looking for another optical system

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soldevilla
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Location: Barcelona, more or less

looking for another optical system

Post by soldevilla »

For a long time, I'm thinking of telecentric lenses. I have not really learned their way of working, but I'm sure as telecentric lenses allow to work with great working distance, the images obtained have fewer artifacts unfocused in the overlapping edges.
So I built a telescope (I've already talked about this before in the forum) from an achromatic pair and a x4 microscope objective. If I put the two lenses to the sum of their focal lengths, I get a telescope focused at infinity, but if I separate the two lenses I get to focus closer.

Of course the quality is horrible now, is a first test. But I've got a field of 3mm (x6 approx.) With a lot of working distance, which is what I wanted to try. The distance between lens and sample in the image is the real working distance!

My vertical photographic system does not provide much the tests. I go continue testing, but if anyone is encouraged to work on this line I would like to hear something about it.

Image

Image

Charles Krebs
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Post by Charles Krebs »

What is your working distance with the 4X in this set-up?

Without looking at the details of your optical arrangement, or even considering that altering the designed working distance can cause optical difficulties, it seems like (on a very simple level) by greatly increasing the working distance you are greatly reducing the numerical aperture. That alone would cause a considerable decrease in resolution.

soldevilla
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Joined: Thu Dec 16, 2010 2:49 pm
Location: Barcelona, more or less

Post by soldevilla »

My setup is very simple. The microscope objective is positioned with extension rings in front of the camera (that's my usual setup). But now I have a positive lens placed well ahead of the microscope objective, in order to form image just in the focusing plane of the microscope objective. I do not think I'm now working on a very small numerical aperture because the exposure time is almost equal to that use in my normal setup.

Here's something interesting: The element that determines the working distance is the front achromatic lens. The microscope objective single "focuses" the image formed by the achromatic. This means that the assembly is virtually parfocal changing the microscope objective. I have found that I can get focus with a x40 normal (no LWD!) with already the same working distance with a 4x. depending on the distance between the microscope objective lens and the front, approximately 80mm.

The image quality is still bad ... but I feel that in a problem finding the correct optical components.

rjlittlefield
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Post by rjlittlefield »

At most, the NA of the combination will be the minimum of the front lens and the objective. Your front lens may be wide enough for its NA to be equal to the 4X objective, but it's very unlikely to be wide enough to equal the 40X. In any event, you'll have front lens aberrations to worry about, and those scale badly with increasing focal length. The result is that the image being seen by the objective will be much less sharp than the real subject.

Consider: if there were a simple way to get huge working distance at high magnification with good resolution, then all the microscope manufacturers would either be using it or be out of business. But neither of those is happening.

--Rik

g4lab
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Post by g4lab »

There was once a gemological rig that did something like that. It had a compound microscope tube and a turret of objectives probably a couple of low mags and a 5x and a 10x and maybe a 20x at most. There was a swing in optic that was about 15mm in diameter. I had the impression it was a negative achromat. The idea was to increase the working distance so that you could focus on gems immersed in a liquid cell filled with liquid in the 1.5 to 1.75 RI range. I am not sure if the lens was negative or positive and I never saw one of these in person. They were made in Germany and were expensive and rare.

soldevilla
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Joined: Thu Dec 16, 2010 2:49 pm
Location: Barcelona, more or less

Post by soldevilla »

Sorry for the rubbish sketch, I do not have much time now.

My scheme is based on adding a "relay lens" in front of the microscope objective. This is often done in the telescope to get the focus is accessible to some accessories.

The sketch 1 is my usual system (and I assume that the system of many fellow forum). The second sketch is ... a normal large-format camera. If the distance is printed before and behind the same, the lens offers 1x. The objective produces a real image at the focal plane.

In the third sketch I combine both sketches, examining the real image formed by the camera lens with the microscope objective. It is not very different from what we do when we place an eyepiece in the microscope or telescope.

I am sure that the system works, but we must find the right elements. The marketing problem is that the size is very large, for about 100 mm. working distance the system will measure more than 350mm. long.

I have to keep trying.

Image

g4lab
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Joined: Fri May 23, 2008 11:07 am

Post by g4lab »

Raynox has a newer macro system the 5320 which is a two and three diopter achromatic system which can be put in series to give five diopters.

They are 72mm in diameter and sell on fleabay for $285 the set.

http://raynox.co.jp/english/dcr/dcr5320pro/index.htm

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