews to be more wishful thinking of people
who paid "X" dollars for a scope and don't want to admit flaws it
might have because they think it would reflect badly on their personal
choices. I've used all the ETXs and had a 105mm ETX that had better
contrast and resolution (by far) than a 78mm Takahashi apo I also
owned. IMO, it was almost as good as a 102mm Takahashi and had purer
colour rendering too. But I can't vouch for it's performance as a
long distance microscope since I never used one like that. I suppose
it is possible the Russian Mak is well-baffled against glare, but it
could also be cause for strong vignetting too. The only way to be
sure is to try it.



.

Relevant Pages

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