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So - you want a splendid holiday microscope? (I)

Well, there are those fascinating, tiny James Bond style field microscopes like the Chinese TWX-1. But, soberly speaking, most of us do not ride on camel backs etc. in extreme expeditions, so only very few will actually need this kind of minimalistic instrument when travelling. Things might be different when moving lightly, for example by airplane. But when travelling by car, a bigger microscope will be more comfortable to use.

[ Microscopic shadow equipment ]

These are the shadows of our microscopic instruments on the wall of a hotel room in Croatia, in autumn 2014.

In this issue we will consider the optical options which we encountered when deciding about the purchase of our own bigger holiday microscopes. From the technical point of view, even a big instrument might still be a compromise. Achromatic microscope objectives will do for most purposes, but some among you possibly will prefer semi-achromatic or plan-achromatic systems. And one should keep in mind that the best corrected microscope objectives like fluorites or apos can be terribly expensive - and a damage or theft might turn out as a perfect spoiler for your holiday.

So we are going to investigate the practical differences with respect to our tardigrade research. As a first example please have a look at a comparison of a perfectly corrected apochromatic objective with a much cheaper so-called "semi-plan" achromatic objective. The object is a micrometer scale, far off from reality but good for a perfectly strict technical comparison:

[ Comparison of 10x microscope objectives ]

Left: Photomicrograph of a micrometer scale, photographed by means of a 10x apochromatic microscope objective (LOMO 10x/N.A. 0.30).
Right: Photomicrograph of the same micrometer scale, taken by means of a "No name" semi-plan microscope object in the 40 $ price range (10x/N.A. 0,25).
Comment: We are showing just a central portion of of the actual field (with a width of 0.3 mm cropped out of a 1.5 mm width). For our tardigrade studies a good central image quality is of utmost importance, the edges are of no concern. Please note that things might be very different when looking at flat cross sections with vast areas where you will need crisp sharpness "from left ear to right ear". Our comparison reveals only a marginal difference, mainly in color correction. With your nose very close to the screen you might note that the apo objective is actually delivering a cleaner black and white at the edges and curves of the black lines, whereas the achromatic counterpart is showing blue edge effects.

These are the portraits of the respective microscope objectives:

[ 10x microscope objectives with different degree of colour correction ]

Left: Apochromatic microscope objective (LOMO 10x/N.A. 0.30).
Right: "No name" semi-plan microscope objective in the 40$ price region (10x/N.A. 0.25).

So it doesn't make a big difference? Every objective will do? Well, not quite. Just forget our stupid micrometer scale and have a look at the tardigrade reality. For exampe tardigrade eggs can turn out as really frustrating, with poor contrast and poor detail. Our 10fold objectives, no matter whether achromatic or apochromatic, will not be sufficient. So we move on one step further to better resolution (better numerical apertures and higher magnification):

[ objective comparisons, N.A. 0.65 ]

Egg of a Macrobiotus sp. tardigrade. Permanent slide. The egg diameter without the protrusions measures 81 µm, with protrusions 90 µm.
Left: Image taken by means of an apochromatic objective (LOMO 20x/N.A. 0.65), detail view with ca. 0.1 mm image width cropped out from a 0.8 mm field.
Right: Image taken by means of a "No name" semi-plan achromatic objective (40x/N.A. 0.65), detail view with ca. 0.1 mm image width cropped out from a 0.4 mm field.

Well, possibly the apo objective was not perfectly focused. In any case the overall image impression is almost equivalent. Some difference in the evenness of the illumination is due to the different illumination quality of the respective stands used.

But now, at home, we might switch to a special optical weapon, e.g. the 40x dry apochromatic LOMO 40/0.95 objective (which has no achromatic sibling):

[ For comparison: image taken by means of 40x apo objective  ]

Image of the same Macrobiotus sp. egg taken by means of a higher resolving apochromatic objective (LOMO 40x/N.A. 0.95). Please note that a 0.95 achromatic counterpart doesn't exist. The image quality is tremendously better than with the 40x/N.A. 0.65 above. Of course e.g. a relatively cheap 60x/0.85 achromatic objective might perform in a similar manner but it will not be easily available for all those modest import microscopes.

And this is a portrait ot the respective apochromatic objective:

[LOMO apochromatic microscope objective (40x/N.A. 0.95)]

LOMO apochromatic microscope objective (40x/N.A. 0.95), Dry objectives of this high aperture as a rule will hava a correcting collar in order to compensate for cover glass thickeness variations. As a consequence they might be trickier to handle. Furthermore the working distance is smaller than in the case of an achromatic objective.

To sum up we can state that even modestly priced optics will perform well for most visual and even most photographic purposes. E.g. many water droplet scenarios can be studied and photographed with a reasonable ease and quality.

But for a few more difficult subjects, e.g. tardigrade eggs, an instrument with achromatic optics might possibly not quite hit the spot. So in some cases your budget could decide upon whether you will be able to adequately study an object with appropriate detail or not.

In the next issue we will look at a complete instrument with bargain optics and discuss its potential and limitations.

© Text, images and video clips by  Martin Mach  (webmaster@baertierchen.de).
Water Bear web base is a licensed and revised version of the German language monthly magazine  Bärtierchen-Journal . Style and grammar amendments by native speakers are warmly welcomed.

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